Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 6,321.6	$ 6,766.5	$ 6,622.4	$ 6,950.7
Residential Mortgage Backed Securities | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	196.6	125.7
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	75.9
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	120.7
Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Prime | Residential Mortgage Backed Securities | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	180.3
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	71.7
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	108.6
Prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.1
Non-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.1
Non-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.2
Sub-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.2
Sub-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0