Basis of Presentation and Summary of Significant Accounting Policies (Unrecognized Revenues Under Contract) (Details) $ in Millions	Mar. 31, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Unrecognized revenue	$ 3,310
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Unrecognized revenue	$ 1,560
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of satisfaction period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Unrecognized revenue	$ 931
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Unrecognized revenue	$ 611
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Unrecognized revenue	$ 199
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of satisfaction period