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Investment Securities - Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,660,220	$ 5,784,754
Unrealized Gains	193,244	197,714
Unrealized Losses	(82,422)	(11,385)
Basis Adjustments (1)	(46,512)	(968)
Fair Value	6,724,530	5,970,115
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	236,427	9,909
Unrealized Gains	10,910	299
Unrealized Losses	(8,337)	0
Basis Adjustments (1)	(10,110)	0
Fair Value	228,890	10,208
U.S. government-sponsored entities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,412,926	841,133
Unrealized Gains	39,527	5,744
Unrealized Losses	(42,518)	(3,921)
Basis Adjustments (1)	(36,402)	(968)
Fair Value	1,373,533	841,988
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,338,842	3,249,002
Unrealized Gains	66,916	91,086
Unrealized Losses	(20,419)	(990)
Basis Adjustments (1)	0	0
Fair Value	3,385,339	3,339,098
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,405,386	1,405,868
Unrealized Gains	66,711	86,325
Unrealized Losses	(4,293)	(31)
Basis Adjustments (1)	0	0
Fair Value	1,467,804	1,492,162
Pooled trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,753	13,763
Unrealized Gains	0	0
Unrealized Losses	(5,543)	(5,850)
Basis Adjustments (1)	0	0
Fair Value	8,210	7,913
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	252,886	265,079
Unrealized Gains	9,180	14,260
Unrealized Losses	(1,312)	(593)
Basis Adjustments (1)	0	0
Fair Value	$ 260,754	$ 278,746

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