Revenue - Change in Deferred Revenue (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Movement in Deferred Revenue [Roll Forward]
Deferral of revenue billed in current period, net of recognition	$ 91,732
Recognition of revenue deferred in prior periods	(76,519)
Foreign currency translation impact	(2,528)
Deferred revenue	157,001
Before ASC 606
Movement in Deferred Revenue [Roll Forward]
Deferred revenue	$ 154,380