Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 2,942,012	$ 3,139,748
Held to Maturity, Gross Unrealized Gains	44,733	27,919
Held to Maturity, Gross Unrealized Losses	15,960	138,004
Held to Maturity, Estimated Fair Value	2,970,785	3,029,663
Available for Sale, Amortized Cost	6,739,379	5,799,135
Available for Sale, Gross Unrealized Gains	173,823	105,276
Available for Sale, Gross Unrealized Losses	1,550	8,975
Available for Sale, Estimated Fair Value	6,911,652	5,895,436
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	248,902	248,592
Held to Maturity, Gross Unrealized Gains	15,844	20,139
Held to Maturity, Gross Unrealized Losses	0	0
Held to Maturity, Estimated Fair Value	264,746	268,731
Available for Sale, Amortized Cost	2,892,208	2,522,159
Available for Sale, Gross Unrealized Gains	16,937	18,395
Available for Sale, Gross Unrealized Losses	889	0
Available for Sale, Estimated Fair Value	2,908,256	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	8,445	9,674
Held to Maturity, Gross Unrealized Gains	67	89
Held to Maturity, Gross Unrealized Losses	20	143
Held to Maturity, Estimated Fair Value	8,492	9,620
Available for Sale, Amortized Cost	1,415,433	1,710,664
Available for Sale, Gross Unrealized Gains	68,839	66,791
Available for Sale, Gross Unrealized Losses	661	1,439
Available for Sale, Estimated Fair Value	1,483,611	1,776,016
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	2,683,315	2,880,482
Held to Maturity, Gross Unrealized Gains	28,822	7,691
Held to Maturity, Gross Unrealized Losses	15,940	137,861
Held to Maturity, Estimated Fair Value	2,696,197	2,750,312
Available for Sale, Amortized Cost	2,388,740	1,476,316
Available for Sale, Gross Unrealized Gains	88,047	20,090
Available for Sale, Gross Unrealized Losses	0	7,492
Available for Sale, Estimated Fair Value	2,476,787	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	42,998	35,972
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,998	$ 35,972