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Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Investment securities |  |  |
Fair value investments | $ 3,208.3 | $ 2,547.3 | $ 1,543.5
Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 880.9 | 746.3 |
AAA | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 1.6 | 0.0 |
AA | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 42.6 | 37.3 |
A | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 192.5 | 212.8 |
BBB | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 465.2 | 335.6 |
BB | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 161.7 | 143.2 |
B | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | $ 17.3 | 17.4 |
Star & Shield Insurance Exchange | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments |  | 6.6 |
Discontinued Operations, Held-for-sale | Star & Shield Insurance Exchange | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments |  | $ 4.2 |

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