| Cover | Oct. 25, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 25, 2021 |
| Entity Registrant Name | Bank of Marin Bancorp |
| Entity Incorporation, State or Country Code | CA |
| Entity File Number | 001-33572 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, no par value and attached Share Purchase Rights |
| Trading Symbol | BMRC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001403475 |
| Amendment Flag | false |
| Document Fiscal Period Focus | Q3 |
| Document Fiscal Year Focus | 2021 |
