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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 196.3 | $ 154.0
Trading Securities, Unrealized Holding Gain | 1.3 | 0.1
Trading Securities, Unrealized Holding Loss | 0.0 | (0.9)
Fixed maturity investments, at fair value | 197.6 | 153.2
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 449.5 | 519.0
Trading Securities, Unrealized Holding Gain | 13.8 | 1.0
Trading Securities, Unrealized Holding Loss | (0.1) | (9.5)
Fixed maturity investments, at fair value | 463.2 | 510.5
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 292.4 | 279.0
Trading Securities, Unrealized Holding Gain | 15.7 | 2.4
Trading Securities, Unrealized Holding Loss | (0.1) | (1.1)
Fixed maturity investments, at fair value | 308.0 | 280.3
Mortgage-backed and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 195.0 | 136.1
Trading Securities, Unrealized Holding Gain | 2.5 | 0.1
Trading Securities, Unrealized Holding Loss | (0.3) | (2.7)
Fixed maturity investments, at fair value | 197.2 | 133.5
Fixed Income Investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 1,133.2 | 1,088.1
Trading Securities, Unrealized Holding Gain | 33.3 | 3.6
Trading Securities, Unrealized Holding Loss | (0.5) | (14.2)
Total fixed maturity investments including assets held for sale | $ 1,166.0 | $ 1,077.5

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