Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Fixed rate CMBS
Investment securities
Average Basis Points of Subordination	25.00
Fair value investments	$ 116.0
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	8.7
Fixed rate CMBS | Senior
Investment securities
Fair value investments	59.1
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	48.2
Floating rate CMBS
Investment securities
Fair value investments	11.5
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	11.5
Commercial
Investment securities
Fair value investments	127.5	$ 140.4
Commercial | Super Senior
Investment securities
Fair value investments	8.7
Commercial | Senior
Investment securities
Fair value investments	59.1
Commercial | Subordinate
Investment securities
Fair value investments	$ 59.7