Description of Business (Policies)	9 Months Ended
Sep. 30, 2018
Description of Business [Abstract]
Nature of Operations	NBT Bancorp Inc. (the “Registrant” or the “Company”) is a registered financial holding company incorporated in the state of Delaware in 1986, with its principal headquarters located in Norwich, New York. The principal assets of the Registrant consist of all of the outstanding shares of common stock of its subsidiaries, including: NBT Bank, National Association (the “Bank”), NBT Financial Services, Inc. (“NBT Financial”), NBT Holdings, Inc. (“NBT Holdings”), Hathaway Agency, Inc. and CNBF Capital Trust I, NBT Statutory Trust I, NBT Statutory Trust II, Alliance Financial Capital Trust I and Alliance Financial Capital Trust II (collectively, the “Trusts”). The Company’s principal sources of revenue are the management fees and dividends it receives from the Bank, NBT Financial and NBT Holdings. The Company’s business, primarily conducted through the Bank but also through its other subsidiaries, consists of providing commercial banking, retail banking and wealth management services primarily to customers in its market area, which includes central and upstate New York, northeastern Pennsylvania, southern New Hampshire, western Massachusetts, Vermont and the southern coastal area of Maine. The Company has been, and intends to continue to be, a community-oriented financial institution offering a variety of financial services. The Company’s business philosophy is to operate as a community bank with local decision-making providing a broad array of banking and financial services to retail, commercial and municipal customers.

NBT Bancorp Inc. (the “Registrant” or the “Company”) is a registered financial holding company incorporated in the state of Delaware in 1986, with its principal headquarters located in Norwich, New York. The principal assets of the Registrant consist of all of the outstanding shares of common stock of its subsidiaries, including: NBT Bank, National Association (the “Bank”), NBT Financial Services, Inc. (“NBT Financial”), NBT Holdings, Inc. (“NBT Holdings”), Hathaway Agency, Inc. and CNBF Capital Trust I, NBT Statutory Trust I, NBT Statutory Trust II, Alliance Financial Capital Trust I and Alliance Financial Capital Trust II (collectively, the “Trusts”). The Company’s principal sources of revenue are the management fees and dividends it receives from the Bank, NBT Financial and NBT Holdings.

The Company’s business, primarily conducted through the Bank but also through its other subsidiaries, consists of providing commercial banking, retail banking and wealth management services primarily to customers in its market area, which includes central and upstate New York, northeastern Pennsylvania, southern New Hampshire, western Massachusetts, Vermont and the southern coastal area of Maine. The Company has been, and intends to continue to be, a community-oriented financial institution offering a variety of financial services. The Company’s business philosophy is to operate as a community bank with local decision-making providing a broad array of banking and financial services to retail, commercial and municipal customers.