Summary of Significant Accounting Policies - Calculation Of Earnings Per Share (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
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
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
Basic weighted average common shares outstanding (in shares)									6,889,621	6,883,663	6,859,209
Dilutive effect of potential common shares from awards granted under equity incentive program (in shares)									88,000	91,000	89,000
Total (in shares)									6,977,910	6,974,864	6,948,474
Earnings per common 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
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