Acquisition (Details) (USD $)	3 Months Ended		9 Months Ended			0 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Mar. 08, 2013 Alliance Financial Corporation [Member] Branch	Sep. 30, 2013 Alliance Financial Corporation [Member]	Sep. 30, 2012 Alliance Financial Corporation [Member]	Sep. 30, 2013 Hampshire First Bank [Member]	Sep. 30, 2013 Acquisition of Financial Services Company [Member]
Business Acquisition [Line Items]
Date of acquisition							Mar. 08, 2013
Number of banking centers operated by acquiree						26
Business acquisition shares converted right to receive entity's common stock (in shares)						2.1779
Number of shares of common stock issued to acquiree shareholders (in shares)							10,300,000
Direct costs related acquisition	$ 326,000	$ 558,000	$ 12,276,000	$ 1,895,000			$ 12,300,000
Core deposit and trust intangible assets useful Live, Minimum							10 years
Core deposit and trust intangible assets useful Lives, Maximum							15 years
Consideration paid [Abstract]
NBT Bancorp common stock issued to Alliance common shareholders						225,551,000
Cash in lieu of fractional shares paid to Alliance common shareholders						11,000
Less treasury shares						5,779,000
Net consideration paid						219,783,000
Recognized Amounts of Identifiable Assets Acquired and (Liabilities Assumed), At Fair Value [Abstract]
Cash and short term investments						81,060,000
Securities						320,618,000
Loans						904,473,000
Intangible assets						13,161,000
Other assets						71,589,000
Deposits						(1,113,420,000)
Borrowings						(126,530,000)
Trust Preferred debentures						(25,774,000)
Other liabilities						(18,908,000)
Total identifiable net assets						106,269,000
Goodwill	265,052,000		265,052,000		152,373,000	113,514,000
Information about acquired loan portfolio [Abstract]
Contractually required principal and interest at acquisition						908,614,000
Contractual cash flows not expected to be collected						(15,466,000)
Expected cash flows at acquisition						893,148,000
Interest component of expected cash flows (accretable premium)						11,325,000
Fair value of acquired loans						904,473,000
Changes in accretable discount [Abstract]
Balance at January 1, 2013							0
Alliance acquisition							11,325,000
Accretion recognized to date							(1,950,000)
Balance at September 30, 2013							9,375,000
Net Income (Loss) [Abstract]
Net interest income							182,610,000	182,081,000
Noninterest income							82,592,000	78,950,000
Net income							52,933,000	48,609,000
Other goodwill adjustments [Abstract]
Goodwill, purchase accounting adjustments									$ (1,000,000)	$ 100,000