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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Investment Securities [Line Items] |  |
Amortized Cost | $ 1,317,318us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 1,350,466us-gaap_AvailableForSaleSecuritiesAmortizedCost
Estimated Fair Value of Available for Sale | 1,309,819us-gaap_AvailableForSaleSecurities | 1,318,365us-gaap_AvailableForSaleSecurities
US Government Agencies Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 23,300us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Estimated Fair Value of Available for Sale | 25,900us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 947,700us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember |
Estimated Fair Value of Available for Sale | $ 951,000us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember |

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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=28360136&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=6871852&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + 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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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