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ACQUISITIONS - Narrative (Details) - SI Financial Group, Inc. $ / shares in Units, shares in Thousands |  | 6 Months Ended
--- | --- | ---
May 17, 2019 USD ($) branch $ / shares shares | Jun. 30, 2019 USD ($) | Jun. 30, 2018 USD ($)
Business Acquisition [Line Items] |  |  |
Equity interest issued or issuable, shares converted (in shares) | shares | 11,858 |  |
Equity interest issued or issuable, share exchange ratio | 0.48 |  |
Equity interest issued or issuable, shares issued (in shares) | shares | 5,691 |  |
Company common stock issued to SIFI common shareholders | $ | $ 175,804,000 |  |
Equity interest issued or issuable, basis for determining value, share price (in dollars per share) | $ / shares | $ 30.89 |  |
Fair value of SIFI stock options converted to Berkshire options | $ | $ 907,000 |  |
Direct acquisition and integration costs | $ |  | $ 11,300,000 | $ 0
Pro forma estimated tax rate |  | 26.83% | 26.83%
Eastern Connecticut |  |  |
Business Acquisition [Line Items] |  |  |
Number of branch banking offices | branch | 18 |  |
Rhode Island |  |  |
Business Acquisition [Line Items] |  |  |
Number of branch banking offices | branch | 5 |  |
Connecticut |  |  |
Business Acquisition [Line Items] |  |  |
Number of branch banking offices | branch | 9 |  |

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