Investment Securities - Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,261,392	$ 5,313,209
Unrealized Gains	195,791	93,542
Unrealized Losses	(9,912)	(21,660)
Allowance for Credit Losses for Investments	0	0
Fair Value	5,447,271	5,385,091
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,388	17,567
Unrealized Gains	343	117
Unrealized Losses	0	(2)
Allowance for Credit Losses for Investments	0	0
Fair Value	35,731	17,682
U.S. government-sponsored entities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	449,789	596,595
Unrealized Gains	3,906	1,027
Unrealized Losses	(194)	(4,638)
Allowance for Credit Losses for Investments	0	0
Fair Value	453,501	592,984
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,199,892	3,151,550
Unrealized Gains	105,353	41,363
Unrealized Losses	(1,191)	(9,052)
Allowance for Credit Losses for Investments	0	0
Fair Value	3,304,054	3,183,861
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,283,874	1,232,497
Unrealized Gains	72,547	44,193
Unrealized Losses	(462)	(1,047)
Allowance for Credit Losses for Investments	0	0
Fair Value	1,355,959	1,275,643
Pooled trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,785	13,811
Unrealized Gains	0	0
Unrealized Losses	(6,600)	(5,589)
Allowance for Credit Losses for Investments	0	0
Fair Value	7,185	8,222
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	278,664	301,189
Unrealized Gains	13,642	6,842
Unrealized Losses	(1,465)	(1,332)
Allowance for Credit Losses for Investments	0	0
Fair Value	$ 290,841	$ 306,699