Basis for Presentation - Backlog (Details) $ in Millions	Mar. 31, 2019 USD ($)
Basis for Presentation
Revenue, Remaining performance obligation	$ 3,796
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining performance obligation (in percent)	30.00%
Contract term	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining performance obligation (in percent)	45.00%
Contract term	24 months