Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (Fair Value, USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 6,777.0	$ 6,766.5	$ 6,565.5	$ 6,950.7
Fixed rate CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	231.5
Fixed rate CMBS | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	119.1
Fixed rate CMBS | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	75.5
Fixed rate CMBS | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	36.9
Floating rate CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	111.7
Floating rate CMBS | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.9
Floating rate CMBS | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Floating rate CMBS | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	106.8
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	343.2	282.3
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	124.0
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	75.5
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 143.7