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Impact of Recent Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Significant Accounting Policies [Line Items] |  |
Federal corporate income tax rate | 21.00% | 35.00%
ASU 2018-02 | Retained Earnings |  |
Significant Accounting Policies [Line Items] |  |
Effect of adopting | $ 2,546 |
ASU 2018-02 | Accumulated Other Comprehensive Loss |  |
Significant Accounting Policies [Line Items] |  |
Effect of adopting | $ (2,546) |

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