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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