Investment Securities (Amortized Cost and Fair Value of Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	$ 1,047,694	$ 958,142
Securities available for sale, Unrealized Gains	5,250	10,611
Securities available for sale, Unrealized Losses	49,247	46,281
Securities available for sale	1,003,697	922,472
Securities held to maturity, Amortized Cost	82,076	84,710
Securities held to maturity, Unrealized Gains	3	28
Securities held to maturity, Unrealized Losses	8,796	8,482
Securities held to maturity, fair value	73,283	76,256
Allowance for credit losses, Amortized Cost	(2)	(2)
Held-to-maturity securities, net , Amortized Cost	82,074	84,708
Residential Mortgage [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	440,697	472,898
Securities available for sale, Unrealized Gains	3,857	8,445
Securities available for sale, Unrealized Losses	21,007	20,775
Securities available for sale	423,547	460,568
Securities held to maturity, Amortized Cost	21,837	22,319
Securities held to maturity, Unrealized Gains	0	0
Securities held to maturity, Unrealized Losses	2,243	2,191
Securities held to maturity, fair value	19,594	20,128
Commercial Loan [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	5,082	5,089
Securities available for sale, Unrealized Gains	72	62
Securities available for sale, Unrealized Losses	0	0
Securities available for sale	5,154	5,151
Securities held to maturity, Amortized Cost	4,958	4,965
Securities held to maturity, Unrealized Gains	0	0
Securities held to maturity, Unrealized Losses	687	693
Securities held to maturity, fair value	4,271	4,272
Residential Collateralized Mortgage [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	126,197	117,118
Securities available for sale, Unrealized Gains	737	1,409
Securities available for sale, Unrealized Losses	3,186	3,095
Securities available for sale	123,748	115,432
Securities held to maturity, Amortized Cost	16,143	17,172
Securities held to maturity, Unrealized Gains	0	0
Securities held to maturity, Unrealized Losses	952	887
Securities held to maturity, fair value	15,191	16,285
Commercial Collateralized Mortgage [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	317,726	318,429
Securities available for sale, Unrealized Gains	15	225
Securities available for sale, Unrealized Losses	24,594	22,174
Securities available for sale	293,147	296,480
Securities held to maturity, Amortized Cost	583	612
Securities held to maturity, Unrealized Gains	0	0
Securities held to maturity, Unrealized Losses	11	10
Securities held to maturity, fair value	572	602
U.S. Government Agency And Government Sponsored Enterprises [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities held to maturity, Amortized Cost	6,851	6,813
Securities held to maturity, Unrealized Gains	0	0
Securities held to maturity, Unrealized Losses	162	124
Securities held to maturity, fair value	6,689	6,689
U.S. Treasury Bills [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	99,791
Securities available for sale, Unrealized Gains	0
Securities available for sale, Unrealized Losses	1
Securities available for sale	99,790
State And Political Subdivisions [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities held to maturity, Amortized Cost	31,704	32,829
Securities held to maturity, Unrealized Gains	3	28
Securities held to maturity, Unrealized Losses	4,741	4,577
Securities held to maturity, fair value	26,966	28,280
Mortgage-Backed Securities [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	889,702	913,534
Securities available for sale, Unrealized Gains	4,681	10,141
Securities available for sale, Unrealized Losses	48,787	46,044
Securities available for sale	845,596	877,631
Securities held to maturity, Amortized Cost	43,521	45,068
Securities held to maturity, Unrealized Gains	0	0
Securities held to maturity, Unrealized Losses	3,893	3,781
Securities held to maturity, fair value	39,628	41,287
Other Debt Obligations [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Amortized Cost	58,201	44,608
Securities available for sale, Unrealized Gains	569	470
Securities available for sale, Unrealized Losses	459	237
Securities available for sale	$ 58,311	$ 44,841

FINANCIAL INSTITUTIONS INC 10-Q