| Document and Entity Information | Jun. 15, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jun. 15, 2021 |
| Entity Registrant Name | ATI PHYSICAL THERAPY, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-39439 |
| Entity Tax Identification Number | 85-1408039 |
| Entity Address, Address Line One | 790 Remington Boulevard |
| Entity Address, City or Town | Bolingbrook |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60440 |
| City Area Code | 630 |
| Local Phone Number | 296-2223 |
| Entity Information, Former Legal or Registered Name | Fortress Value Acquisition Corp. II |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001815849 |
| Former Address [Member] | |
| Entity Information [Line Items] | |
| Entity Address, Address Line One | 1345 Avenue of the Americas |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10105 |
| Common Class A [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Class A common stock, par value of $0.0001 per share |
| Trading Symbol | FAII |
| Security Exchange Name | NYSE |
| Units, each consisting of one share of Class A common stock and one-fifth of one redeemable warrant [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Units, each consisting of one share of Class A common stock and one-fifth of one redeemable warrant |
| Trading Symbol | FAII.U |
| Security Exchange Name | NYSE |
| Redeemable warrants, each whole warrant exercisable for one share of Class A common stock at an exercise price of $11.50 per share [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Redeemable warrants, each whole warrant exercisable for one share of Class A common stock at an exercise price of $11.50 per share |
| Trading Symbol | FAII WS |
| Security Exchange Name | NYSE |
