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Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 2,509.4 | $ 2,547.3 | $ 2,406.9 | $ 1,543.5
Commercial Mortgage Backed Securities Fixed Rate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Average basis points of subordination | 0.3 |  |  |
Fair value investments | $ 17.8 |  |  |
Commercial Mortgage Backed Securities Fixed Rate | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Commercial Mortgage Backed Securities Fixed Rate | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 16.2 |  |  |
Commercial Mortgage Backed Securities Fixed Rate | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 1.6 |  |  |
Commercial Mortgage Backed Securities Floating Rate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Average basis points of subordination | 0.3 |  |  |
Fair value investments | $ 21.2 |  |  |
Commercial Mortgage Backed Securities Floating Rate | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Commercial Mortgage Backed Securities Floating Rate | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Commercial Mortgage Backed Securities Floating Rate | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 21.2 |  |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 39.0 | $ 3.9 |  |
Commercial Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Commercial Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 16.2 |  |  |
Commercial Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 22.8 |  |  |

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