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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2014 | Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 4,785.5 | $ 5,030.5
Fixed maturity investments reclassified to assets held for sale |  | (236.3)
U.S. Government and agency obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 254.4 | 365.5
Trading Securities, Unrealized Holding Gain | 0.2 | 0.5
Trading Securities, Unrealized Holding Loss | (0.5) | (1.0)
Net foreign currency gains (losses) | 5.6 | (2.5)
Fixed maturity investments, at fair value | 259.7 | 362.5
Debt securities issued by corporations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 2,314.6 | 2,330.7
Trading Securities, Unrealized Holding Gain | 48.9 | 44.0
Trading Securities, Unrealized Holding Loss | (6.3) | (13.2)
Net foreign currency gains (losses) | 30.1 | (14.3)
Fixed maturity investments, at fair value | 2,387.3 | 2,347.2
Municipal obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 78.9 | 18.3
Trading Securities, Unrealized Holding Gain | 1.3 | 0
Trading Securities, Unrealized Holding Loss | (0.3) | (0.4)
Net foreign currency gains (losses) | 0 | 0
Fixed maturity investments, at fair value | 79.9 | 17.9
Mortgage-backed and asset-backed securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 1,846.7 | 2,027.3
Trading Securities, Unrealized Holding Gain | 6.8 | 2.4
Trading Securities, Unrealized Holding Loss | (4.6) | (9.9)
Net foreign currency gains (losses) | 16.0 | (5.3)
Fixed maturity investments, at fair value | 1,864.9 | 2,014.5
Foreign government, agency and provincial obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 311.6 | 444.2
Trading Securities, Unrealized Holding Gain | 3.7 | 3.7
Trading Securities, Unrealized Holding Loss | (0.8) | (3.2)
Net foreign currency gains (losses) | (3.3) | (4.8)
Fixed maturity investments, at fair value | 311.2 | 439.9
Preferred Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 79.7 | 79.9
Trading Securities, Unrealized Holding Gain | 6.7 | 5.1
Trading Securities, Unrealized Holding Loss | 0 | 0
Net foreign currency gains (losses) | 0 | (0.2)
Fixed maturity investments, at fair value | 86.4 | 84.8
Fixed Income Investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 4,885.9 | 5,265.9
Trading Securities, Unrealized Holding Gain | 67.6 | 55.7
Trading Securities, Unrealized Holding Loss | (12.5) | (27.7)
Net foreign currency gains (losses) | 48.4 | (27.1)
Total fixed maturity investments including assets held for sale | 4,989.4 | 5,266.8
OneBeacon Runoff |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments reclassified to assets held for sale | $ (203.9) | $ (236.3)

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