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Commitments and Contingencies (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Entity Information [Line Items]    
Net assets of subsidiaries subject to restrictions $ 2,008 $ 1,691
Aggregate statutory capital and surplus 2,152  
Required minimum statutory capital surplus 1,180  
Parent Company    
Entity Information [Line Items]    
Cash, cash equivalents, and investments (excluding restricted investments) $ 350 $ 696