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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 |

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