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Medical Claims and Benefits Payable - Incurred Claims and Allocated Adjustment Expense (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
claim
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Product Information [Line Items]      
Incurred Claims and Allocated Claims Adjustment Expenses $ 17,073 $ 14,774 $ 11,794
Incurred Claims and Allocated Claims Adjustment Expenses 44,069    
Total IBNP 1,717 1,352 1,191
Benefit Year 2015      
Product Information [Line Items]      
Incurred Claims and Allocated Claims Adjustment Expenses 11,939 11,928 $ 12,113
Total IBNP $ 6    
Cumulative number of reported claims (in claim) | claim 84,000,000    
Benefit Year 2016      
Product Information [Line Items]      
Incurred Claims and Allocated Claims Adjustment Expenses $ 15,093 $ 15,064  
Total IBNP $ 46    
Cumulative number of reported claims (in claim) | claim 109,000,000    
Benefit Year 2017      
Product Information [Line Items]      
Incurred Claims and Allocated Claims Adjustment Expenses $ 17,037    
Total IBNP $ 1,665    
Cumulative number of reported claims (in claim) | claim 114,000,000