Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 3,360.2	$ 4,050.9	$ 3,355.3	$ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average Basis Points of Subordination	25.00
Fair value investments	$ 96.1
Commercial Mortgage Backed Securities Fixed Rate [Member] | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.8
Commercial Mortgage Backed Securities Fixed Rate [Member] | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	46.0
Commercial Mortgage Backed Securities Fixed Rate [Member] | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	41.3
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	128.8	$ 140.4
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.8
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	46.0
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 74.0