Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 3,360.2	$ 4,050.9	$ 3,355.3	$ 3,343.6
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	132.3	$ 133.2
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	63.1
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	69.2
Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	124.8
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	63.1
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	61.7
Prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Non-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	7.5
Non-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Non-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	7.5
Non-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0.0