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Benefit Plans - Defined Contribution Plan and Employee Stock Ownership Plan (Details) | 12 Months Ended
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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

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- Definition Schedule of Defined Contribution Plans Disclosures [Line Items] + References No definition available. + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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