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Investment Securities - Mortgage-backed Asset Securities Table (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | --- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 3,232.6 | $ 2,796.2 | $ 2,541.6 | $ 2,746.0
Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 212.6 | 199.4 |  |
Other asset-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 17.3 | 19.1 |  |
Total mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 229.9 | 218.5 |  |
Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 59.4 | 0.0 |  |
Total mortgage and asset-backed securities and collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 289.3 | 218.5 |  |
Level 2 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,378.6 | 1,030.9 | $ 1,090.3 | $ 1,109.6
Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 212.6 | 199.4 |  |
Level 2 | Other asset-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 17.3 | 19.1 |  |
Level 2 | Total mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 229.9 | 218.5 |  |
Level 2 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 59.4 | 0.0 |  |
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 289.3 | 218.5 |  |
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 | Total mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations |  |  |  |
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 | 98.4 | 88.7 |  |
FNMA | Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 98.4 | 88.7 |  |
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 | 76.9 | 70.1 |  |
FHLMC | Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 76.9 | 70.1 |  |
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 | 36.7 | 40.6 |  |
GNMA | Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 36.7 | 40.6 |  |
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 | 212.0 | 199.4 |  |
Total agency | Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 212.0 | 199.4 |  |
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 | 11.3 |  |
Credit card receivables | Level 2 | Other asset-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 11.3 | 11.3 |  |
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 | 6.0 | 7.8 |  |
Vehicle receivables | Level 2 | Other asset-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 6.0 | 7.8 |  |
Vehicle receivables | Level 3 | Other asset-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Non-agency: Residential | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.6 | 0.0 |  |
Non-agency: Residential | Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.6 | 0.0 |  |
Non-agency: Residential | Level 3 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Total non-agency | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.6 | 0.0 |  |
Total non-agency | Level 2 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.6 | 0.0 |  |
Total non-agency | Level 3 | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | $ 0.0 |  |
Collateralized loan obligations | Mortgage-backed securities: |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 59.4 |  |  |

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