Debt (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 11, 2020	Jan. 17, 2020	Dec. 31, 2019	May 24, 2019	Feb. 01, 2019
Principal amount
Total installments of debt	$ 8,011			$ 9,361
Debt due within one year	585			581
Long-term debt	7,426			8,780
Carrying amount
Total debt	8,434			9,261
Debt due within one year	640			568
Long-term debt	7,794			8,693
6.50% Senior Notes due November 2020
Principal amount
Total installments of debt	191			206
Debt due within one year	191			206
Carrying amount
Total debt	191			206
Debt due within one year	$ 191			206
Debt instrument interest rate stated percentage	6.50%
6.375% Senior Notes due December 2021
Principal amount
Total installments of debt	$ 116			222
Carrying amount
Total debt	$ 116			221
Debt instrument interest rate stated percentage	6.375%
5.52% Senior Notes due May 2022
Principal amount
Total installments of debt	$ 134			200
Debt due within one year	92			88
Carrying amount
Total debt	133			198
Debt due within one year	$ 91			87
Debt instrument interest rate stated percentage	5.52%
3.80% Senior Notes due October 2022
Principal amount
Total installments of debt	$ 37			190
Carrying amount
Total debt	$ 37			189
Debt instrument interest rate stated percentage	3.80%
0.50% Exchangeable Senior Bonds due January 2023
Principal amount
Total installments of debt	$ 463			863
Carrying amount
Total debt	$ 462			862
Debt instrument interest rate stated percentage	0.50%
5.375% Senior Secured Notes due May 2023
Principal amount
Total installments of debt	$ 504			525
Debt due within one year	31			16
Carrying amount
Total debt	498			518
Debt due within one year	$ 29			14
Debt instrument interest rate stated percentage	5.375%				5.375%
9.00% Senior Notes due July 2023
Principal amount
Total installments of debt				714
Carrying amount
Total debt				701
Debt instrument interest rate stated percentage	9.00%
5.875% Senior Secured Notes due January 2024
Principal amount
Total installments of debt	$ 585			667
Debt due within one year	83			83
Carrying amount
Total debt	576			656
Debt due within one year	$ 80			79
Debt instrument interest rate stated percentage	5.875%
7.75% Senior Secured Notes due October 2024
Principal amount
Total installments of debt	$ 390			420
Debt due within one year	60			60
Carrying amount
Total debt	383			412
Debt due within one year	$ 58			58
Debt instrument interest rate stated percentage	7.75%
6.25% Senior Notes due December 2024
Principal amount
Total installments of debt	$ 406			437
Debt due within one year	62			62
Carrying amount
Total debt	400			430
Debt due within one year	$ 60			60
Debt instrument interest rate stated percentage	6.25%
6.125% Senior Secured Notes due August 2025
Principal amount
Total installments of debt	$ 468			534
Debt due within one year	66			66
Carrying amount
Total debt	460			525
Debt due within one year	$ 64			64
Debt instrument interest rate stated percentage	6.125%
7.25% Senior Notes due November 2025
Principal amount
Total installments of debt	$ 543			750
Carrying amount
Total debt	$ 535			737
Debt instrument interest rate stated percentage	7.25%
7.50% Senior Notes due January 2026
Principal amount
Total installments of debt	$ 569			750
Carrying amount
Total debt	$ 565			743
Debt instrument interest rate stated percentage	7.50%
2.50% Senior Guaranteed Exchangeable Bonds due January 2027
Principal amount
Total installments of debt	$ 238
Carrying amount
Total debt	277
Debt due within one year	$ 6
Debt instrument interest rate stated percentage	2.50%
11.50% Senior Guaranteed Notes due January 2027
Principal amount
Total installments of debt	$ 687
Carrying amount
Total debt	1,139
Debt due within one year	$ 61
Debt instrument interest rate stated percentage	11.50%	11.50%
6.875% Senior Secured Notes due February 2027
Principal amount
Total installments of debt	$ 550			550
Carrying amount
Total debt	$ 542			541
Debt instrument interest rate stated percentage	6.875%					6.875%
8.00% Senior Notes due February 2027
Principal amount
Total installments of debt	$ 612
Carrying amount
Total debt	$ 606
Debt instrument interest rate stated percentage	8.00%		8.00%
7.45% Notes due April 2027
Principal amount
Total installments of debt	$ 52			88
Carrying amount
Total debt	$ 51			86
Debt instrument interest rate stated percentage	7.45%
8.00% Debentures due April 2027
Principal amount
Total installments of debt	$ 22			57
Carrying amount
Total debt	$ 22			57
Debt instrument interest rate stated percentage	8.00%
7.00% Notes due June 2028
Principal amount
Total installments of debt	$ 261			300
Carrying amount
Total debt	$ 266			306
Debt instrument interest rate stated percentage	7.00%
7.50% Notes due April 2031
Principal amount
Total installments of debt	$ 396			588
Carrying amount
Total debt	$ 394			585
Debt instrument interest rate stated percentage	7.50%
6.80% Senior Notes due March 2038
Principal amount
Total installments of debt	$ 610			1,000
Carrying amount
Total debt	$ 605			991
Debt instrument interest rate stated percentage	6.80%
7.35% Senior Notes due December 2041
Principal amount
Total installments of debt	$ 177			300
Carrying amount
Total debt	$ 176			$ 297
Debt instrument interest rate stated percentage	7.35%