Fair Value and Investment Securities - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 6,400	$ 5,400
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	6,400	5,700
Short-Term Investments | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity Securities	323	700
Short-Term Investments | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity Securities	$ 323	$ 700