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Revenue from Contract with Customers - Remaining Performance Obligation (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction  
Contract operations remaining performance obligations $ 115,746
Performance obligations expected to be satisfied, expected timing 9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction  
Contract operations remaining performance obligations $ 49,182
Performance obligations expected to be satisfied, expected timing 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction  
Contract operations remaining performance obligations $ 12,977
Performance obligations expected to be satisfied, expected timing 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction  
Contract operations remaining performance obligations $ 180,770
Performance obligations expected to be satisfied, expected timing 1 year