Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Fixed maturity investments, at fair value	$ 4,784.3us-gaap_TradingSecuritiesDebt	$ 5,030.5us-gaap_TradingSecuritiesDebt
Sales and maturities of investment securities	6,238.1	4,924.8	6,997.5
Common Stock
Investment securities
Trading Securities, Cost	633.6	890.2
Net foreign currency gains (losses)	(1.9)	(0.8)
Fixed maturity investments, at fair value	801.6	1,156.8
Trading Securities, Unrealized Holding Gain	175.1	271.0
Trading Securities, Unrealized Holding Loss	(5.2)	(3.6)
Convertible fixed maturity investments
Investment securities
Trading Securities, Cost	19.1	71.7
Net foreign currency gains (losses)	0.7	(0.2)
Fixed maturity investments, at fair value	20.5	80.5
Trading Securities, Unrealized Holding Gain	0.9	9.9
Trading Securities, Unrealized Holding Loss	(0.2)	(0.9)
Other long-term investments
Investment securities
Trading Securities, Cost	344.5	238.3
Net foreign currency gains (losses)	1.6	(2.4)
Fixed maturity investments, at fair value	408.2	288.9
Trading Securities, Unrealized Holding Gain	73.0	79.6
Trading Securities, Unrealized Holding Loss	$ (10.9)	$ (26.6)

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Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Amount of investments in debt securities classified as trading.

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Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities.

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Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities.

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Proceeds from Sale and Maturity of Trading Securities Held-for-investment, Excluding Short Term Securities

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The net amount of unrealized gains (losses) from foreign currency transaction adjustments related to trading securities.

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