Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale
Amortized Cost	$ 7,328,737	$ 5,145,866
Gross Unrealized Gains	87,459	192,740
Gross Unrealized Losses	(25,616)	(792)
Fair Value	7,390,580	5,337,814
Held-to-maturity
Amortized Cost	1,128,299	189,836
Gross Unrealized Gains	20,131	13,380
Gross Unrealized Losses	(1,977)	0
Fair Value	1,146,453	203,216
U.S. government and federal agency
Available-for-sale
Amortized Cost	31,449	38,568
Gross Unrealized Gains	207	287
Gross Unrealized Losses	(283)	(267)
Fair Value	31,373	38,588
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	47,324	9,747
Gross Unrealized Gains	6	34
Gross Unrealized Losses	(279)	0
Fair Value	47,051	9,781
State and local governments
Available-for-sale
Amortized Cost	479,116	1,321,763
Gross Unrealized Gains	29,666	94,974
Gross Unrealized Losses	(91)	(54)
Fair Value	508,691	1,416,683
Held-to-maturity
Amortized Cost	1,128,299	189,836
Gross Unrealized Gains	20,131	13,380
Gross Unrealized Losses	(1,977)	0
Fair Value	1,146,453	203,216
Corporate bonds
Available-for-sale
Amortized Cost	191,164	336,867
Gross Unrealized Gains	6,958	12,239
Gross Unrealized Losses	(3)	(8)
Fair Value	198,119	349,098
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	5,498,676	2,261,463
Gross Unrealized Gains	13,982	27,631
Gross Unrealized Losses	(21,313)	(4)
Fair Value	5,491,345	2,289,090
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,081,008	1,177,458
Gross Unrealized Gains	36,640	57,575
Gross Unrealized Losses	(3,647)	(459)
Fair Value	$ 1,114,001	$ 1,234,574

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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Amount, before tax, of unrealized loss 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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Amortized cost excluding accrued interest, before allowance for credit loss, of 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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Amount excluding accrued interest, of 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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