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Investments - Credit Impairment Loss Type (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Jun. 30, 2017
Schedule of Investments [Line Items]            
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Including Credit Impairments $ 0   $ (2)      
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Change in Status 0   0      
Amortized cost 22,143   22,143      
Available-for-sale Securities 21,421   21,421      
Other than temporary impairment, non-credit losses recognized in OCI 0   0      
Asset Backed Securities and Corporate Securities [Member]            
Schedule of Investments [Line Items]            
Amortized cost 0   0      
Available-for-sale Securities $ 0   $ 0      
Predecessor            
Schedule of Investments [Line Items]            
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Including Credit Impairments   $ 0   $ (21)    
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Change in Status   0   0    
Amortized cost         $ 20,847  
Available-for-sale Securities         $ 20,963  
Other than temporary impairment, non-credit losses recognized in OCI   $ 0   $ 0    
Predecessor | Asset Backed Securities and Corporate Securities [Member]            
Schedule of Investments [Line Items]            
Amortized cost           $ 0
Available-for-sale Securities           $ 0