Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Amortization of:
Net periodic benefit cost	$ 19	$ 15	$ 64	$ 38
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	110	97	330	290
Interest cost	161	190	481	569
Expected return on assets	(335)	(317)	(1,003)	(953)
Amortization of:
Prior service cost (credit)	1	1	3	3
Actuarial loss	150	128	449	384
Curtailment benefits			0	0
Settlement charges	12	8	16	14
Net periodic benefit cost	99	107	276	307
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	22	60	67
Interest cost	29	37	86	114
Expected return on assets	(40)	(41)	(119)	(122)
Amortization of:
Prior service cost (credit)	(8)	(30)	(25)	(92)
Actuarial loss	9	12	27	36
Curtailment benefits			(1)	0
Settlement charges	0	0	0	0
Net periodic benefit cost	$ 10	$ 0	$ 28	$ 3