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Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)
1 Months Ended 9 Months Ended 12 Months Ended
Apr. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Amortization expense   $ 14,000,000 $ 13,900,000 $ 18,500,000 $ 18,400,000 $ 18,700,000
Trade Names [Member]            
Impairment indicated         $ 10,200,000  
LifeWorks [Member]            
Impairment indicated $ 0