Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Residential
Investment securities
Fair value investments	205.3	133.2
Commercial
Investment securities
Fair value investments	127.5	140.4
Total Non-agency
Investment securities
Fair value investments	332.8	$ 273.6
Securities Issued in 2004 | Residential
Investment securities
Fair value investments	19.4
Securities Issued in 2004 | Commercial
Investment securities
Fair value investments	0.0
Securities Issued in 2004 | Total Non-agency
Investment securities
Fair value investments	19.4
2005 | Residential
Investment securities
Fair value investments	5.7
2005 | Commercial
Investment securities
Fair value investments	0.0
2005 | Total Non-agency
Investment securities
Fair value investments	5.7
2006 | Residential
Investment securities
Fair value investments	3.0
2006 | Commercial
Investment securities
Fair value investments	0.0
2006 | Total Non-agency
Investment securities
Fair value investments	3.0
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	2.7
2008 | Commercial
Investment securities
Fair value investments	0.0
2008 | Total Non-agency
Investment securities
Fair value investments	2.7
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	7.4
2010 | Commercial
Investment securities
Fair value investments	4.3
2010 | Total Non-agency
Investment securities
Fair value investments	11.7
2011 | Residential
Investment securities
Fair value investments	9.9
2011 | Commercial
Investment securities
Fair value investments	0.0
2011 | Total Non-agency
Investment securities
Fair value investments	9.9
2012 | Residential
Investment securities
Fair value investments	5.0
2012 | Commercial
Investment securities
Fair value investments	18.1
2012 | Total Non-agency
Investment securities
Fair value investments	23.1
2013 | Residential
Investment securities
Fair value investments	15.3
2013 | Commercial
Investment securities
Fair value investments	11.5
2013 | Total Non-agency
Investment securities
Fair value investments	26.8
2014 | Residential
Investment securities
Fair value investments	53.3
2014 | Commercial
Investment securities
Fair value investments	23.4
2014 | Total Non-agency
Investment securities
Fair value investments	76.7
Securities Issued in 2015 | Residential
Investment securities
Fair value investments	48.3
Securities Issued in 2015 | Commercial
Investment securities
Fair value investments	44.4
Securities Issued in 2015 | Total Non-agency
Investment securities
Fair value investments	92.7
Securities Issued in 2016 | Residential
Investment securities
Fair value investments	35.3
Securities Issued in 2016 | Commercial
Investment securities
Fair value investments	25.8
Securities Issued in 2016 | Total Non-agency
Investment securities
Fair value investments	$ 61.1