Acquisition (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 08, 2013 Branch
Business Acquisition [Line Items]
Direct costs related acquisition	$ 10,681	$ 511
Alliance Financial Corporation [Member]
Business Acquisition [Line Items]
Date of acquisition	March 8, 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	10,681
Core deposit and trust intangible assets useful Lives, Minimum (in years)	10
Core deposit and trust intangible assets useful Lives, Maximum (in years)	15
Consideration Paid [Abstract]
NBT Bancorp common stock issued			225,551
Cash in lieu of fractional shares paid to Alliance common shareholders			11
Less treasury shares			5,779
Total consideration paid			219,783
Recognized Amounts of Identifiable Assets Acquired and (Liabilities Assumed), At Fair Value [Abstract]
Cash and short term investments			81,060
Securities			320,618
Loans			904,473
Core deposit intangibles			13,161
Other assets			71,900
Deposits			(1,113,420)
Borrowings			(126,530)
Trust preferred debentures			(25,774)
Other liabilities			(17,952)
Total identifiable net assets			107,536
Goodwill			112,247
Information about acquired loan portfolio [Abstract]
Contractually required principal and interest at acquisition			908,614
Contractual cash flows not expected to be collected			(15,466)
Expected cash flows at acquisition			893,148
Interest component of expected cash flows (accretable premium)			11,325
Fair value of acquired loans			904,473
Net Income (Loss) [Abstract]
Net interest income	59,052	59,544
Noninterest income	29,908	27,532
Net income	$ 16,509	$ 16,158