Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, employer contributions	$ 99	$ 87
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	114	112
Interest cost	309	321
Expected return on plan assets	(471)	(463)
Prior service credit	(15)	(15)
Net loss from previous years	178	178
Net periodic benefit cost	115	133
Defined Benefit Plan, Actual Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	23	27
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	6	8
Interest cost	21	24
Expected return on plan assets	(22)	(21)
Prior service credit	(5)	(6)
Net loss from previous years	2	3
Net periodic benefit cost	2	8
Defined Benefit Plan, Actual Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	$ 11	$ 11