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Related-Party Transactions - Additional Information (Detail) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
May 08, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
ZHG Stock Purchase Agreement [Member]        
Related Party Transaction [Line Items]        
Reimbursement for cost and expenses incurred by company relating to sale       $ 4.0
Revenue recognized   $ 2.5 $ 1.3  
Receivable from related party   $ 1.5    
Stock Purchase Agreement [Member] | ZHG Group [Member]        
Related Party Transaction [Line Items]        
Sale of stock percentage closing date May 08, 2017      
Percentage of common stock outstanding by partnership 21.00%