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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Narrative (Details)
 3 Months Ended 12 Months Ended
Feb. 07, 2017
USD ($)
Dec. 31, 2016
USD ($)
contract
Sep. 30, 2016
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2016
USD ($)
contract
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Derivative [Line Items]                        
Derivative, notional amount   $ 2,199,862,000       $ 1,366,667,000       $ 2,199,862,000 $ 1,366,667,000  
Cash pledged as collateral to derivative counterparties   900,000       16,100,000       900,000 16,100,000  
Amortized cost of securities pledged as collateral to derivative counterparties   47,800,000       24,600,000       47,800,000 24,600,000  
Fair value of securities as pledged collateral to derivative counterparties   47,900,000       24,600,000       47,900,000 24,600,000  
Interest expense   13,276,000 $ 12,540,000 $ 11,577,000 $ 10,779,000 9,676,000 $ 8,481,000 $ 7,766,000 $ 7,258,000 48,172,000 33,181,000 $ 28,351,000
Estimated liability   2,804,000       10,676,000       2,804,000 10,676,000  
Reclassification out of Accumulated Other Comprehensive Income | Subsequent Event | Accumulated Net Gain (Loss) from Cash Flow Hedges                        
Derivative [Line Items]                        
Interest expense $ 6,600,000                      
Interest Rate Swap                        
Derivative [Line Items]                        
Cash pledged as collateral to derivative counterparties   650,000       9,720,000       650,000 9,720,000  
Amount of collateral posted for the net liability positions   19,800,000       28,200,000       19,800,000 28,200,000  
Interest Rate Swap | Commercial counterparties                        
Derivative [Line Items]                        
Cash pledged as collateral to derivative counterparties   0       0       0 0  
Derivative asset, fair value, amount offset against collateral   11,500,000       17,100,000       11,500,000 17,100,000  
Amount of collateral posted for the net liability positions   4,400,000       0       4,400,000 0  
Interest Rate Swap | Financial institutions counterparties                        
Derivative [Line Items]                        
Amount of collateral posted for the net liability positions   15,400,000       28,200,000       15,400,000 28,200,000  
Non-hedging derivatives                        
Derivative [Line Items]                        
Derivative, notional amount   208,145,000       36,043,000       208,145,000 36,043,000  
Estimated liability   $ (4,738,000)       (323,000)       $ (4,738,000) (323,000)  
Cash Flow Hedging | Interest Rate Swap                        
Derivative [Line Items]                        
Number of derivative contracts (contract) | contract   6               6    
Durations of derivative instruments                   3 years    
Cash Flow Hedging | Interest Rate Swap | FHLBB borrowings                        
Derivative [Line Items]                        
Derivative, notional amount   $ 300,000,000               $ 300,000,000    
Period of rolling advances                   1 month    
Maturity period with fixed interest rate                   1 month    
Interest expense                   $ 3,981,000 0  
Cash Flow Hedging | Hedging derivatives                        
Derivative [Line Items]                        
Derivative, notional amount   300,000,000       300,000,000       300,000,000 300,000,000  
Estimated liability   6,573,000       8,532,000       6,573,000 8,532,000  
Cash Flow Hedging | Hedging derivatives | Interest Rate Swap                        
Derivative [Line Items]                        
Derivative, notional amount   300,000,000               300,000,000    
Cash Flow Hedging | Hedging derivatives | Interest Rate Swap | FHLBB borrowings                        
Derivative [Line Items]                        
Derivative, notional amount   300,000,000               300,000,000    
Estimated liability   6,573,000               6,573,000    
Economic Hedging                        
Derivative [Line Items]                        
Credit valuation adjustments   $ 800,000               $ 800,000    
Economic Hedging | Tax advantaged economic development bonds                        
Derivative [Line Items]                        
Fixed rate of interest (percent)   5.09%               5.09%    
Maturity period                   21 years    
Economic Hedging | Interest Rate Swap | Tax advantaged economic development bonds                        
Derivative [Line Items]                        
Derivative, notional amount   $ 11,400,000               $ 11,400,000    
Economic Hedging | Hedging derivatives                        
Derivative [Line Items]                        
Derivative, notional amount   1,691,717,000       1,030,624,000       1,691,717,000 1,030,624,000  
Estimated liability   969,000       2,467,000       969,000 2,467,000  
Economic Hedging | Hedging derivatives | Interest Rate Swap                        
Derivative [Line Items]                        
Derivative, notional amount   1,300,000,000               1,300,000,000    
Economic Hedging | Hedging derivatives | Interest Rate Swap | Tax advantaged economic development bonds                        
Derivative [Line Items]                        
Derivative, notional amount   11,386,000       11,984,000       11,386,000 11,984,000  
Estimated liability   2,021,000       2,450,000       2,021,000 2,450,000  
Economic Hedging | Hedging derivatives | Risk Participation Agreements                        
Derivative [Line Items]                        
Derivative, notional amount   83,360,000       59,016,000       83,360,000 59,016,000  
Estimated liability   (5,000)       56,000       (5,000) 56,000  
Economic Hedging | Hedging derivatives | Forward Commitments                        
Derivative [Line Items]                        
Derivative, notional amount   259,889,000       44,840,000       259,889,000 44,840,000  
Estimated liability   $ (722,000)       $ (53,000)       $ (722,000) $ (53,000)