Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889.0
Residential
Investment securities
Fair value investments	133.2	71.0
Commercial
Investment securities
Fair value investments	140.4	109.3
Total Non-agency
Investment securities
Fair value investments	273.6	$ 180.3
Securities Issued in 2004 [Member] | Residential
Investment securities
Fair value investments	17.3
Securities Issued in 2004 [Member] | Commercial
Investment securities
Fair value investments	0.0
Securities Issued in 2004 [Member] | Total Non-agency
Investment securities
Fair value investments	17.3
2005 | Residential
Investment securities
Fair value investments	8.6
2005 | Commercial
Investment securities
Fair value investments	0.0
2005 | Total Non-agency
Investment securities
Fair value investments	8.6
2006 | Residential
Investment securities
Fair value investments	3.9
2006 | Commercial
Investment securities
Fair value investments	0.0
2006 | Total Non-agency
Investment securities
Fair value investments	3.9
2007 | Residential
Investment securities
Fair value investments	0.0
2007 | Commercial
Investment securities
Fair value investments	0.0
2007 | Total Non-agency
Investment securities
Fair value investments	0.0
2008 | Residential
Investment securities
Fair value investments	3.6
2008 | Commercial
Investment securities
Fair value investments	0.0
2008 | Total Non-agency
Investment securities
Fair value investments	3.6
2009 | Residential
Investment securities
Fair value investments	0.0
2009 | Commercial
Investment securities
Fair value investments	0.0
2009 | Total Non-agency
Investment securities
Fair value investments	0.0
2010 | Residential
Investment securities
Fair value investments	15.2
2010 | Commercial
Investment securities
Fair value investments	5.4
2010 | Total Non-agency
Investment securities
Fair value investments	20.6
2011 | Residential
Investment securities
Fair value investments	13.5
2011 | Commercial
Investment securities
Fair value investments	0.0
2011 | Total Non-agency
Investment securities
Fair value investments	13.5
2012 | Residential
Investment securities
Fair value investments	9.8
2012 | Commercial
Investment securities
Fair value investments	18.0
2012 | Total Non-agency
Investment securities
Fair value investments	27.8
2013 | Residential
Investment securities
Fair value investments	14.0
2013 | Commercial
Investment securities
Fair value investments	17.2
2013 | Total Non-agency
Investment securities
Fair value investments	31.2
2014 | Residential
Investment securities
Fair value investments	47.3
2014 | Commercial
Investment securities
Fair value investments	55.0
2014 | Total Non-agency
Investment securities
Fair value investments	102.3
Securities Issued in 2015 [Member] | Residential
Investment securities
Fair value investments	0.0
Securities Issued in 2015 [Member] | Commercial
Investment securities
Fair value investments	44.8
Securities Issued in 2015 [Member] | Total Non-agency
Investment securities
Fair value investments	$ 44.8