Securities (Amortized Cost and Estimated Fair Value of Securities Excluding Trading Securities Presented by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 161,934
Held to Maturity, Due after one year through five years, Amortized Cost	846,629
Held to Maturity, Due after five years through ten years, Amortized Cost	207,488
Held to Maturity, Due after ten years, Amortized Cost	1,440,502
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	6,456
Held-to-maturity, Equity Securities, Amortized Cost	0
Held to Maturity, Amortized Cost	2,663,009	$ 2,926,486
Held to Maturity, Due in one year or less, Estimated Fair Value	163,320
Held to Maturity, Due after one year through five years, Estimated Fair Value	889,786
Held to Maturity, Due after five years through ten years, Estimated Fair Value	209,616
Held to Maturity, Due after ten years, Estimated Fair Value	1,441,448
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	6,515
Held-to-maturity, Equity Securities, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	2,710,685	2,966,390
Available for Sale, Due in one year or less, Amortized Cost	793,662
Available for Sale, Due after one year through five years, Amortized Cost	2,961,224
Available for Sale, Due after five years through ten years, Amortized Cost	1,391,273
Available for Sale, Due after ten years, Amortized Cost	2,830,940
Available For Sale, Residential mortgage-backed securities, Amortized Cost	1,000,024
Available for Sale, Equity securities, Amortized Cost	42,447
Available for Sale, Amortized Cost	9,019,570	8,261,947
Available for Sale, Due in one year or less, Estimated Fair Value	796,452
Available for Sale, Due after one year through five years, Estimated Fair Value	2,976,538
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,418,590
Available for Sale, Due after ten years, Estimated Fair Value	2,930,899
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	1,041,432
Available for Sale, Equity securities, Estimated Fair Value	42,447
Available for Sale, Estimated Fair Value	$ 9,206,358	$ 8,461,254

Available For Sale Securities Equity Securities Fair Value

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