Accounts Receivable - Activity of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 106	$ 142	$ 164
Additions	189	145	153
Deductions	(104)	(181)	(175)
Ending Balance	191	106	142	$ 164
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity	11,162	13,470	19,828
Income tax expense	450	503	170
Retained Earnings (Accumulated Deficit)
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity	$ (8,031)	(6,814)	(1,643)	$ 1,103
Income tax expense		(2)	37
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings (Accumulated Deficit)
Financing Receivable, Allowance for Credit Loss [Line Items]
Stockholders' equity		$ 9	$ 96