Investment Securities (Non-agency Commercial 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
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	133.7	$ 109.3
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.6
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	41.4
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 82.7
Non-Agency Commercial Mortgage Backed Securities [Member] [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Basis points	2.50%