Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2025
Summary of Significant Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim consolidated financial statements include the accounts of NBT Bancorp Inc. and its wholly-owned subsidiaries mentioned above. In the opinion of management, the interim data includes all adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the results for the interim periods in accordance with U.S. generally accepted accounting principles (“GAAP”) and in accordance with the instructions for the Quarterly Report on Form 10-Q and Article 10 of Regulation S-X as promulgated by the Securities and Exchange Commission (“SEC”). Accordingly, the consolidated financial statements do not include all of the information and notes necessary for complete financial statements in conformity with GAAP. These unaudited interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s 2024 Annual Report on Form 10-K. The results of operations for the interim periods are not necessarily indicative of the results that may be expected for the full year or any other interim period. All material intercompany transactions have been eliminated in consolidation. Amounts previously reported in the consolidated financial statements are reclassified whenever necessary to conform to current period presentation. The Company has evaluated subsequent events for potential recognition and/or disclosure. Refer to Note 16 to the unaudited interim consolidated financial statements in this Quarterly Report on Form 10-Q for the subsequent event related to the Evans Bancorp, Inc. merger.
Use of Estimates in the Preparation of Financial Statements	Use of Estimates in the Preparation of Financial Statements The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results may differ from those estimates and such differences could be material to the financial statements. Estimates associated with the allowance for credit losses are particularly susceptible to material change in the near term.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2025

Summary of Significant Accounting Policies [Abstract]

The accompanying unaudited interim consolidated financial statements include the accounts of NBT Bancorp Inc. and its wholly-owned subsidiaries mentioned above. In the opinion of management, the interim data includes all adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the results for the interim periods in accordance with U.S. generally accepted accounting principles (“GAAP”) and in accordance with the instructions for the Quarterly Report on Form 10-Q and Article 10 of Regulation S-X as promulgated by the Securities and Exchange Commission (“SEC”). Accordingly, the consolidated financial statements do not include all of the information and notes necessary for complete financial statements in conformity with GAAP. These unaudited interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s 2024 Annual Report on Form 10-K. The results of operations for the interim periods are not necessarily indicative of the results that may be expected for the full year or any other interim period. All material intercompany transactions have been eliminated in consolidation. Amounts previously reported in the consolidated financial statements are reclassified whenever necessary to conform to current period presentation. The Company has evaluated subsequent events for potential recognition and/or disclosure. Refer to Note 16 to the unaudited interim consolidated financial statements in this Quarterly Report on Form 10-Q for the subsequent event related to the Evans Bancorp, Inc. merger.

Use of Estimates in the Preparation of Financial Statements

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results may differ from those estimates and such differences could be material to the financial statements. Estimates associated with the allowance for credit losses are particularly susceptible to material change in the near term.