Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Other securities, Amortized cost	$ 469,689	$ 554,059
Other securities, Gross unrealized gains	0	0
Other securities, Gross unrealized losses	0	0
Other securities, Estimated fair value	469,689	554,059
Investment securities available for sale, amortized cost	11,578,829	12,138,636
Investment securities available for sale, gross unrealized gains	365,681	174,699
Investment securities available for sale, gross unrealized losses	25,536	70,664
Investment securities available for sale, estimated fair value	11,918,974	12,242,671
Amortized cost for held to maturity	2,574,421	2,859,709
Gross unrealized gains for held to maturity	90,165	54,043
Gross unrealized losses for held to maturity	41,327	49,605
Estimated fair value for held to maturity	2,623,259	2,864,147
Total Amortized cost	14,622,939	15,552,404
Total for Gross unrealized gains	455,846	228,742
Total for Gross unrealized losses	66,863	120,269
Total for Estimated fair value	15,011,922	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	5,158	5,924
Investment securities available for sale, gross unrealized gains	131	146
Investment securities available for sale, gross unrealized losses	48	42
Investment securities available for sale, estimated fair value	5,241	6,028
Amortized cost for held to maturity	84,614	118,431
Gross unrealized gains for held to maturity	712	1,003
Gross unrealized losses for held to maturity	240	421
Estimated fair value for held to maturity	85,086	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	10,944,676	11,592,959
Investment securities available for sale, gross unrealized gains	323,828	142,370
Investment securities available for sale, gross unrealized losses	1,226	48,701
Investment securities available for sale, estimated fair value	11,267,278	11,686,628
Amortized cost for held to maturity	2,315,012	2,553,612
Gross unrealized gains for held to maturity	88,526	50,936
Gross unrealized losses for held to maturity	350	7,817
Estimated fair value for held to maturity	2,403,188	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	58	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	57	74
Amortized cost for held to maturity	168,784	181,091
Gross unrealized gains for held to maturity	927	2,104
Gross unrealized losses for held to maturity	40,737	41,367
Estimated fair value for held to maturity	128,974	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	135,170	137,556
Investment securities available for sale, gross unrealized gains	1,271	1,514
Investment securities available for sale, gross unrealized losses	21,670	20,190
Investment securities available for sale, estimated fair value	114,771	118,880
Amortized cost for held to maturity	6,011	6,575
Estimated fair value for held to maturity	6,011	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	397,825	299,890
Investment securities available for sale, gross unrealized gains	2,537	294
Investment securities available for sale, gross unrealized losses	14	187
Investment securities available for sale, estimated fair value	400,348	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	28,255	28,438
Investment securities available for sale, gross unrealized gains	17,466	20,143
Investment securities available for sale, gross unrealized losses	2,416	1,188
Investment securities available for sale, estimated fair value	43,305	47,393
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	67,687	73,795
Investment securities available for sale, gross unrealized gains	20,448	10,230
Investment securities available for sale, gross unrealized losses	161	354
Investment securities available for sale, estimated fair value	$ 87,974	$ 83,671