Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Other securities, Amortized cost	$ 554,059	$ 328,742
Other securities, Gross unrealized gains	0	0
Other securities, Gross unrealized losses	0	0
Other securities, Estimated fair value	554,059	328,742
Investment securities available for sale, amortized cost	12,138,636	8,919,324
Investment securities available for sale, gross unrealized gains	174,699	258,851
Investment securities available for sale, gross unrealized losses	70,664	21,243
Investment securities available for sale, estimated fair value	12,242,671	9,156,932
Amortized cost for held to maturity	2,859,709	3,507,868
Gross unrealized gains for held to maturity	54,043	82,179
Gross unrealized losses for held to maturity	49,605	51,765
Estimated fair value for held to maturity	2,864,147	3,538,282
Total Amortized cost	15,552,404	12,755,934
Total for Gross unrealized gains	228,742	341,030
Total for Gross unrealized losses	120,269	73,008
Total for Estimated fair value	15,660,877	13,023,956
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	5,924	8,027
Investment securities available for sale, gross unrealized gains	146	224
Investment securities available for sale, gross unrealized losses	42	53
Investment securities available for sale, estimated fair value	6,028	8,198
Amortized cost for held to maturity	118,431	148,961
Gross unrealized gains for held to maturity	1,003	2,551
Gross unrealized losses for held to maturity	421	189
Estimated fair value for held to maturity	119,013	151,323
Government Issued or Guaranteed [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	11,592,959	8,507,571
Investment securities available for sale, gross unrealized gains	142,370	223,889
Investment securities available for sale, gross unrealized losses	48,701	337
Investment securities available for sale, estimated fair value	11,686,628	8,731,123
Amortized cost for held to maturity	2,553,612	3,149,320
Gross unrealized gains for held to maturity	50,936	78,485
Gross unrealized losses for held to maturity	7,817	7,000
Estimated fair value for held to maturity	2,596,731	3,220,805
Privately Issued [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	74	104
Investment securities available for sale, gross unrealized gains	2	2
Investment securities available for sale, gross unrealized losses	2	3
Investment securities available for sale, estimated fair value	74	103
Amortized cost for held to maturity	181,091	201,733
Gross unrealized gains for held to maturity	2,104	1,143
Gross unrealized losses for held to maturity	41,367	44,576
Estimated fair value for held to maturity	141,828	158,300
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	137,556	138,240
Investment securities available for sale, gross unrealized gains	1,514	1,896
Investment securities available for sale, gross unrealized losses	20,190	18,648
Investment securities available for sale, estimated fair value	118,880	121,488
Amortized cost for held to maturity	6,575	7,854
Estimated fair value for held to maturity	6,575	7,854
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	299,890	161,408
Investment securities available for sale, gross unrealized gains	294	544
Investment securities available for sale, gross unrealized losses	187	5
Investment securities available for sale, estimated fair value	299,997	161,947
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	30,073
Investment securities available for sale, gross unrealized gains	20,143	21,276
Investment securities available for sale, gross unrealized losses	1,188	1,033
Investment securities available for sale, estimated fair value	47,393	50,316
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	73,795	73,901
Investment securities available for sale, gross unrealized gains	10,230	11,020
Investment securities available for sale, gross unrealized losses	354	1,164
Investment securities available for sale, estimated fair value	$ 83,671	$ 83,757