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Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Federal income tax rate | 21.00% | 35.00%
Reclassification of net deferred tax benefits from AOCI to retained earnings |  | $ 22,298
Accounting Standards Update 2018-02 [Member] |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Federal income tax rate | 21.00% | 35.00%
Reclassification of net deferred tax benefits from AOCI to retained earnings | $ 22,300 |

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