Debt - Debt Fair Value (Details) - USD ($) $ in Millions	Feb. 01, 2018	Dec. 31, 2018	Dec. 31, 2017
Line of Credit
Debt Instrument [Line Items]
Authorized debt repayments	$ 1,800
Fair value measurements using significant other observable inputs (Level 2) | Unsecured Notes | 2026 Notes
Debt Instrument [Line Items]
Fair value of notes		$ 800
Fair value measurements using significant other observable inputs (Level 2) | Unsecured Notes | 2027 Notes
Debt Instrument [Line Items]
Fair value of notes		376
Fair value measurements using significant other observable inputs (Level 2) | Unsecured Notes | 2047 Notes
Debt Instrument [Line Items]
Fair value of notes		$ 360	$ 421
Fair value measurements using significant other observable inputs (Level 2) | Unsecured Notes | 2023 Notes
Debt Instrument [Line Items]
Fair value of notes			$ 795