Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Contribution Plan [Abstract]
Defined contribution plan, employer contributions	$ 104	$ 99
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	99	97
Interest cost	290	313
Expected return on plan assets	(529)	(471)
Prior service credit	(15)	(15)
Net loss from previous years	134	191
Net periodic benefit cost	(21)	115
Defined benefit plan, contributions by Employer	22	23
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	5	5
Interest cost	19	21
Expected return on plan assets	(25)	(22)
Prior service credit	(5)	(5)
Net loss from previous years	0	3
Net periodic benefit cost	(6)	2
Defined benefit plan, contributions by Employer	$ 11	11
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
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		13
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