Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Mar. 31, 2020 USD ($) Security	Dec. 31, 2018 USD ($)
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	284
Less than 12 months, Fair Value	$ 387,828	$ 1,322,540
Less than 12 months, Unrealized Losses	$ (7,799)	(8,341)
12 months or more, Count | Security	14
12 months or more, Fair Value	$ 35,385	565,188
12 months or more, Unrealized Losses	$ (508)	(12,149)
Total Count | Security	298
Total Fair Value	$ 423,213	1,887,728
Total Unrealized Losses	$ (8,307)	(20,490)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
12 months or more, Fair Value		19,863
12 months or more, Unrealized Losses		(165)
Total Fair Value		19,863
Total Unrealized Losses		(165)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	1
Less than 12 months, Fair Value	$ 4,406
Total Count | Security	1
Total Fair Value	$ 4,406
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	36
Less than 12 months, Fair Value	$ 159,593	947,415
Less than 12 months, Unrealized Losses	$ (4,079)	(5,236)
12 months or more, Count | Security	10
12 months or more, Fair Value	$ 34,227	517,824
12 months or more, Unrealized Losses	$ (499)	(11,842)
Total Count | Security	46
Total Fair Value	$ 193,820	1,465,239
Total Unrealized Losses	$ (4,578)	(17,078)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	184
Less than 12 months, Fair Value	$ 127,235	361,440
Less than 12 months, Unrealized Losses	$ (1,267)	(3,084)
12 months or more, Count | Security	4
12 months or more, Fair Value	$ 1,158	27,501
12 months or more, Unrealized Losses	$ (9)	(142)
Total Count | Security	188
Total Fair Value	$ 128,393	388,941
Total Unrealized Losses	$ (1,276)	(3,226)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	63
Less than 12 months, Fair Value	$ 96,594	13,685
Less than 12 months, Unrealized Losses	$ (2,453)	(21)
Total Count | Security	63
Total Fair Value	$ 96,594	13,685
Total Unrealized Losses	$ (2,453)	$ (21)