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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 | Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 4,131.5 | $ 2,630.2
Common Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 378.9 | 822.5
Trading Securities, Unrealized Holding Gain | 50.7 | 302.8
Trading Securities, Unrealized Holding Loss | (4.6) | (11.4)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 425.0 | 1,113.9
Other long-term investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 317.1 | 304.5
Trading Securities, Unrealized Holding Gain | 42.7 | 32.0
Trading Securities, Unrealized Holding Loss | (6.4) | (18.4)
Net foreign currency gains (losses) | (2.0) | (2.3)
Fixed maturity investments, at fair value | $ 351.4 | $ 315.8

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