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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 1,490.0 | $ 2,081.1
Common Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 693.8 | 258.6
Trading Securities, Unrealized Holding Gain | 82.6 | 29.0
Trading Securities, Unrealized Holding Loss | (2.0) | (2.0)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 774.4 | 285.6
Other long-term investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 254.5 | 194.0
Trading Securities, Unrealized Holding Gain | 13.1 | 7.9
Trading Securities, Unrealized Holding Loss | (21.2) | (25.2)
Net foreign currency gains (losses) | (16.8) | (3.9)
Fixed maturity investments, at fair value | $ 229.6 | $ 172.8

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