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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fixed maturity investments, at fair value | $ 1,490.0 | $ 2,081.1 |  |
U.S. Government and agency obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 218.1 | 112.1 |  |
Trading Securities, Unrealized Holding Gain | 0.1 | 0.0 |  |
Trading Securities, Unrealized Holding Loss | (0.4) | (1.1) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 217.8 | 111.0 |  |
Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 608.3 | 752.0 |  |
Trading Securities, Unrealized Holding Gain | 3.2 | 2.3 |  |
Trading Securities, Unrealized Holding Loss | (1.0) | (10.1) |  |
Net foreign currency gains (losses) | 14.5 | 2.1 |  |
Fixed maturity investments, at fair value | 625.0 | 746.3 |  |
Municipal obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 252.1 | 238.7 |  |
Trading Securities, Unrealized Holding Gain | 3.1 | 1.1 |  |
Trading Securities, Unrealized Holding Loss | (0.5) | (1.3) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 254.7 | 238.5 |  |
Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 389.1 | 986.9 |  |
Trading Securities, Unrealized Holding Gain | 1.2 | 0.8 |  |
Trading Securities, Unrealized Holding Loss | (2.4) | (7.9) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 387.9 | 979.8 |  |
Foreign government, agency and provincial obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 4.5 | 12.0 |  |
Trading Securities, Unrealized Holding Gain | 0.0 | 0.1 |  |
Trading Securities, Unrealized Holding Loss | (0.1) | 0.0 |  |
Net foreign currency gains (losses) | 0.2 | 0.0 |  |
Fixed maturity investments, at fair value | 4.6 | 12.1 |  |
Fixed Income Investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 1,472.1 | 2,101.7 |  |
Trading Securities, Unrealized Holding Gain | 7.6 | 4.3 |  |
Trading Securities, Unrealized Holding Loss | (4.4) | (20.4) |  |
Net foreign currency gains (losses) | 14.7 | 2.1 |  |
Total fixed maturity investments including assets held for sale | $ 1,490.0 | 2,087.7 |  |
Star & Shield Insurance Exchange |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading securities debt reclassified to assets held-for-sale |  | $ (6.6) | $ (8.3) | $ (9.5)

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