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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Investment Securities [Line Items] |  |
Amortized Cost | $ 1,045,850 | $ 1,317,318
Gross Unrealized Gains | 17,069 | 16,921
Gross Unrealized Losses | (12,186) | (24,420)
Securities available for sale, at fair value | 1,050,733 | 1,309,819
US Government Agencies Residential Mortgage-backed Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 20,902 | 23,344
Gross Unrealized Gains | 2,379 | 2,595
Gross Unrealized Losses | 0 | (3)
Securities available for sale, at fair value | 23,281 | 25,936
Residential Mortgage Backed Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 816,179 | 947,635
Gross Unrealized Gains | 12,819 | 13,076
Gross Unrealized Losses | 4,846 | (9,830)
Securities available for sale, at fair value | 824,152 | 950,881
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 37 | 72
Gross Unrealized Gains | 1 | 2
Gross Unrealized Losses | 0 | 0
Securities available for sale, at fair value | 38 | 74
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 135,853 | 269,181
Gross Unrealized Gains | 19 | 4
Gross Unrealized Losses | 91 | (1,308)
Securities available for sale, at fair value | 135,781 | 267,877
Obligations of States and Political Subdivisions [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 27,064 | 27,058
Gross Unrealized Gains | 397 | 362
Gross Unrealized Losses | 12 | (43)
Securities available for sale, at fair value | 27,449 | 27,377
Corporate Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,895 | 6,682
Gross Unrealized Gains | 438 | 573
Gross Unrealized Losses | 0 | 0
Securities available for sale, at fair value | 2,333 | 7,255
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 42,000 | 41,926
Gross Unrealized Gains | 1,016 | 309
Gross Unrealized Losses | (7,237) | (13,236)
Securities available for sale, at fair value | 35,779 | 28,999
Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,043,930 | 1,315,898
Gross Unrealized Gains | 17,069 | 16,921
Gross Unrealized Losses | (12,186) | (24,420)
Securities available for sale, at fair value | 1,048,813 | 1,308,399
Equity Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,920 | 1,420
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Securities available for sale, at fair value | $ 1,920 | $ 1,420

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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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) -URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562 + Details
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