Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	$ 13,338,301	$ 12,138,636
Investment securities available for sale, gross unrealized gains	135,286	174,699
Investment securities available for sale, gross unrealized losses	141,515	70,664
Investment securities available for sale, estimated fair value	13,332,072	12,242,671
Amortized cost for held to maturity	2,457,278	2,859,709
Gross unrealized gains for held to maturity	38,662	54,043
Gross unrealized losses for held to maturity	44,718	49,605
Estimated fair value for held to maturity	2,451,222	2,864,147
Other securities, Amortized cost	461,118	554,059
Other securities, Estimated fair value	461,118	554,059
Total Amortized cost	16,256,697	15,552,404
Total for Gross unrealized gains	173,948	228,742
Total for Gross unrealized losses	186,233	120,269
Total for Estimated fair value	16,244,412	15,660,877
U.S. Treasury and Federal Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	1,912,110	299,890
Investment securities available for sale, gross unrealized gains	386	294
Investment securities available for sale, gross unrealized losses	9,952	187
Investment securities available for sale, estimated fair value	1,902,544	299,997
Obligations of States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	3,570	5,924
Investment securities available for sale, gross unrealized gains	77	146
Investment securities available for sale, gross unrealized losses	6	42
Investment securities available for sale, estimated fair value	3,641	6,028
Amortized cost for held to maturity	60,858	118,431
Gross unrealized gains for held to maturity	267	1,003
Gross unrealized losses for held to maturity	224	421
Estimated fair value for held to maturity	60,901	119,013
Government Issued or Guaranteed [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	10,980,507	11,592,959
Investment securities available for sale, gross unrealized gains	88,343	142,370
Investment securities available for sale, gross unrealized losses	113,989	48,701
Investment securities available for sale, estimated fair value	10,954,861	11,686,628
Amortized cost for held to maturity	2,233,173	2,553,612
Gross unrealized gains for held to maturity	37,498	50,936
Gross unrealized losses for held to maturity	7,374	7,817
Estimated fair value for held to maturity	2,263,297	2,596,731
Privately Issued [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	45	74
Investment securities available for sale, gross unrealized gains		2
Investment securities available for sale, gross unrealized losses	1	2
Investment securities available for sale, estimated fair value	44	74
Amortized cost for held to maturity	157,704	181,091
Gross unrealized gains for held to maturity	897	2,104
Gross unrealized losses for held to maturity	37,120	41,367
Estimated fair value for held to maturity	121,481	141,828
Collateralized Debt Obligations [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost		28,438
Investment securities available for sale, gross unrealized gains		20,143
Investment securities available for sale, gross unrealized losses		1,188
Investment securities available for sale, estimated fair value		47,393
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	134,105	137,556
Investment securities available for sale, gross unrealized gains	1,407	1,514
Investment securities available for sale, gross unrealized losses	16,996	20,190
Investment securities available for sale, estimated fair value	118,516	118,880
Amortized cost for held to maturity	5,543	6,575
Estimated fair value for held to maturity	5,543	6,575
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	307,964	73,795
Investment securities available for sale, gross unrealized gains	45,073	10,230
Investment securities available for sale, gross unrealized losses	571	354
Investment securities available for sale, estimated fair value	$ 352,466	$ 83,671