| Cover | Sep. 30, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 30, 2021 |
| Entity Registrant Name | Griffin-American Healthcare REIT III, Inc. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 000-55434 |
| Entity Tax Identification Number | 46-1749436 |
| Entity Address, Address Line One | 18191 Von Karman Avenue |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | Irvine |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92612 |
| City Area Code | 949 |
| Local Phone Number | 270-9200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001566912 |
| Current Fiscal Year End Date | --12-31 |
