Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	$ 3,525,188	$ 1,790,321
Less than 12 months, Unrealized Losses	(24,120)	(11,037)
12 months or more, Fair Value	538,079	1,025,349
12 months or more, Unrealized Losses	(18,335)	(23,973)
Total Fair Value	4,063,267	2,815,670
Total Unrealized Losses	(42,455)	(35,010)
U.S. Treasury
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	344,556	236,591
Less than 12 months, Unrealized Losses	(576)	(329)
12 months or more, Fair Value		14,863
12 months or more, Unrealized Losses		(196)
Total Fair Value	344,556	251,454
Total Unrealized Losses	(576)	(525)
U.S. Agencies
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	615,993	387,999
Less than 12 months, Unrealized Losses	(1,032)	(689)
12 months or more, Fair Value		81,593
12 months or more, Unrealized Losses		(486)
Total Fair Value	615,993	469,592
Total Unrealized Losses	(1,032)	(1,175)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	2,056,316	727,142
Less than 12 months, Unrealized Losses	(21,013)	(8,370)
12 months or more, Fair Value	426,959	616,044
12 months or more, Unrealized Losses	(17,076)	(18,504)
Total Fair Value	2,483,275	1,343,186
Total Unrealized Losses	(38,089)	(26,874)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	479,197	401,934
Less than 12 months, Unrealized Losses	(1,316)	(1,406)
12 months or more, Fair Value	60,324	226,678
12 months or more, Unrealized Losses	(779)	(3,760)
Total Fair Value	539,521	628,612
Total Unrealized Losses	(2,095)	(5,166)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	29,126	36,655
Less than 12 months, Unrealized Losses	(183)	(243)
12 months or more, Fair Value	50,796	86,171
12 months or more, Unrealized Losses	(480)	(1,027)
Total Fair Value	79,922	122,826
Total Unrealized Losses	$ (663)	$ (1,270)