Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,534.6	$ 2,328.7	$ 2,532.5	$ 3,208.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	59.8	53.6
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	43.0	38.1
Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	25.6	23.7
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.5	9.2
Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.9	8.9
Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	20.4	18.1
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,237.7	1,160.5	$ 1,405.1	$ 2,105.4
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	59.8	53.6
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	43.0	38.1
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	25.6	23.7
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Level 2 Inputs | Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Level 2 Inputs | Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.5	9.2
Level 2 Inputs | Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.9	8.9
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	20.4	18.1
Level 2 Inputs | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 0.0	$ 0.0