Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Held-to-maturity
Amortized Cost	$ 3,761,094	$ 1,530,500
Allowance for Credit Losses	(1,388)	(1,279)	$ (2,915)
Net Carrying Amount	3,759,706	1,529,221
Gross Unrealized Gains	283	6,398
Gross Unrealized (Losses)	(696,756)	(18,241)
Estimated Fair Value	3,063,233	1,517,378
Available-for-sale
Amortized Cost	4,331,413	7,130,861
Allowance for Credit Losses	0	0	(312)
Gross Unrealized Gains	373	44,005
Gross Unrealized (Losses)	(478,932)	(61,321)
Estimated Fair Value	3,852,854	7,113,545
U.S. Treasury
Available-for-sale
Amortized Cost	2,257	300
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(60)	0
Estimated Fair Value	2,197	300
U.S. Government agencies
Held-to-maturity
Amortized Cost	448,012	232,609
Allowance for Credit Losses	0	0
Net Carrying Amount	448,012	232,609
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(102,558)	(7,914)
Estimated Fair Value	345,454	224,695
Available-for-sale
Amortized Cost	191,498	374,754
Allowance for Credit Losses	0	0
Gross Unrealized Gains	103	495
Gross Unrealized (Losses)	(7,322)	(10,608)
Estimated Fair Value	184,279	364,641
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,190,781	70,342
Allowance for Credit Losses	0	0
Net Carrying Amount	1,190,781	70,342
Gross Unrealized Gains	227	232
Gross Unrealized (Losses)	(118,960)	(1,425)
Estimated Fair Value	1,072,048	69,149
Available-for-sale
Amortized Cost	2,809,319	4,485,548
Allowance for Credit Losses	0	0
Gross Unrealized Gains	20	6,307
Gross Unrealized (Losses)	(266,437)	(43,239)
Estimated Fair Value	2,542,902	4,448,616
State and political subdivisions
Held-to-maturity
Amortized Cost	1,861,102	1,210,248
Allowance for Credit Losses	(110)	(1,197)	(2,307)
Net Carrying Amount	1,860,992	1,209,051
Gross Unrealized Gains	56	6,166
Gross Unrealized (Losses)	(446,198)	(8,462)
Estimated Fair Value	1,414,850	1,206,755
Available-for-sale
Amortized Cost	1,056,124	1,791,097
Allowance for Credit Losses	0	0	(217)
Gross Unrealized Gains	250	30,556
Gross Unrealized (Losses)	(185,300)	(1,995)
Estimated Fair Value	871,074	1,819,658
Other securities
Held-to-maturity
Amortized Cost	261,199	17,301
Allowance for Credit Losses	(1,278)	(82)	(608)
Net Carrying Amount	259,921	17,219
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(29,040)	(440)
Estimated Fair Value	230,881	16,779
Available-for-sale
Amortized Cost	272,215	479,162
Allowance for Credit Losses	0	0	$ (95)
Gross Unrealized Gains	0	6,647
Gross Unrealized (Losses)	(19,813)	(5,479)
Estimated Fair Value	$ 252,402	$ 480,330

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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