Investment Securities - Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,189,363	$ 5,313,209
Unrealized Gains	183,360	93,542
Unrealized Losses	(12,698)	(21,660)
Allowance for Credit Losses for Investments	0	0
Fair Value	5,360,025	5,385,091
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,379	17,567
Unrealized Gains	354	117
Unrealized Losses	0	(2)
Allowance for Credit Losses for Investments	0	0
Fair Value	11,733	17,682
U.S. government-sponsored entities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	515,217	596,595
Unrealized Gains	3,954	1,027
Unrealized Losses	0	(4,638)
Allowance for Credit Losses for Investments	0	0
Fair Value	519,171	592,984
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,089,212	3,151,550
Unrealized Gains	121,282	41,363
Unrealized Losses	(494)	(9,052)
Allowance for Credit Losses for Investments	0	0
Fair Value	3,210,000	3,183,861
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,251,515	1,232,497
Unrealized Gains	51,841	44,193
Unrealized Losses	(961)	(1,047)
Allowance for Credit Losses for Investments	0	0
Fair Value	1,302,395	1,275,643
Pooled trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,798	13,811
Unrealized Gains	0	0
Unrealized Losses	(6,376)	(5,589)
Allowance for Credit Losses for Investments	0	0
Fair Value	7,422	8,222
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	308,242	301,189
Unrealized Gains	5,929	6,842
Unrealized Losses	(4,867)	(1,332)
Allowance for Credit Losses for Investments	0	0
Fair Value	$ 309,304	$ 306,699