Investment Securities (Net Realized and Unrealized 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, Realized Gain (Loss), Total	(7.4)	57.9	49.7	97.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(36.5)	(85.4)	(83.6)	(32.6)
Net Realized and Unrealized Gain (Loss) on Trading Securities	(43.9)	(27.5)	(33.9)	64.4
Income tax benefit (expense) attributable to net realized and unrealized investment gains (losses)	9.4	5.6	6.9	(12.2)
Realized and unrealized gain	(34.5)	(21.9)	(27.0)	52.2
Foreign Currency Transaction Gain (Loss), Realized	0.0	(0.1)	0.4	(0.1)
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	1.5	(7.0)	(15.7)	(13.3)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0	0.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	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
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	0.0	(0.1)	0.4	(0.1)
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
Trading Securities, Realized Gain (Loss), Total	(0.1)	2.2	2.0	4.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	3.3	(10.0)	1.3	12.3
Foreign Currency Transaction Gain (Loss), Realized	0.0	(0.1)	0.0	(0.1)
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
Trading Securities, Realized Gain (Loss), Total	0.0	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	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
Trading Securities, Realized Gain (Loss), Total	(5.9)	51.4	42.0	83.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(30.8)	(69.2)	(58.5)	(48.5)
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.4	0.0
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
Trading Securities, Realized Gain (Loss), Total	0.0	1.6	(4.3)	6.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	1.1	(3.4)	2.0	(5.1)
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
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
Trading Securities, Realized Gain (Loss), Total	(1.4)	2.7	10.0	2.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(10.1)	(2.8)	(28.4)	8.7
Foreign Currency Transaction Gain (Loss), Realized	$ 0.0	$ 0.0	$ 0.0	$ 0.0