Acquisition (Details) (Hampshire First Bank [Member], USD $)	9 Months Ended
	Sep. 30, 2012	Jun. 08, 2012
Hampshire First Bank [Member]
Business Acquisition [Line Items]
Date of acquisition	2012-06-08
Number of banking centers operated by acquiree	5
Number of shares of common stock issued to acquiree shareholders (in shares)	1,300,000
Cash paid		$ 17,200,000
Outstanding common shares on acquisition date (in shares)		2,800,000
Share price of outstanding common shares paid for in cash (in dollars per share)	$ 15.00
Percentage of outstanding common shares paid for in cash (in hundredths)	35.00%
Percentage of outstanding common shares exchanged for Company's common stock	65.00%
Exchange ratio of Company's stock for each share of acquiree's stock (in shares)		0.7019
Stock price of shares issued in exchange (in dollars per share)		$ 20.34
Consideration Paid:
NBT Bancorp common stock issued to Hampshire First common stockholders		25,824,000
Cash consideration paid to Hampshire First common stockholders		14,616,000
Cash consideration paid for Hampshire First employee stock options and warrants		2,583,000
Total consideration paid		43,023,000
Recognized Amounts of Identifiable Assets Acquired and (Liabilities Assumed), At Fair Value:
Cash and short term investments		22,149,000
Loans		218,801,000
Federal Home Loan Bank common stock		1,014,000
Core deposit intangibles		797,000
Other assets		12,926,000
Deposits		(228,198,000)
Borrowings		(41,000)
Other liabilities		(2,848,000)
Total identifiable net assets		24,600,000
Goodwill		18,423,000
Information about acquired loan portfolio [Abstract]
Contractually required principal and interest at acquisition		226,631,000
Contractual cash flows not expected to be collected (nonaccretable discount)		(7,985,000)
Expected cash flows at acquisition		218,646,000
Interest component of expected cash flows (accretable discount)		155,000
Fair value of acquired loans		$ 218,801,000
Estimated useful life of core deposit intangible asset (in years)	10