Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
Fixed rate CMBS
Investment securities
Average basis points of subordination	35.00
Fair value investments	$ 35.1
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	0.0
Fixed rate CMBS | Senior
Investment securities
Fair value investments	27.5
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	7.6
Floating rate CMBS
Investment securities
Fair value investments	35.4
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	35.4
Commercial
Investment securities
Fair value investments	70.5	$ 3.9
Commercial | Super Senior
Investment securities
Fair value investments	0.0
Commercial | Senior
Investment securities
Fair value investments	27.5
Commercial | Subordinate
Investment securities
Fair value investments	$ 43.0