Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (Fair Value, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 6,321.6	$ 6,766.5	$ 6,622.4	$ 6,950.7
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	256.2	282.3
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.2
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	52.3
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 145.7