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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2016 | Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 4,362.8 | $ 2,630.2
Common Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 374.1 | 822.5
Trading Securities, Unrealized Holding Gain | 43.5 | 302.8
Trading Securities, Unrealized Holding Loss | (10.1) | (11.4)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 407.5 | 1,113.9
Other long-term investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 305.0 | 304.5
Trading Securities, Unrealized Holding Gain | 36.7 | 32.0
Trading Securities, Unrealized Holding Loss | (10.3) | (18.4)
Net foreign currency gains (losses) | (2.2) | (2.3)
Fixed maturity investments, at fair value | $ 329.2 | $ 315.8

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