Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777.0	$ 6,766.5
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301.0	$ 236.9
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	84.5
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 198.1
Non-Agency Commercial Mortgage Backed Securities [Member] [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Basis points	2.50%