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Benefit Plans - Defined Contribution Plan (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
date
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
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 $ 1,200 $ 1,200 $ 1,100
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 matching contribution percentage 70.00% 60.00%  
Employer matching contribution maximum amount $ 5 $ 4  
Employer contributions $ 765 $ 589 $ 555