Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,050.9	$ 5,091.8	$ 3,379.6	$ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average Basis Points of Subordination	30.00
Fair value investments		93.2
Commercial Mortgage Backed Securities Fixed Rate [Member] \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		8.9
Commercial Mortgage Backed Securities Fixed Rate [Member] \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		46.1
Commercial Mortgage Backed Securities Fixed Rate [Member] \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		38.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 140.4	104.5
Commercial Mortgage Backed Securities \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		8.9
Commercial Mortgage Backed Securities \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		46.1
Commercial Mortgage Backed Securities \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		$ 49.5