<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>550</ContextCount>
  <ElementCount>240</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>132</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>9952151 - Statement - Condensed Consolidated Statement of Comprehensive Income (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CondensedConsolidatedStatementofComprehensiveIncomeunaudited</Role>
      <ShortName>Condensed Consolidated Statement of Comprehensive Income (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952152 - Statement - Condensed Consolidated Statement of Comprehensive Income (unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CondensedConsolidatedStatementofComprehensiveIncomeunauditedParenthetical</Role>
      <ShortName>Condensed Consolidated Statement of Comprehensive Income (unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Statement of Financial Position (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ConsolidatedStatementofFinancialPositionunaudited</Role>
      <ShortName>Consolidated Statement of Financial Position (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - Condensed Consolidated Statement of Changes in Equity (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CondensedConsolidatedStatementofChangesinEquityunaudited</Role>
      <ShortName>Condensed Consolidated Statement of Changes in Equity (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Statement - Condensed Statement of Cash Flows in the Consolidated Group (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CondensedStatementofCashFlowsintheConsolidatedGroupunaudited</Role>
      <ShortName>Condensed Statement of Cash Flows in the Consolidated Group (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952156 - Disclosure - Accounting policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Accountingpolicies</Role>
      <ShortName>Accounting policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952157 - Disclosure - Net interest income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Netinterestincome</Role>
      <ShortName>Net interest income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952158 - Disclosure - Net results of financial transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Netresultsoffinancialtransactions</Role>
      <ShortName>Net results of financial transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - Impairments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Impairments</Role>
      <ShortName>Impairments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Financial assets and liabilities at fair value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Financialassetsandliabilitiesatfairvalue</Role>
      <ShortName>Financial assets and liabilities at fair value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Derivatives</Role>
      <ShortName>Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - CIRR-system</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CIRRsystem</Role>
      <ShortName>CIRR-system</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Pledged assets and contingent liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Pledgedassetsandcontingentliabilities</Role>
      <ShortName>Pledged assets and contingent liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Capital adequacy and liquidity situation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Capitaladequacyandliquiditysituation</Role>
      <ShortName>Capital adequacy and liquidity situation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Exposures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Exposures</Role>
      <ShortName>Exposures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Transactions with related parties</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Transactionswithrelatedparties</Role>
      <ShortName>Transactions with related parties</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Events after the reporting period</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/Eventsafterthereportingperiod</Role>
      <ShortName>Events after the reporting period</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Accounting policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/AccountingpoliciesPolicies</Role>
      <ShortName>Accounting policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.sek.se/role/Accountingpolicies</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Net interest income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/NetinterestincomeTables</Role>
      <ShortName>Net interest income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Netinterestincome</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Net results of financial transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/NetresultsoffinancialtransactionsTables</Role>
      <ShortName>Net results of financial transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Netresultsoffinancialtransactions</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Impairments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ImpairmentsTables</Role>
      <ShortName>Impairments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Impairments</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Financial assets and liabilities at fair value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueTables</Role>
      <ShortName>Financial assets and liabilities at fair value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Financialassetsandliabilitiesatfairvalue</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/DerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Derivatives</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/DebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Debt</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - CIRR-system (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CIRRsystemTables</Role>
      <ShortName>CIRR-system (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/CIRRsystem</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Pledged assets and contingent liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/PledgedassetsandcontingentliabilitiesTables</Role>
      <ShortName>Pledged assets and contingent liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Pledgedassetsandcontingentliabilities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Capital adequacy and liquidity situation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationTables</Role>
      <ShortName>Capital adequacy and liquidity situation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Capitaladequacyandliquiditysituation</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Exposures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ExposuresTables</Role>
      <ShortName>Exposures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sek.se/role/Exposures</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Net interest income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/NetinterestincomeDetails</Role>
      <ShortName>Net interest income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sek.se/role/NetinterestincomeTables</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Net results of financial transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/NetresultsoffinancialtransactionsDetails</Role>
      <ShortName>Net results of financial transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sek.se/role/NetresultsoffinancialtransactionsTables</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Impairments - Schedule of impairments &amp; schedule of balances in loss allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ImpairmentsScheduleofimpairmentsscheduleofbalancesinlossallowanceDetails</Role>
      <ShortName>Impairments - Schedule of impairments &amp; schedule of balances in loss allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Impairments - Schedule of reconciliation of changes in loss allowances by stages (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ImpairmentsScheduleofreconciliationofchangesinlossallowancesbystagesDetails</Role>
      <ShortName>Impairments - Schedule of reconciliation of changes in loss allowances by stages (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Impairments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ImpairmentsNarrativeDetails</Role>
      <ShortName>Impairments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Financial assets and liabilities at fair value - Financial assets and liabilities at fair value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueFinancialassetsandliabilitiesatfairvalueDetails</Role>
      <ShortName>Financial assets and liabilities at fair value - Financial assets and liabilities at fair value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Financial assets and liabilities at fair value - Financial assets &amp; liabilities reported at fair value in fair value hierarchy (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueFinancialassetsliabilitiesreportedatfairvalueinfairvaluehierarchyDetails</Role>
      <ShortName>Financial assets and liabilities at fair value - Financial assets &amp; liabilities reported at fair value in fair value hierarchy (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Financial assets and liabilities at fair value - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueNarrativeDetails</Role>
      <ShortName>Financial assets and liabilities at fair value - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Financial assets and liabilities at fair value - Financial assets and liabilities at fair value in level 3 (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueFinancialassetsandliabilitiesatfairvalueinlevel3Details</Role>
      <ShortName>Financial assets and liabilities at fair value - Financial assets and liabilities at fair value in level 3 (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Financial assets and liabilities at fair value - Assets and liabilities Sensitivity analysis - level 3 (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueAssetsandliabilitiesSensitivityanalysislevel3Details</Role>
      <ShortName>Financial assets and liabilities at fair value - Assets and liabilities Sensitivity analysis - level 3 (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Financial assets and liabilities at fair value - Fair value related to credit risk (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/FinancialassetsandliabilitiesatfairvalueFairvaluerelatedtocreditriskDetails</Role>
      <ShortName>Financial assets and liabilities at fair value - Fair value related to credit risk (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/DerivativesDetails</Role>
      <ShortName>Derivatives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sek.se/role/DerivativesTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/DebtDetails</Role>
      <ShortName>Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sek.se/role/DebtTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - CIRR-system (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CIRRsystemDetails</Role>
      <ShortName>CIRR-system (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sek.se/role/CIRRsystemTables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Pledged assets and contingent liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/PledgedassetsandcontingentliabilitiesDetails</Role>
      <ShortName>Pledged assets and contingent liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sek.se/role/PledgedassetsandcontingentliabilitiesTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Capital adequacy and liquidity situation - Capital adequacy (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationCapitaladequacyDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Capital adequacy (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Capital adequacy and liquidity situation - Leverage ratio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationLeverageratioDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Leverage ratio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Capital adequacy and liquidity situation - Own funds (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationOwnfundsDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Own funds (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Capital adequacy and liquidity situation - Minimum capital requirements exclusive of buffers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationMinimumcapitalrequirementsexclusiveofbuffersDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Minimum capital requirements exclusive of buffers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Capital adequacy and liquidity situation - Operational risk and capital buffer requirements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationOperationalriskandcapitalbufferrequirementsNarrativeDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Operational risk and capital buffer requirements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Capital adequacy and liquidity situation - Liquidity coverage ratio and Net stable funding ratio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationLiquiditycoverageratioandNetstablefundingratioDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Liquidity coverage ratio and Net stable funding ratio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Capital adequacy and liquidity situation - Liquidity reserve (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/CapitaladequacyandliquiditysituationLiquidityreserveDetails</Role>
      <ShortName>Capital adequacy and liquidity situation - Liquidity reserve (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Exposures - Total net exposure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ExposuresTotalnetexposureDetails</Role>
      <ShortName>Exposures - Total net exposure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Exposures - Net exposure by region and class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ExposuresNetexposurebyregionandclassDetails</Role>
      <ShortName>Exposures - Net exposure by region and class (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="sek-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Exposures - Net exposures European countries, excluding Sweden by exposure classes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sek.se/role/ExposuresNetexposuresEuropeancountriesexcludingSwedenbyexposureclassesDetails</Role>
      <ShortName>Exposures - Net exposures European countries, excluding Sweden by exposure classes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="6-K" isDefinitelyFs="true" isIfrs="true" original="sek-20260630.htm">sek-20260630.htm</File>
    <File>sek-20260630.xsd</File>
    <File>sek-20260630_cal.xml</File>
    <File>sek-20260630_def.xml</File>
    <File>sek-20260630_lab.xml</File>
    <File>sek-20260630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>sek-20260630_g1.jpg</File>
    <File>sek-20260630_g10.gif</File>
    <File>sek-20260630_g11.gif</File>
    <File>sek-20260630_g12.gif</File>
    <File>sek-20260630_g13.gif</File>
    <File>sek-20260630_g14.gif</File>
    <File>sek-20260630_g15.jpg</File>
    <File>sek-20260630_g16.jpg</File>
    <File>sek-20260630_g2.jpg</File>
    <File>sek-20260630_g3.jpg</File>
    <File>sek-20260630_g4.gif</File>
    <File>sek-20260630_g5.gif</File>
    <File>sek-20260630_g6.gif</File>
    <File>sek-20260630_g7.gif</File>
    <File>sek-20260630_g8.jpg</File>
    <File>sek-20260630_g9.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="8">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="676">https://xbrl.ifrs.org/taxonomy/2025-03-27/ifrs-full</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
