|**Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)**                                                                       | 9 Months Ended                                                                                             |                                                                                                           
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------
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|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Securities available for sale, at fair value                                                                                                                                                                     |                                                                                                            | $ 1,050,733,000                                                                                           
|Assets                                                                                                                                                                                                           |                                                                                                            | $ 1,125,998,000                                                                                           
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Minimum [Member] | Discount rates [Member]                                                                                                     |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            | [1]                                                                                                        | 5.25%                                                                                                     
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Maximum [Member] | Discount rates [Member]                                                                                                     |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            | [1]                                                                                                        | 12.00%                                                                                                    
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Minimum [Member] | Probability of default [Member]                                                             |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 0.00%                                                                                                     
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Minimum [Member] | Prepayment rates [Member]                                                                   |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 0.00%                                                                                                     
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Maximum [Member] | Probability of default [Member]                                                             |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 100.00%                                                                                                   
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Maximum [Member] | Prepayment rates [Member]                                                                   |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 73.52%                                                                                                    
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Weighted average [Member] | Probability of default [Member]                                                    |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 13.72%                                                                                                    
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Weighted average [Member] | Prepayment rates [Member]                                                          |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 5.09%                                                                                                     
|Equity Securities [Member]                                                                                                                                                                                       |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Securities available for sale, at fair value                                                                                                                                                                     |                                                                                                            | $ 1,920,000                                                                                               
|Impaired Loans [Member] | Discounted Cash Flow [Member]                                                                                                                                                          |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Assets                                                                                                                                                                                                           |                                                                                                            | $ 520,000                                                                                                 
|Impaired Loans [Member] | Discounted Cash Flow [Member] | Weighted average [Member] | Discount rates [Member]                                                                                                    |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 1.90%                                                                                                     
|Impaired Loans [Member] | Reserve Study [Member]                                                                                                                                                                 |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Assets                                                                                                                                                                                                           |                                                                                                            | $ 2,903,000                                                                                               
|Impaired Loans [Member] | Reserve Study [Member] | NGL per gallon [Member]                                                                                                                                       |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Fair Value Inputs Monetary                                                                                                                                                                                       | [2]                                                                                                        |                                                                                                           
|Impaired Loans [Member] | Reserve Study [Member] | Minimum [Member] | Gas per MCF [Member]                                                                                                                       |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Fair Value Inputs Monetary                                                                                                                                                                                       | [2]                                                                                                        | $ 2.71                                                                                                    
|Impaired Loans [Member] | Reserve Study [Member] | Minimum [Member] | Oil per BBL/d [Member]                                                                                                                     |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Fair Value Inputs Monetary                                                                                                                                                                                       | [2]                                                                                                        | 55.00                                                                                                     
|Impaired Loans [Member] | Reserve Study [Member] | Maximum [Member] | Gas per MCF [Member]                                                                                                                       |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Fair Value Inputs Monetary                                                                                                                                                                                       | [2]                                                                                                        | 3.48                                                                                                      
|Impaired Loans [Member] | Reserve Study [Member] | Maximum [Member] | Oil per BBL/d [Member]                                                                                                                     |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Fair Value Inputs Monetary                                                                                                                                                                                       | [2]                                                                                                        | $ 90.00                                                                                                   
|Impaired Loans [Member] | Reserve Study [Member] | Weighted average [Member] | Discount rates [Member]                                                                                                           |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Weighted average rate                                                                                                                                                                                            |                                                                                                            | 10.00%                                                                                                    
|Level 3 [Member]                                                                                                                                                                                                 |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Securities available for sale, at fair value                                                                                                                                                                     |                                                                                                            | $ 37,699,000                                                                                              
|Assets                                                                                                                                                                                                           |                                                                                                            | 37,699,000                                                                                                
|Level 3 [Member] | Equity Securities [Member]                                                                                                                                                                    |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Securities available for sale, at fair value                                                                                                                                                                     |                                                                                                            | 1,920,000                                                                                                 
|Pooled Trust Preferred Collateralized Debt Obligations [Member]                                                                                                                                                  |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Securities available for sale, at fair value                                                                                                                                                                     |                                                                                                            | 35,779,000                                                                                                
|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                                                                                                                                                                     |                                                                                                            |                                                                                                           
|Pooled Trust Preferred Collateralized Debt Obligations [Member] | Level 3 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]                                                             |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Securities available for sale, at fair value                                                                                                                                                                     |                                                                                                            | 35,779,000                                                                                                
|Fair Value, Measurements, Nonrecurring [Member]                                                                                                                                                                  |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Other Real Estate Owned Assets Fair Value                                                                                                                                                                        |                                                                                                            | 12,394,000                                                                                                
|Assets                                                                                                                                                                                                           |                                                                                                            | 48,966,000                                                                                                
|Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]                                                                                                                                               |                                                                                                            |                                                                                                           
|**Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                 |                                                                                                            |                                                                                                           
|Other Real Estate Owned Assets Fair Value                                                                                                                                                                        |                                                                                                            | 8,000                                                                                                     
|Assets                                                                                                                                                                                                           |                                                                                                            | 9,664,000                                                                                                 
|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]**                                                                                                                 |                                                                                                            |                                                                                                           
|Other Real Estate Owned Assets Fair Value                                                                                                                                                                        |                                                                                                            | $ 8,000                                                                                                   
|                                                                                                                                                                                                                
|[1]
incorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.[2]
unobservable inputs are defined as follows: MCF - million cubic feet; BBL/d - barrels per day. | [1]                                                                                                        | incorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.
|[1]                                                                                                                                                                                                              | incorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.
|[2]                                                                                                                                                                                                              | unobservable inputs are defined as follows: MCF - million cubic feet; BBL/d - barrels per day.            

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