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Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value of Financial Instruments [Line Items]
Noncurrent liabilities - held for sale (Note 10)	$ 718	$ 691
Investments - held for sale (Note 10)	881	882
Investments not required to be disclosed	$ 301	$ 299
NUG contracts
Fair Value of Financial Instruments [Line Items]
Period of future observable data to determine contract price	2 years

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