Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	$ 25.7	$ (16.1)
Trading securities, Tax on Realized Holding (Losses) Gains on Investments	(1.7)	(0.6)
Trading securities realized holding (losses) gain on investments after tax	24.0	(16.7)
Foreign Currency Transaction Gains (Losses), Realized	0.0	11.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gains (Losses) Tax	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	0.0	11.0
Net realized investment gains (losses), pre-tax	25.7	(5.1)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(1.7)	(0.6)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	24.0	(5.7)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	(0.9)	(13.7)
Foreign Currency Transaction Gains (Losses), Realized	0.0	18.2
Net realized investment gains (losses), pre-tax	(0.9)	4.5
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	0.0	(0.1)
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0
Net realized investment gains (losses), pre-tax	0.0	(0.1)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	25.8	1.2
Foreign Currency Transaction Gains (Losses), Realized	0.0	0.0
Net realized investment gains (losses), pre-tax	25.8	1.2
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	0.8	(3.5)
Foreign Currency Transaction Gains (Losses), Realized	0.0	(7.2)
Net realized investment gains (losses), pre-tax	$ 0.8	$ (10.7)