Fair Value of Financial Instruments - Liabilities, Recurring (Details) - Fair Value, Measurements, Recurring - Significant Other Observable Inputs (Level 2) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities-Long-term debt, excluding finance lease and other obligations	$ 10,665	$ 10,681
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities-Long-term debt, excluding finance lease and other obligations	$ 10,583	$ 10,089