Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Interest rate swaps | Paying a Fixed Rate | United States of America, Dollars
Derivative [Line Items]
Derivative, notional amount	$ 27,900,000	$ 78,400,000
Interest rate swaps | Receiving a Fixed Rate | United States of America, Dollars
Derivative [Line Items]
Derivative, notional amount	25,500,000	32,100,000
Long | Interest rate futures
Derivative [Line Items]
Derivative, notional amount	2,500,000,000	1,900,000,000
Long | Foreign currency forward contracts
Derivative [Line Items]
Derivative, notional amount	722,600,000	354,100,000
Long | Foreign currency forward contracts
Derivative [Line Items]
Derivative, notional amount	195,600,000	121,300,000
Long | Credit default swaps
Derivative [Line Items]
Derivative, notional amount	500,000	1,000,000.0
Long | Total return swaps
Derivative [Line Items]
Derivative, notional amount	173,500,000
Long | Equity futures
Derivative [Line Items]
Derivative, notional amount	122,000,000.0	44,700,000
Short | Interest rate futures
Derivative [Line Items]
Derivative, notional amount	1,000,000,000.0	545,800,000
Short | Foreign currency forward contracts
Derivative [Line Items]
Derivative, notional amount	1,200,000,000	601,200,000
Short | Foreign currency forward contracts
Derivative [Line Items]
Derivative, notional amount	61,000,000.0	42,900,000
Short | Credit default swaps
Derivative [Line Items]
Derivative, notional amount	143,400,000	126,200,000
Short | Total return swaps
Derivative [Line Items]
Derivative, notional amount	0
Short | Equity futures
Derivative [Line Items]
Derivative, notional amount	$ 0	$ 0