| Document and Entity Information Document | Jan. 24, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 24, 2021 |
| Entity Registrant Name | FIRST SOLAR, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-33156 |
| Entity Tax Identification Number | 20-4623678 |
| Entity Address, Address Line One | 350 West Washington Street, Suite 600 |
| Entity Address, City or Town | Tempe |
| Entity Address, State or Province | AZ |
| Entity Address, Postal Zip Code | 85281 |
| City Area Code | 602 |
| Local Phone Number | 414-9300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, $0.001 par value |
| Trading Symbol | FSLR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001274494 |
| Amendment Flag | false |
