Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 16	$ 12
Pension obligations	3,366	1,876
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less net plan assets)	3,382	1,888
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	25	23
Pension obligations	0	0
Non-pension postretirement benefit obligations	1,742	2,190
Unfunded status (net benefit obligation less net plan assets)	$ 1,767	$ 2,213