Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Interest rate futures | Long
Derivative [Line Items]
Notional amount	$ 1,100,000,000	$ 1,200,000,000
Interest rate futures | Short
Derivative [Line Items]
Notional amount	399,700,000	727,900,000
Interest rate swaps | Paying a Fixed Rate | US Dollars
Derivative [Line Items]
Notional amount	51,300,000
Interest rate swaps | Receiving a Fixed Rate | US Dollars
Derivative [Line Items]
Notional amount	5,300,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Long
Derivative [Line Items]
Notional amount	191,100,000	184,200,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Short
Derivative [Line Items]
Notional amount	68,500,000	91,400,000
Foreign Currency Forward Contracts, Investment Operations | Long
Derivative [Line Items]
Notional amount	17,600,000	26,900,000
Foreign Currency Forward Contracts, Investment Operations | Short
Derivative [Line Items]
Notional amount	4,500,000	57,300,000
Credit default swaps | Long
Derivative [Line Items]
Notional amount	15,500,000	0
Credit default swaps | Short
Derivative [Line Items]
Notional amount	$ 13,300,000	$ 75,200,000