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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Held-to-maturity
Amortized Cost	$ 932,484		$ 335,946
Allowance for credit losses on HTM	(1,132)	$ (1,618)	(2,915)	$ (307)	$ (409)	$ 0
Net Carrying Amount	931,352		333,031
Gross Unrealized Gains	9,550		8,924
Gross Unrealized (Losses)	(5,306)		(30)
Estimated Fair Value	935,596		341,925
Available-for-sale
Amortized Cost	6,543,022		3,397,043
Allowance for credit losses on AFS	0	(2,454)	(312)	(609)	(269)	0
Gross Unrealized Gains	59,361		79,663
Gross Unrealized (Losses)	(45,802)		(2,796)
Estimated Fair Value	6,556,581		3,473,598
U.S. Treasury
Available-for-sale
Amortized Cost	600
Allowance for credit losses on AFS	0
Gross Unrealized Gains	0
Gross Unrealized (Losses)	0
Estimated Fair Value	600
U.S. Government agencies
Held-to-maturity
Amortized Cost	77,396
Allowance for credit losses on HTM	0
Net Carrying Amount	77,396
Gross Unrealized Gains	0
Gross Unrealized (Losses)	(2,054)
Estimated Fair Value	75,342
Available-for-sale
Amortized Cost	567,722		477,693
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	720		844
Gross Unrealized (Losses)	(13,505)		(1,300)
Estimated Fair Value	554,937		477,237
Mortgage-backed securities
Held-to-maturity
Amortized Cost	60,649		22,354
Allowance for credit losses on HTM	0		0
Net Carrying Amount	60,649		22,354
Gross Unrealized Gains	474		683
Gross Unrealized (Losses)	(729)		0
Estimated Fair Value	60,394		23,037
Available-for-sale
Amortized Cost	3,996,823		1,374,769
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	13,405		21,261
Gross Unrealized (Losses)	(23,019)		(1,094)
Estimated Fair Value	3,987,209		1,394,936
State and political subdivisions
Held-to-maturity
Amortized Cost	794,178		312,416
Allowance for credit losses on HTM	(871)	(1,042)	(2,307)	(95)	(97)	0
Net Carrying Amount	793,307		310,109
Gross Unrealized Gains	8,118		8,148
Gross Unrealized (Losses)	(2,523)		(30)
Estimated Fair Value	798,902		318,227
Available-for-sale
Amortized Cost	1,526,833		1,416,136
Allowance for credit losses on AFS	0	(64)	(217)	(371)	(95)	0
Gross Unrealized Gains	36,945		55,111
Gross Unrealized (Losses)	(6,281)		(307)
Estimated Fair Value	1,557,497		1,470,723
Other securities
Held-to-maturity
Amortized Cost	261		1,176
Allowance for credit losses on HTM	(261)	(576)	(608)	(212)	(312)	0
Net Carrying Amount	0		568
Gross Unrealized Gains	958		93
Gross Unrealized (Losses)	0		0
Estimated Fair Value	958		661
Available-for-sale
Amortized Cost	451,044		128,445
Allowance for credit losses on AFS	0	$ (2,390)	(95)	$ (238)	$ (174)	$ 0
Gross Unrealized Gains	8,291		2,447
Gross Unrealized (Losses)	(2,997)		(95)
Estimated Fair Value	$ 456,338		$ 130,702

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