Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Other Assets
Derivative [Line Items]
Derivative Asset	$ 6,382,000	$ 5,306,000
Interest Rate Futures | Other Assets
Derivative [Line Items]
Derivative Asset	1,136,000	122,000
Interest Rate Futures | Long
Derivative [Line Items]
Derivative, Notional Amount	952,300,000	1,000,000,000
Interest Rate Futures | Short
Derivative [Line Items]
Derivative, Notional Amount	772,500,000	1,100,000,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Other Assets
Derivative [Line Items]
Derivative Asset	4,169,000	4,563,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Long
Derivative [Line Items]
Derivative, Notional Amount	205,100,000	172,400,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Short
Derivative [Line Items]
Derivative, Notional Amount	150,700,000	101,500,000
Foreign Currency Forward Contracts, Investment Operations | Other Assets
Derivative [Line Items]
Derivative Asset	76,000	408,000
Foreign Currency Forward Contracts, Investment Operations | Long
Derivative [Line Items]
Derivative, Notional Amount	30,100,000	31,300,000
Foreign Currency Forward Contracts, Investment Operations | Short
Derivative [Line Items]
Derivative, Notional Amount	62,000,000	143,400,000
Credit Default Swaps | Other Assets
Derivative [Line Items]
Derivative Asset	1,001,000	213,000
Credit Default Swaps | Long
Derivative [Line Items]
Derivative, Notional Amount	0	0
Credit Default Swaps | Short
Derivative [Line Items]
Derivative, Notional Amount	$ 45,700,000	$ 46,100,000