Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,091.8	$ 4,050.9	$ 3,379.6	$ 3,343.6
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.3	$ 133.2
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	133.6
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	60.7
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	187.4
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	133.6
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	53.8
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	6.9
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	6.9
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