Securities (Amortized Cost and Estimated Fair Value of Securities Excluding Trading Securities Presented by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 738,493
Held to Maturity, Due after one year through five years, Amortized Cost	297,244
Held to Maturity, Due after five years through ten years, Amortized Cost	312,357
Held to Maturity, Due after ten years, Amortized Cost	897,855
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	4,511
Held-to-maturity, Equity Securities, Amortized Cost	0
Held to Maturity, Amortized Cost	2,250,460	$ 2,663,009
Held to Maturity, Due in one year or less, Estimated Fair Value	745,518
Held to Maturity, Due after one year through five years, Estimated Fair Value	311,767
Held to Maturity, Due after five years through ten years, Estimated Fair Value	311,980
Held to Maturity, Due after ten years, Estimated Fair Value	888,932
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	4,550
Held-to-maturity, Equity Securities, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	2,262,747	2,710,685
Available for Sale, Due in one year or less, Amortized Cost	90,632
Available for Sale, Due after one year through five years, Amortized Cost	4,632,290
Available for Sale, Due after five years through ten years, Amortized Cost	348,731
Available for Sale, Due after ten years, Amortized Cost	4,335,957
Available For Sale, Residential mortgage-backed securities, Amortized Cost	756,072
Available for Sale, Equity securities, Amortized Cost	42,494
Available for Sale, Amortized Cost	10,206,176	9,019,570
Available for Sale, Due in one year or less, Estimated Fair Value	91,753
Available for Sale, Due after one year through five years, Estimated Fair Value	4,649,251
Available for Sale, Due after five years through ten years, Estimated Fair Value	357,762
Available for Sale, Due after ten years, Estimated Fair Value	4,276,850
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	785,167
Available for Sale, Equity securities, Estimated Fair Value	42,494
Available for Sale, Estimated Fair Value	$ 10,203,277	$ 9,206,358

Available For Sale Securities Equity Securities Fair Value

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