| Document and Entity Information | Oct. 26, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 26, 2020 |
| Entity Registrant Name | K12 Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-33883 |
| Entity Tax Identification Number | 95-4774688 |
| Entity Address, Address Line One | 2300 Corporate Park Drive |
| Entity Address, City or Town | Herndon |
| Entity Address, State or Province | VA |
| Entity Address, Postal Zip Code | 20171 |
| City Area Code | 703 |
| Local Phone Number | 483-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001157408 |
| Title of 12(b) Security | Common Stock, $0.0001 par value |
| Trading Symbol | LRN |
| Security Exchange Name | NYSE |
