Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Held-to-maturity
Amortized Cost	$ 3,710,472	$ 3,729,502
Allowance for credit losses on HTM	(3,214)	(3,214)	$ (1,888)	$ (1,388)
Net Carrying Amount	3,707,258	3,726,288
Gross Unrealized Gains	324	638
Gross Unrealized (Losses)	(658,300)	(591,556)
Estimated Fair Value	3,049,282	3,135,370
Available-for-sale
Amortized Cost	3,394,971	3,509,709
Allowance for Credit Losses	0	0	(5,800)	0
Gross Unrealized Gains	232	230
Gross Unrealized (Losses)	(367,645)	(357,786)
Estimated Fair Value	3,027,558	3,152,153
U.S. Government agencies
Held-to-maturity
Amortized Cost	453,805	453,121
Allowance for credit losses on HTM	0	0
Net Carrying Amount	453,805	453,121
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(94,887)	(89,203)
Estimated Fair Value	358,918	363,918
Available-for-sale
Amortized Cost	71,768	74,460
Allowance for Credit Losses	0	0
Gross Unrealized Gains	28	35
Gross Unrealized (Losses)	(1,995)	(1,993)
Estimated Fair Value	69,801	72,502
U.S. Treasury
Available-for-sale
Amortized Cost	1,990	2,285
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(26)	(31)
Estimated Fair Value	1,964	2,254
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,142,352	1,161,694
Allowance for credit losses on HTM	0	0
Net Carrying Amount	1,142,352	1,161,694
Gross Unrealized Gains	238	354
Gross Unrealized (Losses)	(126,553)	(107,834)
Estimated Fair Value	1,016,037	1,054,214
Available-for-sale
Amortized Cost	2,048,895	2,138,652
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3	8
Gross Unrealized (Losses)	(203,534)	(198,353)
Estimated Fair Value	1,845,364	1,940,307
State and political subdivisions
Held-to-maturity
Amortized Cost	1,857,894	1,858,680
Allowance for credit losses on HTM	(2,252)	(2,006)	(362)	(110)
Net Carrying Amount	1,855,642	1,856,674
Gross Unrealized Gains	86	284
Gross Unrealized (Losses)	(410,749)	(369,509)
Estimated Fair Value	1,444,979	1,487,449
Available-for-sale
Amortized Cost	1,013,190	1,035,147
Allowance for Credit Losses	0	0	0	0
Gross Unrealized Gains	142	187
Gross Unrealized (Losses)	(138,483)	(132,541)
Estimated Fair Value	874,849	902,793
Other securities
Held-to-maturity
Amortized Cost	256,421	256,007
Allowance for credit losses on HTM	(962)	(1,208)	(1,526)	(1,278)
Net Carrying Amount	255,459	254,799
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(26,111)	(25,010)
Estimated Fair Value	229,348	229,789
Available-for-sale
Amortized Cost	259,128	259,165
Allowance for Credit Losses	0	0	$ (5,800)	$ 0
Gross Unrealized Gains	59	0
Gross Unrealized (Losses)	(23,607)	(24,868)
Estimated Fair Value	$ 235,580	$ 234,297