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Summary of Significant Accounting Policies - Credit loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Allowance activity
Allowance for doubtful accounts, beginning balance	$ 28	$ 29
Additions charged to expense	51	39
Accounts written-off, net of recoveries	(44)	(43)
Acquisitions, divestitures and other, net	(1)	3
Allowance for doubtful accounts, ending balance	33	28
Accounts receivable, net of allowance	2,097	1,949
Total allowance increase (decrease)	$ 5
Receivables outstanding less than 60 days (as a percent)	90.00%
Threshold period to measure receivables	60 days
Notes and other receivables, current and noncurrent	$ 703	544
Allowance for notes and other receivables, current and noncurrent	$ 8	$ 1
Receivables classified as current (as a percent)	75.00%	70.00%
Receivables related to income tax payments in excess of our provision for income taxes	$ 414	$ 231
Federal Fuel Credits Receivable	20	70
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Allowance activity
Allowance for doubtful accounts, beginning balance	(1)
Allowance for doubtful accounts, ending balance	(1)	$ (1)
Allowance for notes and other receivables, current and noncurrent	$ 4

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Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

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Carrying amount as of the balance sheet date of income taxes previously overpaid to tax authorities (such as U.S. Federal, state and local tax authorities) representing refunds of overpayments or recoveries based on agreed-upon resolutions of disputes. Also called income tax refund receivable.

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