Revenue - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Capitalized Contract Cost [Line Items]
Costs in excess of billings	$ 24,951		$ 22,634
Amounts included in billings in excess of cost at the beginning of the period	14,137	$ 9,294
Amounts included in unearned revenue at the beginning of the period	11,052	$ 2,977
Billings in excess of cost
Capitalized Contract Cost [Line Items]
Contract with customer liability	(38,672)		(17,857)
Unearned revenue
Capitalized Contract Cost [Line Items]
Contract with customer liability	$ (21,860)		$ (12,028)