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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fixed maturity investments, at fair value | $ 4,362.8 | $ 2,630.2 |  |
U.S. Government and agency obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 261.2 | 160.4 |  |
Trading Securities, Unrealized Holding Gain | 0.7 | 0.0 |  |
Trading Securities, Unrealized Holding Loss | 0.0 | (0.4) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 261.9 | 160.0 |  |
Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 1,046.9 | 1,001.0 |  |
Trading Securities, Unrealized Holding Gain | 19.3 | 4.3 |  |
Trading Securities, Unrealized Holding Loss | (0.1) | (5.3) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 1,066.1 | 1,000.0 |  |
Municipal obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 294.8 | 227.8 |  |
Trading Securities, Unrealized Holding Gain | 8.2 | 2.2 |  |
Trading Securities, Unrealized Holding Loss | (0.1) | (1.2) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 302.9 | 228.8 |  |
Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 2,646.8 | 1,170.6 |  |
Trading Securities, Unrealized Holding Gain | 10.4 | 2.0 |  |
Trading Securities, Unrealized Holding Loss | (2.0) | (5.6) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 2,655.2 | 1,167.0 |  |
Foreign government, agency and provincial obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 1.0 | 1.0 |  |
Trading Securities, Unrealized Holding Gain | 0.2 | 0.2 |  |
Trading Securities, Unrealized Holding Loss | 0.0 | 0.0 |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 1.2 | 1.2 |  |
Preferred Stock |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 78.3 | 78.3 |  |
Trading Securities, Unrealized Holding Gain | 6.4 | 4.4 |  |
Trading Securities, Unrealized Holding Loss | 0.0 | 0.0 |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Fixed maturity investments, at fair value | 84.7 | 82.7 |  |
Fixed Income Investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Cost or amortized cost | 4,329.0 | 2,639.1 |  |
Trading Securities, Unrealized Holding Gain | 45.2 | 13.1 |  |
Trading Securities, Unrealized Holding Loss | (2.2) | (12.5) |  |
Net foreign currency gains (losses) | 0.0 | 0.0 |  |
Total fixed maturity investments including assets held for sale | 4,372.0 | 2,639.7 |  |
Star & Shield Insurance Exchange |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities Debt Reclassified to Assets Held-for-sale | $ 9.2 | $ 9.5 | $ 9.1 | $ 10.1

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