Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,633.8	$ 2,547.3	$ 2,604.6	$ 1,543.5
Commercial Mortgage Backed Securities Fixed Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average Basis Points of Subordination	0.0030
Fair value investments	$ 7.8
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	6.2
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]
Fair value investments	2.6
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	2.6
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.4	$ 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	6.2
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4.2