Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	$ 52.8	$ 21.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	50.6	42.1
Net Realized and Unrealized Gain (Loss) on Trading Securities	103.4	63.8
Income tax expense attributable to net realized and unrealized investment gains	(26.3)	(15.7)
Realized and unrealized gain	77.1	48.1
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	31.0	0.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	52.3	31.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	3.5	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	25.5	18.8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(3.4)	4.5
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	(4.2)	2.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.8	0.7
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	(3.0)	(0.3)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.9	5.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	0	(0.1)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0

The sum of the realized and unrealized net gain (loss) on trading securities during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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