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Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Interest rate futures | Long    
Derivative [Line Items]    
Notional amount $ 1,500,000,000 $ 1,500,000,000
Interest rate futures | Short    
Derivative [Line Items]    
Notional amount 812,700,000 801,100,000
Interest rate swaps | US Dollars | Paying a Fixed Rate    
Derivative [Line Items]    
Notional amount 22,900,000 40,300,000
Interest rate swaps | US Dollars | Receiving a Fixed Rate    
Derivative [Line Items]    
Notional amount 6,900,000 0
Foreign currency forward contracts, underwriting and non-investment operations | Long    
Derivative [Line Items]    
Notional amount 218,600,000 215,400,000
Foreign currency forward contracts, underwriting and non-investment operations | Short    
Derivative [Line Items]    
Notional amount 25,400,000 44,200,000
Foreign currency forward contracts, investment operations | Long    
Derivative [Line Items]    
Notional amount 66,400,000 16,600,000
Foreign currency forward contracts, investment operations | Short    
Derivative [Line Items]    
Notional amount 40,500,000 5,100,000
Credit default swaps | Long    
Derivative [Line Items]    
Notional amount 1,000,000 1,000,000
Credit default swaps | Short    
Derivative [Line Items]    
Notional amount $ 41,100,000 $ 18,800,000