Revenue Contract balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Jan. 01, 2018
Revenue Recognition [Abstract]
Increase (Decrease) in Unbilled Receivables	$ 1,600
Unbilled Receivables, Current	170,369	$ 0		$ 171,956
Deferred Revenue	30,552	0		$ 37,590
Billing for the reporting period	406,000
Revenue recognized but unbilled	404,000
Revenues	416,413	$ 484,175	$ 446,176
Increase or decrease in deferred revenue	7,000
Deferred Revenue, Revenue Recognized	12,000
Deferrals of customer payments	5,000
revenue recognized, included in beginning balance of contract liability	$ 12,500