Securities - Amortized Cost and Fair Value of Held to Maturity Securities by Final Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Amortized Cost And Estimated Fair Value Of Debt Securities By Contractual Maturity [Abstract]
Securities Held to Maturity, Due in one year or less, Amortized Cost	$ 169,617
Securities Held to Maturity, Due after one year through five years, Amortized Cost	453,352
Securities Held to Maturity, Due after five years through ten years, Amortized Cost	115,748
Securities Held to Maturity, Due after ten years, Amortized Cost	1,427,572
Securities Held to Maturity, Amortized Cost	2,166,289us-gaap_HeldToMaturitySecurities	2,611,352us-gaap_HeldToMaturitySecurities
Securities Held to Maturity, Due in one year or less, Fair Value	170,786
Securities Held to Maturity, Due after one year through five years, Fair Value	446,039
Securities Held to Maturity, Due after five years through ten years, Fair Value	115,195
Securities Held to Maturity, Due after ten years, Fair Value	1,454,320
Total Held to Maturity Securities, Fair Value	$ 2,186,340	$ 2,576,584

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of held-to-maturity debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the fifth through tenth fiscal year following the latest fiscal year.

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Fair value of held-to-maturity debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the next fiscal year through the fifth fiscal year following the latest fiscal year.

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Fair value of held-to-maturity debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing after the tenth fiscal year following the latest fiscal year.

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Fair value of held-to-maturity debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of investments in debt securities classified as held-to-maturity.

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