Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	$ 11,551,614	$ 12,138,636
Investment securities available for sale, gross unrealized gains	330,225	174,699
Investment securities available for sale, gross unrealized losses	19,272	70,664
Investment securities available for sale, estimated fair value	11,862,567	12,242,671
Amortized cost for held to maturity	2,409,950	2,859,709
Gross unrealized gains for held to maturity	85,704	54,043
Gross unrealized losses for held to maturity	39,557	49,605
Estimated fair value for held to maturity	2,456,097	2,864,147
Other securities, Amortized cost	461,057	554,059
Other securities, Gross unrealized gains	0	0
Other securities, Gross unrealized losses	0	0
Other, fair value	461,057	554,059
Total Amortized cost	14,422,621	15,552,404
Total for Gross unrealized gains	415,929	228,742
Total for Gross unrealized losses	58,829	120,269
Total for Estimated fair value	14,779,721	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	724,540	299,890
Investment securities available for sale, gross unrealized gains	1,864	294
Investment securities available for sale, gross unrealized losses	3	187
Investment securities available for sale, estimated fair value	726,401	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	4,912	5,924
Investment securities available for sale, gross unrealized gains	108	146
Investment securities available for sale, gross unrealized losses	5	42
Investment securities available for sale, estimated fair value	5,015	6,028
Amortized cost for held to maturity	72,258	118,431
Gross unrealized gains for held to maturity	458	1,003
Gross unrealized losses for held to maturity	220	421
Estimated fair value for held to maturity	72,496	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	10,617,605	11,592,959
Investment securities available for sale, gross unrealized gains	314,088	142,370
Investment securities available for sale, gross unrealized losses	1,031	48,701
Investment securities available for sale, estimated fair value	10,930,662	11,686,628
Amortized cost for held to maturity	2,168,460	2,553,612
Gross unrealized gains for held to maturity	83,630	50,936
Gross unrealized losses for held to maturity	215	7,817
Estimated fair value for held to maturity	2,251,875	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	52	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	51	74
Amortized cost for held to maturity	163,489	181,091
Gross unrealized gains for held to maturity	1,616	2,104
Gross unrealized losses for held to maturity	39,122	41,367
Estimated fair value for held to maturity	125,983	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	134,393	137,556
Investment securities available for sale, gross unrealized gains	1,802	1,514
Investment securities available for sale, gross unrealized losses	17,969	20,190
Investment securities available for sale, estimated fair value	118,226	118,880
Amortized cost for held to maturity	5,743	6,575
Estimated fair value for held to maturity	5,743	6,575
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	70,112	73,795
Investment securities available for sale, gross unrealized gains	12,363	10,230
Investment securities available for sale, gross unrealized losses	263	354
Investment securities available for sale, estimated fair value	$ 82,212	$ 83,671