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Debt (Summary of Debt) (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019 | Dec. 18, 2019 | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |
Balance outstanding |  | $ 79.9 | $ 0.0 |  |  |
Secured Debt |  | 3,957.6 | 3,339.7 |  |  |
Balance outstanding |  | 789.2 | 1,187.8 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | 408.1 | 330.3 |  | $ 417.9 | $ 303.6
Mortgage note | [1] | 11.1 | 11.3 |  |  |
Revolving Secured Line of Credit [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Balance outstanding |  | 79.9 | 0.0 |  |  |
Amount available for borrowing | [2] | $ 260.1 | $ 340.0 |  |  |
Interest rate |  | 2.87% | 0.00% |  |  |
Warehouse Facility II [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 201.0 | $ 0.0 |  |  |
Amount available for borrowing | [2] | 199.0 | 400.0 |  |  |
Loans pledged as collateral |  | 248.9 | 0.0 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 4.8 | $ 1.0 |  |  |
Interest rate |  | 3.33% | 0.00% |  |  |
Warehouse Facility IV [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 0.0 | $ 0.0 |  |  |
Amount available for borrowing | [2] | 300.0 | 300.0 |  |  |
Loans pledged as collateral |  | 0.0 | 0.0 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 1.0 | $ 1.0 |  |  |
Interest rate |  | 0.00% | 0.00% |  |  |
Warehouse Facility V [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 50.0 | $ 0.0 |  |  |
Amount available for borrowing | [2] | 50.0 | 100.0 |  |  |
Loans pledged as collateral |  | 63.2 | 0.0 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 2.3 | $ 1.0 |  |  |
Interest rate |  | 3.48% | 0.00% |  |  |
Warehouse Facility VI [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 0.0 | $ 0.0 |  |  |
Amount available for borrowing | [2] | 75.0 | 75.0 |  |  |
Loans pledged as collateral |  | 0.0 |  |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 0.0 | $ 0.0 |  |  |
Interest rate |  | 0.00% | 0.00% |  |  |
Warehouse Facility VII [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 100.0 | $ 0.0 |  |  |
Amount available for borrowing | [2] | 50.0 | 150.0 |  |  |
Loans pledged as collateral |  | 123.8 | 0.0 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 3.4 | $ 1.0 |  |  |
Interest rate |  | 3.64% | 0.00% |  |  |
Warehouse Facility VIII [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 49.0 | $ 0.0 |  |  |
Amount available for borrowing | [2] | 151.0 | 200.0 |  |  |
Loans pledged as collateral |  | 64.0 | 0.0 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 2.2 | $ 0.0 |  |  |
Interest rate |  | 2.82% | 0.00% |  |  |
Term ABS 2016-3 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 0.0 | $ 51.8 |  |  |
Loans pledged as collateral |  | 0.0 | 219.5 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 0.0 | $ 23.5 |  |  |
Interest rate |  | 0.00% | 3.60% |  |  |
Term ABS 2017-1 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 66.8 | $ 120.9 |  |  |
Loans pledged as collateral |  | 248.5 | 292.8 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 27.4 | $ 26.1 |  |  |
Interest rate |  | 3.36% | 3.19% |  |  |
Term ABS 2017-2 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 202.9 | $ 277.2 |  |  |
Loans pledged as collateral |  | 357.3 | 426.7 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 37.2 | $ 35.1 |  |  |
Interest rate |  | 2.94% | 2.83% |  |  |
Term ABS 2017-3 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 238.6 | $ 303.2 |  |  |
Loans pledged as collateral |  | 332.6 | 393.0 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 31.6 | $ 29.3 |  |  |
Interest rate |  | 2.98% | 2.91% |  |  |
Term ABS 2018-1 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 463.6 | $ 500.0 |  |  |
Loans pledged as collateral |  | 570.0 | 609.5 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 48.3 | $ 43.8 |  |  |
Interest rate |  | 3.26% | 3.24% |  |  |
Term ABS 2018-2 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 450.0 | $ 450.0 |  |  |
Loans pledged as collateral |  | 528.7 | 550.4 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 43.4 | $ 37.6 |  |  |
Interest rate |  | 3.68% | 3.68% |  |  |
Term ABS 2018-3 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 398.3 | $ 398.3 |  |  |
Loans pledged as collateral |  | 469.8 | 487.7 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 37.8 | $ 32.3 |  |  |
Interest rate |  | 3.72% | 3.72% |  |  |
Term ABS 2019-1 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 402.5 | $ 402.5 |  |  |
Loans pledged as collateral |  | 473.0 | 490.2 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 36.7 | $ 31.9 |  |  |
Interest rate |  | 3.53% | 3.53% |  |  |
Term ABS 2019-2 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 500.0 | $ 500.0 |  |  |
Loans pledged as collateral |  | 642.0 | 628.5 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 45.2 | $ 38.6 |  |  |
Interest rate |  | 3.13% | 3.13% |  |  |
Term ABS 2019-3 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 351.7 | $ 351.7 |  |  |
Loans pledged as collateral |  | 515.4 | 428.6 |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 36.4 | $ 27.2 |  |  |
Interest rate |  | 2.56% | 2.56% |  |  |
Term ABS 2020-1 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Secured Debt |  | $ 500.0 |  |  |  |
Loans pledged as collateral |  | 836.9 |  |  |  |
Restricted cash and cash equivalents pledged as collateral |  | $ 49.5 |  |  |  |
Interest rate |  | 2.18% |  |  |  |
2021 Senior Notes [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Balance outstanding |  | $ 0.0 | $ 151.8 |  |  |
Interest rate |  | 0.00% | 6.125% | 6.125% |  |
2023 Senior Notes [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Balance outstanding |  | $ 0.0 | $ 250.0 |  |  |
Interest rate |  | 0.00% | 7.375% | 7.375% |  |
2024 Senior Notes [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Balance outstanding |  | $ 400.0 | $ 400.0 | $ 400.0 |  |
Interest rate |  | 5.125% | 5.125% | 5.125% |  |
2026 Senior Notes [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Balance outstanding |  | $ 400.0 | $ 400.0 |  |  |
Interest rate |  | 6.625% | 6.625% |  |  |
Mortgages [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rate |  | 3.08% | 3.21% |  |  |
Mortgage note |  | $ 11.1 | $ 11.3 |  |  |
[1] | Measured at amortized cost with fair value disclosed.
[2] | Availability may be limited by the amount of assets pledged as collateral.

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