General - Accounts Receivable, Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
General
Accounts receivable	$ 797,938	$ 786,395
Change in accounts receivable (in dollars)	$ 11,543
Change in accounts receivable (as a percent)	1.50%
Contract assets	$ 8,055	4,647
Change in contract assets (in dollars)	$ 3,408
Change in contract assets (as a percent)	73.30%
Contract liabilities - current	$ 8,071	16,853
Change in contract liabilities - current (in dollars)	$ (8,782)
Change in contract liabilities - current (as a percent)	(52.10%)
Contract liabilities - long-term	$ 12,963	$ 13,441
Change in contract liabilities - long-term (in dollars)	$ (478)
Change in contract liabilities - long-term (as a percent)	(3.60%)
Revenue recognized previously included in current contract liabilities	$ 12,200