Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Long | Interest Rate Futures
Derivative [Line Items]
Derivative, Notional Amount	$ 1,208,300,000	$ 1,012,500,000
Long | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations
Derivative [Line Items]
Derivative, Notional Amount	184,200,000	172,400,000
Long | Foreign Currency Forward Contracts, Investment Operations
Derivative [Line Items]
Derivative, Notional Amount	26,900,000	31,300,000
Long | Credit Default Swaps
Derivative [Line Items]
Derivative, Notional Amount	0	0
Short | Interest Rate Futures
Derivative [Line Items]
Derivative, Notional Amount	727,900,000	1,115,900,000
Short | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations
Derivative [Line Items]
Derivative, Notional Amount	91,400,000	101,500,000
Short | Foreign Currency Forward Contracts, Investment Operations
Derivative [Line Items]
Derivative, Notional Amount	57,300,000	143,400,000
Short | Credit Default Swaps
Derivative [Line Items]
Derivative, Notional Amount	$ 75,200,000	$ 46,100,000