Revenue - Performance Obligation (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 151,032
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 37,322
Performance obligation
Unsatisfied performance obligation, period	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 62,679
Performance obligation
Unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 24,151
Performance obligation
Unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 11,738
Performance obligation
Unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 6,046
Performance obligation
Unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue
Revenue, Remaining Performance Obligation, Amount	$ 9,096
Performance obligation
Unsatisfied performance obligation, period