| Document and Entity Information | Feb. 26, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001610114 |
| Document Type | 8-K |
| Document Period End Date | Feb. 26, 2021 |
| Entity Registrant Name | New Senior Investment Group Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-36499 |
| Entity Tax Identification Number | 80-0912734 |
| Entity Address, Address Line One | 55 West 46th Street |
| Entity Address, Address Line Two | Suite 2204 |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10036 |
| City Area Code | 646 |
| Local Phone Number | 822-3700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, par value $0.01 per share |
| Trading Symbol | SNR |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
