Investment Securities (Non-agency Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 6,321.6	$ 6,766.5	$ 6,622.4	$ 6,950.7
Fair Value | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	149.3
Fair Value | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	106.9
Fair Value | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	196.6	125.7
Fair Value | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	256.2	282.3
Fair Value | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	452.8	408.0
Fair Value | Securities Issued in 2004 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.5
Fair Value | Securities Issued in 2004 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | Securities Issued in 2004 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.5
Fair Value | Securities Issued in 2005 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.1
Fair Value | Securities Issued in 2005 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.1
Fair Value | Securities Issued In 2006 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.6
Fair Value | Securities Issued In 2006 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.5
Fair Value | Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	19.1
Fair Value | Securities Issued In 2007 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | Securities Issued In 2007 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.8
Fair Value | Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.8
Fair Value | Securities Issued In 2008 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	17.2
Fair Value | Securities Issued In 2008 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	17.2
Fair Value | Securities Issued In 2009 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | Securities Issued In 2009 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | 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
Fair Value | Securities Issued In 2010 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.3
Fair Value | Securities Issued In 2010 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.0
Fair Value | Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	28.3
Fair Value | Securities Issued In 2011 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	23.5
Fair Value | Securities Issued In 2011 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair Value | Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	23.5
Fair Value | Securities Issued in 2012 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.2
Fair Value | Securities Issued in 2012 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.6
Fair Value | Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	26.8
Fair Value | Securities Issued in 2013 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	42.6
Fair Value | Securities Issued in 2013 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	94.5
Fair Value | Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	137.1
Fair Value | Securities Issued in 2014 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	54.6
Fair Value | Securities Issued in 2014 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.8
Fair Value | Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	172.4
Subordinate | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Subordinate | Fair Value | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	41.6
Subordinate | Fair Value | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	104.1
Subordinate | Fair Value | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	145.7
Super Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	75.9
Super Senior | Fair Value | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	55.4
Super Senior | Fair Value | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.8
Super Senior | Fair Value | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.2
Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	120.7
Senior | Fair Value | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	52.3
Senior | Fair Value | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Senior | Fair Value | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 52.3