Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (Fair Value, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Investment securities
Fair value investments	$ 6,736.7	$ 6,766.5	$ 6,580.6	$ 6,950.7
Fixed rate CMBS
Investment securities
Fair value investments	225.1
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	123.5
Fixed rate CMBS | Senior
Investment securities
Fair value investments	76.0
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	25.6
Floating rate CMBS
Investment securities
Fair value investments	104.6
Floating rate CMBS | Super Senior
Investment securities
Fair value investments	5.0
Floating rate CMBS | Senior
Investment securities
Fair value investments	26.4
Floating rate CMBS | Subordinate
Investment securities
Fair value investments	73.2
Commercial Mortgage Backed Securities
Investment securities
Fair value investments	329.7	282.3
Commercial Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	128.5
Commercial Mortgage Backed Securities | Senior
Investment securities
Fair value investments	102.4
Commercial Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	$ 98.8