Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Contribution Plan [Abstract]
Defined contribution plan, employer contributions	$ 88	$ 77	$ 192	$ 176
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	100	97	199	194
Interest cost	300	312	590	625
Expected return on plan assets	(544)	(472)	(1,073)	(943)
Prior service credit	(14)	(14)	(29)	(29)
Net loss from previous years	133	191	267	382
Net periodic benefit cost	(25)	114	(46)	229
Defined benefit plan, contributions by Employer	23	28	45	51
Medical and Life Benefits
Components of Net Periodic Benefit Cost
Service cost	5	6	10	11
Interest cost	19	21	38	42
Expected return on plan assets	(24)	(23)	(49)	(45)
Prior service credit	(6)	(6)	(11)	(11)
Net loss from previous years	0	4	0	7
Net periodic benefit cost	(6)	2	(12)	4
Defined benefit plan, contributions by Employer	$ 11	13	$ 22	24
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		8
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		26
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		$ 2