ACQUISITIONS (Details) (USD $)	1 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2012 item	Sep. 30, 2012	Dec. 31, 2011	Apr. 20, 2012	Apr. 19, 2012
ACQUISITIONS
Outstanding common shares		22,212,764	21,147,736
Carrying value of assets		$ 4,634,234,000	$ 3,991,204,000
Carrying value of loans outstanding		3,385,302,000	2,924,126,000
Carrying value of deposits		3,449,808,000	3,101,175,000
Recognized Amounts of Identifiable Assets and (Liabilities Assumed), At Fair Value:
Cash and short term investments				10,567,000
Investment securities				41,382,000
Loans				209,592,000
Premises and equipment				1,393,000
Core deposit intangibles				1,000,000
Other intangibles				(238,000)
Other assets				10,749,000
Deposits				(210,135,000)
Borrowings				(38,885,000)
Other Liabilities				(2,187,000)
Total identifiable net assets				23,238,000
Goodwill				14,146,000
Carrying amount of impaired loans		23,800,000
Carrying amount of nonimpaired loans		192,000,000
Deferred tax assets carryforwards recognized		4,800,000
Information about the acquired loan portfolio
Loans				209,592,000
As Acquired
Recognized Amounts of Identifiable Assets and (Liabilities Assumed), At Fair Value:
Cash and short term investments				10,567,000
Investment securities				41,428,000
Loans				215,773,000
Premises and equipment				1,393,000
Other assets				3,081,000
Deposits				(209,707,000)
Borrowings				(35,865,000)
Other Liabilities				(1,978,000)
Total identifiable net assets				24,692,000
Information about the acquired loan portfolio
Loans				215,773,000
Fair Value Adjustments
Recognized Amounts of Identifiable Assets and (Liabilities Assumed), At Fair Value:
Investment securities				(46,000)
Loans				(6,181,000)
Core deposit intangibles				1,000,000
Other intangibles				(238,000)
Other assets				7,668,000
Deposits				(428,000)
Borrowings				(3,020,000)
Other Liabilities				(209,000)
Total identifiable net assets				(1,454,000)
Information about the acquired loan portfolio
Loans				(6,181,000)
CBT
ACQUISITIONS
Number of banking offices operated	8
Number of shares of common stock issued to acquiree's shareholders	965,000
Outstanding common shares				3,600,000
Cash paid per share (in dollars per share)				$ 8.25
Percentage of the outstanding common stock paid in cash				30.00%
Percentage of the outstanding common stock exchanged for entity's stock				70.00%
Stock exchange ratio for each share of acquiree's stock				0.381
Share price per share (in dollars per share)					$ 22.80
Carrying value of assets				268,800,000
Carrying value of loans outstanding				215,800,000
Carrying value of deposits				209,700,000
Consideration Paid:
Berkshire Hills Bancorp common stock issued to CBT common stockholders				21,992,000
Cash consideration paid				8,952,000
Repurchase of CBT's preferred stock and warrant	6,290,000
Cash consideration paid for CBT employee stock options				150,000
Total consideration paid				37,384,000
Recognized Amounts of Identifiable Assets and (Liabilities Assumed), At Fair Value:
Loans				15,347,000
Information about the acquired loan portfolio
Contractually required principal and interest at acquisition				23,726,000
Contractually cash flows not expected to be collected (nonaccretable discount)				(5,563,000)
Expected cash flows at acquisition				18,163,000
Interest component of expected cash flows (accretable discount)				(2,816,000)
Loans				15,347,000
Estimated useful life of core deposits intangible asset	8 years
Acquisition related cost incurred		3,800,000	300,000
CBT | Non-Cumulative Perpetual Preferred Stock, Series A
Consideration Paid:
Repurchase of CBT's preferred stock and warrant	5,400,000
CBT | Warrants
Consideration Paid:
Repurchase of CBT's preferred stock and warrant	800,000
Greenpark
Consideration Paid:
Cash consideration paid	4,000,000
Cash paid for certain prepaid assets	58,000
Payoff of Greenpark's line of credit	46,496,000
Premiums paid for loans, and loan commitments	2,770,000
Contingent purchase price	1,087,000
Total consideration paid	54,411,000
Recognized Amounts of Identifiable Assets and (Liabilities Assumed), At Fair Value:
Loans	48,408,000
Premises and equipment	98,000
Other assets	2,621,000
Other Liabilities	(862,000)
Total identifiable net assets	50,265,000
Goodwill	4,146,000
Information about the acquired loan portfolio
Loans	48,408,000
Acquisition related cost incurred		500,000
Period over which earn out payments will be made to seller	5 years
Greenpark | Minimum
Information about the acquired loan portfolio
Expected annual earn out payments to estimate fair value of the contingent liability	200,000
Greenpark | Maximum
Information about the acquired loan portfolio
Expected annual earn out payments to estimate fair value of the contingent liability	$ 300,000