Summary of Significant Accounting Policies - Narrative (Details)		3 Months Ended								6 Months Ended	12 Months Ended			19 Months Ended
	Dec. 01, 2014 USD ($) owner	Dec. 31, 2017 USD ($) trust borrower	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment trust borrower shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Jun. 30, 2016 USD ($)
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											$ 2,566,000	$ 2,449,000	$ 2,728,000
Anti-dilutive shares (in shares) | shares											0	0	54,903
Commercial and construction loans		$ 425,100,000				$ 376,400,000				$ 376,400,000	$ 425,100,000	$ 376,400,000
Large borrowing relationships | borrower		26									26
Unfunded commitments to large borrowers		$ 99,100,000									$ 99,100,000
Goodwill		15,017,000				15,017,000				15,017,000	15,017,000	15,017,000
Compensation expense - RML acquisition payments		(193,000)	$ 149,000	$ 0	$ 174,000	708,000	$ 3,250,000	$ 687,000	$ 130,000		130,000	4,775,000	$ 4,094,000
Reduction in net income		$ (214,000)	$ (5,601,000)	$ (3,741,000)	$ (3,922,000)	$ (3,695,000)	$ (3,283,000)	$ (4,506,000)	$ (3,506,000)		(13,478,000)	(14,990,000)	$ (18,334,000)
RML
Accounting Policy [Line Items]
Number of former equity owners | owner	4
Voting interests acquired	76.50%
Number of former equity owners with continuing employment contracts | owner	3
Number of former equity owners that must continue employment under terms of acquisition agreement | owner	2
Accrued compensation expense related to acquisition payments											$ 0
Accounting for Payments Made to Former Equity Owners | RML
Accounting Policy [Line Items]
Total purchase price	$ 22,200,000
Goodwill	7,500,000
Accounting for Payments Made to Former Equity Owners | Previously Reported | RML
Accounting Policy [Line Items]
Total purchase price	29,500,000
Goodwill	$ 14,800,000
Accounting for Payments Made to Former Equity Owners | Restatement Adjustment | RML
Accounting Policy [Line Items]
Compensation expense - RML acquisition payments										765,000				$ 2,300,000
Reduction in net income										$ 438,000		$ 1,800,000		$ 1,400,000
Lender Concentration Risk
Accounting Policy [Line Items]
Concentration risk, percent											36.00%
Buildings
Accounting Policy [Line Items]
Premises and equipment, useful life											39 years
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%									50.10%