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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 2,602.2 | $ 2,422.0
Common Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 453.0 | 495.3
Trading Securities, Unrealized Holding Gain | 52.0 | 127.4
Trading Securities, Unrealized Holding Loss | (22.9) | (3.6)
Net foreign currency gains (losses) | 0.0 | (7.4)
Fixed maturity investments, at fair value | 482.1 | 611.7
Convertible Debt Securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 3.1 | 13.4
Trading Securities, Unrealized Holding Gain | 2.5 | 0.8
Trading Securities, Unrealized Holding Loss | 0.0 | (0.3)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 5.6 | 13.9
Other long-term investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 271.9 | 273.3
Trading Securities, Unrealized Holding Gain | 37.7 | 52.5
Trading Securities, Unrealized Holding Loss | (12.9) | (6.5)
Net foreign currency gains (losses) | (2.1) | (1.3)
Fixed maturity investments, at fair value | $ 294.6 | $ 318.0

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