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Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other Assets $ 11,273,000 $ 11,186,000
Estimated Fair Value of Available for Sale 886,560,000 1,309,819,000
Total Debt Securities 884,390,000 1,308,399,000
Discounted Cash Flow at Fair Value 2,963,000  
Fair Value $ 966,548,000 1,368,052,000
Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 1.90%  
Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 4.81%  
Weighted Average [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 10.00%  
Probability of default [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 0.00%  
Probability of default [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 100.00%  
Probability of default [Member] | Weighted Average [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 13.38%  
Prepayment rates [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Prepayment rates  
Prepayment rates [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 0.00%  
Prepayment rates [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 73.28%  
Prepayment rates [Member] | Weighted Average [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 5.07%  
Discount rates [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Discount rates  
Counterparty credit risk [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Counterparty credit risk  
Gas per MCF [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Gas per MCF  
Gas per MCF [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average $ 2.34  
Gas per MCF [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average $ 3.71  
Oil per BBL/d [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Oil per BBL/d  
Oil per BBL/d [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average $ 42.56  
Oil per BBL/d [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average $ 65.13  
Option Model [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation Technique Option model  
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Probability of default [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Probability of default  
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discount rates [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 5.25%  
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discount rates [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average rate 12.00%  
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation Technique Discounted Cash Flow  
Other Investments [Member] | Par Value [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation Technique Par Value  
Impaired Loans [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 2,859,000  
Impaired Loans [Member] | Discount rates [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unobservable Inputs Discount rate  
Impaired Loans [Member] | Reserve study [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation Technique Gas Reserve study  
Other Real Estate Owned [Member] | Internal Valuation [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation Technique Internal Valuation  
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other Assets $ 0 0
Estimated Fair Value of Available for Sale 37,828,000 30,419,000
Total Debt Securities 35,658,000 28,999,000
Fair Value 37,828,000 30,419,000
Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Estimated Fair Value of Available for Sale 2,170,000 1,420,000
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Estimated Fair Value of Available for Sale   1,420,000
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Other Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Estimated Fair Value of Available for Sale 2,170,000  
Fair Value, Measurements, Nonrecurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other real estate owned 10,047,000 7,981,000
Fair Value 53,846,000 53,771,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other real estate owned 8,000 153,000
Fair Value $ 12,828,000 $ 11,079,000