Debt (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Long-term debt
Debt at face value	$ 26,175		$ 23,778
Capitalized leases	852		818
Net unamortized premiums and discounts	248		284
Total debt	27,275		24,880
Short-term debt	(1,089)		(1,427)
Long-term debt	26,186		23,453
9.125% Debentures due 2021 [Member]
Long-term debt
Debt at face value	$ 150		$ 150
Debt (Additional Textual) [Abstract]
Stated percentage of debt	9.125%		9.125%
8.20% Debentures due 2025 [Member]
Long-term debt
Debt at face value	$ 150		$ 150
Debt (Additional Textual) [Abstract]
Stated percentage of debt	8.20%		8.20%
8.125% Notes due 2030 [Member]
Long-term debt
Debt at face value	$ 600		$ 600
Debt (Additional Textual) [Abstract]
Stated percentage of debt	8.125%		8.125%
7.9% Debentures due 2047 [Member]
Long-term debt
Debt at face value	$ 100		$ 100
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.90%		7.90%
7.8% Debentures due 2027 [Member]
Long-term debt
Debt at face value	$ 300		$ 300
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.80%		7.80%
7.65% Debentures due 2023 [Member]
Long-term debt
Debt at face value	$ 88		$ 88
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.65%		7.65%
7.40% Notes due 2031 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.40%		7.40%
7.375% Debentures due 2029 [Member]
Long-term debt
Debt at face value	$ 92		$ 92
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.375%		7.375%
7.25% Notes due 2031 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.25%		7.25%
7.20% Notes due 2031 [Member]
Long-term debt
Debt at face value	$ 575		$ 575
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.20%		7.20%
7% Debentures due 2029 [Member]
Long-term debt
Debt at face value	$ 200		$ 200
Debt (Additional Textual) [Abstract]
Stated percentage of debt	7.00%		7.00%
6.95% Notes due 2029 [Member]
Long-term debt
Debt at face value	$ 1,549		$ 1,549
Debt (Additional Textual) [Abstract]
Stated percentage of debt	6.95%		6.95%
6.875% Debentures due 2026 [Member]
Long-term debt
Debt at face value	$ 67		$ 67
Debt (Additional Textual) [Abstract]
Stated percentage of debt	6.875%		6.875%
6.65% Debentures due 2018 [Member]
Long-term debt
Debt at face value	$ 297		$ 297
Debt (Additional Textual) [Abstract]
Stated percentage of debt	6.65%		6.65%
6.50% Notes due 2039 [Member]
Long-term debt
Debt at face value	$ 2,750		$ 2,750
Debt (Additional Textual) [Abstract]
Stated percentage of debt	6.50%		6.50%
6.00% Notes due 2020 [Member]
Long-term debt
Debt at face value	$ 1,000		$ 1,000
Debt (Additional Textual) [Abstract]
Stated percentage of debt	6.00%		6.00%
5.951% Notes due 2037 [Member]
Long-term debt
Debt at face value	$ 645		$ 645
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.951%		5.951%
5.95% Notes due 2036 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.95%		5.95%
5.95% Notes due 2046 [Member]
Long-term debt
Debt at face value	$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.95%		5.95%
5.90% Notes due 2032 [Member]
Long-term debt
Debt at face value	$ 505		$ 505
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.90%		5.90%
5.90% Notes due 2038 [Member]
Long-term debt
Debt at face value	$ 600		$ 600
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.90%		5.90%
5.75% Notes due 2019 [Member]
Long-term debt
Debt at face value	$ 2,250		$ 2,250
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.75%		5.75%
5.625% Notes due 2016 [Member]
Long-term debt
Debt at face value	$ 0		$ 1,250
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.625%		5.625%
5.20% Notes due 2018 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	5.20%		5.20%
4.95% Notes due 2026 [Member]
Long-term debt
Debt at face value	$ 1,250		$ 0
Debt (Additional Textual) [Abstract]
Stated percentage of debt	4.95%		4.95%
4.3% Notes due 2044 [Member]
Long-term debt
Debt at face value	$ 750		$ 750
Debt (Additional Textual) [Abstract]
Stated percentage of debt	4.30%		4.30%
Other Debt [Member]
Long-term debt
Debt at face value	$ 24		$ 24
4.2% Notes due 2021 [Member]
Long-term debt
Debt at face value	$ 1,250		$ 0
Debt (Additional Textual) [Abstract]
Stated percentage of debt	4.20%		4.20%
4.15% Notes due 2034 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	4.15%		4.15%
3.35% Notes due 2024 [Member]
Long-term debt
Debt at face value	$ 1,000		$ 1,000
Debt (Additional Textual) [Abstract]
Stated percentage of debt	3.35%		3.35%
3.35% Notes due 2025 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	3.35%		3.35%
2.875% Notes due 2021 [Member]
Long-term debt
Debt at face value	$ 750		$ 750
Debt (Additional Textual) [Abstract]
Stated percentage of debt	2.875%		2.875%
2.4% Notes due 2022 [Member]
Long-term debt
Debt at face value	$ 1,000		$ 1,000
Debt (Additional Textual) [Abstract]
Stated percentage of debt	2.40%		2.40%
2.2% Notes due 2020 [Member]
Long-term debt
Debt at face value	$ 500		$ 500
Debt (Additional Textual) [Abstract]
Stated percentage of debt	2.20%		2.20%
1.5% Notes due 2018 [Member]
Long-term debt
Debt at face value	$ 750		$ 750
Debt (Additional Textual) [Abstract]
Stated percentage of debt	1.50%		1.50%
1.05% Notes due 2017 [Member]
Long-term debt
Debt at face value	$ 1,000		$ 1,000
Debt (Additional Textual) [Abstract]
Stated percentage of debt	1.05%		1.05%
Floating rate term loan due 2019 at 1.94% - 2.31% during 2016 [Member]
Long-term debt
Debt at face value	$ 1,450	$ 1,600
Floating term loan due 2018 at .69% - 1.24% during 2016 [Member]
Long-term debt
Debt at face value	250		$ 250
Floating Rate Notes Due 2022 at 1.26% - 1.81% during 2016[Member]
Long-term debt
Debt at face value	500		500
Variable rate debt bonds [Member]
Long-term debt
Debt at face value	283
Commercial paper at 0.16% - 0.80% at year-end 2015 [Member]
Long-term debt
Debt at face value	0		803
Marine Terminal Revenue Refunding Bonds due 2031 at 0.01% - 0.95% at year-end 2016 and 0.01% - 0.14% at year-end 2015
Long-term debt
Debt at face value	265		265
Industrial Development Bonds due 2015 through 2038 at 0.01% - 0.91% at year-end 2016 and 0.01% - 0.13% at year-end 2015
Long-term debt
Debt at face value	$ 18		$ 18