Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) bank	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment bank	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment			2
Interest income	$ 16,580	$ 15,519	$ 46,970	$ 44,876
Interest expense	761	602	1,899	1,882
Net Interest Income	15,819	14,917	45,071	42,994
Benefit for loan losses	0	2,500	(300)	3,200
Other operating income	8,673	13,855	24,449	32,522
Compensation expense - RML acquisition payments	0	149	0	323
Other operating expense	18,099	17,542	51,500	52,493
Income Before Provision for Income Taxes	6,393	8,581	18,320	19,500
Provision for income taxes	1,129	2,980	3,164	6,236
Net Income	5,264	5,601	15,156	13,264
Less: Net income attributable to the noncontrolling interest	0	78	0	327	$ 327
Net Income Attributable to Northrim BanCorp, Inc.	5,264	5,523	15,156	12,937	13,151
Total assets	1,502,673		1,502,673		1,518,596
Loans held for sale	$ 56,636		$ 56,636		43,979
Community Banking
Segment Reporting Information [Line Items]
Number of stores | bank	14		14
Interest income	$ 15,923	14,953	$ 45,443	43,550
Interest expense	565	388	1,435	1,483
Net Interest Income	15,358	14,565	44,008	42,067
Benefit for loan losses	0	2,500	(300)	3,200
Other operating income	2,770	7,636	8,124	14,502
Compensation expense - RML acquisition payments		149		323
Other operating expense	12,204	12,252	36,319	37,158
Income Before Provision for Income Taxes	5,924	7,300	16,113	15,888
Provision for income taxes	996	2,452	2,537	4,745
Net Income		4,848		11,143
Less: Net income attributable to the noncontrolling interest		78		327
Net Income Attributable to Northrim BanCorp, Inc.	4,928	4,770	13,576	10,816
Total assets	1,421,407		1,421,407		1,452,602
Loans held for sale	0		0		0
Home Mortgage Lending
Segment Reporting Information [Line Items]
Interest income	657	566	1,527	1,326
Interest expense	196	214	464	399
Net Interest Income	461	352	1,063	927
Benefit for loan losses	0	0	0	0
Other operating income	5,903	6,219	16,325	18,020
Compensation expense - RML acquisition payments		0		0
Other operating expense	5,895	5,290	15,181	15,335
Income Before Provision for Income Taxes	469	1,281	2,207	3,612
Provision for income taxes	133	528	627	1,491
Net Income		753		2,121
Less: Net income attributable to the noncontrolling interest		0		0
Net Income Attributable to Northrim BanCorp, Inc.	336	$ 753	1,580	$ 2,121
Total assets	81,266		81,266		65,994
Loans held for sale	$ 56,636		$ 56,636		$ 43,979