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Intangible Assets - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]      
Amortization expense related to intangible assets $ 12.9 $ 18.2 $ 24.4