Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	$ 3,642,815	$ 3,104,606
Less than 12 months, Unrealized Losses	(78,219)	(27,391)
12 months or more, Fair Value	1,776,555	1,967,685
12 months or more, Unrealized Losses	(98,381)	(61,929)
Total Fair Value	5,419,370	5,072,291
Total Unrealized Losses	(176,600)	(89,320)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	9,802	9,851
Less than 12 months, Unrealized Losses	(134)	(64)
12 months or more, Fair Value	28,288	28,792
12 months or more, Unrealized Losses	(1,874)	(1,385)
Total Fair Value	38,090	38,643
Total Unrealized Losses	(2,008)	(1,449)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	198	14,553
Less than 12 months, Unrealized Losses	(2)	(10)
Total Fair Value	198	14,553
Total Unrealized Losses	(2)	(10)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	2,143,258	1,990,006
Less than 12 months, Unrealized Losses	(57,180)	(19,980)
12 months or more, Fair Value	1,407,240	1,562,333
12 months or more, Unrealized Losses	(79,847)	(52,060)
Total Fair Value	3,550,498	3,552,339
Total Unrealized Losses	(137,027)	(72,040)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	1,488,089	1,076,930
Less than 12 months, Unrealized Losses	(20,901)	(7,325)
12 months or more, Fair Value	341,027	376,560
12 months or more, Unrealized Losses	(16,660)	(8,484)
Total Fair Value	1,829,116	1,453,490
Total Unrealized Losses	(37,561)	(15,809)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	1,468	13,266
Less than 12 months, Unrealized Losses	(2)	(12)
Total Fair Value	1,468	13,266
Total Unrealized Losses	$ (2)	$ (12)