Document and Entity Information (USD $)	9 Months Ended
	Sep. 30, 2012	Oct. 31, 2012	Jun. 30, 2011
Document and Entity Information [Abstract]
Entity Registrant Name	NBT BANCORP INC
Entity Central Index Key	0000790359
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 704,029,832
Entity Common Stock, Shares Outstanding		33,745,327
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q3
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2012