Junior Subordinated Debentures (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2005	May 31, 2003
Investment in unconsolidated trust	$ 56,141				$ 56,128				$ 56,141	$ 56,128
Junior subordinated debentures	10,310				18,558				10,310	18,558
Interest expense on deposits, borrowings and junior subordinated debentures	548	$ 602	$ 648	$ 632	$ 631	$ 647	$ 639	$ 644	2,430	2,561	$ 2,871
Northrim Capital Trust 1
Investment in unconsolidated trust													$ 8,000
Junior subordinated debentures	$ 8,200								$ 8,200
LIBOR period									90 days
Basis plus LIBOR									3.15%
Liquidation preference per capital security (in USD per share)	$ 1,000								$ 1,000
Interest expense on deposits, borrowings and junior subordinated debentures									$ 219	319	289
Northrim Statutory Trust 2
Investment in unconsolidated trust												$ 10,000
Junior subordinated debentures	$ 10,300								$ 10,300
LIBOR period									90 days
Basis plus LIBOR									1.37%
Liquidation preference per capital security (in USD per share)	$ 1,000								$ 1,000
Interest rate	2.96%								2.96%
Interest expense on deposits, borrowings and junior subordinated debentures									$ 268	$ 215	$ 174