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Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Accounting Standards Update 2017-07 [Member] |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
New accounting pronouncement, effect of change on operating income | $ (2.7) | $ 3.4

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