Contingencies and Commitments​ (Details) $ in Millions								1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended
	Dec. 23, 2020 claimant defendant	Jun. 26, 2020 claimant	Jun. 12, 2020 USD ($) firefighter claimant police_officer	Jun. 11, 2020 firefighter	Mar. 17, 2020 subsidiary	Jun. 06, 2019 plaintiff	Sep. 15, 2017 executive	Jun. 30, 2019 USD ($)	Sep. 30, 2021 USD ($)	Mar. 31, 2021	Jun. 30, 2018 request	Sep. 30, 2021 USD ($) claimant firefighter police_officer	Sep. 30, 2018 request	Dec. 31, 2020 USD ($)
Site Contingency [Line Items]
Environmental remediation | $									$ 103			$ 103		$ 156
Environmental remediation, current | $									56			56		90
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | $									8			7
Accrual for Environmental Loss Contingencies, Increase (Decrease) for Other Items | $									1			2
Environmental remediation, payments | $									23			$ 44
Revenue | Geographic Concentration Risk | RUSSIAN FEDERATION
Site Contingency [Line Items]
Concentration risk, percentage										12.00%
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Birnberg Peirce Ltd | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	118		4									113
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Howe & Co Solicitors | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs			2
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Russell-Cooke LLP | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	24	3										34
Grenfell Tower Fire, Survivors And Estates Of Decedents | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Loss Contingency, Number of Defendants | defendant	23
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Deighton Pierce Glynn | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	7
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By SMQ Legal Services | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	5
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Scott Moncrieff | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	4
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Saunders Law | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	6											31
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Imran Khan And Partners | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	40											47
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Howe And Co | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	67											61
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Hodge Jones And Allen Solicitor | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	114
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Hickman And Rose | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	19											23
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Duncan Lewis Solicitors | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	5
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Bindmans LLP | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	341
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Bhatt Murphy Ltd | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs	82											76
Grenfell Tower Fire, Survivors And Estates Of Decedents, Represented By Slater & Gordon | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs												27
Grenfell Tower Fire, Emergency Responders, Represented By Thompsons Solicitors | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs | firefighter				80								98
Loss Contingency, Number Of Additional Plaintiffs | firefighter				5								7
Grenfell Tower Fire, Emergency Responders, Represented By Penningtons Manches Cooper LLP | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs | police_officer			27									36
Grenfell Tower Fire, Emergency Responders, Represented By Pattinson And Brewer | Damage from Fire, Explosion or Other Hazard
Site Contingency [Line Items]
Number of plaintiffs | firefighter			2
Loss, Contingency, Number Of Plaintiffs Withdrawn | firefighter			1
Behrens v. Arconic
Site Contingency [Line Items]
Number of plaintiffs | plaintiff						247
Sullivan v. Arconic
Site Contingency [Line Items]
Number of former executives | executive							3
Airbus Matters | Damages from Product Defects
Site Contingency [Line Items]
Loss Contingency, New Claims Filed, Number | request											3		3
Loss Contingency, Damages Sought, Value | $			$ 200
Federal Antimonopoly Services Of The Russian Federation Litigation | Unfavorable Regulatory Action
Site Contingency [Line Items]
Loss Contingency, Number Of Subsidiaries | subsidiary					2
Massena West, NY
Site Contingency [Line Items]
Environmental remediation | $									64			$ 64		$ 115
Accrual for Environmental Loss Contingencies, Revision in Estimates | $									11			11
Environmental remediation, additions | $								$ (25)
Massena West, NY | Legal Matters, Other
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Revision in Estimates | $									2
Massena West, NY | Construction in Progress
Site Contingency [Line Items]
Environmental remediation | $									40			40
Massena West, NY | Maintenance
Site Contingency [Line Items]
Environmental remediation | $									$ 24			$ 24