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Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Interest rate futures | Long    
Derivative [Line Items]    
Notional amount $ 3,700,000,000 $ 1,900,000,000
Interest rate futures | Short    
Derivative [Line Items]    
Notional amount 202,300,000 545,800,000
Interest rate swaps | US Dollars | Paying a Fixed Rate    
Derivative [Line Items]    
Notional amount 32,200,000 78,400,000
Interest rate swaps | US Dollars | Receiving a Fixed Rate    
Derivative [Line Items]    
Notional amount 23,500,000 32,100,000
Foreign currency forward contracts, underwriting and non-investment operations | Long    
Derivative [Line Items]    
Notional amount 786,000,000.0 354,100,000
Foreign currency forward contracts, underwriting and non-investment operations | Short    
Derivative [Line Items]    
Notional amount 820,400,000 601,200,000
Foreign currency forward contracts, investment operations | Long    
Derivative [Line Items]    
Notional amount 128,100,000 121,300,000
Foreign currency forward contracts, investment operations | Short    
Derivative [Line Items]    
Notional amount 40,700,000 42,900,000
Credit default swaps | Long    
Derivative [Line Items]    
Notional amount 0 1,000,000.0
Credit default swaps | Short    
Derivative [Line Items]    
Notional amount 88,500,000 126,200,000
Total return swaps | Long    
Derivative [Line Items]    
Notional amount 225,000,000.0  
Total return swaps | Short    
Derivative [Line Items]    
Notional amount 0  
Equity futures | Long    
Derivative [Line Items]    
Notional amount 0 44,700,000
Equity futures | Short    
Derivative [Line Items]    
Notional amount $ 0 $ 0