Derivative Instruments - Volume of Activity, in terms of Notional Amount, Related to Derivative Instruments (Details) - USD ($)
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Derivative [Line Items]    
Beginning of Period $ 0 $ 0
Additions 2,965,000,000 1,075,000,000
Scheduled Settlements (2,965,000,000) (1,075,000,000)
Early Terminations 0 0
End of Period 0 0
Interest Rate Swap    
Derivative [Line Items]    
Beginning of Period 275,000,000 2,985,000,000
Additions 750,000,000 275,000,000
Scheduled Settlements 0 (100,000,000)
Early Terminations (875,000,000) (2,885,000,000)
End of Period 150,000,000 275,000,000
2-year U.S. Treasury Note Futures    
Derivative [Line Items]    
Beginning of Period 0 0
Additions 50,000,000 1,150,000,000
Scheduled Settlements (50,000,000) 0
Early Terminations 0 (1,150,000,000)
End of Period 0 0
10-year U.S. Treasury Note Futures    
Derivative [Line Items]    
Beginning of Period 0 0
Additions 590,100,000 765,000,000
Scheduled Settlements (365,000,000) 0
Early Terminations (200,100,000) (765,000,000)
End of Period 25,000,000 0
Purchased Call Options on Ten Year U.S. Treasury Note Futures    
Derivative [Line Items]    
Beginning of Period 0  
Additions 165,200,000  
Scheduled Settlements (65,200,000)  
Early Terminations (75,000,000)  
End of Period 25,000,000 0
Put Options on S&P 500 ETF    
Derivative [Line Items]    
Beginning of Period $ 0 0
Additions   1,850,000
Scheduled Settlements   (1,850,000)
Early Terminations   0
End of Period   $ 0