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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2020 | Dec. 31, 2019
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 195.5 | $ 231.7
Trading Securities, Unrealized Holding Gain | 4.9 | 1.0
Trading Securities, Unrealized Holding Loss | 0.0 | (0.2)
Fixed maturity investments, at fair value | 200.4 | 232.5
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 480.2 | 454.9
Trading Securities, Unrealized Holding Gain | 26.1 | 12.5
Trading Securities, Unrealized Holding Loss | (0.5) | (0.2)
Fixed maturity investments, at fair value | 505.8 | 467.2
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 248.3 | 284.7
Trading Securities, Unrealized Holding Gain | 16.7 | 12.5
Trading Securities, Unrealized Holding Loss | (0.2) | (0.1)
Fixed maturity investments, at fair value | 264.8 | 297.1
Mortgage-backed and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 213.2 | 206.6
Trading Securities, Unrealized Holding Gain | 7.4 | 2.7
Trading Securities, Unrealized Holding Loss | 0.0 | (0.3)
Fixed maturity investments, at fair value | 220.6 | 209.0
Fixed Income Investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 1,137.2 | 1,177.9
Trading Securities, Unrealized Holding Gain | 55.1 | 28.7
Trading Securities, Unrealized Holding Loss | (0.7) | (0.8)
Total fixed maturity investments including assets held for sale | $ 1,191.6 | $ 1,205.8

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