Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Amortization of:
Net periodic benefit cost	$ (10)	$ (5)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	97	89
Interest cost	189	221
Expected return on assets	(318)	(307)
Amortization of:
Prior service cost (benefit)	1	0
Actuarial loss	128	104
Net periodic benefit cost	97	107
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	23	24
Interest cost	38	47
Expected return on assets	(41)	(38)
Amortization of:
Prior service cost (benefit)	(31)	(45)
Actuarial loss	12	11
Net periodic benefit cost	$ 1	$ (1)