Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Interest rate swaps | Paying a Fixed Rate | United States of America, Dollars
Derivative [Line Items]
Derivative notional amount	$ 78,400,000	$ 40,300,000
Interest rate swaps | Receiving a Fixed Rate | United States of America, Dollars
Derivative [Line Items]
Derivative notional amount	32,100,000	0
Long | Interest rate futures
Derivative [Line Items]
Derivative notional amount	1,900,000,000	1,500,000,000
Long | Foreign currency forward contracts
Derivative [Line Items]
Derivative notional amount	354,100,000	215,400,000
Long | Foreign currency forward contracts
Derivative [Line Items]
Derivative notional amount	121,300,000	16,600,000
Long | Credit default swaps
Derivative [Line Items]
Derivative notional amount	1,000,000	1,000,000
Short | Interest rate futures
Derivative [Line Items]
Derivative notional amount	545,800,000	801,100,000
Short | Foreign currency forward contracts
Derivative [Line Items]
Derivative notional amount	601,200,000	44,200,000
Short | Foreign currency forward contracts
Derivative [Line Items]
Derivative notional amount	42,900,000	5,100,000
Short | Credit default swaps
Derivative [Line Items]
Derivative notional amount	$ 126,200,000	$ 18,800,000