Loans and liquidity investments - Difference between book value and amount required to be paid at maturity (Details) - SEK (kr) kr in Millions	Dec. 31, 2025	Dec. 31, 2024
Loans and liquidity investments [Abstract]
Sum of amounts exceeding nominal	kr 119	kr 74
Sum of amounts falling below nominal	kr (328)	kr (640)

Difference between book value amount and amount contractually required to be paid at maturity for interest-bearing securities not carried at fair value - sum of amounts falling below nominal.

No definition available.

Name:	sek_InterestBearingSecuritiesFallingBelowNominal
Namespace Prefix:	sek_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant