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Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Investment securities |  |  |
Fair value investments | $ 3,208.3 | $ 2,547.3 | $ 1,543.5
Fixed rate CMBS |  |  |
Investment securities |  |  |
Average basis points of subordination | 35.00 |  |
Fair value investments | $ 35.1 |  |
Fixed rate CMBS | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Fixed rate CMBS | Senior |  |  |
Investment securities |  |  |
Fair value investments | 27.5 |  |
Fixed rate CMBS | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | 7.6 |  |
Floating rate CMBS |  |  |
Investment securities |  |  |
Fair value investments | 35.4 |  |
Floating rate CMBS | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Floating rate CMBS | Senior |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Floating rate CMBS | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | 35.4 |  |
Commercial |  |  |
Investment securities |  |  |
Fair value investments | 70.5 | $ 3.9 |
Commercial | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Commercial | Senior |  |  |
Investment securities |  |  |
Fair value investments | 27.5 |  |
Commercial | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | $ 43.0 |  |

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