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Contract Balances - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Contract With Customer Asset And Liability [Line Items]          
Bad debt expense and write-offs (reversals) $ 100,000 $ (100,000) $ 1,100,000 $ 1,700,000  
Prepaid Expenses and Other Current Assets          
Contract With Customer Asset And Liability [Line Items]          
Assets impairment charges 0 0 0 0  
Contract assets 400,000   400,000   $ 1,200,000
Current Deferred Revenue and Advanced Payments          
Contract With Customer Asset And Liability [Line Items]          
Revenue recognized from beginning deferred revenue $ 39,200,000 $ 40,400,000 $ 170,600,000 $ 170,900,000