Summary of Accounting Policies Summary of Accounting Policies (Revenue Recognition) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Revenue Recognition [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 791
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue Recognition [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 204
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue Recognition [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 239
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue Recognition [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 148
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year