Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 Fixed rate CMBS	Mar. 31, 2014 Maximum Commercial Mortgage Backed Securities	Mar. 31, 2014 Fair Value	Dec. 31, 2013 Fair Value	Mar. 31, 2013 Fair Value	Dec. 31, 2012 Fair Value	Mar. 31, 2014 Fair Value Fixed rate CMBS	Mar. 31, 2014 Fair Value Floating rate CMBS	Mar. 31, 2014 Fair Value Commercial Mortgage Backed Securities	Dec. 31, 2013 Fair Value Commercial Mortgage Backed Securities	Mar. 31, 2014 Fair Value Non-Agency Residential Mortgage Resecuritization
Investment securities
Average basis points of subordination	0.25
Percentage of underlying loans reported as non-performing		1.00%
Fair value investments			$ 6,736.7	$ 6,766.5	$ 6,580.6	$ 6,950.7	$ 225.1	$ 104.6	$ 329.7	$ 282.3	$ 37.8