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Variable Interest Entities (VIEs) (Details) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2011
Variable Interest Entity [Line Items]          
Administrative fee revenue   $ 50 $ 55 $ 69  
New Mexico Data Center          
Variable Interest Entity [Line Items]          
Tax credit claimed as a percentage of original investment amount         39.00%
Tax credit claimed, term         7 years
Percentage of credit claimed for first three years         5.00%
Percentage of credit claimed for remaining four years         6.00%
Capital contribution by related party         $ 6
Principal amount of loan by party         16
Proceeds from other debt     6   $ 21
Variable Interest Entity, Primary Beneficiary          
Variable Interest Entity [Line Items]          
Amount paid for health care services   119 122 $ 117  
Variable interest entity, total assets   8 18    
Variable interest entity, total liabilities   $ 8 $ 18    
Variable Interest Entity, Primary Beneficiary | Subsequent Event          
Variable Interest Entity [Line Items]          
Payments for purchase of assets $ 2