Fair Value of Financial Instruments (Details) - Fair value measurements determined on a nonrecurring basis - Fair value, Input Level 2 - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Carrying Amount
Liabilities
Liabilities-Long-term debt, excluding finance lease and other obligations	$ 31,542	$ 34,472
Interest rate swap contracts	107	51
Fair Value
Liabilities
Liabilities-Long-term debt, excluding finance lease and other obligations	33,217	35,737
Interest rate swap contracts	$ 107	$ 51

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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