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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2020 | Dec. 31, 2019
Common Stock |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 502.8 | $ 553.3
Trading Securities, Unrealized Holding Gain | 69.3 | 130.6
Trading Securities, Unrealized Holding Loss | (4.9) | 0.0
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 567.2 | 683.9
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 677.9 | 667.4
Trading Securities, Unrealized Holding Gain | 289.6 | 255.2
Trading Securities, Unrealized Holding Loss | (67.1) | (64.1)
Net foreign currency gains (losses) | (2.3) | (2.2)
Fixed maturity investments, at fair value | $ 898.1 | $ 856.3

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