Investment Securities (Net 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, Change in Unrealized Holding Gain (Loss)	$ 9.5	$ 29.0
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(2.8)	(9.1)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	6.7	19.9
Foreign Currency Transaction Gain (Loss), Unrealized	41.1	13.1
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	(10.3)	(2.9)
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	30.8	10.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	50.6	42.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(13.1)	(12.0)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	37.5	30.1
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	12.1	19.4
Foreign Currency Transaction Gain (Loss), Unrealized	40.2	12.2
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, Change in Unrealized Holding Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	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, Change in Unrealized Holding Gain (Loss)	(4.2)	4.0
Foreign Currency Transaction Gain (Loss), Unrealized	0.8	0.5
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, Change in Unrealized Holding Gain (Loss)	0.7	0.7
Foreign Currency Transaction Gain (Loss), Unrealized	0.1	0
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, Change in Unrealized Holding Gain (Loss)	0.9	4.9
Foreign Currency Transaction Gain (Loss), Unrealized	0	0.4
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, Change in Unrealized Holding Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0

Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

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Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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