DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative instruments and hedging activities
Cash pledged as collateral to derivative counterparties	$ 3,400,000								$ 3,400,000
Amortized cost of securities pledged as collateral to derivative counterparties	22,800,000								22,800,000
Fair value of securities as pledged collateral to derivative counterparties	23,100,000								23,100,000
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	1,034,667,000				905,071,000				1,034,667,000	905,071,000
Estimated Fair Value Asset (Liability)	(5,815,000)				(3,652,000)				(5,815,000)	(3,652,000)
Interest rate swaps
Reclassification of unrealized loss from accumulated other comprehensive loss to interest expense	(7,369,000)	(7,107,000)	(6,846,000)	(7,029,000)	(8,817,000)	(8,635,000)	(8,593,000)	(8,944,000)	(28,351,000)	(34,989,000)	(32,551,000)
Net interest expense recognized in interest expense									9,166,000	14,130,000	10,069,000
FHLBB borrowings
Interest rate swaps
Extinguishment of FHLB advances				215,000,000
Loss reclassified from the effective portion of the unrealized changes in the fair value of the derivatives				8,600,000
Cash flow hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	300,000,000				425,000,000				300,000,000	425,000,000
Estimated Fair Value Asset (Liability)	(3,299,000)				(2,290,000)				(3,299,000)	(2,290,000)
Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	695,078,000				459,872,000				695,078,000	459,872,000
Estimated Fair Value Asset (Liability)	(3,141,000)				(1,620,000)				(3,141,000)	(1,620,000)
Interest rate swaps
Credit valuation adjustments	37,300								37,300
Economic hedges | Tax advantaged economic development bonds
Interest rate swaps
Fixed rate of interest (as a percent)	5.09%								5.09%
Life of the bond									21 years
Non-hedging derivatives
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	39,589,000				20,199,000				39,589,000	20,199,000
Estimated Fair Value Asset (Liability)	625,000				258,000				625,000	258,000
Interest rate swaps | FHLBB borrowings
Interest rate swaps
Notional amount of derivative contract terminated				30,000,000
Interest rate swaps | 90 day rolling FHLB advances
Interest rate swaps
Notional amount of derivative contract terminated				145,000,000
Interest rate swaps | Cash flow hedges | FHLBB borrowings
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount					150,000,000					150,000,000
Weighted Average Maturity										2 years 6 months
Weighted Average Rate, Received (as a percent)					0.25%					0.25%
Weighted Average Rate, Contract pay rate (as a percent)					2.61%					2.61%
Estimated Fair Value Asset (Liability)					(3,102,000)					(3,102,000)
Cash flow hedges:
Maturity period with fixed interest rate									1 month
Period of rolling advances									1 month
Interest rate swaps
Unrealized (loss) gain recognized in accumulated other comprehensive loss									(6,405,000)	11,783,000
Reclassification of unrealized loss from accumulated other comprehensive loss to interest expense										(3,620,000)
Reclassification of unrealized loss from accumulated other comprehensive loss to other non-interest expense for termination of swaps									8,630,000	943,000
Reclassification of unrealized deferred tax (benefit) from accumulated other comprehensive loss to tax expense for terminated swaps									(3,611,000)	(489,000)
Net tax benefit (expense) on items recognized in accumulated other comprehensive loss									2,583,000	(3,276,000)
Net interest expense recognized in interest expense										4,374,000
Interest rate swaps | Cash flow hedges | Junior subordinated debentures
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount					15,000,000					15,000,000
Weighted Average Maturity										4 months 24 days
Weighted Average Rate, Received (as a percent)					2.09%					2.09%
Weighted Average Rate, Contract pay rate (as a percent)					5.54%					5.54%
Estimated Fair Value Asset (Liability)					(203,000)					(203,000)
Interest rate swaps
Unrealized (loss) gain recognized in accumulated other comprehensive loss									(1,000)	(18,000)
Reclassification of unrealized loss from accumulated other comprehensive loss to interest expense									(204,000)	(519,000)
Net tax benefit (expense) on items recognized in accumulated other comprehensive loss									(80,000)	(197,000)
Other comprehensive income recorded in accumulated other comprehensive loss, net of reclassification adjustments and tax effects									1,320,000	5,645,000
Net interest expense recognized in interest expense									204,000	519,000
Interest rate swaps | Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	606,900,000								606,900,000
Interest rate swaps | Economic hedges | Tax advantaged economic development bonds
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	12,554,000				13,095,000				12,554,000	13,095,000
Weighted Average Maturity									14 years 10 months 24 days	15 years 10 months 24 days
Weighted Average Rate, Received (as a percent)	0.52%				0.54%				0.52%	0.54%
Weighted Average Rate, Contract pay rate (as a percent)	5.09%				5.09%				5.09%	5.09%
Estimated Fair Value Asset (Liability)	(2,578,000)				(1,889,000)				(2,578,000)	(1,889,000)
Interest rate swaps | Economic hedges | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	297,158,000				206,933,000				297,158,000	206,933,000
Weighted Average Maturity									6 years	5 years 4 months 24 days
Weighted Average Rate, Received (as a percent)	2.23%				2.44%				2.23%	2.44%
Weighted Average Rate, Contract pay rate (as a percent)	4.54%				4.68%				4.54%	4.68%
Estimated Fair Value Asset (Liability)	(12,183,000)				(6,278,000)				(12,183,000)	(6,278,000)
Economic hedges and non-hedging derivatives
Unrealized (loss) gain recognized in other non-interest income									(4,514,000)	(6,004,000)
Interest rate swaps | Economic hedges | Industrial revenue bond
Economic hedges and non-hedging derivatives
Unrealized (loss) gain recognized in other non-interest income									(1,333,000)	972,000
Forward-starting interest rate swaps | 90 day rolling FHLB advances
Interest rate swaps
Notional amount of derivative contract terminated				235,000,000
Forward-starting interest rate swaps | Cash flow hedges
Cash flow hedges:
Number of instruments	6								6
Forward-starting interest rate swaps | Cash flow hedges | 2016 and 2017
Cash flow hedges:
Durations of derivative instruments									3 years
Forward-starting interest rate swaps | Cash flow hedges | FHLBB borrowings
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	300,000,000				260,000,000				300,000,000	260,000,000
Weighted Average Maturity									4 years 3 months 18 days	4 years 6 months
Weighted Average Rate, Contract pay rate (as a percent)	2.29%				1.88%				2.29%	1.88%
Estimated Fair Value Asset (Liability)	(3,299,000)				1,015,000				(3,299,000)	1,015,000
Reverse interest rate swaps | Economic hedges | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	297,158,000				206,933,000				297,158,000	206,933,000
Weighted Average Maturity									6 years	5 years 4 months 24 days
Weighted Average Rate, Received (as a percent)	4.54%				4.68%				4.54%	4.68%
Weighted Average Rate, Contract pay rate (as a percent)	2.23%				2.44%				2.23%	2.44%
Estimated Fair Value Asset (Liability)	12,221,000				6,286,000				12,221,000	6,286,000
Economic hedges and non-hedging derivatives
Unrealized (loss) gain recognized in other non-interest income									4,514,000	6,004,000
Favorable change in credit valuation adjustment recognized in other non-interest income									20,000	450,000
Risk participation agreements | Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	45,842,000								45,842,000
Weighted Average Maturity									16 years 7 months 6 days
Estimated Fair Value Asset (Liability)	(91,000)								(91,000)
Economic hedges and non-hedging derivatives
Unrealized (loss) gain recognized in other non-interest income									(91,000)
Interest rate lock commitments | Non-hedging derivatives
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	39,589,000				20,199,000				39,589,000	20,199,000
Weighted Average Maturity									2 months 12 days	2 months 12 days
Estimated Fair Value Asset (Liability)	625,000				258,000				625,000	258,000
Economic hedges and non-hedging derivatives
Unrealized (loss) gain recognized in other non-interest income									625,000	258,000
Realized gain (loss) in other non-interest income									3,938,000	(2,467,000)
Forward sale commitments | Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional Amount	42,366,000				32,911,000				42,366,000	32,911,000
Weighted Average Maturity									2 months 12 days	2 months 12 days
Estimated Fair Value Asset (Liability)	(510,000)				261,000				(510,000)	261,000
Economic hedges and non-hedging derivatives
Unrealized (loss) gain recognized in other non-interest income									(510,000)	261,000
Realized gain (loss) in other non-interest income									$ (1,494,000)	$ 7,501,000