Contract Balances - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Contract With Customer Asset And Liability [Line Items]
Bad debt expense	$ 800,000	$ 800,000	$ 948,000	$ 1,835,000
Prepaid Expenses and Other Current Assets
Contract With Customer Asset And Liability [Line Items]
Assets impairment charges	0	0	0	0
Contract assets	500,000		500,000		$ 1,200,000
Current Deferred Revenue and Advanced Payments
Contract With Customer Asset And Liability [Line Items]
Revenue recognized from beginning deferred revenue	$ 56,800,000	$ 55,700,000	$ 131,300,000	$ 130,500,000