Revenue - Contract Liability (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Changes in Contract Liability [Roll Forward]
Contract with customer, liability, beginning	$ 153,180
Increases to the contract liability balance due to cash received	46,959
Revenue recognized in the period	(43,652)
Contract with customer, liability, ending	$ 156,487