Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 7.4	$ 0.7	$ 21.0	$ 265.0
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(0.6)	(0.1)	(3.6)	(45.1)
Trading securities realized holding gain (loss) on investments after tax	6.8	0.6	17.4	219.9
Foreign Currency Transaction Gain (Loss), Realized	(0.6)	0.0	(0.2)	0.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	(0.6)	0.0	(0.2)	0.0
Net realized investment gains, pre-tax	6.8	0.7	20.8	265.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.6)	(0.1)	(3.6)	(45.1)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	6.2	0.6	17.2	219.9
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.6	0.3	(0.4)	2.0
Foreign Currency Transaction Gain (Loss), Realized	1.2	0.0	2.7	0.0
Net realized investment gains, pre-tax	1.8	0.3	2.3	2.0
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.1)	0.2	(0.1)	0.4
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Net realized investment gains, pre-tax	(0.1)	0.2	(0.1)	0.4
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	4.9	0.2	18.5	262.6
Foreign Currency Transaction Gain (Loss), Realized	5.4	0.0	6.0	0.0
Net realized investment gains, pre-tax	10.3	0.2	24.5	262.6
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	2.0	0.0	3.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	(7.2)	0.0	(8.9)	0.0
Net realized investment gains, pre-tax	$ (5.2)	$ 0.0	$ (5.9)	$ 0.0