Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Interest Rate Futures | Long
Derivative [Line Items]
Notional amount	$ 1,400,000,000	$ 1,200,000,000
Interest Rate Futures | Short
Derivative [Line Items]
Notional amount	824,200,000	727,900,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Long
Derivative [Line Items]
Notional amount	123,300,000	184,200,000
Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Short
Derivative [Line Items]
Notional amount	121,500,000	91,400,000
Foreign Currency Forward Contracts, Investment Operations | Long
Derivative [Line Items]
Notional amount	39,100,000	26,900,000
Foreign Currency Forward Contracts, Investment Operations | Short
Derivative [Line Items]
Notional amount	51,700,000	57,300,000
Credit Default Swaps | Long
Derivative [Line Items]
Notional amount	0	0
Credit Default Swaps | Short
Derivative [Line Items]
Notional amount	$ 63,000,000	$ 75,200,000