Investment Securities - Amortized Cost and Fair Value of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,163,880	$ 3,410,272
Allowance for Credit Losses	0	(2,195)	$ (2,195)	$ (2,525)
Net Carrying Amount	3,163,880	3,408,077
Gross Unrealized Gains	5,622	3,166
Gross Unrealized (Losses)	(245,006)	(338,604)
Estimated Fair Value	2,924,496	3,072,639
U.S. government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	254,741	297,698
Allowance for Credit Losses	0	0
Net Carrying Amount	254,741	297,698
Gross Unrealized Gains	968	1,164
Gross Unrealized (Losses)	(8,394)	(14,072)
Estimated Fair Value	247,315	284,790
U.S. government-sponsored mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,378,237	1,527,463
Allowance for Credit Losses	0	0
Net Carrying Amount	1,378,237	1,527,463
Gross Unrealized Gains	1,360	760
Gross Unrealized (Losses)	(138,541)	(203,539)
Estimated Fair Value	1,241,056	1,324,684
Private mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	155,155	184,643
Allowance for Credit Losses	0	0
Net Carrying Amount	155,155	184,643
Gross Unrealized Gains	163	0
Gross Unrealized (Losses)	(7,417)	(13,249)
Estimated Fair Value	147,901	171,394
Non-government-sponsored asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	171,752	228,751
Allowance for Credit Losses	0
Net Carrying Amount	171,752	228,751
Gross Unrealized Gains	348	331
Gross Unrealized (Losses)	(1,067)	(3,434)
Estimated Fair Value	171,033	225,648
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	959,533	956,055
Allowance for Credit Losses	0	0
Net Carrying Amount	959,533	956,055
Gross Unrealized Gains	1,015	335
Gross Unrealized (Losses)	(78,219)	(86,029)
Estimated Fair Value	882,329	870,361
Other securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	244,462	215,662
Allowance for Credit Losses	0	(2,195)
Net Carrying Amount	244,462	213,467
Gross Unrealized Gains	1,768	576
Gross Unrealized (Losses)	(11,368)	(18,281)
Estimated Fair Value	$ 234,862	$ 195,762

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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