Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 21	$ 16
Pension obligations	3,385	3,366
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less net plan assets)	3,406	3,382
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	27	25
Pension obligations	0	0
Non-pension postretirement benefit obligations	1,618	1,742
Unfunded status (net benefit obligation less net plan assets)	$ 1,645	$ 1,767