Significant accounting policies - Financial instruments (Details)	12 Months Ended
Dec. 31, 2025 item
Significant accounting policies [Abstract]
Number of scenarios prepared for each PD curve	3
Number of credit conversion factors	2

The number of scenarios prepared for each PD curve

No definition available.

Name:	sek_NumberOfScenariosPreparedForEachPdCurve
Namespace Prefix:	sek_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration