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Commitments and Contingencies (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Entity Information [Line Items]    
Net assets of subsidiaries subject to restrictions $ 1,777 $ 1,691
Aggregate statutory capital and surplus 1,904  
Required minimum statutory capital surplus 1,181  
Parent Company    
Entity Information [Line Items]    
Cash, cash equivalents, and investments (excluding restricted investments) $ 706 $ 696