Investment Securities - Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Available-for-Sale
Amortized Cost	$ 9,218,249	$ 7,384,033
Unrealized Gains	54,315	112,282
Unrealized Losses	(467,917)	(118,235)
Basis Adjustments	(54,043)	3,986
Fair Value	8,750,604	7,382,066
Held-to-Maturity
Amortized Cost	2,082,242	0
Unrealized Gains	42
Unrealized Losses	(126,688)
Basis Adjustments	0
Allowance for securities held-to-maturity	(160)
Fair Value	1,955,596	0
Total held-to-maturity securities	2,082,242
U.S. Treasury
Available-for-Sale
Amortized Cost	440,461	234,555
Unrealized Gains	0	1,233
Unrealized Losses	(8,887)	(7,751)
Basis Adjustments	(12,550)	7,547
Fair Value	419,024	235,584
U.S. government-sponsored entities and agencies
Available-for-Sale
Amortized Cost	1,730,259	1,575,994
Unrealized Gains	41,507	7,354
Unrealized Losses	(150,253)	(37,014)
Basis Adjustments	(41,493)	(3,561)
Fair Value	1,580,020	1,542,773
Held-to-Maturity
Amortized Cost	528,524
Unrealized Gains	0
Unrealized Losses	(48,115)
Basis Adjustments	0
Fair Value	480,409
Mortgage-backed securities
Available-for-Sale
Amortized Cost	5,904,666	3,737,484
Unrealized Gains	2,535	27,421
Unrealized Losses	(283,459)	(66,074)
Basis Adjustments	0	0
Fair Value	5,623,742	3,698,831
Held-to-Maturity
Amortized Cost	463,111
Unrealized Gains	0
Unrealized Losses	(22,057)
Basis Adjustments	0
Fair Value	441,054
States and political subdivisions
Available-for-Sale
Amortized Cost	750,442	1,587,172
Unrealized Gains	9,499	69,696
Unrealized Losses	(9,885)	(1,882)
Basis Adjustments	0	0
Fair Value	750,056	1,654,986
Held-to-Maturity
Amortized Cost	1,090,667
Unrealized Gains	42
Unrealized Losses	(56,516)
Basis Adjustments	0
Fair Value	1,034,193
Pooled trust preferred securities
Available-for-Sale
Amortized Cost	13,762	13,756
Unrealized Gains	0	0
Unrealized Losses	(4,097)	(4,260)
Basis Adjustments	0	0
Fair Value	9,665	9,496
Other securities
Available-for-Sale
Amortized Cost	378,659	235,072
Unrealized Gains	774	6,578
Unrealized Losses	(11,336)	(1,254)
Basis Adjustments	0	0
Fair Value	368,097	$ 240,396
Held-to-Maturity
Amortized Cost	100
Unrealized Gains	0
Unrealized Losses	0
Basis Adjustments	0
Fair Value	$ 100

Debt Securities, Available-For-Sale, Accumulated Basis Adjustments, Before Tax

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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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