Revenues (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Accounts receivable, net	$ 82,447	$ 82,447	$ 83,328
Unearned income	9,600	9,600
Unearned income - long term	24,000	24,000
Revenue recognized that was included in unearned income	2,000	5,700
Transaction price allocated to performance obligations that are unsatisfied or partially satisfied	$ 237,800	$ 237,800
Revenue, remaining performance obligation, period of expected timing of satisfaction		7 years
Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, period of expected timing of satisfaction		2 years
Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, period of expected timing of satisfaction		5 years