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Revenue From Contracts With Customers - Schedule of Deferred Revenue by Arrangement (Detail)
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
Deferred Revenue [Abstract]  
Beginning balance, January 1, $ 14,448
Amount of deferred revenue recognized in income (10,092)
Additions to deferred revenue 9,924
Ending balance, June 30 $ 14,280