Revenue - Contract Assets and Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Costs in Excess of Billings
Beginning balance, January 1, 2018	$ 16,532
Costs in excess of billings to receivables	(15,450)
Costs in excess of billings recognized, net of reclassification to receivables	20,440
Ending balance, September 30, 2018	21,522
Billings in Excess of Cost
Beginning balance, January 1, 2018	(12,779)
Billings in excess of cost to revenue	9,294
Net billings in advance and cash payments not recognized as revenue	(18,415)
Ending balance, September 30, 2018	$ (21,900)