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Revenues - Performance Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Revenue from Contract with Customer [Abstract]      
Accounts receivable, net $ 82,865 $ 82,865 $ 83,328
Unearned income 14,017 14,017 7,737
Unearned income - long term 24,217 24,217 $ 23,486
Revenue recognized that was included in unearned income 1,200 6,900  
Transaction price allocated to performance obligations that are unsatisfied or partially satisfied $ 249,000 $ 249,000  
Professional Services, All Other Contracts | Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, remaining performance obligation, period of expected timing of satisfaction 2 years 2 years  
Professional Services, All Other Contracts | Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, remaining performance obligation, period of expected timing of satisfaction 5 years 5 years  
Professional Services, One Contract Representing Majority of Performance Obligations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, remaining performance obligation, period of expected timing of satisfaction 7 years 7 years