Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 3,365.4	$ 3,343.6	$ 3,610.7	$ 3,889.0
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	40.5
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	123.7	71.0
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	133.7	109.3
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	257.4	$ 180.3
Securities Issued in 2004 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.3
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	18.3
Securities Issued in 2005 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.9
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	2.9
Securities Issued In 2006 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.0
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	4.0
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	4.2
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	4.2
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	17.1
Securities Issued In 2010 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5.7
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	22.8
Securities Issued In 2011 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14.6
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	14.6
Securities Issued in 2012 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13.4
Securities Issued in 2012 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.6
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	22.0
Securities Issued in 2013 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14.7
Securities Issued in 2013 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.1
Securities Issued in 2014 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	34.5
Securities Issued in 2014 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	55.6
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.1
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	45.4
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	45.4
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	42.2
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	40.5
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	82.7
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.6
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	35.1
Super Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.6
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	41.4
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	88.6
Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 41.4
Non-Agency Commercial Mortgage Backed Securities [Member] [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Basis points	2.50%