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Benefit Plans - Defined Contribution Plan and Employee Stock Ownership Plan (Details)
 12 Months Ended
Dec. 31, 2020
USD ($)
date
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Defined Contribution Plan Disclosure [Line Items]      
Employer matching contribution percentage 70.00%    
Employer matching contribution maximum amount $ 5,000    
Defined Contribution Plan (the 401k Plan)      
Defined Contribution Plan Disclosure [Line Items]      
Minimum age of eligible employee for 401(k) plan 18 years    
Minimum employment period to qualify for 401(k) plan 90 days    
Number of open enrollment dates for 401(k) plan | date 4    
Minimum annual contribution per employee, percent of eligible compensation 1.00%    
Maximum annual contribution per employee, percent of eligible compensation 50.00%    
Annual vesting percentage 20.00%    
Defined contribution plan, number of years to be fully vested 5 years    
Employer contributions $ 929,000 $ 874,000 $ 851,000
Bank of Marin Employee Stock Ownership and Savings Plan (the Plan)      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, number of years to be fully vested 5 years    
Cash contributed to the ESOP $ 1,300,000 $ 1,200,000 $ 1,200,000