Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale
Amortized Cost	$ 5,750,190	$ 5,145,866
Gross Unrealized Gains	131,176	192,740
Gross Unrealized Losses	(28,051)	(792)
Fair Value	5,853,315	5,337,814
Held-to-maturity
Amortized Cost	588,751	189,836
Gross Unrealized Gains	10,937	13,380
Gross Unrealized Losses	(728)	0
Fair Value	598,960	203,216
U.S. government and federal agency
Available-for-sale
Amortized Cost	36,122	38,568
Gross Unrealized Gains	231	287
Gross Unrealized Losses	(490)	(267)
Fair Value	35,863	38,588
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	5,231	9,747
Gross Unrealized Gains	22	34
Gross Unrealized Losses	0	0
Fair Value	5,253	9,781
State and local governments
Available-for-sale
Amortized Cost	914,572	1,321,763
Gross Unrealized Gains	64,593	94,974
Gross Unrealized Losses	(513)	(54)
Fair Value	978,652	1,416,683
Held-to-maturity
Amortized Cost	588,751	189,836
Gross Unrealized Gains	10,937	13,380
Gross Unrealized Losses	(728)	0
Fair Value	598,960	203,216
Corporate bonds
Available-for-sale
Amortized Cost	278,914	336,867
Gross Unrealized Gains	9,952	12,239
Gross Unrealized Losses	(85)	(8)
Fair Value	288,781	349,098
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	3,369,683	2,261,463
Gross Unrealized Gains	16,420	27,631
Gross Unrealized Losses	(22,476)	(4)
Fair Value	3,363,627	2,289,090
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,145,668	1,177,458
Gross Unrealized Gains	39,958	57,575
Gross Unrealized Losses	(4,487)	(459)
Fair Value	$ 1,181,139	$ 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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