Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Amortization of:
Net periodic benefit cost	$ (5)	$ 11	$ (10)	$ 21
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	89	102	178	202
Interest cost	221	200	442	401
Expected return on assets	(306)	(313)	(612)	(626)
Amortization of:
Prior service cost (benefit)	0	0	0	1
Actuarial loss	103	157	206	314
Settlement charges	0	1	0	1
Net periodic benefit cost	107	147	214	293
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	24	28	47	56
Interest cost	47	44	94	88
Expected return on assets	(38)	(43)	(77)	(87)
Amortization of:
Prior service cost (benefit)	(44)	(47)	(89)	(93)
Actuarial loss	10	16	23	33
Settlement charges	0	0	0	0
Net periodic benefit cost	$ (1)	$ (2)	$ (2)	$ (3)