Earnings Per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
	Oct. 15, 2019	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income from continuing operations attributable to common shareholders:		$ 106	$ 36	$ (84)	$ 153	$ 118	$ 58	$ (136)	$ 86	$ 211	$ 126	$ 309
Less: preferred stock dividends declared										2	2	2
Net income from continuing operations attributable to common shareholders:										209	124	307
Income from discontinued operations										50	344	333
Net income available to Howmet Aerospace common shareholders - basic										259	468	640
Add: interest expense related to convertible notes										0	9	11
Net income available to Howmet common shareholders - diluted										$ 259	$ 477	$ 651
Average shares outstanding - basic (in shares)										435	446	483
Effect of dilutive securities:
Stock options (in shares)										0	1	1
Stock and performance awards (in shares)										4	5	5
Convertible notes (in shares)										0	11	14
Average shares outstanding - diluted (in shares)										439	463	503
Convertible notes
Effect of dilutive securities:
Number of anti-dilutive securities (in shares)	15									0	0	0