Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Amortization of:
Net periodic benefit cost	$ (14)	$ (5)	$ (24)	$ (10)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	96	89	193	178
Interest cost	190	221	379	442
Expected return on assets	(318)	(306)	(636)	(612)
Amortization of:
Prior service cost (benefit)	1	0	2	0
Actuarial loss	128	103	256	206
Settlement charges	6	0	6	0
Net periodic benefit cost	103	107	200	214
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	24	45	47
Interest cost	39	47	77	94
Expected return on assets	(40)	(38)	(81)	(77)
Amortization of:
Prior service cost (benefit)	(31)	(44)	(62)	(89)
Actuarial loss	12	10	24	23
Settlement charges	0	0	0	0
Net periodic benefit cost	$ 2	$ (1)	$ 3	$ (2)