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Revenues - Summary of Deferred Revenue and Customer Advances (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
Change in Contract with Customer, Liability [Roll Forward]  
Opening balance January 1, 2018 $ 17,194
Additional cash advances 2,107
Less amounts recognized in revenues (2,175)
Ending balance March 31, 2018 $ 17,126