Segment Information - Schedule of Segment Information (Details) - USD ($) $ 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
Segment Reporting Information [Line Items]
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
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 (benefit) 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
Total assets	1,519,109				1,526,540				1,519,109	1,526,540	1,499,492
Loans held for sale	43,778				43,596				43,778	43,596	50,553
Community Banking
Segment Reporting Information [Line Items]
Total interest income									58,308	56,986	57,729
Interest expense									1,860	1,739	1,796
Net Interest Income									56,448	55,247	55,933
Provision (benefit) for loan losses									3,200	2,298	1,754
Other operating income									17,187	13,756	14,995
Compensation expense - RML acquisition payments									130	4,775	4,094
Other operating expense									50,271	48,610	47,070
Income Before Provision for Income Taxes									20,034	13,320	18,010
Provision for income taxes									9,499	2,867	5,024
Net Income									10,535	10,453	12,986
Less: Net income attributable to the noncontrolling interest									327	579	551
Net Income Attributable to Northrim BanCorp, Inc.									10,208	9,874	12,435
Total assets	1,453,115				1,459,950				1,453,115	1,459,950	1,431,759
Loans held for sale	0				0				0	0	0
Home Mortgage Lending
Segment Reporting Information [Line Items]
Total interest income									1,800	1,932	2,051
Interest expense									570	822	1,075
Net Interest Income									1,230	1,110	976
Provision (benefit) for loan losses									0	0	0
Other operating income									23,287	29,507	29,613
Compensation expense - RML acquisition payments									0	0	0
Other operating expense									20,752	22,895	21,481
Income Before Provision for Income Taxes									3,765	7,722	9,108
Provision for income taxes									822	3,185	3,760
Net Income									2,943	4,537	5,348
Less: Net income attributable to the noncontrolling interest									0	0	0
Net Income Attributable to Northrim BanCorp, Inc.									2,943	4,537	5,348
Total assets	65,994				66,590				65,994	66,590	67,733
Loans held for sale	$ 43,778				$ 43,596				$ 43,778	$ 43,596	$ 50,553