Retirement Benefits Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 26	$ 28
Pension obligations	3,988	3,736
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less plan assets)	4,014	3,764
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	33	31
Pension obligations	0	0
Non-pension postretirement benefit obligations	1,928	2,093
Unfunded status (net benefit obligation less plan assets)	$ 1,961	$ 2,124