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Variable Interest Entities (VIEs) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Variable Interest Entity [Line Items]          
Administrative fee revenue $ 12 $ 13 $ 38 $ 40  
Variable Interest Entity, Primary Beneficiary          
Variable Interest Entity [Line Items]          
Amount paid for health care services 29 $ 31 89 $ 92  
Total carrying amount of assets 20   20   $ 18
Total amount of liabilities $ 24   $ 24   $ 18