Retirement Programs - Pension and Postretirement Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended					12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Curtailment and termination benefits								$ (9)
Settlement charges			$ 6	$ 40	$ 51			0
Amount recognized in Net pension and OPEB cost (benefit), excluding service cost						$ (192)	$ (177)	(32)
Termination charge								17
Payment for plan settlement					$ 91
Pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost						155	150	142
Interest cost						151	208	261
Expected return on plan assets						(521)	(482)	(462)
Net amortization and deferral						176	90	61
Curtailment and termination benefits						0	0	8
Settlement charges	$ 4	$ 6				4	6	97
Amount recognized in Net pension and OPEB cost (benefit), excluding service cost						(190)	(178)	(35)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total						(35)	(28)	107
OPEB
Defined Benefit Plan Disclosure [Line Items]
Service cost						2	2	2
Interest cost						3	5	7
Expected return on plan assets						0
Net amortization and deferral						(5)	(4)	(4)
Curtailment and termination benefits						0	0	0
Settlement charges						0
Amount recognized in Net pension and OPEB cost (benefit), excluding service cost						(2)	1	3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total						0	3	$ 5
Payment for plan settlement						$ 0	$ 0