Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Amortization of:
Net periodic benefit cost	$ 26	$ 14	$ 46	$ 24
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	111	96	220	193
Interest cost	160	190	320	379
Expected return on assets	(333)	(318)	(667)	(636)
Amortization of:
Prior service cost (credit)	0	1	1	2
Actuarial loss	149	128	299	256
Curtailment benefits			0	0
Settlement charges	4	6	4	6
Net periodic benefit cost	91	103	177	200
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	22	40	45
Interest cost	28	39	57	77
Expected return on assets	(39)	(40)	(79)	(81)
Amortization of:
Prior service cost (credit)	(9)	(31)	(17)	(62)
Actuarial loss	10	12	19	24
Curtailment benefits			(1)	0
Settlement charges	0	0	0	0
Net periodic benefit cost	$ 10	$ 2	$ 19	$ 3