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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 2,613.2 | $ 2,639.7
U.S. Government and agency obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 88.4 | 160.4
Trading Securities, Unrealized Holding Gain | 0.4 | 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 | 88.8 | 160.0
Debt securities issued by corporations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 899.0 | 1,001.0
Trading Securities, Unrealized Holding Gain | 12.5 | 4.3
Trading Securities, Unrealized Holding Loss | (1.1) | (5.3)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 910.4 | 1,000.0
Municipal obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 264.3 | 227.8
Trading Securities, Unrealized Holding Gain | 4.6 | 2.2
Trading Securities, Unrealized Holding Loss | (0.2) | (1.2)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 268.7 | 228.8
Mortgage-backed and asset-backed securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 1,259.9 | 1,170.6
Trading Securities, Unrealized Holding Gain | 5.1 | 2.0
Trading Securities, Unrealized Holding Loss | (4.1) | (5.6)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 1,260.9 | 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 | 4.9 | 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 | 83.2 | 82.7
Fixed Income Investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 2,590.9 | 2,639.1
Trading Securities, Unrealized Holding Gain | 27.7 | 13.1
Trading Securities, Unrealized Holding Loss | (5.4) | (12.5)
Net foreign currency gains (losses) | 0.0 | 0.0
Total fixed maturity investments including assets held for sale | $ 2,613.2 | $ 2,639.7

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