Fair Value of Financial Instruments - Liabilities, Recurring (Details) - Fair Value, Measurements, Recurring - Significant Other Observable Inputs (Level 2) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities-Long-term debt, excluding capital lease and other obligations	$ 10,696	$ 10,711
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities-Long-term debt, excluding capital lease and other obligations	$ 10,413	$ 10,528