Regulatory Matters - Summary of Capital Requirements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Northrim Bancorp, Inc.
Tier One Common Equity Based Capital [Abstract]
Common equity tier 1 capital (to risk-weighted assets), actual amount	$ 177,005	$ 171,190
Common equity tier 1 capital (to risk-weighted assets), actual ratio	13.89%	13.20%
Common equity tier 1 capital (to risk-weighted assets), adequately-capitalized amount	$ 57,345	$ 58,360
Common equity tier 1 capital (to risk-weighted assets), adequately-capitalized ratio	4.50%	4.50%
Capital [Abstract]
Total Capital (to risk-weighted assets), actual amount	$ 202,638	$ 204,927
Total Capital (to risk-weighted assets), actual ratio	15.90%	15.80%
Total Capital (to risk-weighted assets), adequately-capitalized amount	$ 101,956	$ 103,761
Total Capital (to risk-weighted assets), adequately-capitalized ratio	8.00%	8.00%
Tier One Risk Based Capital [Abstract]
Tier 1 Capital (to risk-weighted assets), actual amount	$ 186,637	$ 188,658
Tier 1 Capital (to risk-weighted assets), actual ratio	14.65%	14.54%
Tier 1 Capital (to risk-weighted assets), adequately-capitalized amount	$ 76,438	$ 77,851
Tier 1 capital (to risk-weighted assets), adequately capitalized ratio	6.00%	6.00%
Tier One Leverage Capital [Abstract]
Tier I Capital (to average assets), actual amount	$ 186,637	$ 188,658
Tier I Capital (to average assets), actual ratio	12.41%	12.59%
Tier I Capital (to average assets), adequately-capitalized amount	$ 60,157	$ 59,939
Tier 1 capital (to average assets), adequately-capitalized ratio	4.00%	4.00%
Northrim Bank
Tier One Common Equity Based Capital [Abstract]
Common equity tier 1 capital (to risk-weighted assets), actual amount	$ 161,996	$ 168,529
Common equity tier 1 capital (to risk-weighted assets), actual ratio	12.83%	13.07%
Common equity tier 1 capital (to risk-weighted assets), adequately-capitalized amount	$ 56,819	$ 58,025
Common equity tier 1 capital (to risk-weighted assets), adequately-capitalized ratio	4.50%	4.50%
Common equity tier 1 capital (to risk-weighted assets), well-capitalized amount	$ 82,071	$ 83,813
Common equity tier 1 capital (to risk-weighted assets), well-capitalized ratio	6.50%	6.50%
Capital [Abstract]
Total Capital (to risk-weighted assets), actual amount	$ 177,857	$ 184,695
Total Capital (to risk-weighted assets), actual ratio	14.08%	14.33%
Total Capital (to risk-weighted assets), adequately-capitalized amount	$ 101,055	$ 103,110
Total Capital (to risk-weighted assets), adequately-capitalized ratio	8.00%	8.00%
Total Capital (to risk-weighted assets), well-capitalized amount	$ 126,319	$ 128,887
Total Capital (to risk-weighted assets), well-capitalized ratio	10.00%	10.00%
Tier One Risk Based Capital [Abstract]
Tier 1 Capital (to risk-weighted assets), actual amount	$ 161,996	$ 168,529
Tier 1 Capital (to risk-weighted assets), actual ratio	12.83%	13.07%
Tier 1 Capital (to risk-weighted assets), adequately-capitalized amount	$ 75,758	$ 77,366
Tier 1 capital (to risk-weighted assets), adequately capitalized ratio	6.00%	6.00%
Tier 1 Capital (to risk-weighted assets), well-capitalized amount	$ 101,011	$ 103,155
Tier 1 Capital (to risk-weighted assets), well-capitalized ratio	8.00%	8.00%
Tier One Leverage Capital [Abstract]
Tier I Capital (to average assets), actual amount	$ 161,996	$ 168,529
Tier I Capital (to average assets), actual ratio	10.87%	11.30%
Tier I Capital (to average assets), adequately-capitalized amount	$ 59,612	$ 59,656
Tier 1 capital (to average assets), adequately-capitalized ratio	4.00%	4.00%
Tier I Capital (to average assets), well-capitalized amount	$ 74,515	$ 74,570
Tier I Capital (to average assets), well-capitalized ratio	5.00%	5.00%