Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	$ 5,784,754	$ 5,313,209
Unrealized Gains	197,714	93,542
Unrealized Losses	(11,385)	(21,660)
Basis Adjustments	(968)	0
Fair Value	5,970,115	5,385,091
U.S. Treasury
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	9,909	17,567
Unrealized Gains	299	117
Unrealized Losses	0	(2)
Basis Adjustments	0	0
Fair Value	10,208	17,682
U.S. government-sponsored entities and agencies
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	841,133	596,595
Unrealized Gains	5,744	1,027
Unrealized Losses	(3,921)	(4,638)
Basis Adjustments	(968)	0
Fair Value	841,988	592,984
Mortgage-backed securities
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	3,249,002	3,151,550
Unrealized Gains	91,086	41,363
Unrealized Losses	(990)	(9,052)
Basis Adjustments	0	0
Fair Value	3,339,098	3,183,861
States and political subdivisions
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	1,405,868	1,232,497
Unrealized Gains	86,325	44,193
Unrealized Losses	(31)	(1,047)
Basis Adjustments	0	0
Fair Value	1,492,162	1,275,643
Pooled trust preferred securities
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	13,763	13,811
Unrealized Gains	0	0
Unrealized Losses	(5,850)	(5,589)
Basis Adjustments	0	0
Fair Value	7,913	8,222
Other securities
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Amortized Cost	265,079	301,189
Unrealized Gains	14,260	6,842
Unrealized Losses	(593)	(1,332)
Basis Adjustments	0	0
Fair Value	$ 278,746	$ 306,699

Schedule of available for sale and held to maturity securities.

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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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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 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 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 of allowance for credit loss 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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