Acquisition (Details) (USD $)	3 Months Ended		6 Months Ended				0 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Mar. 08, 2013	Dec. 31, 2012	Mar. 08, 2013 Alliance Financial Corporation [Member] Branch	Jun. 30, 2013 Alliance Financial Corporation [Member]	Jun. 30, 2013 Hampshire First Bank [Member]	Jun. 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	$ 1,269,000	$ 826,000	$ 11,950,000	$ 1,337,000				$ 11,950,000
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							225,551,000
Cash in lieu of fractional shares paid to Alliance common shareholders							11,000
Less treasury shares							5,779,000
Total 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
Core deposit intangibles							13,161,000
Other assets							71,823,000
Deposits							(1,113,420,000)
Borrowings							(126,530,000)
Trust Preferred debentures							(25,774,000)
Other liabilities							(18,466,000)
Total identifiable net assets							106,945,000
Goodwill	264,376,000		264,376,000			152,373,000	112,838,000
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			(994,000)
Balance at June 30, 2013	10,331,000		10,331,000
Net Income (Loss) [Abstract]
Net interest income			120,593,000	119,471,000
Noninterest income			55,445,000	52,738,000
Net income			33,911,000	32,104,000
Other goodwill adjustments [Abstract]
Goodwill, Purchase Accounting Adjustments									$ (1,000,000)	$ 100,000