Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Held-to-maturity			$ 50
Fair Value, Inputs, Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Held-to-maturity	$ 50
Senior Notes | Fair Value, Inputs, Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of long-term debt	5,310	$ 4,455
Senior Notes | Fair Value, Inputs, Level 2 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of long-term debt	$ 7,295	$ 7,285