Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average Basis Points of Subordination	0.0025
Fair value investments	$ 115.2
Commercial Mortgage Backed Securities Fixed Rate [Member] | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.6
Commercial Mortgage Backed Securities Fixed Rate [Member] | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1
Commercial Mortgage Backed Securities Fixed Rate [Member] | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	43.5
Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7
Commercial Mortgage Backed Securities Floating Rate [Member] | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Floating Rate [Member] | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Floating Rate [Member] | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	126.9	$ 127.5
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.6
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 55.2