Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025
Cash flows from operating activities:
Net income	$ 42,169	$ 34,410
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	3,684	3,825
Net accretion of discounts on securities	(4,229)	(2,235)
Provision for credit losses	5,347	5,490
Share-based compensation	2,173	1,460
Deferred income tax expense	3,698	843
Proceeds from sale of loans held for sale	22,198	16,049
Originations of loans held for sale	(21,029)	(15,868)
Income on company owned life insurance	(5,656)	(5,742)
Net gain on sale of loans held for sale	(306)	(257)
Net gain on investment securities	(328)	(3)
Net loss on other assets	454	0
Decrease in other assets	3,187	15,854
Decrease in other liabilities	(5,013)	(49,546)
Net cash provided by operating activities	46,349	4,280
Cash flows from investing activities:
Purchases of available for sale securities	(267,583)	(93,434)
Purchases of held to maturity securities	(1,229)	(2,045)
Proceeds from principal payments, maturities and calls on available for sale securities	259,199	40,833
Proceeds from principal payments, maturities and calls on held to maturity securities	6,637	25,868
Proceeds from sales of securities available for sale	18,379	64,955
Net increase in loans	(101,476)	(63,217)
Purchase of company owned life insurance	(29)	(72,905)
Proceeds received from surrender of company owned life insurance	0	73,958
Purchases of premises and equipment	(1,498)	(2,412)
Net cash used in investing activities	(87,600)	(28,399)
Cash flows from financing activities:
Net increase in deposits	93,117	51,283
Net increase in short-term borrowings	73,000	2,000
Repayment of long-term borrowings	(115,000)	(10,000)
Purchases of common stock for treasury	(6,195)	(481)
Cash dividends paid to common and preferred shareholders	(13,248)	(12,970)
Net cash provided by financing activities	31,674	29,832
Net (decrease) increase in cash and cash equivalents	(9,577)	5,713
Cash and cash equivalents, beginning of period	108,751	87,321
Cash and cash equivalents, end of period	$ 99,174	$ 93,034

(Benefit) provision for credit losses.

No definition available.

Name:	fisi_BenefitProvisionForCreditLosses
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.