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Fair Value of Instruments - Additional Information (Detail) - USD ($)
 6 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2015
Sep. 30, 2015
Dec. 31, 2014
Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Interest-bearing deposits maturing date 90 days      
Carrying amounts of dividends payable, number of days   Forty days    
Collateral dependent impaired loans categorized as Level 3   $ 2,541,000 $ 1,843,000 $ 1,686,000
Specific allocation for impaired loans   465,000 387,000 $ 252,000
Asset impairment charges   0 0  
Level 3 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Collateral dependent impaired loans categorized as Level 3   600,000 1,500,000  
Fair Value on a Recurring Basis [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Specific allocation for impaired loans   $ 465,000 $ 387,000