Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Fair Value
Investment securities
Fair value investments	$ 6,736.7	$ 6,766.5	$ 6,580.6	$ 6,950.7
Residential Mortgage Backed Securities | Fair Value
Investment securities
Fair value investments	132.3	125.7
Residential Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	39.6
Residential Mortgage Backed Securities | Senior
Investment securities
Fair value investments	92.7
Residential Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	0
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Fair Value
Investment securities
Fair value investments	7.2
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	7.2
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Senior
Investment securities
Fair value investments	0
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	0
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Fair Value
Investment securities
Fair value investments	19.2
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	0
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Senior
Investment securities
Fair value investments	19.2
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	0
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Fair Value
Investment securities
Fair value investments	105.9
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	32.4
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Senior
Investment securities
Fair value investments	73.5
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	$ 0