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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Common Stock |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 798.6 | $ 904.7
Trading Securities, Unrealized Holding Gain | 107.3 | 51.0
Trading Securities, Unrealized Holding Loss | (0.9) | (30.1)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 905.0 | 925.6
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 389.8 | 330.3
Trading Securities, Unrealized Holding Gain | 164.3 | 52.2
Trading Securities, Unrealized Holding Loss | (54.3) | (54.9)
Net foreign currency gains (losses) | (2.2) | (2.0)
Fixed maturity investments, at fair value | $ 497.6 | $ 325.6

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