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Recently Issued Accounting Pronouncements - Additional Information (Detail) - Accounting Standards Update 2017-07 [Member]
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Operating Income (Loss) [Member]  
Change in Accounting Estimate [Line Items]  
Impact on operating results $ (5.4)
Other Nonoperating Income (Expense) [Member]  
Change in Accounting Estimate [Line Items]  
Impact on operating results $ 4.7