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Commitments and Contingencies - Narrative (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Loss Contingencies [Line Items]    
Fair value of assets pledged as collateral $ 915 $ 771
Acquisition of mortgage loans    
Loss Contingencies [Line Items]    
Amount of purchase commitment 29  
Purchase of limited partnerships and private placement investments    
Loss Contingencies [Line Items]    
Amount of purchase commitment 511  
Federal Home Loan Bank of Boston | Line of Credit    
Loss Contingencies [Line Items]    
Non-putable funding agreements issued to FHLB 788 657
Fair value of assets pledged as collateral $ 915 $ 771