Investment Securities (Mortgage-backed Asset Securities Table) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Level 2 Inputs
Investment securities
Fair value investments	$ 4,727.5	$ 4,982.2	$ 4,795.9	$ 5,206.1
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Investment securities
Fair value investments	464.8	512.3
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Investment securities
Fair value investments	167.3	81.2
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Investment securities
Fair value investments	85.6	91.3
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investment securities
Fair value investments	717.7	684.8
Level 2 Inputs | Residential Mortgage Backed Securities
Investment securities
Fair value investments	132.3	125.7
Level 2 Inputs | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	310.1	282.3
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	442.4	408.0
Level 2 Inputs | Collateralized Mortgage Backed Securities
Investment securities
Fair value investments	1,160.1	1,092.8
Level 2 Inputs | Collateralized Credit Card Securities
Investment securities
Fair value investments	207.7	289.4
Level 2 Inputs | Collateralized Auto Loans
Investment securities
Fair value investments	231.5	365.0
Level 2 Inputs | Collateralized Securities, Other
Investment securities
Fair value investments	201.3	245.3
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Investment securities
Fair value investments	640.5	899.7
Level 2 Inputs | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	1,800.6	1,992.5
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Investment securities
Fair value investments	0	0
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Investment securities
Fair value investments	0	0
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Investment securities
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investment securities
Fair value investments	0	0
Level 3 Inputs | Residential Mortgage Backed Securities
Investment securities
Fair value investments	0	0
Level 3 Inputs | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	19.6	0
Level 3 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	19.6	0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Investment securities
Fair value investments	19.6	0
Level 3 Inputs | Collateralized Credit Card Securities
Investment securities
Fair value investments	22.0	22.0
Level 3 Inputs | Collateralized Auto Loans
Investment securities
Fair value investments	0	0
Level 3 Inputs | Collateralized Securities, Other
Investment securities
Fair value investments	0	0
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Investment securities
Fair value investments	22.0	22.0
Level 3 Inputs | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	41.6	22.0
Fair Value
Investment securities
Fair value investments	6,736.7	6,766.5	6,580.6	6,950.7
Fair Value | Government National Mortgage Association Certificates and Obligations (GNMA)
Investment securities
Fair value investments	464.8	512.3
Fair Value | Federal National Mortgage Association Certificates and Obligations (FNMA)
Investment securities
Fair value investments	167.3	81.2
Fair Value | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Investment securities
Fair value investments	85.6	91.3
Fair Value | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investment securities
Fair value investments	717.7	684.8
Fair Value | Residential Mortgage Backed Securities
Investment securities
Fair value investments	132.3	125.7
Fair Value | Commercial Mortgage Backed Securities
Investment securities
Fair value investments	329.7	282.3
Fair Value | Mortgage-backed Securities, Issued by Private Enterprises
Investment securities
Fair value investments	462.0	408.0
Fair Value | Collateralized Mortgage Backed Securities
Investment securities
Fair value investments	1,179.7	1,092.8
Fair Value | Collateralized Credit Card Securities
Investment securities
Fair value investments	229.7	311.4
Fair Value | Collateralized Auto Loans
Investment securities
Fair value investments	231.5	365.0
Fair Value | Collateralized Securities, Other
Investment securities
Fair value investments	201.3	245.3
Fair Value | Asset-backed Securities, Securitized Loans and Receivables
Investment securities
Fair value investments	662.5	921.7
Fair Value | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	$ 1,842.2	$ 2,014.5