Revenue - Contract Assets and Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Costs in Excess of Billings
Beginning balance, January 1, 2018	$ 16,532
Costs in excess of billings to receivables	(11,647)
Costs in excess of billings recognized, net of reclassification to receivables	17,329
Ending balance, March 31, 2018	22,214
Billings in Excess of Cost
Beginning balance, January 1, 2018	(12,779)
Billings in excess of cost to revenue	8,340
Net billings in advance and cash payments not recognized as revenue	(7,133)
Ending balance, March 31, 2018	$ (11,572)