Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889.0
Fixed rate CMBS
Investment securities
Average Basis Points of Subordination	0.25
Fair value investments	$ 101.1
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	8.7
Fixed rate CMBS | Senior
Investment securities
Fair value investments	48.6
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	43.8
Floating rate CMBS
Investment securities
Fair value investments	39.3
Floating rate CMBS | Super Senior
Investment securities
Fair value investments	0.0
Floating rate CMBS | Senior
Investment securities
Fair value investments	0.0
Floating rate CMBS | Subordinate
Investment securities
Fair value investments	39.3
Commercial
Investment securities
Fair value investments	140.4	$ 109.3
Commercial | Super Senior
Investment securities
Fair value investments	8.7
Commercial | Senior
Investment securities
Fair value investments	48.6
Commercial | Subordinate
Investment securities
Fair value investments	$ 83.1
Maximum | Commercial
Investment securities
Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans	1.00%