Segment Information - Schedule of Segment Information (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) segment bank	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment	2
Interest income	$ 14,795	$ 14,465
Interest expense	532	632
Net Interest Income	14,263	13,833
Provision for loan losses	0	400
Other operating income	7,462	8,905
Compensation expense - RML acquisition payments	0	174
Other operating expense	16,795	16,441
Income Before Provision for Income Taxes	4,930	5,723
Provision for income taxes	868	1,801
Net Income	4,062	3,922
Less: Net income attributable to the noncontrolling interest	0	97	$ 327
Net Income Attributable to Northrim BanCorp, Inc.	4,062	3,825	13,151
Total assets	1,524,741		1,518,596
Loans held for sale	$ 41,216		43,979
Community Banking
Segment Reporting Information [Line Items]
Number of branches | bank	14
Interest income	$ 14,463	14,115
Interest expense	427	566
Net Interest Income	14,036	13,549
Provision for loan losses	0	400
Other operating income	2,518	3,455
Compensation expense - RML acquisition payments	0	174
Other operating expense	12,394	11,622
Income Before Provision for Income Taxes	4,160	4,808
Provision for income taxes	659	1,422
Net Income		3,386
Less: Net income attributable to the noncontrolling interest		97
Net Income Attributable to Northrim BanCorp, Inc.	3,501	3,289
Total assets	1,460,720		1,452,602
Loans held for sale	0		0
Home Mortgage Lending
Segment Reporting Information [Line Items]
Interest income	332	350
Interest expense	105	66
Net Interest Income	227	284
Provision for loan losses	0	0
Other operating income	4,944	5,450
Compensation expense - RML acquisition payments	0	0
Other operating expense	4,401	4,819
Income Before Provision for Income Taxes	770	915
Provision for income taxes	209	379
Net Income		536
Less: Net income attributable to the noncontrolling interest		0
Net Income Attributable to Northrim BanCorp, Inc.	561	$ 536
Total assets	64,021		65,994
Loans held for sale	$ 41,216		$ 43,979