Retirement Benefits and Trusteed Assets Retirement Benefits and Trusteed Assets (VEBA Trust) (Details) - USD ($) $ / shares in Units, $ in Millions		9 Months Ended
	Feb. 17, 2015	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contribution of common stock to VEBA Trust		$ 117
Other Postretirement Benefits
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contribution of common stock to VEBA Trust		$ 117
VEBA Trust | Other Postretirement Benefits
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contributions to Master VEBA Trust, Number of shares	1,427,835
Price Per Share	$ 81.91
Contribution of common stock to VEBA Trust	$ 117