Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Jun. 30, 2020 USD ($) Security	Dec. 31, 2019 USD ($)
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	69
Less than 12 months, Fair Value	$ 438,557	$ 1,322,540
Less than 12 months, Unrealized Losses	$ (752)	(8,341)
12 months or more, Count | Security	3
12 months or more, Fair Value	$ 1,060	565,188
12 months or more, Unrealized Losses	$ (5)	(12,149)
Total Count | Security	72
Total Fair Value	$ 439,617	1,887,728
Total Unrealized Losses	$ (757)	(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	12
Less than 12 months, Fair Value	$ 314,609
Less than 12 months, Unrealized Losses	$ (50)
Total Count | Security	12
Total Fair Value	$ 314,609
Total Unrealized Losses	$ (50)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	14
Less than 12 months, Fair Value	$ 80,375	947,415
Less than 12 months, Unrealized Losses	$ (371)	(5,236)
12 months or more, Fair Value		517,824
12 months or more, Unrealized Losses		(11,842)
Total Count | Security	14
Total Fair Value	$ 80,375	1,465,239
Total Unrealized Losses	$ (371)	(17,078)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	43
Less than 12 months, Fair Value	$ 43,573	361,440
Less than 12 months, Unrealized Losses	$ (331)	(3,084)
12 months or more, Count | Security	3
12 months or more, Fair Value	$ 1,060	27,501
12 months or more, Unrealized Losses	$ (5)	(142)
Total Count | Security	46
Total Fair Value	$ 44,633	388,941
Total Unrealized Losses	$ (336)	(3,226)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value		13,685
Less than 12 months, Unrealized Losses		(21)
Total Fair Value		13,685
Total Unrealized Losses		$ (21)