Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5
Commercial Mortgage Backed Securities Fixed Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average basis points of subordination	0.3
Fair value investments	$ 17.8
Commercial Mortgage Backed Securities Fixed Rate | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Fixed Rate | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.2
Commercial Mortgage Backed Securities Fixed Rate | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 1.6
Commercial Mortgage Backed Securities Floating Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average basis points of subordination	0.3
Fair value investments	$ 21.2
Commercial Mortgage Backed Securities Floating Rate | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Floating Rate | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Floating Rate | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	21.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39.0	$ 3.9
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.2
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 22.8