Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ (16.1)	$ 0.4
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(0.6)	(0.2)
Trading securities realized holding gain (loss) on investments after tax	(16.7)	0.2
Foreign Currency Transaction Gain (Loss), Realized	11.0	0.2
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	11.0	0.2
Net realized investment (losses) gains, pre-tax	(5.1)	0.6
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.6)	(0.2)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(5.7)	0.4
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(13.7)	(1.0)
Foreign Currency Transaction Gain (Loss), Realized	18.2	0.1
Net realized investment (losses) gains, pre-tax	4.5	(0.9)
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.1)	0.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0
Net realized investment (losses) gains, pre-tax	(0.1)	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	1.2	0.8
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.1
Net realized investment (losses) gains, pre-tax	1.2	0.9
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(3.5)	0.6
Foreign Currency Transaction Gain (Loss), Realized	(7.2)	0.0
Net realized investment (losses) gains, pre-tax	$ (10.7)	$ 0.6