Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 91,330
Due after one year through five years, Amortized Cost	159,946
Due after five years through ten years, Amortized Cost	302,843
Due after ten years, Amortized Cost	205,042
Residential mortgage-backed securities, Amortized Cost	859,305
Held to Maturity, Amortized Cost	1,618,466	$ 1,596,385
Due in one year, Fair Value	91,344
Due after one year through five years, Fair Value	171,623
Due after five years through ten years, Fair Value	325,292
Due after ten years, Fair Value	201,254
Residential mortgage-backed securities, Fair Value	870,711
Fair Value	$ 1,660,224	$ 1,621,039

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
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-Paragraph 1
-Subparagraph (SX 210.7-03.1)
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-Publisher FASB
-Name Accounting Standards Codification
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 3
-Subparagraph (b)
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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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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Subparagraph (a)
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Amount of held-to-maturity debt securities at fair value that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the fair value over several maturity groupings.

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

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