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Investment Securities (Investment Holdings) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Investment securities |  |
Fixed maturity investments, at fair value |  | $ 2,081,100,000
U.S. Government and agency obligations |  |
Investment securities |  |
Cost or amortized cost | $ 297,800,000 | 112,100,000
Gross unrealized gains | 0 | 0
Gross unrealized losses | (1,300,000) | (1,100,000)
Net foreign currency gains | 0 | 0
Fixed maturity investments, at fair value | 296,500,000 | 111,000,000
Debt securities issued by corporations |  |
Investment securities |  |
Cost or amortized cost | 867,600,000 | 752,000,000
Gross unrealized gains | 2,900,000 | 2,300,000
Gross unrealized losses | (4,300,000) | (10,100,000)
Net foreign currency gains | 14,700,000 | 2,100,000
Fixed maturity investments, at fair value | 880,900,000 | 746,300,000
Mortgage and asset-backed securities |  |
Investment securities |  |
Cost or amortized cost | 697,200,000 | 986,900,000
Gross unrealized gains | 1,600,000 | 800,000
Gross unrealized losses | (4,100,000) | (7,900,000)
Net foreign currency gains | 0 | 0
Fixed maturity investments, at fair value | 694,700,000 | 979,800,000
Municipal obligations |  |
Investment securities |  |
Cost or amortized cost | 252,000,000 | 238,700,000
Gross unrealized gains | 3,700,000 | 1,100,000
Gross unrealized losses | (800,000) | (1,300,000)
Net foreign currency gains | 0 | 0
Fixed maturity investments, at fair value | 254,900,000 | 238,500,000
Foreign government, agency and provincial obligations |  |
Investment securities |  |
Cost or amortized cost | 2,600,000 | 12,000,000
Gross unrealized gains | 0 | 100,000
Gross unrealized losses | 0 | 0
Net foreign currency gains | 100,000 | 0
Fixed maturity investments, at fair value | 2,700,000 | 12,100,000
Fixed income investments |  |
Investment securities |  |
Cost or amortized cost | 2,117,200,000 | 2,101,700,000
Gross unrealized gains | 8,200,000 | 4,300,000
Gross unrealized losses | (10,500,000) | (20,400,000)
Net foreign currency gains | 14,800,000 | 2,100,000
Total fixed maturity investments | $ 2,129,700,000 | 2,087,700,000
Including Short Term Fixed |  |
Investment securities |  |
Duration of fixed maturities (years) | 3 years 4 months 26 days |
Excluding Short Term Investments |  |
Investment securities |  |
Duration of fixed maturities (years) | 3 years 8 months 15 days |
Star & Shield Insurance Exchange |  |
Investment securities |  |
Fixed maturity investments, at fair value |  | 6,600,000.0
Less: Fixed maturity investments reclassified to assets held for sale |  | $ 6,600,000

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