| Cover Page | Nov. 05, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 5, 2020 |
| Entity Registrant Name | FIVE STAR SENIOR LIVING INC. |
| Entity Central Index Key | 0001159281 |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | FVE |
| Security Exchange Name | NASDAQ |
| Entity File Number | 1-16817 |
| Entity Incorporation, State or Country Code | MD |
| Entity Tax Identification Number | 04-3516029 |
| Entity Address, Address Line One | 400 Centre Street |
| Entity Address, City or Town | Newton |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02458 |
| City Area Code | 617 |
| Local Phone Number | 796-8387 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
