Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	$ 11,911,151	$ 13,338,301
Investment securities available for sale, gross unrealized gains	118,505	135,286
Investment securities available for sale, gross unrealized losses	100,791	141,515
Investment securities available for sale, estimated fair value	11,928,865	13,332,072
Amortized cost for held to maturity	3,388,268	2,457,278
Gross unrealized gains for held to maturity	37,547	38,662
Gross unrealized losses for held to maturity	45,283	44,718
Estimated fair value for held to maturity	3,380,532	2,451,222
Other securities, Amortized cost	498,927	461,118
Other securities, Estimated fair value	498,927	461,118
Total Amortized cost	15,798,346	16,256,697
Total gross unrealized gains	156,052	173,948
Total gross unrealized losses	146,074	186,233
Total estimated fair value	15,808,324	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	2,118,665	1,912,110
Investment securities available for sale, gross unrealized gains	39	386
Investment securities available for sale, gross unrealized losses	10,838	9,952
Investment securities available for sale, estimated fair value	2,107,866	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	2,849	3,570
Investment securities available for sale, gross unrealized gains	66	77
Investment securities available for sale, gross unrealized losses	2	6
Investment securities available for sale, estimated fair value	2,913	3,641
Amortized cost for held to maturity	38,958	60,858
Gross unrealized gains for held to maturity	211	267
Gross unrealized losses for held to maturity	125	224
Estimated fair value for held to maturity	39,044	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	9,601,810	10,980,507
Investment securities available for sale, gross unrealized gains	84,797	88,343
Investment securities available for sale, gross unrealized losses	78,425	113,989
Investment securities available for sale, estimated fair value	9,608,182	10,954,861
Amortized cost for held to maturity	3,196,908	2,233,173
Gross unrealized gains for held to maturity	35,630	37,498
Gross unrealized losses for held to maturity	11,886	7,374
Estimated fair value for held to maturity	3,220,652	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	36	45
Investment securities available for sale, gross unrealized losses	1	1
Investment securities available for sale, estimated fair value	35	44
Amortized cost for held to maturity	147,189	157,704
Gross unrealized gains for held to maturity	1,706	897
Gross unrealized losses for held to maturity	33,272	37,120
Estimated fair value for held to maturity	115,623	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	133,959	134,105
Investment securities available for sale, gross unrealized gains	2,923	1,407
Investment securities available for sale, gross unrealized losses	11,059	16,996
Investment securities available for sale, estimated fair value	125,823	118,516
Amortized cost for held to maturity	5,213	5,543
Estimated fair value for held to maturity	5,213	5,543
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Investment securities available for sale, amortized cost	53,832	307,964
Investment securities available for sale, gross unrealized gains	30,680	45,073
Investment securities available for sale, gross unrealized losses	466	571
Investment securities available for sale, estimated fair value	$ 84,046	$ 352,466