Fair Value of Financial Instruments (Details) - Fair Value Measurements Determined on a Nonrecurring Basis - Fair Value Inputs, Level 2 - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Carrying Amount
Fair value disclosure
Long-term debt, excluding finance lease and other obligations	$ 29,409	$ 31,542
Interest rate swap contracts (see Note 13)	47	107
Carrying Amount | Disposal Group, Held-for-sale, Not Discontinued Operations
Fair value disclosure
Long-term debt, excluding finance lease and other obligations	1,400
Fair Value
Fair value disclosure
Long-term debt, excluding finance lease and other obligations	30,309	33,217
Interest rate swap contracts (see Note 13)	47	$ 107
Fair Value | Disposal Group, Held-for-sale, Not Discontinued Operations
Fair value disclosure
Long-term debt, excluding finance lease and other obligations	$ 1,500