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Document and Entity Information - USD ($)
 12 Months Ended
Jun. 30, 2018
Jul. 31, 2018
Dec. 31, 2017
Document and Entity Information      
Entity Registrant Name K12 INC    
Entity Central Index Key 0001157408    
Document Type 10-K    
Document Period End Date Jun. 30, 2018    
Amendment Flag false    
Current Fiscal Year End Date --06-30    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Accelerated Filer    
Entity Public Float     $ 415,602,150
Entity Common Stock, Shares Outstanding   39,572,956  
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY