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Investment Securities (Debt securities issued by corporation) (Details) - USD ($) | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fixed maturity investments, at fair value | $ 1,490,000,000 | $ 2,081,100,000 |  |
Fair value investments | 2,509,400,000 | 2,547,300,000 | $ 2,406,900,000 | $ 1,543,500,000
Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fixed maturity investments, at fair value | 625,000,000 | 746,300,000 |  |
Fair value investments | 625,000,000 | 746,300,000 |  |
AA | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 26,700,000 | 37,300,000 |  |
A | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 119,400,000 | 212,800,000 |  |
BBB | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 291,900,000 | 335,600,000 |  |
BB | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 165,400,000 | 143,200,000 |  |
B | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 21,600,000 | 17,400,000 |  |
Star & Shield Insurance Exchange | Fixed maturity investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fixed maturity investments, at fair value |  | 4.2 |  |
Fair value investments |  | $ 6,600,000 |  |

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