Investment Securities - Unrealized Losses and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	$ 72,674	$ 134,631
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(1,076)	(3,744)
Fair Value of Available-for-Sale Securities, 12 Months or More	2,405,247	2,566,329
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(243,930)	(334,860)
Fair Value of Available-for-Sale Securities, Total	2,477,921	2,700,960
Unrealized Losses of Available-for-Sale Securities, Total	(245,006)	(338,604)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	19,050	77,866
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	(524)	(3,016)
Fair Value of Held-to-Maturity Securities, 12 Months or More	1,099,378	1,064,094
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(107,001)	(129,287)
Fair Value of Held-to-Maturity Securities, Total	1,118,428	1,141,960
Unrealized Losses of Held-to-Maturity Securities	(107,525)	(132,303)
U.S. government-sponsored enterprises
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	16,429	25,946
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(64)	(326)
Fair Value of Available-for-Sale Securities, 12 Months or More	164,831	161,759
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(8,330)	(13,746)
Fair Value of Available-for-Sale Securities, Total	181,260	187,705
Unrealized Losses of Available-for-Sale Securities, Total	(8,394)	(14,072)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	0	0
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	0	0
Fair Value of Held-to-Maturity Securities, 12 Months or More	42,247	40,539
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(1,523)	(3,021)
Fair Value of Held-to-Maturity Securities, Total	42,247	40,539
Unrealized Losses of Held-to-Maturity Securities	(1,523)	(3,021)
U.S. government-sponsored mortgage-backed securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	15,657	34,597
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(22)	(1,088)
Fair Value of Available-for-Sale Securities, 12 Months or More	1,128,946	1,215,317
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(138,519)	(202,451)
Fair Value of Available-for-Sale Securities, Total	1,144,603	1,249,914
Unrealized Losses of Available-for-Sale Securities, Total	(138,541)	(203,539)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	6,583	48,254
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	(42)	(1,979)
Fair Value of Held-to-Maturity Securities, 12 Months or More	74,627	69,220
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(3,324)	(4,716)
Fair Value of Held-to-Maturity Securities, Total	81,210	117,474
Unrealized Losses of Held-to-Maturity Securities	(3,366)	(6,695)
Private mortgage-backed securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	0	9,491
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	0	(129)
Fair Value of Available-for-Sale Securities, 12 Months or More	138,047	161,903
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(7,417)	(13,120)
Fair Value of Available-for-Sale Securities, Total	138,047	171,394
Unrealized Losses of Available-for-Sale Securities, Total	(7,417)	(13,249)
Non-government-sponsored asset backed securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	642	10,849
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(1)	(60)
Fair Value of Available-for-Sale Securities, 12 Months or More	56,007	92,857
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(1,066)	(3,374)
Fair Value of Available-for-Sale Securities, Total	56,649	103,706
Unrealized Losses of Available-for-Sale Securities, Total	(1,067)	(3,434)
State and political subdivisions
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	28,597	46,591
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(911)	(1,230)
Fair Value of Available-for-Sale Securities, 12 Months or More	750,624	761,289
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(77,308)	(84,799)
Fair Value of Available-for-Sale Securities, Total	779,221	807,880
Unrealized Losses of Available-for-Sale Securities, Total	(78,219)	(86,029)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	12,467	29,612
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	(482)	(1,037)
Fair Value of Held-to-Maturity Securities, 12 Months or More	982,504	954,335
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(102,154)	(121,550)
Fair Value of Held-to-Maturity Securities, Total	994,971	983,947
Unrealized Losses of Held-to-Maturity Securities	(102,636)	(122,587)
Other securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	11,349	7,157
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(78)	(911)
Fair Value of Available-for-Sale Securities, 12 Months or More	166,792	173,204
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(11,290)	(17,370)
Fair Value of Available-for-Sale Securities, Total	178,141	180,361
Unrealized Losses of Available-for-Sale Securities, Total	$ (11,368)	$ (18,281)

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