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Benefits Payable - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]      
Favorable medical claims reserve development related to prior fiscal years $ 518hum_FavorableMedicalClaimsReserveDevelopmentRelatedToPriorFiscalYears $ 474hum_FavorableMedicalClaimsReserveDevelopmentRelatedToPriorFiscalYears $ 257hum_FavorableMedicalClaimsReserveDevelopmentRelatedToPriorFiscalYears