Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Other securities, Amortized cost	$ 536,062	$ 554,059
Other securities, Gross unrealized gains	0	0
Other securities, Gross unrealized losses	0	0
Other securities, Estimated fair value	536,062	554,059
Investment securities available for sale, amortized cost	11,937,326	12,138,636
Investment securities available for sale, gross unrealized gains	297,018	174,699
Investment securities available for sale, gross unrealized losses	33,697	70,664
Investment securities available for sale, estimated fair value	12,200,647	12,242,671
Amortized cost for held to maturity	2,730,611	2,859,709
Gross unrealized gains for held to maturity	81,182	54,043
Gross unrealized losses for held to maturity	42,450	49,605
Estimated fair value for held to maturity	2,769,343	2,864,147
Total Amortized cost	15,203,999	15,552,404
Total for Gross unrealized gains	378,200	228,742
Total for Gross unrealized losses	76,147	120,269
Total for Estimated fair value	15,506,052	15,660,877
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	5,356	5,924
Investment securities available for sale, gross unrealized gains	138	146
Investment securities available for sale, gross unrealized losses	46	42
Investment securities available for sale, estimated fair value	5,448	6,028
Amortized cost for held to maturity	103,408	118,431
Gross unrealized gains for held to maturity	886	1,003
Gross unrealized losses for held to maturity	332	421
Estimated fair value for held to maturity	103,962	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	11,490,181	11,592,959
Investment securities available for sale, gross unrealized gains	265,879	142,370
Investment securities available for sale, gross unrealized losses	5,998	48,701
Investment securities available for sale, estimated fair value	11,750,062	11,686,628
Amortized cost for held to maturity	2,445,563	2,553,612
Gross unrealized gains for held to maturity	78,448	50,936
Gross unrealized losses for held to maturity	2,070	7,817
Estimated fair value for held to maturity	2,521,941	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	65	74
Investment securities available for sale, gross unrealized gains	2	2
Investment securities available for sale, gross unrealized losses	2	2
Investment securities available for sale, estimated fair value	65	74
Amortized cost for held to maturity	175,467	181,091
Gross unrealized gains for held to maturity	1,848	2,104
Gross unrealized losses for held to maturity	40,048	41,367
Estimated fair value for held to maturity	137,267	141,828
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	136,968	137,556
Investment securities available for sale, gross unrealized gains	1,407	1,514
Investment securities available for sale, gross unrealized losses	25,667	20,190
Investment securities available for sale, estimated fair value	112,708	118,880
Amortized cost for held to maturity	6,173	6,575
Estimated fair value for held to maturity	6,173	6,575
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	201,002	299,890
Investment securities available for sale, gross unrealized gains	1,197	294
Investment securities available for sale, gross unrealized losses	7	187
Investment securities available for sale, estimated fair value	202,192	299,997
Collateralized Debt Obligations [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	28,483	28,438
Investment securities available for sale, gross unrealized gains	18,170	20,143
Investment securities available for sale, gross unrealized losses	1,613	1,188
Investment securities available for sale, estimated fair value	45,040	47,393
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	75,271	73,795
Investment securities available for sale, gross unrealized gains	10,225	10,230
Investment securities available for sale, gross unrealized losses	364	354
Investment securities available for sale, estimated fair value	$ 85,132	$ 83,671