Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Non-controlling Interest
Beginning balance (in shares) at Dec. 31, 2016		6,898,000
Beginning balance, value at Dec. 31, 2016	$ 186,712	$ 6,898	$ 62,952	$ 117,141	$ (397)	$ 118
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividend declared	(5,970)			(5,970)
Stock-based compensation expense	665		665
Exercise of stock options and vesting of restricted stock units, net (in shares)		32,000
Exercise of stock options and vesting of restricted stock units, net	(243)	$ 32	(275)
Treasury stock buy-back (in shares)		(58,000)
Treasury stock buy-back	(1,607)	$ (58)	(1,549)
Distributions to noncontrolling interest	(445)					(445)
Other comprehensive income (loss), net of tax	212				212
Reclassification for remeasuring of deferred tax assets related to investment securities	0			85	(85)
Net income attributable to the noncontrolling interest	327					327
Net income attributable to Northrim BanCorp, Inc.	$ 13,151			13,151
Ending balance (in shares) at Dec. 31, 2017	6,871,963	6,872,000
Ending balance, value at Dec. 31, 2017	$ 192,802	$ 6,872	61,793	124,407	(270)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle | Accounting Standards Update 2017-08	(62)			(62)
Cumulative effect of new accounting principle | Accounting Standards Update 2016-01	0			191	(191)
Cash dividend declared	(1,664)			(1,664)
Stock-based compensation expense	253		253
Other comprehensive income (loss), net of tax	(418)				(418)
Net income attributable to the noncontrolling interest	0
Net income attributable to Northrim BanCorp, Inc.	$ 4,062			4,062
Ending balance (in shares) at Mar. 31, 2018	6,871,963	6,872,000
Ending balance, value at Mar. 31, 2018	$ 194,973	$ 6,872	$ 62,046	126,934	$ (879)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle | Accounting Standards Update 2017-08				$ (62)