Accounts Receivable, Net - Summary (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended	12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts	$ (62)			$ (63)			$ (62)	$ (69)
Accounts receivable, net	891						891	883
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	62			63			$ 62	$ 69
Provision for receivables	0			7
Charge-offs	(5)			(6)
Recoveries and other	(1)			$ 2
Credit Concentration Risk | Accounts Receivable | Customers
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Concentration risk (percentage)							6.50%	7.20%
Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Invoiced	717						$ 717	$ 735
Accrued	236						236	217
Allowance for doubtful accounts	(62)	$ (68)	$ (68)		$ (60)	$ (60)	(62)	(69)	$ (55)
Accounts receivable, net	891						891	883
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	$ 62	68	68		60	60	$ 62	$ 69
Provision for receivables		1	4		9	8
Charge-offs		(2)	(5)		(8)	(2)
Recoveries and other		$ 1	$ 0		$ (1)	$ (1)