Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Interest rate futures | Long
Derivative [Line Items]
Notional amount	$ 2,200,000,000	$ 2,500,000,000
Interest rate futures | Short
Derivative [Line Items]
Notional amount	935,700,000	1,000,000,000.0
Interest rate swaps | US Dollars | Paying a Fixed Rate
Derivative [Line Items]
Notional amount	2,000,000.0	27,900,000
Interest rate swaps | US Dollars | Receiving a Fixed Rate
Derivative [Line Items]
Notional amount	25,500,000	25,500,000
Foreign currency forward contracts, underwriting and non-investment operations | Long
Derivative [Line Items]
Notional amount	502,400,000	722,600,000
Foreign currency forward contracts, underwriting and non-investment operations | Short
Derivative [Line Items]
Notional amount	651,500,000	1,200,000,000
Foreign currency forward contracts, investment operations | Long
Derivative [Line Items]
Notional amount	384,800,000	195,600,000
Foreign currency forward contracts, investment operations | Short
Derivative [Line Items]
Notional amount	255,500,000	61,000,000.0
Credit default swaps | Long
Derivative [Line Items]
Notional amount	500,000	500,000
Credit default swaps | Short
Derivative [Line Items]
Notional amount	117,100,000	143,400,000
Total return swaps | Long
Derivative [Line Items]
Notional amount	173,500,000	173,500,000
Total return swaps | Short
Derivative [Line Items]
Notional amount	0	0
Equity futures | Long
Derivative [Line Items]
Notional amount	50,400,000	122,000,000.0
Equity futures | Short
Derivative [Line Items]
Notional amount	$ 0	$ 0