Investment Securities (Non-agency Mortgage-backed Securities) (Details) (Fair Value, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Investment securities
Fair value investments	$ 6,736.7	$ 6,766.5	$ 6,580.6	$ 6,950.7
Residential Mortgage Backed Securities
Investment securities
Fair value investments	132.3	125.7
Commercial Mortgage Backed Securities
Investment securities
Fair value investments	329.7	282.3
Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	462.0	408.0
Securities Issued in 2004 [Member] | Residential Mortgage Backed Securities
Investment securities
Fair value investments	11.3
Securities Issued in 2004 [Member] | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued in 2004 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	11.3
Securities Issued in 2005 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	20.2
Securities Issued in 2005 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	20.2
Securities Issued In 2006 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	11.0
Securities Issued In 2006 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	8.5
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	19.5
Securities Issued In 2007 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued In 2007 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	5.0
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	5.0
Securities Issued In 2008 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	19.6
Securities Issued In 2008 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	26.4
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	46.0
Securities Issued In 2009 [Member] | Residential Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued In 2009 [Member] | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued In 2009 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	0
Securities Issued In 2010 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	26.4
Securities Issued In 2010 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	13.0
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	39.4
Securities Issued In 2011 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued In 2011 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	27.0
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	27.0
Securities Issued in 2012 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	0
Securities Issued in 2012 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	131.8
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	131.8
Securities Issued in 2013 | Residential Mortgage Backed Securities
Investment securities
Fair value investments	32.8
Securities Issued in 2013 | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	94.3
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	127.1
Securities Issued in 2014 [Member] | Residential Mortgage Backed Securities
Investment securities
Fair value investments	11.0
Securities Issued in 2014 [Member] | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	23.7
Securities Issued in 2014 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	$ 34.7