Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Amortization of:
Net periodic benefit cost	$ (20)	$ (10)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	110	97
Interest cost	161	189
Expected return on assets	(336)	(318)
Amortization of:
Prior service cost (credit)	1	1
Actuarial loss	150	128
Curtailment benefits	0	0
Net periodic benefit cost	86	97
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	21	23
Interest cost	28	38
Expected return on assets	(40)	(41)
Amortization of:
Prior service cost (credit)	(8)	(31)
Actuarial loss	9	12
Curtailment benefits	(1)	0
Net periodic benefit cost	$ 9	$ 1