Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	$ 10,938,796	$ 13,338,301
Investment securities available for sale, gross unrealized gains	85,154	135,286
Investment securities available for sale, gross unrealized losses	127,666	141,515
Investment securities available for sale, estimated fair value	10,896,284	13,332,072
Amortized cost for held to maturity	3,353,213	2,457,278
Gross unrealized gains for held to maturity	29,919	38,662
Gross unrealized losses for held to maturity	41,370	44,718
Estimated fair value for held to maturity	3,341,762	2,451,222
Other securities, Amortized cost	415,028	461,118
Other securities, Estimated fair value	415,028	461,118
Total Amortized cost	14,707,037	16,256,697
Total for Gross unrealized gains	115,073	173,948
Total for Gross unrealized losses	169,036	186,233
Total for Estimated fair value	14,653,074	16,244,412
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	1,965,665	1,912,110
Investment securities available for sale, gross unrealized gains		386
Investment securities available for sale, gross unrealized losses	18,178	9,952
Investment securities available for sale, estimated fair value	1,947,487	1,902,544
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	2,555	3,570
Investment securities available for sale, gross unrealized gains	36	77
Investment securities available for sale, gross unrealized losses	2	6
Investment securities available for sale, estimated fair value	2,589	3,641
Amortized cost for held to maturity	24,562	60,858
Gross unrealized gains for held to maturity	109	267
Gross unrealized losses for held to maturity	49	224
Estimated fair value for held to maturity	24,622	60,901
Government Issued or Guaranteed [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	8,755,482	10,980,507
Investment securities available for sale, gross unrealized gains	59,497	88,343
Investment securities available for sale, gross unrealized losses	98,587	113,989
Investment securities available for sale, estimated fair value	8,716,392	10,954,861
Amortized cost for held to maturity	3,187,953	2,233,173
Gross unrealized gains for held to maturity	27,236	37,498
Gross unrealized losses for held to maturity	13,746	7,374
Estimated fair value for held to maturity	3,201,443	2,263,297
Privately Issued [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	28	45
Investment securities available for sale, gross unrealized losses		1
Investment securities available for sale, estimated fair value	28	44
Amortized cost for held to maturity	135,688	157,704
Gross unrealized gains for held to maturity	2,574	897
Gross unrealized losses for held to maturity	27,575	37,120
Estimated fair value for held to maturity	110,687	121,481
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	136,905	134,105
Investment securities available for sale, gross unrealized gains	2,402	1,407
Investment securities available for sale, gross unrealized losses	10,475	16,996
Investment securities available for sale, estimated fair value	128,832	118,516
Amortized cost for held to maturity	5,010	5,543
Estimated fair value for held to maturity	5,010	5,543
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	78,161	307,964
Investment securities available for sale, gross unrealized gains	23,219	45,073
Investment securities available for sale, gross unrealized losses	424	571
Investment securities available for sale, estimated fair value	$ 100,956	$ 352,466