| **Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $)**                                                                                                                                                                                                                                                                                                                                                                     |                 |                 | 12 Months Ended |         | 12 Months Ended |                       |        |        |        |                     |             |        |     |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|---------|-----------------|-----------------------|--------|--------|--------|---------------------|-------------|--------|-----|
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| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                             |            |            |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |  |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Other real estate owned                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |            |            |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |  |   |   |   |   |   |   |   |   |   |   |   |   | $ 12,924,000 | $ 12,228,000 | $ 172,000 | $ 247,000 |
| Other Assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 | 14,358,000      | 16,480,000      |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |  |   |   |   |   |   |   | 0 | 0 |   |   |   |   |   |   |   |   |
| Available-for-sale Securities, Fair Value Disclosure                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 1,318,365,000   | 1,171,303,000   |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |  |   |   |   |   |   | 24,943,000 | 24,793,000 |   | 1,420,000 | 1,975,000 | 1,420,000 | 1,420,000 |   |   |   |   |
| Total Debt Securities                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 1,316,945,000   | 1,169,328,000   |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |  |   |   |   |   |   | 23,523,000 | 23,373,000 |   |   |   |   |   |   |   |   |   |
| Weighted average rate                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |            |            | 0.00%           | 100.00% | 19.08%          |                  | 0.00%  | 74.35% | 7.50%  |                |        |   |     |   |  |   |   |  |   |  |   |   |   | 5.75% | [1] | 15.50% | [1] |   |   |   | 7.89% | [2] | 8.92% | [2] |   |   |   |  | 10.00% |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Weighted average                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |            |            |            |    |            |                  |   |   |   |                |        | 3.56   | [3] | 7.60 | [3] |   | 79.27 | [3] | 106.00 | [3] |   | 1.54 |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |  |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Unobservable Inputs                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |            |            |            |    |            | Prepayment rates |   |   |   | Discount rates | Gas per MCF |   |     |   |  | Oil per BBL/d |   |  |   |  | NGL per gallon |   | Probability of default |   |  |   |  |   |    |   |   |  |   |  | Counterparty credit risk |   |   |  |   | Discount rate |   |    |   |   |   |   |   |   |   |   |   |   |   |   |
| Valuation Technique                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |            |            |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  | Discounted Cash Flow |   | Par Value |   |  |   |  |   | Option model |   |  |   |   | Gas Reserve study |   | Internal Valuation |   |   |   |   |   |   |   |   |   |   |   |
| Fair Value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | $ 1,368,167,000 | $ 1,216,011,000 |            |    |            |                  |   |   |   |                |        |   |     |   |  |   |   |  |   |  |   |   |   |   |  |   |  |   |   |   |   |  |   |  |   |   | $ 6,868,000 | [4] |   |   |   |   |   | $ 24,943,000 | $ 24,793,000 |   |   |   |   |   | $ 63,483,000 | $ 102,004,000 | $ 13,828,000 | $ 7,074,000 |
|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| [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.[4]the remainder of impaired loans valued using Level 3 inputs are not included in this disclosure as the values of those loans are based on bankruptcy agreement documentation. |

| X                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Weighted Average Inputs Price
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other real estate owned assets fair value.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized 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 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.
+ References
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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
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of significant assumptions used by management to measure fair value.
+ References
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| - Definition
Description of the inputs and valuation technique(s) used to measure fair value.
+ References
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| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of assets categorized as other which are not in and of themselves material enough to require separate disclosure.
+ References
No definition available.
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