Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale
Amortized Cost	$ 6,069,544	$ 5,145,866
Gross Unrealized Gains	97,052	192,740
Gross Unrealized Losses	(19,453)	(792)
Fair Value	6,147,143	5,337,814
Held-to-maturity
Amortized Cost	1,024,730	189,836
Gross Unrealized Gains	22,542	13,380
Gross Unrealized Losses	(259)	0
Fair Value	1,047,013	203,216
U.S. government and federal agency
Available-for-sale
Amortized Cost	34,544	38,568
Gross Unrealized Gains	243	287
Gross Unrealized Losses	(306)	(267)
Fair Value	34,481	38,588
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	48,303	9,747
Gross Unrealized Gains	11	34
Gross Unrealized Losses	(298)	0
Fair Value	48,016	9,781
State and local governments
Available-for-sale
Amortized Cost	504,524	1,321,763
Gross Unrealized Gains	32,164	94,974
Gross Unrealized Losses	(141)	(54)
Fair Value	536,547	1,416,683
Held-to-maturity
Amortized Cost	1,024,730	189,836
Gross Unrealized Gains	22,542	13,380
Gross Unrealized Losses	(259)	0
Fair Value	1,047,013	203,216
Corporate bonds
Available-for-sale
Amortized Cost	249,058	336,867
Gross Unrealized Gains	8,824	12,239
Gross Unrealized Losses	(4)	(8)
Fair Value	257,878	349,098
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	4,139,439	2,261,463
Gross Unrealized Gains	14,819	27,631
Gross Unrealized Losses	(14,673)	(4)
Fair Value	4,139,585	2,289,090
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,093,676	1,177,458
Gross Unrealized Gains	40,991	57,575
Gross Unrealized Losses	(4,031)	(459)
Fair Value	$ 1,130,636	$ 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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Amount excluding accrued interest, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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