Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ (7.4)	$ 58.0	$ 49.3	$ 97.1
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	1.5	(7.0)	(15.7)	(13.3)
Trading securities realized holding gain (loss) on investments after tax	(5.9)	51.0	33.6	83.8
Foreign Currency Transaction Gain (Loss), Realized	0.0	(0.1)	0.4	(0.1)
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	0.0	(0.1)	0.4	(0.1)
Trading Securities, Realized Gain (Loss), Total	(7.4)	57.9	49.7	97.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	1.5	(7.0)	(15.7)	(13.3)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(5.9)	50.9	34.0	83.7
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.1)	2.3	2.0	4.7
Foreign Currency Transaction Gain (Loss), Realized	0.0	(0.1)	0.0	(0.1)
Trading Securities, Realized Gain (Loss), Total	(0.1)	2.2	2.0	4.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Trading Securities, Realized Gain (Loss), Total	0.0	0.0	0.0	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(5.9)	51.4	41.6	83.2
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.4	0.0
Trading Securities, Realized Gain (Loss), Total	(5.9)	51.4	42.0	83.2
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.0	1.6	(4.3)	6.5
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Trading Securities, Realized Gain (Loss), Total	0.0	1.6	(4.3)	6.5
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(1.4)	2.7	10.0	2.7
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Trading Securities, Realized Gain (Loss), Total	$ (1.4)	$ 2.7	$ 10.0	$ 2.7