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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Net income attributable to Northrim BanCorp, Inc.	$ 4,580	$ 7,538	$ 4,261	$ 4,312	$ 4,848	$ 5,264	$ 5,830	$ 4,062	$ 20,691	$ 20,004	$ 13,151
Basic weighted average common shares outstanding (in shares)									6,708,622	6,877,573	6,889,621
Dilutive effect of potential common shares from awards granted under equity incentive program (in shares)									99,000	104,000	88,000
Total (in shares)									6,808,209	6,981,557	6,977,910
Earnings per common share
Basic (in USD per share)	$ 0.70	$ 1.13	$ 0.62	$ 0.63	$ 0.70	$ 0.77	$ 0.85	$ 0.59	$ 3.08	$ 2.91	$ 1.91
Diluted (in USD per share)	$ 0.69	$ 1.11	$ 0.62	$ 0.62	$ 0.69	$ 0.75	$ 0.84	$ 0.58	$ 3.04	$ 2.86	$ 1.88