Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Other securities, Amortized cost	$ 336,544	$ 328,742
Other securities, Gross unrealized gains	0	0
Other securities, Gross unrealized losses	0	0
Other securities, Estimated fair value	336,544	328,742
Investment securities available for sale, amortized cost	10,923,234	8,919,324
Investment securities available for sale, gross unrealized gains	274,610	258,851
Investment securities available for sale, gross unrealized losses	38,335	21,243
Investment securities available for sale, estimated fair value	11,159,509	9,156,932
Amortized cost for held to maturity	2,998,486	3,507,868
Gross unrealized gains for held to maturity	72,509	82,179
Gross unrealized losses for held to maturity	45,308	51,765
Estimated fair value for held to maturity	3,025,687	3,538,282
Total Amortized cost	14,258,264	12,755,934
Total for Gross unrealized gains	347,119	341,030
Total for Gross unrealized losses	83,643	73,008
Total for Estimated fair value	14,521,740	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	6,174	8,027
Investment securities available for sale, gross unrealized gains	170	224
Investment securities available for sale, gross unrealized losses	48	53
Investment securities available for sale, estimated fair value	6,296	8,198
Amortized cost for held to maturity	125,251	148,961
Gross unrealized gains for held to maturity	1,395	2,551
Gross unrealized losses for held to maturity	347	189
Estimated fair value for held to maturity	126,299	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	10,504,756	8,507,571
Investment securities available for sale, gross unrealized gains	228,333	223,889
Investment securities available for sale, gross unrealized losses	19,041	337
Investment securities available for sale, estimated fair value	10,714,048	8,731,123
Amortized cost for held to maturity	2,679,546	3,149,320
Gross unrealized gains for held to maturity	69,486	78,485
Gross unrealized losses for held to maturity	4,787	7,000
Estimated fair value for held to maturity	2,744,245	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	82	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	82	103
Amortized cost for held to maturity	186,883	201,733
Gross unrealized gains for held to maturity	1,628	1,143
Gross unrealized losses for held to maturity	40,174	44,576
Estimated fair value for held to maturity	148,337	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	136,793	138,240
Investment securities available for sale, gross unrealized gains	1,650	1,896
Investment securities available for sale, gross unrealized losses	17,975	18,648
Investment securities available for sale, estimated fair value	120,468	121,488
Amortized cost for held to maturity	6,806	7,854
Estimated fair value for held to maturity	6,806	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	197,764	161,408
Investment securities available for sale, gross unrealized gains	1,630	544
Investment securities available for sale, gross unrealized losses		5
Investment securities available for sale, estimated fair value	199,394	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	28,467	30,073
Investment securities available for sale, gross unrealized gains	22,465	21,276
Investment securities available for sale, gross unrealized losses	1,056	1,033
Investment securities available for sale, estimated fair value	49,876	50,316
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	49,198	73,901
Investment securities available for sale, gross unrealized gains	20,360	11,020
Investment securities available for sale, gross unrealized losses	213	1,164
Investment securities available for sale, estimated fair value	$ 69,345	$ 83,757