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Medical Claims Payable ((Favorable) Unfavorable Developments By Changes In Key Assumptions) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Medical Claims Payable [Abstract]      
Assumed trend factors $ (399.5)antm_FavorableUnfavorableDevelopmentsByChangesInTrendFactorsAssumptions $ (428.4)antm_FavorableUnfavorableDevelopmentsByChangesInTrendFactorsAssumptions $ (394.4)antm_FavorableUnfavorableDevelopmentsByChangesInTrendFactorsAssumptions
Assumed completion factors (142.4)antm_FavorableUnfavorableDevelopmentsByChangesInCompletionFactorsAssumptions (170.7)antm_FavorableUnfavorableDevelopmentsByChangesInCompletionFactorsAssumptions (119.2)antm_FavorableUnfavorableDevelopmentsByChangesInCompletionFactorsAssumptions
Total $ (541.9)antm_FavorableUnfavorableDevelopmentsByChangesInTotalAssumptions $ (599.1)antm_FavorableUnfavorableDevelopmentsByChangesInTotalAssumptions $ (513.6)antm_FavorableUnfavorableDevelopmentsByChangesInTotalAssumptions