DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative financial instruments and hedging activities
Cash pledged as collateral to derivative counterparties	$ 5,300,000		$ 5,300,000
Amortized cost of securities pledged as collateral to derivative counterparties	34,300,000		34,300,000
Fair value of securities as pledged collateral to derivative counterparties	35,200,000		35,200,000
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	1,287,322,000		1,287,322,000		577,950,000
Estimated Fair Value Asset (Liability)	(13,356,000)		(13,356,000)		(13,003,000)
Interest rate derivatives designated as hedges of cash flows
Net interest expense recognized in interest expense	2,270,000	2,131,000	6,416,000	6,267,000
Economic hedges and non-hedging derivatives
Net interest expense recognized in interest and dividend income on securities	2,270,000	2,131,000	6,416,000	6,267,000
FHLBB borrowings
Cash flow hedges
Amount of borrowings	13,700,000		13,700,000
Cash flow hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	270,000,000		270,000,000		200,000,000
Estimated Fair Value Asset (Liability)	(12,258,000)		(12,258,000)		(9,007,000)
Interest rate derivatives designated as hedges of cash flows
Amount reclassified as an increase to interest expense during the next twelve months	5,300,000		5,300,000
Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	741,566,000		741,566,000		356,412,000
Estimated Fair Value Asset (Liability)	(8,931,000)		(8,931,000)		(3,930,000)
Interest rate derivatives designated as hedges of cash flows
Credit valuation adjustments	502,000		502,000
Economic hedges | Tax advantaged economic development bonds
Interest rate derivatives designated as hedges of cash flows
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	275,756,000		275,756,000		21,538,000
Estimated Fair Value Asset (Liability)	7,833,000		7,833,000		(66,000)
Interest rate swaps | Cash flow hedges
Interest rate derivatives designated as hedges of cash flows
Other comprehensive income recorded in accumulated other comprehensive loss, net of reclassification adjustments and tax effects	(454,000)	(470,000)	(1,350,000)	72,000
Interest rate swaps | Cash flow hedges | FHLBB borrowings
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	115,000,000		115,000,000		105,000,000
Weighted Average Maturity			1 year 9 months 18 days		1 year 8 months 12 days
Weighted Average Rate, Received (as a percent)	0.44%		0.44%		0.49%
Weighted Average Rate, Paid (as a percent)	3.47%		3.47%		4.00%
Estimated Fair Value Asset (Liability)	(5,509,000)		(5,509,000)		(5,888,000)
Interest rate derivatives designated as hedges of cash flows
Unrealized (loss) gain recognized in accumulated other comprehensive loss	(1,086,000)	(987,000)	(3,446,000)	(544,000)
Reclassification of unrealized loss from accumulated other comprehensive loss to non-interest income for hedge ineffectiveness	11,000	11,000	31,000	31,000
Reclassification of realized gain from accumulated other comprehensive loss to other non-interest income for termination of swaps	235,000	235,000	706,000	706,000
Reclassification of unrealized deferred tax benefit from accumulated other comprehensive loss to tax expense for terminated swaps	(98,000)	(98,000)	(226,000)	(293,000)
Net tax benefit (expense) on items recognized in accumulated other	441,000	407,000	1,448,000	210,000
Net interest expense recognized in interest expense	1,121,000	1,239,000	3,327,000	3,663,000
Economic hedges and non-hedging derivatives
Net interest expense recognized in interest and dividend income on securities	1,121,000	1,239,000	3,327,000	3,663,000
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		15,000,000
Weighted Average Maturity			1 year 7 months 6 days		2 years 4 months 24 days
Weighted Average Rate, Received (as a percent)	2.28%		2.28%		2.35%
Weighted Average Rate, Paid (as a percent)	5.54%		5.54%		5.54%
Estimated Fair Value Asset (Liability)	(829,000)		(829,000)		(1,055,000)
Interest rate derivatives designated as hedges of cash flows
Unrealized (loss) gain recognized in accumulated other comprehensive loss	71,000	(65,000)	226,000	(62,000)
Net tax benefit (expense) on items recognized in accumulated other	(28,000)	27,000	(89,000)	24,000
Net interest expense recognized in interest expense	148,000	131,000	363,000	386,000
Economic hedges and non-hedging derivatives
Net interest expense recognized in interest and dividend income on securities	148,000	131,000	363,000	386,000
Interest rate swaps | Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	410,900,000		410,900,000
Interest rate swaps | Economic hedges | Industrial revenue bond
Interest rate derivatives designated as hedges of cash flows
Net interest expense recognized in interest expense	(158,000)	(1,812,000)	(473,000)	(2,137,000)
Economic hedges and non-hedging derivatives
Net interest expense recognized in interest and dividend income on securities	(158,000)	(1,812,000)	(473,000)	(2,137,000)
Unrealized gain (loss) recognized in other non-interest income	(24,000)		(1,118,000)	(69,000)
Interest rate swaps | Economic hedges | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	198,600,000		198,600,000		160,389,000
Weighted Average Maturity			5 years 2 months 12 days		5 years 7 months 6 days
Weighted Average Rate, Received (as a percent)	2.58%		2.58%		2.73%
Weighted Average Rate, Paid (as a percent)	5.35%		5.35%		5.77%
Estimated Fair Value Asset (Liability)	(16,290,000)		(16,290,000)		(14,351,000)
Economic hedges and non-hedging derivatives
Unrealized gain (loss) recognized in other non-interest income	859,000	6,133,000	4,119,000	7,073,000
Interest rate swaps | Economic hedges | Tax advantaged economic development bonds
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	13,734,000		13,734,000		14,096,000
Weighted Average Maturity			17 years 2 months 12 days		17 years 10 months 24 days
Weighted Average Rate, Received (as a percent)	0.60%		0.60%		0.64%
Weighted Average Rate, Paid (as a percent)	5.09%		5.09%		5.09%
Estimated Fair Value Asset (Liability)	(3,719,000)		(3,719,000)		(3,505,000)
Forward-starting interest rate swaps | Cash flow hedges
Cash flow hedges
Number of instruments	13		13
Forward-starting interest rate swaps | Cash flow hedges | 2013
Cash flow hedges
Number of instruments	2		2
Forward-starting interest rate swaps | Cash flow hedges | 2014
Cash flow hedges
Number of instruments	7		7
Forward-starting interest rate swaps | Cash flow hedges | 2015
Cash flow hedges
Number of instruments	2		2
Forward-starting interest rate swaps | Cash flow hedges | FHLBB borrowings
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	140,000,000		140,000,000		80,000,000
Weighted Average Maturity			5 years 8 months 12 days		3 years 6 months
Weighted Average Rate, Paid (as a percent)	2.37%		2.37%		2.56%
Estimated Fair Value Asset (Liability)	(5,920,000)		(5,920,000)		(2,064,000)
Forward-starting interest rate swaps 1 | Cash flow hedges | 2013
Cash flow hedges
Number of instruments	2		2
Durations of derivative instruments			4 years
Forward-starting interest rate swaps 1 | Cash flow hedges | 2014
Cash flow hedges
Number of instruments	1		1
Durations of derivative instruments			3 years
Forward-starting interest rate swaps 1 | Cash flow hedges | 2015
Cash flow hedges
Number of instruments	1		1
Durations of derivative instruments			4 years
Forward-starting interest rate swaps 2 | Cash flow hedges | 2013
Cash flow hedges
Number of instruments	1		1
Durations of derivative instruments			5 years
Forward-starting interest rate swaps 2 | Cash flow hedges | 2014
Cash flow hedges
Number of instruments	4		4
Durations of derivative instruments			4 years
Forward-starting interest rate swaps 2 | Cash flow hedges | 2015
Cash flow hedges
Number of instruments	1		1
Durations of derivative instruments			7 years
Forward-starting interest rate swaps 3 | Cash flow hedges | 2014
Cash flow hedges
Number of instruments	2		2
Durations of derivative instruments			5 years
Reverse interest rate swaps | Economic hedges | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	198,600,000		198,600,000		160,389,000
Weighted Average Maturity			5 years 2 months 12 days		5 years 7 months 6 days
Weighted Average Rate, Received (as a percent)	5.35%		5.35%		5.77%
Weighted Average Rate, Paid (as a percent)	2.58%		2.58%		2.73%
Estimated Fair Value Asset (Liability)	15,747,000		15,747,000		13,871,000
Economic hedges and non-hedging derivatives
Unrealized gain (loss) recognized in other non-interest income	(859,000)	(6,133,000)	(4,119,000)	(7,073,000)
Favorable (unfavorable) change in credit valuation adjustment recognized in other non-interest income	(31,000)	(281,000)	(22,000)	(464,000)
Interest rate lock commitments | Non-hedging derivatives
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	275,756,000		275,756,000		21,538,000
Weighted Average Maturity			2 months 12 days		2 months 12 days
Estimated Fair Value Asset (Liability)	7,833,000		7,833,000		(66,000)
Economic hedges and non-hedging derivatives
Unrealized gain (loss) recognized in other non-interest income	10,858,000	182,000	15,195,000	256,000
Forward commitments | Economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	330,632,000		330,632,000		21,538,000
Weighted Average Maturity			2 months 12 days		2 months 12 days
Estimated Fair Value Asset (Liability)	(4,669,000)		(4,669,000)		55,000
Economic hedges and non-hedging derivatives
Unrealized gain (loss) recognized in other non-interest income	$ (6,832,000)	$ (182,000)	$ (8,963,000)	$ (292,000)