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Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions | Jun. 30, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 5,091.8 | $ 4,050.9 | $ 3,379.6 | $ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 93.2 |  |  |
Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.3 |  |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 194.3 | 133.2 |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 104.5 | 140.4 |  |
Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 298.8 | $ 273.6 |  |
Securities Issued in 2004 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 21.9 |  |  |
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 | 21.9 |  |  |
Securities Issued in 2005 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 6.4 |  |  |
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 | 6.4 |  |  |
Securities Issued In 2006 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 3.4 |  |  |
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 | 3.4 |  |  |
Securities Issued In 2007 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2007 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2008 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 3.2 |  |  |
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 | 3.2 |  |  |
Securities Issued In 2009 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2009 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2009 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Securities Issued In 2010 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 10.8 |  |  |
Securities Issued In 2010 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 5.0 |  |  |
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 15.8 |  |  |
Securities Issued In 2011 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.8 |  |  |
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 | 11.8 |  |  |
Securities Issued in 2012 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 6.9 |  |  |
Securities Issued in 2012 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 18.4 |  |  |
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 25.3 |  |  |
Securities Issued in 2013 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 12.3 |  |  |
Securities Issued in 2013 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.6 |  |  |
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 23.9 |  |  |
Securities Issued in 2014 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 65.3 |  |  |
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 | 88.7 |  |  |
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 52.3 |  |  |
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 46.1 |  |  |
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 | 98.4 |  |  |
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 38.2 |  |  |
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.3 |  |  |
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 | 49.5 |  |  |
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 8.9 |  |  |
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 | 133.6 |  |  |
Super Senior | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 8.9 |  |  |
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 46.1 |  |  |
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 | 60.7 |  |  |
Senior | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 46.1 |  |  |

X
- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 + Details
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