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Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 5,097.6 | $ 5,039.7 | $ 3,360.2 | $ 4,050.9
Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 115.2 |  |  |
Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.7 |  |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 309.9 | 205.3 |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 126.9 | 127.5 |  |
Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 436.8 | $ 332.8 |  |
Securities Issued in 2004 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 18.1 |  |  |
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 | 18.1 |  |  |
Securities Issued in 2005 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 5.3 |  |  |
Securities Issued in 2005 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 5.3 |  |  |
Securities Issued In 2006 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2.8 |  |  |
Securities Issued In 2006 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2.8 |  |  |
Securities Issued In 2008 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2.4 |  |  |
Securities Issued In 2008 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2.4 |  |  |
Securities Issued In 2010 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 6.0 |  |  |
Securities Issued In 2010 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4.1 |  |  |
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 10.1 |  |  |
Securities Issued In 2011 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 8.4 |  |  |
Securities Issued In 2011 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 8.4 |  |  |
Securities Issued in 2012 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4.2 |  |  |
Securities Issued in 2012 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 18.1 |  |  |
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 22.3 |  |  |
Securities Issued in 2013 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 19.8 |  |  |
Securities Issued in 2013 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.5 |  |  |
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 31.3 |  |  |
Securities Issued in 2014 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 49.4 |  |  |
Securities Issued in 2014 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 23.4 |  |  |
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 72.8 |  |  |
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 104.2 |  |  |
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 44.2 |  |  |
Securities Issued in 2015 [Member] | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 148.4 |  |  |
Securities Issued in 2016 [Member] | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 34.5 |  |  |
Securities Issued in 2016 [Member] | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 25.6 |  |  |
Securities Issued in 2016 [Member] | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 60.1 |  |  |
Securities Issued in 2017 [Member] | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 54.8 |  |  |
Securities Issued in 2017 [Member] | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued in 2017 [Member] | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 54.8 |  |  |
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 43.5 |  |  |
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.7 |  |  |
Subordinate | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Subordinate | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 55.2 |  |  |
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1.6 |  |  |
Super Senior | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Super Senior | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 104.9 |  |  |
Super Senior | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1.6 |  |  |
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.1 |  |  |
Senior | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Senior | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 205.0 |  |  |
Senior | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 70.1 |  |  |

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