Amortized cost of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.

No definition available.

Name:	nbtb_FinancingReceivableModifiedSubsequentDefault
Namespace Prefix:	nbtb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Allowance for Credit Losses and Credit Quality of Loans, Financing Receivables with Payment Default that Were Modified to Borrowers Experiencing Financial Difficulty (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract]
Financing receivables with payment default that were modified to borrowers experiencing financial difficulty	$ 0	$ 0
Term Extension [Member] \| Residential [Member]
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract]
Financing receivables with payment default that were modified to borrowers experiencing financial difficulty			$ 59	$ 171