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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Held-to-maturity
Amortized Cost	$ 3,821,063		$ 1,530,500
Allowance for credit losses on HTM	(1,381)	$ (1,377)	(1,279)	$ (1,132)	$ (1,618)	$ (2,915)
Net Carrying Amount	3,819,682		1,529,221
Gross Unrealized Gains	4,276		6,398
Gross Unrealized (Losses)	(544,996)		(18,241)
Estimated Fair Value	3,278,962		1,517,378
Available-for-sale
Amortized Cost	4,730,178		7,130,861
Allowance for Credit Losses	0		0	0	(2,454)	(312)
Gross Unrealized Gains	1,506		44,005
Gross Unrealized (Losses)	(390,037)		(61,321)
Estimated Fair Value	4,341,647		7,113,545
U.S. Treasury
Available-for-sale
Amortized Cost	1,447		300
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(6)		0
Estimated Fair Value	1,441		300
U.S. Government agencies
Held-to-maturity
Amortized Cost	446,789		232,609
Allowance for credit losses on HTM	0		0
Net Carrying Amount	446,789		232,609
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(67,286)		(7,914)
Estimated Fair Value	379,503		224,695
Available-for-sale
Amortized Cost	202,528		374,754
Allowance for Credit Losses	0		0
Gross Unrealized Gains	339		495
Gross Unrealized (Losses)	(4,534)		(10,608)
Estimated Fair Value	198,333		364,641
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,244,713		70,342
Allowance for credit losses on HTM	0		0
Net Carrying Amount	1,244,713		70,342
Gross Unrealized Gains	3,951		232
Gross Unrealized (Losses)	(61,982)		(1,425)
Estimated Fair Value	1,186,682		69,149
Available-for-sale
Amortized Cost	3,150,519		4,485,548
Allowance for Credit Losses	0		0
Gross Unrealized Gains	4		6,307
Gross Unrealized (Losses)	(186,589)		(43,239)
Estimated Fair Value	2,963,934		4,448,616
State and political subdivisions
Held-to-maturity
Amortized Cost	1,869,027		1,210,248
Allowance for credit losses on HTM	(103)	(1,285)	(1,197)	(871)	(1,042)	(2,307)
Net Carrying Amount	1,868,924		1,209,051
Gross Unrealized Gains	240		6,166
Gross Unrealized (Losses)	(397,081)		(8,462)
Estimated Fair Value	1,472,083		1,206,755
Available-for-sale
Amortized Cost	1,103,260		1,791,097
Allowance for Credit Losses	0		0	0	(64)	(217)
Gross Unrealized Gains	518		30,556
Gross Unrealized (Losses)	(188,523)		(1,995)
Estimated Fair Value	915,255		1,819,658
Other securities
Held-to-maturity
Amortized Cost	260,534		17,301
Allowance for credit losses on HTM	(1,278)	$ (92)	(82)	(261)	(576)	(608)
Net Carrying Amount	259,256		17,219
Gross Unrealized Gains	85		0
Gross Unrealized (Losses)	(18,647)		(440)
Estimated Fair Value	240,694		16,779
Available-for-sale
Amortized Cost	272,424		479,162
Allowance for Credit Losses	0		0	$ 0	$ (2,390)	$ (95)
Gross Unrealized Gains	645		6,647
Gross Unrealized (Losses)	(10,385)		(5,479)
Estimated Fair Value	$ 262,684		$ 480,330