Merger and Acquisition Activity (Details) (USD $)	12 Months Ended					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Alliance Financial Corporation [Member]	Dec. 31, 2012 Hampshire First Bank [Member]	Jun. 08, 2012 Hampshire First Bank [Member]	Dec. 31, 2012 Other Acquisitions [Member]
Business Acquisition [Line Items]
Date of acquisition				2012-06-08
Date of acquisition agreement			2012-10-07
Assets of entity to be acquired			$ 1,400,000,000
Number of banking centers operated by acquiree			26	5
Cost of entity to be acquired contracted purchase price			233,400,000
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 (in hundredths)				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
Goodwill acquired during period	20,344,000	17,188,000				1,900,000
Other intangible assets acquired during period						1,400,000
Consideration Paid [Abstract]
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 [Abstract]
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					(7,985,000)
Expected cash flows at acquisition					218,646,000
Interest component of expected cash flows (accretable premium)					155,000
Fair value of acquired loans					218,801,000
Loans acquired during period fair value discount					$ 5,700,000