Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Non-Agency Residential Mortgage [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.9
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 126.9	$ 127.5