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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Held-to-maturity
Amortized Cost	$ 1,518,076		$ 335,946
Allowance for credit losses on HTM	(1,279)	$ (1,132)	(2,915)	$ (373)	$ (307)	$ 0
Net Carrying Amount	1,516,797		333,031
Gross Unrealized Gains	2,623		8,924
Gross Unrealized (Losses)	(31,504)		(30)
Estimated Fair Value	1,487,916		341,925
Available-for-sale
Amortized Cost	6,838,743		3,397,043
Allowance for credit losses on AFS	0	0	(312)	(1,208)	(609)	0
Gross Unrealized Gains	40,581		79,663
Gross Unrealized (Losses)	(57,121)		(2,796)
Estimated Fair Value	6,822,203		3,473,598
U.S. Treasury
Available-for-sale
Amortized Cost	300
Allowance for credit losses on AFS	0
Gross Unrealized Gains	0
Gross Unrealized (Losses)	0
Estimated Fair Value	300
U.S. Government agencies
Held-to-maturity
Amortized Cost	232,549
Allowance for credit losses on HTM	0
Net Carrying Amount	232,549
Gross Unrealized Gains	0
Gross Unrealized (Losses)	(5,179)
Estimated Fair Value	227,370
Available-for-sale
Amortized Cost	361,452		477,693
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	635		844
Gross Unrealized (Losses)	(7,705)		(1,300)
Estimated Fair Value	354,382		477,237
Mortgage-backed securities
Held-to-maturity
Amortized Cost	57,930		22,354
Allowance for credit losses on HTM	0		0
Net Carrying Amount	57,930		22,354
Gross Unrealized Gains	417		683
Gross Unrealized (Losses)	(1,024)		0
Estimated Fair Value	57,323		23,037
Available-for-sale
Amortized Cost	4,434,981		1,374,769
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	14,741		21,261
Gross Unrealized (Losses)	(28,102)		(1,094)
Estimated Fair Value	4,421,620		1,394,936
State and political subdivisions
Held-to-maturity
Amortized Cost	1,210,287		312,416
Allowance for credit losses on HTM	(1,196)	(871)	(2,307)	(73)	(95)	0
Net Carrying Amount	1,209,091		310,109
Gross Unrealized Gains	2,206		8,148
Gross Unrealized (Losses)	(24,962)		(30)
Estimated Fair Value	1,186,335		318,227
Available-for-sale
Amortized Cost	1,574,674		1,416,136
Allowance for credit losses on AFS	0	0	(217)	(1,148)	(371)	0
Gross Unrealized Gains	17,877		55,111
Gross Unrealized (Losses)	(17,343)		(307)
Estimated Fair Value	1,575,208		1,470,723
Other securities
Held-to-maturity
Amortized Cost	17,310		1,176
Allowance for credit losses on HTM	(83)	(261)	(608)	(300)	(212)	0
Net Carrying Amount	17,227		568
Gross Unrealized Gains	0		93
Gross Unrealized (Losses)	(339)		0
Estimated Fair Value	16,888		661
Available-for-sale
Amortized Cost	467,336		128,445
Allowance for credit losses on AFS	0	$ 0	(95)	$ (60)	$ (238)	$ 0
Gross Unrealized Gains	7,328		2,447
Gross Unrealized (Losses)	(3,971)		(95)
Estimated Fair Value	$ 470,693		$ 130,702