Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	$ 395,084	$ 3,525,188
Less than 12 months, Unrealized Losses	(963)	(24,120)
12 months or more, Fair Value	365,254	538,079
12 months or more, Unrealized Losses	(6,312)	(18,335)
Total Fair Value	760,338	4,063,267
Total Unrealized Losses	(7,275)	(42,455)
U.S. Treasury
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	5,025	344,556
Less than 12 months, Unrealized Losses	(5)	(576)
Total Fair Value	5,025	344,556
Total Unrealized Losses	(5)	(576)
U.S. Agencies
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	45,635	615,993
Less than 12 months, Unrealized Losses	(24)	(1,032)
12 months or more, Fair Value	3,036
12 months or more, Unrealized Losses	(7)
Total Fair Value	48,671	615,993
Total Unrealized Losses	(31)	(1,032)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	204,221	2,056,316
Less than 12 months, Unrealized Losses	(744)	(21,013)
12 months or more, Fair Value	312,994	426,959
12 months or more, Unrealized Losses	(6,186)	(17,076)
Total Fair Value	517,215	2,483,275
Total Unrealized Losses	(6,930)	(38,089)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	131,455	479,197
Less than 12 months, Unrealized Losses	(188)	(1,316)
12 months or more, Fair Value	10,155	60,324
12 months or more, Unrealized Losses	(55)	(779)
Total Fair Value	141,610	539,521
Total Unrealized Losses	(243)	(2,095)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	8,748	29,126
Less than 12 months, Unrealized Losses	(2)	(183)
12 months or more, Fair Value	39,069	50,796
12 months or more, Unrealized Losses	(64)	(480)
Total Fair Value	47,817	79,922
Total Unrealized Losses	$ (66)	$ (663)