Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	$ 19.6	$ (10.2)	$ 90.8	$ (24.9)
Trading securities, Tax on Realized Holding (Losses) Gains on Investments	(0.2)	2.3	(9.0)	11.2
Trading securities realized holding (losses) gain on investments after tax	19.4	(7.9)	81.8	(13.7)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	11.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gains (Losses) Tax	0.0	0.0	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	0.0	0.0	0.0	11.0
Net realized investment gains (losses), pre-tax (1)	19.6	(10.2)	90.8	(13.9)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.2)	2.3	(9.0)	11.2
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	19.4	(7.9)	81.8	(2.7)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	1.4	(5.1)	3.5	(27.6)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	18.2
Net realized investment gains (losses), pre-tax (1)	1.4	(5.1)	3.5	(9.4)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains			0.1	(0.8)
Foreign Currency Transaction Gains (Losses), Realized			0.0	0.0
Net realized investment gains (losses), pre-tax (1)			0.1	(0.8)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	14.0	(1.5)	81.4	10.5
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	0.0
Net realized investment gains (losses), pre-tax (1)	14.0	(1.5)	81.4	10.5
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	4.2	(3.6)	5.8	(7.0)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	(7.2)
Net realized investment gains (losses), pre-tax (1)	$ 4.2	$ (3.6)	$ 5.8	$ (14.2)