Derivatives (Details) - SEK (kr) kr in Millions	Jun. 30, 2026	Dec. 31, 2025
Disclosure of detailed information about financial instruments [line items]
Assets Fair value	kr 7,208	kr 6,721
Liabilities Fair value	3,816	8,988
Derivatives
Disclosure of detailed information about financial instruments [line items]
Nominal amounts	684,211	650,807
Interest rate-related contracts
Disclosure of detailed information about financial instruments [line items]
Assets Fair value	2,784	4,624
Liabilities Fair value	1,310	1,332
Interest rate-related contracts \| Derivatives
Disclosure of detailed information about financial instruments [line items]
Nominal amounts	533,585	510,865
Currency-related contracts
Disclosure of detailed information about financial instruments [line items]
Assets Fair value	4,423	2,097
Liabilities Fair value	2,392	7,550
Currency-related contracts \| Derivatives
Disclosure of detailed information about financial instruments [line items]
Nominal amounts	147,643	137,052
Contracts related to commodities, credit risk, etc.
Disclosure of detailed information about financial instruments [line items]
Assets Fair value	0	0
Liabilities Fair value	114	105
Contracts related to commodities, credit risk, etc. \| Derivatives
Disclosure of detailed information about financial instruments [line items]
Nominal amounts	kr 2,983	kr 2,890

The amount of financial assets classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 55
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_55&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_DerivativeFinancialAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of financial liabilities classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

+ References

+ Details

Name:	ifrs-full_DerivativeFinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 112
-Subparagraph c
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_112_c&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_NotionalAmount
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=ifrs-full_DerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=sek_InterestRateRelatedContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=sek_CurrencyRelatedContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=sek_CommoditiesRiskCreditRiskAndOtherRiskContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: