Securities (Amortized Cost and Estimated Fair Value of Securities Excluding Trading Securities Presented by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 264,587
Held to Maturity, Due after one year through five years, Amortized Cost	159,765
Held to Maturity, Due after five years through ten years, Amortized Cost	386,711
Held to Maturity, Due after ten years, Amortized Cost	617,425
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	3,610
Held-to-maturity, Equity Securities, Amortized Cost	0
Held to Maturity, Amortized Cost	1,432,098	$ 2,250,460
Held to Maturity, Due in one year or less, Estimated Fair Value	267,924
Held to Maturity, Due after one year through five years, Estimated Fair Value	163,819
Held to Maturity, Due after five years through ten years, Estimated Fair Value	391,784
Held to Maturity, Due after ten years, Estimated Fair Value	628,677
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	3,587
Held-to-maturity, Equity Securities, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	1,455,791	2,262,747
Available for Sale, Due in one year or less, Amortized Cost	57,390
Available for Sale, Due after one year through five years, Amortized Cost	4,063,797
Available for Sale, Due after five years through ten years, Amortized Cost	425,148
Available for Sale, Due after ten years, Amortized Cost	5,092,767
Available For Sale, Residential mortgage-backed securities, Amortized Cost	648,288
Available for Sale, Equity securities, Amortized Cost	42,561
Available for Sale, Amortized Cost	10,329,951	10,206,176
Available for Sale, Due in one year or less, Estimated Fair Value	58,057
Available for Sale, Due after one year through five years, Estimated Fair Value	4,056,508
Available for Sale, Due after five years through ten years, Estimated Fair Value	439,008
Available for Sale, Due after ten years, Estimated Fair Value	5,227,789
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	665,086
Available for Sale, Equity securities, Estimated Fair Value	42,561
Available for Sale, Estimated Fair Value	$ 10,489,009	$ 10,203,277

Available For Sale Securities Equity Securities Fair Value

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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