Revenues - Contract Assets (Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from External Customer [Line Items]
Contract assets – current	$ 59,422	$ 62,467
Contract liabilities – current	(27,060)	(32,339)	$ (51,600)
Net contract assets	32,362	30,128
Contract assets, held-for-sale	2,900	1,800
Contract liabilities, held-for-sale	1,100	$ 100
Difference Between Work Performed On Open Contracts And Contractual Billing Terms
Revenue from External Customer [Line Items]
Changes in contract assets (liabilities)	(3,000)
Difference Related To Customer Advances On Certain Contract
Revenue from External Customer [Line Items]
Changes in contract assets (liabilities)	$ 5,300