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ACQUISITIONS - HAMPDEN BANCORP NARRATIVE (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Apr. 17, 2015
USD ($)
subsidiary
branch
$ / shares
shares
Dec. 31, 2015
USD ($)
shares
Dec. 31, 2014
USD ($)
Business Acquisition [Line Items]      
Stock issued during period, shares, acquisitions (shares)   5,630,000.00  
Hampden Bancorp Inc      
Business Acquisition [Line Items]      
Number of banking subsidiaries (subsidiary) | subsidiary 1    
Number of retail bank branches acquired (branch) | branch 10    
Business combination, number of acquiree shares outstanding, net of shares owned by acquirer (shares) 5,167,000.000    
Business combination, number of acquiree shares outstanding, owned by acquirer (shares) 209,000    
Stock issued during period, shares, acquisitions (shares) 4,355,000.000    
Business combination, number of shares issued, conversion of acquiree stock 0.81    
Shares issued, price per share (USD per share) | $ / shares $ 27.38    
Berkshire Hills Bancorp common stock issued to Hampden common stockholders | $ $ 114,600    
Reduction to preliminary consideration | $ 130    
Payments to acquire businesses | $ $ 114,470    
Number of shares held and not released to common stockholders (shares) 6,000    
Number of shares subsequently converted, which were previously not released (shares) 5,000    
Fair value of Hampden shares previously owned by the Company prior to acquisition | $ $ 4,600    
Business combination, step acquisition, equity interest in acquiree, remeasurement gain | $ $ 2,200 $ 2,200  
Acquisition related costs | $   $ 11,100 $ 1,500
Hampden Bancorp Inc | Hampden Bank Shareholders      
Business Acquisition [Line Items]      
Stock issued during period, shares, acquisitions (shares) 4,186,000.000    
Hampden Bancorp Inc | Berkshire Bank      
Business Acquisition [Line Items]      
Stock issued during period, shares, acquisitions (shares) 169,000