Investment Securities - Mortgage-backed Asset Securities Table (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,533.4	$ 2,746.0	$ 2,569.4	$ 2,328.7
Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	189.0	179.9
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	21.9	29.1
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	210.9	209.0
Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,066.8	1,109.6	$ 1,092.0	$ 1,160.5
Level 2 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	189.0	179.9
Level 2 | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	21.9	29.1
Level 2 | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	210.9	209.0
Level 3 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Level 3 | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
FNMA | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	84.5	88.6
FNMA | Level 2 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	84.5	88.6
FNMA | Level 3 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
FHLMC | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	67.1	60.5
FHLMC | Level 2 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	67.1	60.5
FHLMC | Level 3 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
GNMA | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	37.4	30.8
GNMA | Level 2 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	37.4	30.8
GNMA | Level 3 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Total agency | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	189.0	179.9
Total agency | Level 2 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	189.0	179.9
Total agency | Level 3 | Mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Credit card receivables | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.3	14.0
Credit card receivables | Level 2 | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.3	14.0
Credit card receivables | Level 3 | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Vehicle receivables | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	10.6	15.1
Vehicle receivables | Level 2 | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	10.6	15.1
Vehicle receivables | Level 3 | Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 0.0	$ 0.0