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Cover - shares
 9 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Cover page.    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2019  
Document Transition Report false  
Entity File Number 001-33572  
Entity Registrant Name Bank of Marin Bancorp  
Entity Incorporation, State or Country Code CA  
Entity Tax Identification Number 20-8859754  
Entity Address, Address Line One 504 Redwood Blvd.  
Entity Address, Address Line Two  Suite 100  
Entity Address, City or Town Novato  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94947  
City Area Code 415  
Local Phone Number 763-4520  
Title of 12(b) Security Common stock, no par value  
Trading Symbol BMRC  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Smaller Reporting Company true  
Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding (in shares)   13,588,234
Entity Central Index Key 0001403475  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Amendment Flag false