Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, employer contributions	$ 91	$ 83	$ 267	$ 241
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	94	112	322	335
Interest cost	308	321	925	963
Expected return on plan assets	(471)	(464)	(1,414)	(1,390)
Prior service credit	(14)	(15)	(43)	(45)
Net loss from previous years	178	179	534	536
Net periodic benefit cost	95	133	324	399
Defined Benefit Plan, Actual Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	20	14	71	61
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	6	8	19	23
Interest cost	21	24	63	71
Expected return on plan assets	(22)	(21)	(67)	(64)
Prior service credit	(4)	(6)	(15)	(17)
Net loss from previous years	1	3	6	11
Net periodic benefit cost	2	8	6	24
Defined Benefit Plan, Actual Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	$ 11	$ 17	$ 35	$ 46