Basis of Presentation - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 9,936		$ 22,171
Cash, cash equivalents and restricted cash	11,111	$ 10,183	23,498	$ 3,917
Decrease in cash, cash equivalents, restricted cash and assets held for sale	(12,387)	$ 6,266
Cash and cash equivalents
Cash and Cash Equivalents [Line Items]
Decrease in cash, cash equivalents, restricted cash and assets held for sale	(12,235)
Prepaid expenses and other
Cash and Cash Equivalents [Line Items]
Restricted cash	1,074		1,195
Decrease in cash, cash equivalents, restricted cash and assets held for sale	(121)
Other assets
Cash and Cash Equivalents [Line Items]
Restricted cash	101		$ 132
Decrease in cash, cash equivalents, restricted cash and assets held for sale	$ (31)