Summary of Significant Accounting Policies - Narrative (Details)	3 Months Ended								12 Months Ended			117 Months Ended	123 Months Ended	213 Months Ended
	Dec. 31, 2019 USD ($) trust borrower	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment trust borrower shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Aug. 31, 2017	Dec. 31, 2008 USD ($)
Accounting Policy [Line Items]
Number of wholly-owned Trusts | trust	1								1
Number of operating segments | segment									2
Maturity period of securities at acquisition									90 days
Threshold for nonperforming loans individually evaluated for impairment									$ 50,000
Loan loss history used									5 years
Average holding of purchased receivables									1 year
Advertising, promotion and marketing expenses									$ 2,373,000	$ 2,318,000	$ 2,566,000
Anti-dilutive shares (in shares) | shares									0	44,721	0
Commercial and construction loans	$ 513,300,000				$ 451,800,000				$ 513,300,000	$ 451,800,000			$ 451,800,000	$ 451,800,000
Large borrowing relationships | borrower	31								31
Unfunded commitments to large borrowers	$ 137,400,000								$ 137,400,000
Commercial servicing revenue									624,000	1,422,000	$ 372,000
Assets	1,643,996,000				1,502,988,000				1,643,996,000	1,502,988,000	1,518,596,000		1,502,988,000	1,502,988,000
Net income	4,580,000	$ 7,538,000	$ 4,261,000	$ 4,312,000	4,848,000	$ 5,264,000	$ 5,830,000	$ 4,062,000	20,691,000	20,004,000	13,151,000
Total premises and equipment	62,281,000				59,948,000				62,281,000	59,948,000			59,948,000	59,948,000
Depreciation									3,000,000	2,300,000	$ 2,900,000
Operating lease right-of-use asset	14,306,000								14,306,000
Operating lease liability	14,229,000								$ 14,229,000
Revenue Recognized From Commercial Servicing Activities
Accounting Policy [Line Items]
Commercial servicing revenue										1,000,000
Assets					737,000					737,000			737,000	737,000
Net income														737,000
Misclassification
Accounting Policy [Line Items]
Assets					480,000					480,000			480,000	480,000
Net income													480,000
Depreciation												$ 670,000
Lender Concentration Risk
Accounting Policy [Line Items]
Concentration risk, percent									40.00%
Land
Accounting Policy [Line Items]
Total premises and equipment	5,137,000				5,137,000				$ 5,137,000	5,137,000			5,137,000	5,137,000
Land | Misclassification
Accounting Policy [Line Items]
Total premises and equipment																	$ 2,600,000
Furniture and equipment
Accounting Policy [Line Items]
Total premises and equipment	11,778,000				10,917,000				11,778,000	10,917,000			10,917,000	10,917,000
Leasehold improvements
Accounting Policy [Line Items]
Total premises and equipment	9,161,000				8,268,000				$ 9,161,000	8,268,000			8,268,000	8,268,000
Buildings
Accounting Policy [Line Items]
Premises and equipment, useful life									39 years
Total premises and equipment	$ 36,205,000				$ 35,626,000				$ 36,205,000	$ 35,626,000			$ 35,626,000	$ 35,626,000
Buildings | Misclassification
Accounting Policy [Line Items]
Total premises and equipment																	$ (2,600,000)
Minimum | Furniture and equipment
Accounting Policy [Line Items]
Premises and equipment, useful life									3 years
Minimum | Leasehold improvements
Accounting Policy [Line Items]
Premises and equipment, useful life									2 years
Maximum | Furniture and equipment
Accounting Policy [Line Items]
Premises and equipment, useful life									7 years
Maximum | Leasehold improvements
Accounting Policy [Line Items]
Premises and equipment, useful life									15 years
Northrim Benefits Group
Accounting Policy [Line Items]
Equity interest percentage																50.10%
Accounting Standards Update 2016-02
Accounting Policy [Line Items]
Operating lease right-of-use asset															$ 15,900,000
Operating lease liability															$ 15,900,000