Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Contribution Plan [Abstract]
Defined contribution plan, employer contributions	$ 104	$ 91	$ 296	$ 267
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	102	97	301	291
Interest cost	316	312	906	937
Expected return on plan assets	(571)	(471)	(1,644)	(1,414)
Prior service credit	(15)	(14)	(44)	(43)
Net loss from previous years	135	171	402	553
Net periodic benefit cost	(33)	95	(79)	324
Defined benefit plan, contributions by Employer	273	20	318	71
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	5	5	15	16
Interest cost	20	21	58	63
Expected return on plan assets	(26)	(22)	(75)	(67)
Prior service credit	(5)	(4)	(16)	(15)
Net loss from previous years	(2)	2	(2)	9
Net periodic benefit cost	(8)	2	(20)	6
Defined benefit plan, contributions by Employer	10	11	32	35
Defined Benefit Plan, Interest on Service Cost is reclassified to Interest Cost [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior Period Reclassification Adjustment		4		12
Defined Benefit Plan, Administrative Expenses of the Service Cost reclassified to Net Actuarial Loss [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior Period Reclassification Adjustment		$ (7)		19
Defined Benefit Plan, Administrative Expenses of the Service Cost reclassified to Net Actuarial Loss [Member] | Medical and Life Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior Period Reclassification Adjustment	$ 1			$ 3
Voluntary Contributions [Member] | Pension Benefits
Components of Net Periodic Benefit Cost
Defined benefit plan, contributions by Employer			$ 250