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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Narrative (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Feb. 07, 2017
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
contract
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Derivative [Line Items]                          
Derivative, notional amount   $ 4,070,729       $ 3,303,570         $ 4,070,729 $ 3,303,570  
Cash pledged as collateral to derivative counterparties   96,300       25,400         96,300 25,400  
Amortized cost of securities pledged as collateral to derivative counterparties   25,700       13,100         25,700 13,100  
Fair value of securities as pledged collateral to derivative counterparties   25,800       12,800         25,800 12,800  
Interest expense   34,108 $ 36,854 $ 37,643 $ 35,650 33,929 $ 29,184 $ 25,192 $ 21,389   144,255 109,694 $ 64,113
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Cash Flow Hedges                          
Derivative [Line Items]                          
Interest expense $ 6,600                        
Interest Rate Swap                          
Derivative [Line Items]                          
Cash pledged as collateral to derivative counterparties   96,310       25,412         96,310 25,412  
Amount of collateral posted for the net liability positions   122,100       38,200         122,100 38,200  
Interest Rate Swap | Financial institutions counterparties                          
Derivative [Line Items]                          
Derivative asset, fair value, amount offset against collateral   600       5,900         600 5,900  
Amount of collateral posted for the net liability positions   78,800       18,800         78,800 18,800  
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   76,400       21,200         76,400 21,200  
Amount of collateral posted for the net liability positions   1,100       9,700         1,100 9,700  
Non-hedging derivatives                          
Derivative [Line Items]                          
Derivative, notional amount   168,997       165,079         168,997 165,079  
Economic Hedging                          
Derivative [Line Items]                          
Credit valuation adjustments   $ 1,200                 $ 1,200    
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   $ 9,400                 $ 9,400    
Economic Hedging | Hedging derivatives                          
Derivative [Line Items]                          
Derivative, notional amount   3,901,732       3,138,491         3,901,732 3,138,491  
Economic Hedging | Hedging derivatives | Interest Rate Swap                          
Derivative [Line Items]                          
Derivative, notional amount   3,300,000                 3,300,000    
Economic Hedging | Hedging derivatives | Interest Rate Swap | Tax advantaged economic development bonds                          
Derivative [Line Items]                          
Derivative, notional amount   9,390       10,090         9,390 10,090  
Economic Hedging | Hedging derivatives | Risk Participation Agreements                          
Derivative [Line Items]                          
Derivative, notional amount   315,140       243,806         315,140 243,806  
Economic Hedging | Hedging derivatives | Forward Commitments                          
Derivative [Line Items]                          
Derivative, notional amount   $ 237,412       $ 190,807         $ 237,412 $ 190,807  
Cash Flow Hedging                          
Derivative [Line Items]                          
Durations of derivative instruments                   3 years      
Cash Flow Hedging | Interest Rate Swap                          
Derivative [Line Items]                          
Number of interest rate swap contracts (contract) | contract                   6      
Cash Flow Hedging | Interest Rate Swap | FHLBB borrowings                          
Derivative [Line Items]                          
Derivative, notional amount $ 300,000                 $ 300,000