Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, employer contributions	$ 71	$ 65		$ 158	$ 150
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	111	121		223	242
Interest cost	321	306		642	612
Expected return on plan assets	(463)	(493)		(926)	(987)
Prior service credit	(15)	(15)		(30)	(30)
Net loss from previous years	179	170		357	341
Net periodic benefit cost	133	89		266	178
Defined Benefit Plan, Expected Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	20	19		47	544
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	7	9		15	18
Interest cost	23	23		47	47
Expected return on plan assets	(22)	(23)		(43)	(45)
Prior service credit	(5)	(7)		(11)	(14)
Net loss from previous years	5	8		8	14
Net periodic benefit cost	8	10		16	20
Defined Benefit Plan, Expected Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer	$ 18	$ 19		$ 29	$ 25
Voluntary Contributions [Member] | Pension Benefits
Defined Benefit Plan, Expected Contributions in Current Fiscal Year [Abstract]
Defined benefit plan, contributions by Employer			$ 500