Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,275.4	$ 3,208.3	$ 2,633.8	$ 2,547.3
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	25.9	46.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	53.9	84.5
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	40.8	62.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	120.6	192.8
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Private debt instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	120.6	263.3
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9	206.0
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.2	142.4
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	83.0
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.1	431.4
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	132.7	694.7
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,254.6	2,105.4	$ 1,649.7	$ 2,093.8
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	25.9	46.3
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	53.9	84.5
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	40.8	62.0
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	120.6	192.8
Level 2 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Level 2 Inputs | Private debt instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Level 2 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	120.6	263.3
Level 2 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9	206.0
Level 2 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.2	142.4
Level 2 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	83.0
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.1	431.4
Level 2 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	132.7	694.7
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Private debt instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0.0	$ 0.0