Quarterly Results of Operations (Unaudited) (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
Quarterly Financial Data [Abstract]
Total interest income	$ 15,232	$ 15,519	$ 14,892	$ 14,465	$ 14,522	$ 14,860	$ 14,718	$ 14,818	$ 60,108	$ 58,918	$ 59,780
Total interest expense	548	602	648	632	631	647	639	644	2,430	2,561	2,871
Net Interest Income	14,684	14,917	14,244	13,833	13,891	14,213	14,079	14,174	57,678	56,357	56,909
Provision for loan losses	0	2,500	300	400	743	652	200	703	3,200	2,298	1,754
Other operating income	7,952	13,855	9,762	8,905	10,359	11,935	11,864	9,105	40,474	43,263	44,608
Compensation expense - RML acquisition payments	(193)	149	0	174	708	3,250	687	130	130	4,775	4,094
Other operating expense	18,530	17,542	18,510	16,441	17,646	17,936	18,682	17,241	71,023	71,505	68,551
Income Before Provision for Income Taxes	4,299	8,581	5,196	5,723	5,153	4,310	6,374	5,205	23,799	21,042	27,118
Provision for income taxes	4,085	2,980	1,455	1,801	1,458	1,027	1,868	1,699	10,321	6,052	8,784
Net Income	214	5,601	3,741	3,922	3,695	3,283	4,506	3,506	13,478	14,990	18,334
Less: Net income attributable to the noncontrolling interest	0	78	152	97	105	188	156	130	327	579	551
Net Income Attributable to Northrim BanCorp, Inc.	$ 214	$ 5,523	$ 3,589	$ 3,825	$ 3,590	$ 3,095	$ 4,350	$ 3,376	$ 13,151	$ 14,411	$ 17,783
Earnings per share, basic (in USD per share)	$ 0.03	$ 0.80	$ 0.52	$ 0.55	$ 0.52	$ 0.45	$ 0.63	$ 0.49	$ 1.91	$ 2.09	$ 2.59
Earnings per share, diluted (in USD per share)	$ 0.03	$ 0.79	$ 0.51	$ 0.55	$ 0.51	$ 0.44	$ 0.63	$ 0.48	$ 1.88	$ 2.06	$ 2.56