Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	$ 45.5	$ 1.4	$ 71.2	$ (14.7)
Trading securities, Tax on Realized Holding (Losses) Gains on Investments	(7.1)	9.4	(8.8)	8.9
Trading securities realized holding (losses) gain on investments after tax	38.4	10.8	62.4	(5.8)
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	45.5	1.4	71.2	(3.7)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(7.1)	9.4	(8.8)	8.9
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	38.4	10.8	62.4	5.2
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	3.0	(8.8)	2.1	(22.5)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	18.2
Net realized investment gains (losses), pre-tax	3.0	(8.8)	2.1	(4.3)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	0.1	(0.7)	0.1	(0.8)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	0.0
Net realized investment gains (losses), pre-tax	0.1	(0.7)	0.1	(0.8)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	41.6	10.8	67.4	12.0
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	0.0
Net realized investment gains (losses), pre-tax	41.6	10.8	67.4	12.0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	0.8	0.1	1.6	(3.4)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0	0.0	(7.2)
Net realized investment gains (losses), pre-tax	$ 0.8	$ 0.1	$ 1.6	$ (10.6)