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Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $) |  |  | 3 Months Ended |  | 3 Months Ended |  | 3 Months Ended |  | 3 Months Ended |  |  |  |  |  |  |  |
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Mar. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Maximum [Member] Discount rates [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Probability of default [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Prepayment rates [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Probability of default [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Prepayment rates [Member] | Dec. 31, 2013 Interest Rate Swap [Member] | Mar. 31, 2014 Interest Rate Swap [Member] Option model [Member] Minimum [Member] Counterparty credit risk [Member] | Mar. 31, 2014 Interest Rate Swap [Member] Option model [Member] Maximum [Member] Counterparty credit risk [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mar. 31, 2014 Impaired Loans [Member] Reserve Study [Member] NGL per gallon [Member] | Mar. 31, 2014 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Gas per MCF [Member] | Mar. 31, 2014 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Oil per BBL/d [Member] | Mar. 31, 2014 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Gas per MCF [Member] | Mar. 31, 2014 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Oil per BBL/d [Member] | Mar. 31, 2014 impaired Loans Valued Using a Reserve Study [Domain] | Mar. 31, 2014 impaired Loans Valued Using a Reserve Study [Domain] Reserve Study [Member] Weighted average [Member] Discount rates [Member] | Mar. 31, 2014 Impaired Loans Valued Using Discounted Cash Flows [Domain] Reserve Study [Member] Weighted average [Member] Discount rates [Member] | Mar. 31, 2014 Level 3 [Member] | Dec. 31, 2013 Level 3 [Member] | Mar. 31, 2014 Level 3 [Member] Equity Securities [Member] | Dec. 31, 2013 Level 3 [Member] Equity Securities [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] | Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mar. 31, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member] | Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Securities available for sale, at fair value | $ 1,351,039,000 | $ 1,318,365,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 29,296,000 | $ 24,943,000 | $ 1,420,000 | $ 1,420,000 | $ 27,876,000 | $ 23,523,000 | $ 27,876,000 | $ 23,523,000
Assets |  |  |  |  |  |  |  |  | 0 |  |  |  |  | 113,000 |  |  |  |  |  |  |  |  |  |  | 5,848,000 |  |  |  |  | 29,296,000 | 24,943,000 |  |  |  |  |  |
Weighted average rate |  |  | 15.50% | [1] | 100.00% | 100.00% | 18.22% | 7.48% |  | 6.33% | [2] | 8.12% | [2] |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% | [2] | 9.00% | [2] |  |  |  |  |  |  |  |
Fair Value Inputs Monetary |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.83 | [3] | $ 3.84 | [3] | $ 78.30 | [3] | $ 6.45 | [3] | $ 107.00 | [3] |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | (a)incorporates spread over risk free rate related primarily to credit quality and illiquidity of secu
[2] | represents the range of the credit spread curve used in valuation.
[3] | unobservable inputs are defined as follows: MCF - million cubic feet; BBL/d - barrels per day; NGL - natural gas liquid.

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + 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=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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