Debt (Schedule of Debt) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2020	Feb. 29, 2020	Nov. 30, 2019	May 31, 2019	Oct. 31, 2018	Sep. 30, 2017	Aug. 31, 2017	Mar. 01, 2017	Feb. 28, 2017	Nov. 30, 2016	May 31, 2016	Jun. 30, 2008
Debt Instrument [Line Items]
Long-term debt	$ 9,755,000,000		$ 9,755,000,000
Finance leases	135,000,000	$ 127,000,000	135,000,000	$ 127,000,000
Total debt	9,682,000,000	11,428,000,000	9,682,000,000	11,428,000,000
Less short-term portion	(704,000,000)	(997,000,000)	(704,000,000)	(997,000,000)
Total long-term debt	8,978,000,000	10,431,000,000	8,978,000,000	10,431,000,000
Loss on extinguishment of debt			183,000,000	61,000,000	$ 0
Line of credit
Debt Instrument [Line Items]
Maximum borrowing capacity										$ 1,000,000,000
Accounts receivable securitization facility expiring 2019 | Line of credit
Debt Instrument [Line Items]
Maximum borrowing capacity	800,000,000		800,000,000
Long-term debt	634,000,000	929,000,000	634,000,000	929,000,000
Total debt	634,000,000		634,000,000
Borrowing capacity, net of letters of credit	$ 166,000,000		$ 166,000,000
Interest rate at December 31, 2020	1.50%		1.50%
Average month-end debt outstanding			$ 667,000,000
Weighted-average interest rate on average debt outstanding	1.80%		1.80%
Maximum month-end debt outstanding			$ 811,000,000
$3.75 billion ABL facility expiring 2024 | Line of credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 3,750,000,000		3,750,000,000														$ 1,250,000,000
Long-term debt	977,000,000	1,638,000,000	977,000,000	1,638,000,000
Borrowing capacity, net of letters of credit	2,705,000,000		2,705,000,000
Letters of credit	$ 60,000,000		$ 60,000,000
Interest rate at December 31, 2020	1.40%		1.40%
Average month-end debt outstanding			$ 794,000,000
Weighted-average interest rate on average debt outstanding	1.90%		1.90%
Maximum month-end debt outstanding			$ 1,494,000,000
Term loan facility expiring 2025
Debt Instrument [Line Items]
Long-term debt	$ 971,000,000	979,000,000	$ 971,000,000	979,000,000
Debt repayment installment rate	1.00%		1.00%
Term loan facility expiring 2025 | Line of credit
Debt Instrument [Line Items]
Borrowing capacity, net of letters of credit	$ 0		$ 0
Interest rate at December 31, 2020	1.90%		1.90%
Average month-end debt outstanding			$ 983,000,000
Weighted-average interest rate on average debt outstanding	2.20%		2.20%
Maximum month-end debt outstanding			$ 988,000,000
5 1/2 percent Senior Notes due 2025 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	5.50%		5.50%
Long-term debt	$ 0	795,000,000	$ 0	795,000,000
Loss on extinguishment of debt			$ 27,000,000
4 5/8 Senior Notes due 2025 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	4.625%		4.625%
Long-term debt	$ 0	742,000,000	$ 0	742,000,000
Loss on extinguishment of debt	$ 24,000,000		$ 24,000,000
5 7/8 percent Senior Notes due 2026 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	5.875%		5.875%
Long-term debt	$ 999,000,000	999,000,000	$ 999,000,000	999,000,000
6 1/2 percent Senior Notes due 2026 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	6.50%		6.50%
Long-term debt	$ 0	1,089,000,000	$ 0	1,089,000,000
Loss on extinguishment of debt			$ 132,000,000
5 1/2 percent Senior Notes due 2027 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	5.50%		5.50%
Long-term debt	$ 994,000,000	992,000,000	$ 994,000,000	992,000,000
3 7/8 percent Senior Secured Notes due 2027 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	3.875%		3.875%					3.875%
Long-term debt	$ 742,000,000	741,000,000	$ 742,000,000	741,000,000
Loss on extinguishment of debt		29,000,000
4 7/8 percent Senior Notes due 2028 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	4.875%		4.875%
Long-term debt	$ 1,654,000,000	1,652,000,000	$ 1,654,000,000	1,652,000,000
4 7/8 percent Senior Notes due 2028 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	4.875%		4.875%
Long-term debt	$ 4,000,000	4,000,000	$ 4,000,000	4,000,000
5 1/4 percent Senior Notes due 2030 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	5.25%		5.25%
Long-term debt	$ 742,000,000	741,000,000	$ 742,000,000	741,000,000
4 percent Senior Notes due 2030 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	4.00%		4.00%
Long-term debt	$ 742,000,000	0	$ 742,000,000	0
3 7/8 percent Senior Notes due 2031 | Senior notes
Debt Instrument [Line Items]
Stated interest rate	3.875%		3.875%
Long-term debt	$ 1,088,000,000	0	$ 1,088,000,000	0
Subsidiaries | 5 7/8 percent Senior Notes due 2026 | Senior notes
Debt Instrument [Line Items]
Debt instrument, face amount														$ 250,000,000		$ 750,000,000
Subsidiaries | 5 1/2 percent Senior Notes due 2027 | Senior notes
Debt Instrument [Line Items]
Debt instrument, face amount													$ 1,000,000,000.0	$ 250,000,000	$ 750,000,000
Subsidiaries | 3 7/8 percent Senior Secured Notes due 2027 | Senior notes
Debt Instrument [Line Items]
Debt instrument, face amount		$ 750,000,000		$ 750,000,000				$ 750,000,000
Subsidiaries | 4 7/8 percent Senior Notes due 2028 | Senior notes
Debt Instrument [Line Items]
Long-term debt	1,669,000,000		1,669,000,000
Debt instrument, face amount												$ 925,000,000
Subsidiaries | 4 7/8 percent Senior Notes due 2028 | Senior notes
Debt Instrument [Line Items]
Long-term debt	$ 4,000,000		$ 4,000,000
Debt instrument, face amount											$ 750,000,000
Subsidiaries | 5 1/4 percent Senior Notes due 2030 | Senior notes
Debt Instrument [Line Items]
Debt instrument, face amount									$ 750,000,000
Subsidiaries | 4 percent Senior Notes due 2030 | Senior notes
Debt Instrument [Line Items]
Stated interest rate							4.00%
Debt instrument, face amount							$ 750,000,000
Subsidiaries | 3 7/8 percent Senior Notes due 2031 | Senior notes
Debt Instrument [Line Items]
Debt instrument, face amount						$ 1,100,000,000