Revenue - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Capitalized Contract Cost [Line Items]
Costs in excess of billings	$ 25,519		$ 22,634
Amounts included in billings in excess of cost at the beginning of the period	9,697	$ 8,340
Amounts included in unearned revenue at the beginning of the period	4,661	$ 1,836
Billings in excess of cost
Capitalized Contract Cost [Line Items]
Contract with customer liability	18,259		17,857
Unearned revenue
Capitalized Contract Cost [Line Items]
Contract with customer liability	$ 12,917		$ 12,028