Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, employer contributions	$ 83	$ 77		$ 241	$ 227
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	112	121		335	363
Interest cost	321	306		963	918
Expected return on plan assets	(464)	(494)		(1,390)	(1,481)
Prior service credit	(15)	(15)		(45)	(45)
Net loss from previous years	179	170		536	511
Net periodic benefit cost	133	88		399	266
Defined Benefit Plan, Expected Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	14	20		61	564
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	8	8		23	26
Interest cost	24	24		71	71
Expected return on plan assets	(21)	(22)		(64)	(67)
Prior service credit	(6)	(7)		(17)	(21)
Net loss from previous years	3	6		11	20
Net periodic benefit cost	8	9		24	29
Defined Benefit Plan, Expected Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	$ 17	$ 19		$ 46	$ 44
Voluntary Contributions [Member] | Pension Benefits
Defined Benefit Plan, Expected Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer			$ 500