Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, employer contributions	$ 77	$ 71	$ 176	$ 158
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	114	111	228	223
Interest cost	308	321	617	642
Expected return on plan assets	(472)	(463)	(943)	(926)
Prior service credit	(14)	(15)	(29)	(30)
Net loss from previous years	178	179	356	357
Net periodic benefit cost	114	133	229	266
Defined Benefit Plan, Actual Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	28	20	51	47
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	7	7	13	15
Interest cost	21	23	42	47
Expected return on plan assets	(23)	(22)	(45)	(43)
Prior service credit	(6)	(5)	(11)	(11)
Net loss from previous years	3	5	5	8
Net periodic benefit cost	2	8	4	16
Defined Benefit Plan, Actual Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	$ 13	$ 18	$ 24	$ 29