Debt (Long-term Debt) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Current portion of long-term debt	$ 11	$ 11	$ 4
Finance Leases	280	280
Total long-term debt	7,740	7,740	5,998
Less current portion of long-term debt	11	11	4
Long-term debt	$ 7,729	$ 7,729	5,994
Debentures, 4.25%, Due 2020 [Member]
Debentures			$ 291
Interest rate on debt	4.25%	4.25%	4.25%
Debt maturity date	2020		2020
Debentures, 8.875%, Due 2021 [Member]
Debentures	$ 64	$ 64	$ 65
Interest rate on debt	8.875%	8.875%	8.875%
Debt maturity date		2021	2021
Debentures, 2.90%, Due 2022 [Member]
Debentures	$ 374	$ 374	$ 373
Interest rate on debt	2.90%	2.90%	2.90%
Debt maturity date		2022	2022
Debentures, 3.70%, due 2023 [Member]
Debentures	$ 249	$ 249	$ 249
Interest rate on debt	3.70%	3.70%	3.70%
Debt maturity date		2023	2023
Medium-Term Notes, Average Rate 7.66%, Due Through 2023 [Member]
Debentures	$ 45	$ 45	$ 45
Average rate	7.66%	7.66%	7.66%
Debt maturity date		2023	2023
Debentures, 7.00%, Due 2024 [Member]
Debentures	$ 100	$ 100	$ 100
Interest rate on debt	7.00%	7.00%
Debt maturity date		2024	2024
Debentures, 3.9%, Due 2049 [Member]
Debentures	$ 395	$ 395
Interest rate on debt	3.90%	3.90%
Debt maturity date		2049
Debentures, 5.45%, Due 2079 [Member]
Debentures	$ 1,085	$ 1,085
Interest rate on debt	5.45%	5.45%
Debt maturity date		2079
Yen-denominated Debentures, .698%, due 2024 [Member]
Debentures	$ 192	$ 192	$ 191
Interest rate on debt	0.698%	0.698%	0.698%
Debt maturity date		2024	2024
Yen-denominated Debentures, .722%, due 2025 [Member]
Debentures	$ 91	$ 91	$ 90
Interest rate on debt	0.722%	0.722%	0.722%
Debt maturity date		2025	2025
Yen-denominated Debentures, .992%, due 2027 [Member]
Debentures	$ 430	$ 430	$ 426
Interest rate on debt	0.992%	0.992%	0.992%
Debt maturity date		2027	2027
Yen-denominated Debentures, 1.043%, due 2028 [Member]
Debentures	$ 278	$ 278	$ 276
Interest rate on debt	1.043%	1.043%	1.043%
Debt maturity date		2028	2028
Yen-denominated Debentures, 1.153%, due 2031 [Member]
Debentures	$ 285	$ 285
Interest rate on debt	1.153%	1.153%		1.153%
Debt maturity date		2031
Yen-denominated Debentures, 1.513%, due 2039 [Member]
Debentures	$ 54	$ 54
Interest rate on debt	1.513%	1.513%		1.513%
Debt maturity date		2039
Debentures, 6.85%, Due 2029 [Member]
Debentures	$ 163	$ 163	$ 164
Interest rate on debt	6.85%	6.85%	6.85%
Debt maturity date		2029	2029
Yen-denominated Debentures, 1.219%, due 2030 [Member]
Debentures	$ 227	$ 227	$ 226
Interest rate on debt	1.219%	1.219%	1.219%
Debt maturity date		2030	2030
Debentures, Callable, 7.25%, Due 2036 [Member]
Debentures	$ 249	$ 249	$ 248
Interest rate on debt	7.25%	7.25%	7.25%
Debt maturity date		2036	2036
Debentures, 4.70%, Due 2037 [Member]
Debentures	$ 295	$ 295	$ 295
Interest rate on debt	4.70%	4.70%	4.70%
Debt maturity date		2037	2037
Yen-denominated Debentures, 1.583%, due 2037 [Member]
Debentures	$ 91	$ 91	$ 91
Interest rate on debt	1.583%	1.583%	1.583%
Debt maturity date		2037	2037
Debentures, 5.75%, Due 2040 [Member]
Debentures	$ 395	$ 395	$ 395
Interest rate on debt	5.75%	5.75%	5.75%
Debt maturity date		2040	2040
Debentures, 4.75%, Due 2042 [Member]
Debentures	$ 496	$ 496	$ 496
Interest rate on debt	4.75%	4.75%	4.75%
Debt maturity date		2042	2042
Debentures, 5.35%, due 2048 [Member]
Debentures	$ 543	$ 543	$ 543
Interest rate on debt	5.35%	5.35%	5.35%
Debt maturity date		2048	2048
Debentures, 4.375%, due 2057 [Member]
Debentures	$ 742	$ 742	$ 742
Interest rate on debt	4.375%	4.375%	4.375%
Debt maturity date		2057	2057
Debentures, 5.85%, due 2068 [Member]
Debentures	$ 296	$ 296	$ 296
Interest rate on debt	5.85%	5.85%	5.85%
Debt maturity date		2068	2068
Financing Leases, Average Discount Rate 5.47%, Due Through 2044 [Member]
Finance Leases	$ 280	$ 280
Average rate	5.47%	5.47%
Debt maturity date		2044
Other, Average Rate 4.27%, Due Through 2043 [Member]
Debentures	$ 321	$ 321	$ 396
Average rate	4.27%	4.27%	4.27%
Debt maturity date		2043	2043