Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 3,065,867	$ 3,139,748
Held to Maturity, Gross Unrealized Gains	38,762	27,919
Held to Maturity, Gross Unrealized Losses	32,285	138,004
Held to Maturity, Estimated Fair Value	3,072,344	3,029,663
Available for Sale, Amortized Cost	6,160,101	5,799,135
Available for Sale, Gross Unrealized Gains	163,776	105,276
Available for Sale, Gross Unrealized Losses	934	8,975
Available for Sale, Estimated Fair Value	6,322,943	5,895,436
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	248,797	248,592
Held to Maturity, Gross Unrealized Gains	18,000	20,139
Held to Maturity, Gross Unrealized Losses	0	0
Held to Maturity, Estimated Fair Value	266,797	268,731
Available for Sale, Amortized Cost	2,449,708	2,522,159
Available for Sale, Gross Unrealized Gains	19,080	18,395
Available for Sale, Gross Unrealized Losses	20	0
Available for Sale, Estimated Fair Value	2,468,768	2,540,554
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Amortized Cost	0	54,024
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	44
Available for Sale, Estimated Fair Value	0	53,980
Residential mortgage- backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	9,330	9,674
Held to Maturity, Gross Unrealized Gains	91	89
Held to Maturity, Gross Unrealized Losses	0	143
Held to Maturity, Estimated Fair Value	9,421	9,620
Available for Sale, Amortized Cost	1,514,030	1,710,664
Available for Sale, Gross Unrealized Gains	77,194	66,791
Available for Sale, Gross Unrealized Losses	663	1,439
Available for Sale, Estimated Fair Value	1,590,561	1,776,016
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	2,806,390	2,880,482
Held to Maturity, Gross Unrealized Gains	20,671	7,691
Held to Maturity, Gross Unrealized Losses	32,285	137,861
Held to Maturity, Estimated Fair Value	2,794,776	2,750,312
Available for Sale, Amortized Cost	2,153,351	1,476,316
Available for Sale, Gross Unrealized Gains	67,502	20,090
Available for Sale, Gross Unrealized Losses	251	7,492
Available for Sale, Estimated Fair Value	2,220,602	1,488,914
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,350	1,000
Held to Maturity, Gross Unrealized Gains	0	0
Held to Maturity, Gross Unrealized Losses	0	0
Held to Maturity, Estimated Fair Value	1,350	1,000
Available for Sale, Amortized Cost	43,012	35,972
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 43,012	$ 35,972