Revenue Recognition - Performance Obligations (Details) $ in Millions	Sep. 30, 2018 USD ($)
Revenue from Contract with Customer [Abstract]
Remaining performance obligation amount	$ 277
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation amount	$ 143
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of remaining performance obligation, period	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation amount	$ 113
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of remaining performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation amount	$ 21
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of remaining performance obligation, period