Document and Entity Information - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2018
Feb. 22, 2019
Jun. 30, 2018
Document and Entity Information      
Entity Registrant Name MAGELLAN HEALTH INC    
Entity Central Index Key 0000019411    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 2.4
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Ex Transition Period false    
Entity Common Stock, Shares Outstanding   23,925,342  
Entity Shell Company false    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY