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Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $) |  |  | 9 Months Ended |  | 9 Months Ended |  | 9 Months Ended |  | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |
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Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Minimum [Member] Discount rates [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Maximum [Member] Discount rates [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Probability of default [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Prepayment rates [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Probability of default [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Prepayment rates [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Probability of default [Member] | Sep. 30, 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] | Sep. 30, 2014 Interest Rate Swap [Member] Option model [Member] Minimum [Member] Counterparty credit risk [Member] | Sep. 30, 2014 Interest Rate Swap [Member] Option model [Member] Maximum [Member] Counterparty credit risk [Member] | Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] NGL per gallon [Member] | Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Gas per MCF [Member] | Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Oil per BBL/d [Member] | Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Gas per MCF [Member] | Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Oil per BBL/d [Member] | Sep. 30, 2014 impaired Loans Valued Using a Reserve Study [Domain] | Sep. 30, 2014 impaired Loans Valued Using a Reserve Study [Domain] Reserve Study [Member] Weighted average [Member] Discount rates [Member] | Sep. 30, 2014 impaired Loans Valued Using Discounted Cash Flows [Member] | Sep. 30, 2014 Impaired Loans Valued Using Discounted Cash Flows [Domain] Reserve Study [Member] Weighted average [Member] Discount rates [Member] | Sep. 30, 2014 Level 3 [Member] | Dec. 31, 2013 Level 3 [Member] | Sep. 30, 2014 Level 3 [Member] Equity Securities [Member] | Dec. 31, 2013 Level 3 [Member] Equity Securities [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] | Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] | Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member] | Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member] | Sep. 30, 2014 Fair Value, Measurements, Nonrecurring [Member] | Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] | Sep. 30, 2014 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] | Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] | Sep. 30, 2014 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] Other Real Estate Owned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Securities available for sale, at fair value | $ 1,332,774,000 | $ 1,318,365,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,876,000 | $ 24,943,000 | $ 1,420,000 | $ 1,420,000 | $ 29,456,000 | $ 23,523,000 | $ 23,523,000 | $ 29,456,000 |  |  |  |  |
Other Real Estate Owned Assets Fair Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,309,000 | 12,924,000 | 190,000 | 172,000 | 190,000
Assets |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,934,000 |  | 113,000 |  | 30,876,000 | 24,943,000 |  |  |  |  |  |  | 48,935,000 | 63,483,000 | 13,779,000 | 13,828,000 |
Weighted average rate |  |  | 5.50% | [1] | 14.50% | [1] | 0.00% | 0.00% | 100.00% | 100.00% | 17.48% | 6.35% |  | 3.74% | [2] | 6.62% | [2] |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  | 9.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value Inputs Monetary |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.83 | [3] | $ 3.84 | [3] | $ 83.64 | [3] | $ 6.45 | [3] | $ 107.00 | [3] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | incorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.
[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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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + 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 (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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