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Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Interest rate futures | Long    
Derivative [Line Items]    
Notional amount $ 2,100,000,000 $ 2,500,000,000
Interest rate futures | Short    
Derivative [Line Items]    
Notional amount 913,400,000 1,000,000,000.0
Interest rate swaps | US Dollars | Paying a Fixed Rate    
Derivative [Line Items]    
Notional amount 0 27,900,000
Interest rate swaps | US Dollars | Receiving a Fixed Rate    
Derivative [Line Items]    
Notional amount 23,500,000 25,500,000
Foreign currency forward contracts, underwriting and non-investment operations | Long    
Derivative [Line Items]    
Notional amount 464,000,000.0 722,600,000
Foreign currency forward contracts, underwriting and non-investment operations | Short    
Derivative [Line Items]    
Notional amount 514,000,000.0 1,200,000,000
Foreign currency forward contracts, investment operations | Long    
Derivative [Line Items]    
Notional amount 219,900,000 195,600,000
Foreign currency forward contracts, investment operations | Short    
Derivative [Line Items]    
Notional amount 64,200,000 61,000,000.0
Credit default swaps | Long    
Derivative [Line Items]    
Notional amount 0 500,000
Credit default swaps | Short    
Derivative [Line Items]    
Notional amount 121,200,000 143,400,000
Total return swaps | Long    
Derivative [Line Items]    
Notional amount 26,100,000 173,500,000
Total return swaps | Short    
Derivative [Line Items]    
Notional amount 0 0
Equity futures | Long    
Derivative [Line Items]    
Notional amount 0 122,000,000.0
Equity futures | Short    
Derivative [Line Items]    
Notional amount $ 44,000,000.0 $ 0