Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	$ 12,640,244	$ 13,338,301
Investment securities available for sale, gross unrealized gains	125,261	135,286
Investment securities available for sale, gross unrealized losses	134,505	141,515
Investment securities available for sale, estimated fair value	12,631,000	13,332,072
Amortized cost for held to maturity	2,876,119	2,457,278
Gross unrealized gains for held to maturity	36,201	38,662
Gross unrealized losses for held to maturity	46,435	44,718
Estimated fair value for held to maturity	2,865,885	2,451,222
Other securities, Amortized cost	461,296	461,118
Other securities, Estimated fair value	461,296	461,118
Total Amortized cost	15,977,659	16,256,697
Total for Gross unrealized gains	161,462	173,948
Total for Gross unrealized losses	180,940	186,233
Total for Estimated fair value	15,958,181	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	1,907,573	1,912,110
Investment securities available for sale, gross unrealized gains	181	386
Investment securities available for sale, gross unrealized losses	9,504	9,952
Investment securities available for sale, estimated fair value	1,898,250	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	3,051	3,570
Investment securities available for sale, gross unrealized gains	77	77
Investment securities available for sale, gross unrealized losses	4	6
Investment securities available for sale, estimated fair value	3,124	3,641
Amortized cost for held to maturity	46,836	60,858
Gross unrealized gains for held to maturity	233	267
Gross unrealized losses for held to maturity	156	224
Estimated fair value for held to maturity	46,913	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	10,525,601	10,980,507
Investment securities available for sale, gross unrealized gains	87,830	88,343
Investment securities available for sale, gross unrealized losses	109,117	113,989
Investment securities available for sale, estimated fair value	10,504,314	10,954,861
Amortized cost for held to maturity	2,670,966	2,233,173
Gross unrealized gains for held to maturity	34,777	37,498
Gross unrealized losses for held to maturity	10,577	7,374
Estimated fair value for held to maturity	2,695,166	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	42	45
Investment securities available for sale, gross unrealized losses	1	1
Investment securities available for sale, estimated fair value	41	44
Amortized cost for held to maturity	153,036	157,704
Gross unrealized gains for held to maturity	1,191	897
Gross unrealized losses for held to maturity	35,702	37,120
Estimated fair value for held to maturity	118,525	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	133,778	134,105
Investment securities available for sale, gross unrealized gains	1,300	1,407
Investment securities available for sale, gross unrealized losses	15,311	16,996
Investment securities available for sale, estimated fair value	119,767	118,516
Amortized cost for held to maturity	5,281	5,543
Estimated fair value for held to maturity	5,281	5,543
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	70,199	307,964
Investment securities available for sale, gross unrealized gains	35,873	45,073
Investment securities available for sale, gross unrealized losses	568	571
Investment securities available for sale, estimated fair value	$ 105,504	$ 352,466