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Acquisitions - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Business Combinations [Abstract]      
Acquisition costs related to assets   $ 0.4  
Revenues from the date of acquisition $ 6.9 $ 6.8 $ 9.5