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Deferred Revenue and Other Contract-Related Liabilities - Changes in Deferred Revenue (Details)
3 Months Ended
Mar. 31, 2018
USD ($)
Movement in Deferred Revenue [Roll Forward]  
Deferred revenue, beginning of the period $ 11,589,000
Revenue deferred during the period 2,873,000
Recognition of revenue deferred in prior periods (2,077,000)
Deferred revenue, end of the period $ 12,385,000