Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Amortized Cost	$ 891,111	$ 1,317,318
Gross Unrealized Gains	11,926	16,921
Gross Unrealized Losses	(16,477)	(24,420)
Estimated Fair Value of Available for Sale	886,560	1,309,819
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	27,066	27,058
Gross Unrealized Gains	532	362
Gross Unrealized Losses	0	(43)
Estimated Fair Value of Available for Sale	27,598	27,377
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,897	6,682
Gross Unrealized Gains	422	573
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	2,319	7,255
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	42,239	41,926
Gross Unrealized Gains	916	309
Gross Unrealized Losses	(7,497)	(13,236)
Estimated Fair Value of Available for Sale	35,658	28,999
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	888,941	1,315,898
Gross Unrealized Gains	11,926	16,921
Gross Unrealized Losses	(16,477)	(24,420)
Estimated Fair Value of Available for Sale	884,390	1,308,399
Mortgage - Backed Securities - Residential [Member] \| US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	20,034	23,344
Gross Unrealized Gains	2,071	2,595
Gross Unrealized Losses	(13)	(3)
Estimated Fair Value of Available for Sale	22,092	25,936
Mortgage - Backed Securities - Residential [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	778,476	947,635
Gross Unrealized Gains	7,983	13,076
Gross Unrealized Losses	(8,882)	(9,830)
Estimated Fair Value of Available for Sale	777,577	950,881
Mortgage - Backed Securities - Commercial [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	28	72
Gross Unrealized Gains	0	2
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	28	74
Other Government - Sponsored Enterprises [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	19,201	269,181
Gross Unrealized Gains	2	4
Gross Unrealized Losses	(85)	(1,308)
Estimated Fair Value of Available for Sale	19,118	267,877
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	2,170	1,420
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	$ 2,170	$ 1,420

Investment Securities [Line Items]

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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