Shareholders' Equity	3 Months Ended
Mar. 31, 2019
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders’ Equity Dividend On April 18, 2019, BMBC’s Board of Directors declared a regular quarterly dividend of $0.25 per share payable June 1, 2019 to shareholders of record as of May 1, 2019. During the first quarter of 2019, the Corporation paid or accrued, as applicable, a regular quarterly dividend of $0.25 per share. This dividend totaled $5.1 million, based on outstanding shares and restricted stock units as of February 1, 2019 of 20,381,859 shares. S-3 Shelf Registration Statement and Offerings Thereunder In May 2018, BMBC filed a shelf registration statement on Form S-3, SEC File No. 333-224849 (the “Shelf Registration Statement”). The Shelf Registration Statement allows BMBC to raise additional capital from time to time through offers and sales of registered securities consisting of common stock, debt securities, warrants, purchase contracts, rights and units or units consisting of any combination of the foregoing securities. BMBC may sell these securities using the prospectus in the Shelf Registration Statement, together with applicable prospectus supplements, from time to time, in one or more offerings. In addition, BMBC has in place a Dividend Reinvestment and Stock Purchase Plan (the “Plan”), which allows it to issue up to 1,500,000 shares of registered common stock. The Plan allows for the grant of a request for waiver (“RFW”) above the Plan’s maximum investment of $120 thousand per account per year. A RFW is granted based on a variety of factors, including BMBC’s current and projected capital needs, prevailing market prices of BMBC’s common stock and general economic and market conditions. For the three months ended March 31, 2019, BMBC did not issue any shares under the Plan. The Plan administrator conducted dividend reinvestments for Plan participants through open market purchases. No RFWs were approved during the three months ended March 31, 2019. No other sales of equity securities were executed under the Shelf Registration Statement during the three months ended March 31, 2019. Option Exercises and Vesting of Restricted Stock Units ("RSUs") and Performance Stock Units ("PSUs") In addition to shares that may be issued through the Plan, BMBC also issues shares through the exercise of stock options and the vesting of RSUs and PSUs. During the three months ended March 31, 2019, 29,600 shares were issued pursuant to the exercise of stock options, increasing shareholders’ equity by $540 thousand. The increase in shareholders’ equity related to the vesting of RSUs and PSUs, which is recognized over the vesting period through stock based compensation expense, was $1.1 million for the three months ended March 31, 2019. Stock Repurchases On August 6, 2015, BMBC announced a stock repurchase program (the “2015 Program”) under which the Corporation may repurchase up to 1,200,000 shares of BMBC’s common stock, at an aggregate purchase price not to exceed $40 million. During the three months ended March 31, 2019, 12,702 shares were repurchased under the 2015 Program. As of March 31, 2019, the maximum number of shares remaining authorized for repurchase under the 2015 Program was 27,314. In addition to the 2015 Program, it is BMBC’s practice to retire shares to its treasury account upon the vesting of stock awards to certain officers in order to cover the statutory income tax withholdings related to such vestings. On April 18, 2019, BMBC announced a new stock repurchase program (the "2019 Program") under which the Corporation may repurchase up to 1,000,000 shares of BMBC's common stock. Under the 2019 Program, the Corporation may repurchase BMBC's common stock at any price, but the aggregate purchase price is not to exceed $45 million. The 2019 Program will become effective upon the completion of BMBC’s existing 2015 Program.

Shareholders’ Equity

Dividend

On April 18, 2019, BMBC’s Board of Directors declared a regular quarterly dividend of $0.25 per share payable June 1, 2019 to shareholders of record as of May 1, 2019. During the first quarter of 2019, the Corporation paid or accrued, as applicable, a regular quarterly dividend of $0.25 per share. This dividend totaled $5.1 million, based on outstanding shares and restricted stock units as of February 1, 2019 of 20,381,859 shares.

S-3 Shelf Registration Statement and Offerings Thereunder

In May 2018, BMBC filed a shelf registration statement on Form S-3, SEC File No. 333-224849 (the “Shelf Registration Statement”). The Shelf Registration Statement allows BMBC to raise additional capital from time to time through offers and sales of registered securities consisting of common stock, debt securities, warrants, purchase contracts, rights and units or units consisting of any combination of the foregoing securities. BMBC may sell these securities using the prospectus in the Shelf Registration Statement, together with applicable prospectus supplements, from time to time, in one or more offerings.

In addition, BMBC has in place a Dividend Reinvestment and Stock Purchase Plan (the “Plan”), which allows it to issue up to 1,500,000 shares of registered common stock. The Plan allows for the grant of a request for waiver (“RFW”) above the Plan’s maximum investment of $120 thousand per account per year. A RFW is granted based on a variety of factors, including BMBC’s current and projected capital needs, prevailing market prices of BMBC’s common stock and general economic and market conditions.

For the three months ended March 31, 2019, BMBC did not issue any shares under the Plan. The Plan administrator conducted dividend reinvestments for Plan participants through open market purchases. No RFWs were approved during the three months ended March 31, 2019. No other sales of equity securities were executed under the Shelf Registration Statement during the three months ended March 31, 2019.

Option Exercises and Vesting of Restricted Stock Units ("RSUs") and Performance Stock Units ("PSUs")

In addition to shares that may be issued through the Plan, BMBC also issues shares through the exercise of stock options and the vesting of RSUs and PSUs. During the three months ended March 31, 2019, 29,600 shares were issued pursuant to the exercise of stock options, increasing shareholders’ equity by $540 thousand. The increase in shareholders’ equity related to the vesting of RSUs and PSUs, which is recognized over the vesting period through stock based compensation expense, was $1.1 million for the three months ended March 31, 2019.

Stock Repurchases

On August 6, 2015, BMBC announced a stock repurchase program (the “2015 Program”) under which the Corporation may repurchase up to 1,200,000 shares of BMBC’s common stock, at an aggregate purchase price not to exceed $40 million. During the three months ended March 31, 2019, 12,702 shares were repurchased under the 2015 Program. As of March 31, 2019, the maximum number of shares remaining authorized for repurchase under the 2015 Program was 27,314. In addition to the 2015 Program, it is BMBC’s practice to retire shares to its treasury account upon the vesting of stock awards to certain officers in order to cover the statutory income tax withholdings related to such vestings.

On April 18, 2019, BMBC announced a new stock repurchase program (the "2019 Program") under which the Corporation may repurchase up to 1,000,000 shares of BMBC's common stock. Under the 2019 Program, the Corporation may repurchase BMBC's common stock at any price, but the aggregate purchase price is not to exceed $45 million. The 2019 Program will become effective upon the completion of BMBC’s existing 2015 Program.