Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Fair value investments	$ 5,991.4	$ 6,766.5	$ 6,950.7
Fixed rate CMBS
Investment securities
Average Basis Points of Subordination	0.25
Fair value investments	116.2
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	13.1
Fixed rate CMBS | Senior
Investment securities
Fair value investments	61.6
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	41.5
Floating rate CMBS
Investment securities
Fair value investments	120.7
Floating rate CMBS | Super Senior
Investment securities
Fair value investments	0
Floating rate CMBS | Senior
Investment securities
Fair value investments	0
Floating rate CMBS | Subordinate
Investment securities
Fair value investments	120.7
Commercial
Investment securities
Fair value investments	236.9	282.3
Commercial | Super Senior
Investment securities
Fair value investments	13.1
Commercial | Senior
Investment securities
Fair value investments	61.6
Commercial | Subordinate
Investment securities
Fair value investments	$ 162.2
Maximum | Commercial
Investment securities
Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans	1.00%