Line of credit facility, date of last required payment.

No definition available.

Name:	kpti_LineOfCreditFacilityDateOfLastRequiredPayment
Namespace Prefix:	kpti_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum liquidity covenant amount.

+ References

No definition available.

+ Details

Name:	kpti_MinimumLiquidityCovenantAmount
Namespace Prefix:	kpti_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of issuance of indebtedness for borrowed money or capital stock.

+ References

No definition available.

+ Details

Name:	kpti_PercentageOfIssuanceOfIndebtednessForBorrowedMoneyOrCapitalStock
Namespace Prefix:	kpti_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the first payment under the line of credit must be paid, in YYYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_LineOfCreditFacilityDateOfFirstRequiredPayment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the frequency of the required periodic payments of interest, principal, or both, and the amount, if set, or a description of a formula upon which payment is based.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityFrequencyOfPaymentAndPaymentTerms
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-14

+ Details

Name:	us-gaap_ProceedsFromDebtNetOfIssuanceCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-14

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481674/830-30-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483399/855-10-50-2

+ Details

Name:	us-gaap_SubsequentEventLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=kpti_SecuredOvernightFinancingRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=kpti_AmendedCreditAgreementAndAmendedTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=kpti_AmendedTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Subsequent Event - Senior Secured Term Loan - Additional Information (Detail) - USD ($) $ in Millions		1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Oct. 11, 2026	Oct. 31, 2025	Sep. 30, 2025	Sep. 30, 2025	Oct. 10, 2026
Scenario, Forecast [Member]
Subsequent Event [Line Items]
Minimum liquidity covenant amount	$ 25.0				$ 10.0
Percentage of net cash proceeds from issuance of indebtedness					50.00%
Line of Credit [Member] \| Secured Overnight Financing Rate [Member]
Subsequent Event [Line Items]
Rate margin			10.25%	9.25%
Line of Credit [Member] \| Base Rate [Member]
Subsequent Event [Line Items]
Rate margin			3.00%	3.00%
Subsequent Event [Member] \| Amended Credit Agreement and Amended Term Loan [Member] \| Line of Credit [Member]
Subsequent Event [Line Items]
Line of credit facility		$ 112.5
Subsequent Event [Member] \| Amended Term Loan [Member]
Subsequent Event [Line Items]
Additional borrowings		12.5
Received net cash proceeds		$ 12.3
Interest, payment terms		Interest on all borrowings under the Amended Term Loan incurred from July 1, 2025 to March 31, 2026 will be paid in-kind. Interest incurred after March 31, 2026 will be paid in cash on the applicable payment date.
Date of first required payment		Jul. 01, 2025
Date of last required payment		Mar. 31, 2026