Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	$ 76.0	$ 11.7	$ 128.3	$ 60.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(55.2)	16.5	70.2	5.9
Net Realized and Unrealized Gain (Loss) on Trading Securities	20.8	28.2	198.5	66.1
Income tax expense attributable to net realized and unrealized investment gains	(6.6)	(1.2)	(45.4)	(2.5)
Realized and unrealized gain	14.2	27.0	153.1	63.6
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	14.5	(9.4)	24.2	(1.5)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	23.3	(21.2)	108.0	(88.6)
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	1.1	(0.6)	1.1	0.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0.1
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	54.7	20.5	92.9	57.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(69.9)	33.6	(43.0)	84.2
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	2.1	(0.2)	5.9	(0.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(3.9)	2.3	(5.4)	(0.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	3.3	1.9	4.0	4.8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(4.7)	1.7	10.6	10.8
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	$ 0.3	$ (0.5)	$ 0.2	$ (0.3)