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Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
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
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.1 | 70.3 |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 39.0 | 3.9 |  |
Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 109.1 | $ 74.2 |  |
Securities Issued in 2004 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.3 |  |  |
Securities Issued in 2004 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2004 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.3 |  |  |
Securities Issued in 2013 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1.3 |  |  |
Securities Issued in 2013 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1.3 |  |  |
Securities Issued in 2014 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 20.6 |  |  |
Securities Issued in 2014 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 20.6 |  |  |
Securities Issued in 2015 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 47.9 |  |  |
Securities Issued in 2015 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2015 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 47.9 |  |  |
Securities Issued in 2016 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2016 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 3.7 |  |  |
Securities Issued in 2016 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 3.7 |  |  |
Securities Issued in 2017 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2017 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 35.3 |  |  |
Securities Issued in 2017 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 35.3 |  |  |

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