Debt (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Principal amount
Total installments of debt	$ 7,002	$ 7,376
Debt due within one year	510	449
Long-term debt	6,492	6,927
Carrying amount
Total debt	7,348	7,807
Debt due within one year	575	505
Long-term debt	6,773	7,302
6.375% Senior Notes due December 2021
Principal amount
Total installments of debt	38	38
Debt due within one year	38	38
Carrying amount
Total debt	38	38
Debt due within one year	$ 38	38
Debt instrument interest rate stated percentage	6.375%
5.52% Senior Secured Notes due May 2022
Principal amount
Total installments of debt	$ 42	111
Debt due within one year	42	93
Carrying amount
Total debt	42	111
Debt due within one year	$ 42	92
Debt instrument interest rate stated percentage	5.52%
3.80% Senior Notes due October 2022
Principal amount
Total installments of debt	$ 27	27
Carrying amount
Total debt	$ 27	27
Debt instrument interest rate stated percentage	3.80%
0.50% Exchangeable Senior Bonds due January 2023
Principal amount
Total installments of debt	$ 140	463
Carrying amount
Total debt	$ 140	462
Debt instrument interest rate stated percentage	0.50%
5.375% Senior Secured Notes due May 2023
Principal amount
Total installments of debt	$ 338	364
Debt due within one year	63	47
Carrying amount
Total debt	336	360
Debt due within one year	$ 62	46
Debt instrument interest rate stated percentage	5.375%
5.875% Senior Secured Notes due January 2024
Principal amount
Total installments of debt	$ 462	585
Debt due within one year	110	83
Carrying amount
Total debt	457	577
Debt due within one year	$ 107	80
Debt instrument interest rate stated percentage	5.875%
7.75% Senior Secured Notes due October 2024
Principal amount
Total installments of debt	$ 330	360
Debt due within one year	60	60
Carrying amount
Total debt	325	354
Debt due within one year	$ 58	58
Debt instrument interest rate stated percentage	7.75%
6.25% Senior Secured Notes due December 2024
Principal amount
Total installments of debt	$ 344	375
Debt due within one year	62	62
Carrying amount
Total debt	339	369
Debt due within one year	$ 61	60
Debt instrument interest rate stated percentage	6.25%
4.00% Senior Guaranteed Exchangeable Bonds due December 2025
Principal amount
Total installments of debt	$ 294
Carrying amount
Total debt	$ 263
Debt instrument interest rate stated percentage	4.00%
6.125% Senior Secured Notes due August 2025
Principal amount
Total installments of debt	$ 402	468
Debt due within one year	66	66
Carrying amount
Total debt	396	461
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	$ 411	411
Carrying amount
Total debt	$ 406	405
Debt instrument interest rate stated percentage	7.25%
7.50% Senior Notes due January 2026
Principal amount
Total installments of debt	$ 569	569
Carrying amount
Total debt	$ 565	565
Debt instrument interest rate stated percentage	7.50%
2.50% Senior Guaranteed Exchangeable Bonds due January 2027
Principal amount
Total installments of debt	$ 238	238
Carrying amount
Total debt	271	277
Debt due within one year	$ 6	6
Debt instrument interest rate stated percentage	2.50%
11.50% Senior Guaranteed Notes due January 2027
Principal amount
Total installments of debt	$ 687	687
Carrying amount
Total debt	1,078	1,139
Debt due within one year	$ 70	61
Debt instrument interest rate stated percentage	11.50%
6.875% Senior Secured Notes due February 2027
Principal amount
Total installments of debt	$ 550	550
Debt due within one year	69
Carrying amount
Total debt	544	542
Debt due within one year	$ 67
Debt instrument interest rate stated percentage	6.875%
8.00% Senior Notes due February 2027
Principal amount
Total installments of debt	$ 612	612
Carrying amount
Total debt	$ 607	606
Debt instrument interest rate stated percentage	8.00%
7.45% Notes due April 2027
Principal amount
Total installments of debt	$ 52	52
Carrying amount
Total debt	$ 52	51
Debt instrument interest rate stated percentage	7.45%
8.00% Debentures due April 2027
Principal amount
Total installments of debt	$ 22	22
Carrying amount
Total debt	$ 22	22
Debt instrument interest rate stated percentage	8.00%
7.00% Notes due June 2028
Principal amount
Total installments of debt	$ 261	261
Carrying amount
Total debt	$ 265	266
Debt instrument interest rate stated percentage	7.00%
7.50% Notes due April 2031
Principal amount
Total installments of debt	$ 396	396
Carrying amount
Total debt	$ 394	394
Debt instrument interest rate stated percentage	7.50%
6.80% Senior Notes due March 2038
Principal amount
Total installments of debt	$ 610	610
Carrying amount
Total debt	$ 605	605
Debt instrument interest rate stated percentage	6.80%
7.35% Senior Notes due December 2041
Principal amount
Total installments of debt	$ 177	177
Carrying amount
Total debt	$ 176	$ 176
Debt instrument interest rate stated percentage	7.35%