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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2016 | Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fixed maturity investments, at fair value | $ 4,175.4 | $ 4,250.2 |  |
U.S. Government and agency obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 110.7 | 281.7 |  |
Trading Securities, Unrealized Holding Gain | 0.0 | 0.1 |  |
Trading Securities, Unrealized Holding Loss | (0.4) | (3.5) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 110.3 | 278.3 |  |
Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 1,680.2 | 1,512.6 |  |
Trading Securities, Unrealized Holding Gain | 10.5 | 8.4 |  |
Trading Securities, Unrealized Holding Loss | (7.6) | (13.7) |  |
Net foreign currency gains (losses) | 3.7 | 2.1 |  |
Fixed maturity investments, at fair value | 1,686.8 | 1,509.4 |  |
Municipal obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 328.9 | 308.8 |  |
Trading Securities, Unrealized Holding Gain | 2.8 | 1.9 |  |
Trading Securities, Unrealized Holding Loss | (1.4) | (1.7) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 330.3 | 309.0 |  |
Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 2,020.0 | 2,141.7 |  |
Trading Securities, Unrealized Holding Gain | 5.1 | 2.6 |  |
Trading Securities, Unrealized Holding Loss | (8.7) | (11.4) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 2,016.4 | 2,132.9 |  |
Foreign government, agency and provincial obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 17.3 | 12.9 |  |
Trading Securities, Unrealized Holding Gain | 0.4 | 0.3 |  |
Trading Securities, Unrealized Holding Loss | 0.0 | 0.0 |  |
Net foreign currency gains (losses) | 0.2 | 0.0 |  |
Fixed maturity investments, at fair value | 17.9 | 13.2 |  |
Preferred Stock |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 8.3 | 8.3 |  |
Trading Securities, Unrealized Holding Gain | 5.4 | 5.7 |  |
Trading Securities, Unrealized Holding Loss | 0.0 | 0.0 |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 13.7 | 14.0 |  |
Fixed Income Investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 4,165.4 | 4,266.0 |  |
Trading Securities, Unrealized Holding Gain | 24.2 | 19.0 |  |
Trading Securities, Unrealized Holding Loss | (18.1) | (30.3) |  |
Net foreign currency gains (losses) | 3.9 | 2.1 |  |
Total fixed maturity investments including assets held for sale | $ 4,175.4 | 4,256.8 |  |
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) | $ (9.4) | $ (9.5)

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