Revenue - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Capitalized Contract Cost [Line Items]
Costs in excess of billings	$ 23,562		$ 22,634
Amounts included in billings in excess of cost at the beginning of the period	11,357	$ 9,044
Amounts included in unearned revenue at the beginning of the period	6,153	$ 2,581
Billings in excess of cost
Capitalized Contract Cost [Line Items]
Contract with customer liability	(38,133)		(17,857)
Unearned revenue
Capitalized Contract Cost [Line Items]
Contract with customer liability	$ (9,971)		$ (12,028)