Revenues - Contract Assets, Contract Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract asset	$ 11,021
Contract liabilities (deferred revenue)	138,039	$ 125,282
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	43,282
Contract asset, long-term	$ 600