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Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Investment securities |  |  |
Fair value investments | $ 5,039.7 | $ 4,050.9 | $ 3,343.6
Fixed rate CMBS |  |  |
Investment securities |  |  |
Average Basis Points of Subordination | 25.00 |  |
Fair value investments | $ 116.0 |  |
Fixed rate CMBS | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 8.7 |  |
Fixed rate CMBS | Senior |  |  |
Investment securities |  |  |
Fair value investments | 59.1 |  |
Fixed rate CMBS | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | 48.2 |  |
Floating rate CMBS |  |  |
Investment securities |  |  |
Fair value investments | 11.5 |  |
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 | 11.5 |  |
Commercial |  |  |
Investment securities |  |  |
Fair value investments | 127.5 | $ 140.4 |
Commercial | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 8.7 |  |
Commercial | Senior |  |  |
Investment securities |  |  |
Fair value investments | 59.1 |  |
Commercial | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | $ 59.7 |  |

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