Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Dec. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($)
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	44
Less than 12 months, Fair Value	$ 235,222	$ 1,322,540
Less than 12 months, Unrealized Losses	$ (1,452)	(8,341)
12 months or more, Count | Security	6
12 months or more, Fair Value	$ 2,833	565,188
12 months or more, Unrealized Losses	$ (259)	(12,149)
Total Count | Security	50
Total Fair Value	$ 238,055	1,887,728
Total Unrealized Losses	$ (1,711)	(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)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	16
Less than 12 months, Fair Value	$ 174,234	947,415
Less than 12 months, Unrealized Losses	$ (1,157)	(5,236)
12 months or more, Fair Value		517,824
12 months or more, Unrealized Losses		(11,842)
Total Count | Security	16
Total Fair Value	$ 174,234	1,465,239
Total Unrealized Losses	$ (1,157)	(17,078)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	24
Less than 12 months, Fair Value	$ 55,653	361,440
Less than 12 months, Unrealized Losses	$ (279)	(3,084)
12 months or more, Count | Security	6
12 months or more, Fair Value	$ 2,833	27,501
12 months or more, Unrealized Losses	$ (259)	(142)
Total Count | Security	30
Total Fair Value	$ 58,486	388,941
Total Unrealized Losses	$ (538)	(3,226)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	4
Less than 12 months, Fair Value	$ 5,335	13,685
Less than 12 months, Unrealized Losses	$ (16)	(21)
Total Count | Security	4
Total Fair Value	$ 5,335	13,685
Total Unrealized Losses	$ (16)	$ (21)