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Related-Party Transactions - Additional Information (Detail) - USD ($)
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Dec. 01, 2009
Term B-2 Loans [Member]      
Related Party Transaction [Line Items]      
Debt instrument, balance     $ 1,405,000,000
ZHG Stock Purchase Agreement [Member]      
Related Party Transaction [Line Items]      
Reimbursement for cost and expenses incurred by company relating to sale $ 4,000,000    
Revenue recognized 2,600,000    
Blackstone and Affiliates [Member] | Term B-2 Loans [Member]      
Related Party Transaction [Line Items]      
Debt instrument, balance $ 0 $ 25,000,000