Debt (Summary of Debt) (Details) - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Revolving secured line of credit balance outstanding		$ 169.5	$ 79.2
Secured Debt		4,444.1	3,990.9
Senior notes balance outstanding		989.6	989.0
Restricted cash and cash equivalents pledged as collateral		559.2	457.7	$ 480.0	$ 410.0
Mortgage note	[1]	8.3	8.4
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Revolving secured line of credit balance outstanding		169.5	79.2
Amount available for borrowing	[2]	$ 240.5	$ 330.8
Interest rate		7.22%	7.33%
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Secured Debt		$ 0.0	$ 0.0
Amount available for borrowing	[2]	400.0	400.0
Loans pledged as collateral		0.0	0.0
Restricted cash and cash equivalents pledged as collateral		$ 2.0	$ 1.0
Interest rate		0.00%	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	$ 1.5
Interest rate		0.00%	0.00%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Secured Debt		$ 0.0	$ 0.0
Amount available for borrowing	[2]	200.0	200.0
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 VI [Member]
Debt Instrument [Line Items]
Secured Debt		$ 60.0	$ 0.0
Amount available for borrowing	[2]	15.0	75.0
Loans pledged as collateral		76.5	0.0
Restricted cash and cash equivalents pledged as collateral		$ 2.2	$ 0.0
Interest rate		7.37%	0.00%
Warehouse Facility VIII [Member]
Debt Instrument [Line Items]
Secured Debt		$ 0.0	$ 0.0
Amount available for borrowing	[2]	200.0	200.0
Loans pledged as collateral		0.0	0.0
Restricted cash and cash equivalents pledged as collateral		$ 1.0	$ 0.8
Interest rate		0.00%	0.00%
Term ABS 2019-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 500.0	$ 500.0
Loans pledged as collateral		547.4	597.3
Restricted cash and cash equivalents pledged as collateral		$ 49.1	$ 47.6
Interest rate		5.15%	5.15%
Term ABS 2020-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 0.0	$ 110.3
Loans pledged as collateral		0.0	418.4
Restricted cash and cash equivalents pledged as collateral		$ 0.0	$ 42.3
Interest rate		0.00%	2.06%
Term ABS 2021-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 100.0	$ 100.0
Loans pledged as collateral		114.1	112.8
Restricted cash and cash equivalents pledged as collateral		$ 10.1	$ 8.8
Interest rate		7.53%	7.56%
Term ABS 2021-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 109.3	$ 188.2
Loans pledged as collateral		370.5	415.5
Restricted cash and cash equivalents pledged as collateral		$ 39.2	$ 37.3
Interest rate		1.58%	1.38%
Term ABS 2021-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 193.2	$ 265.0
Loans pledged as collateral		351.6	396.3
Restricted cash and cash equivalents pledged as collateral		$ 35.2	$ 33.8
Interest rate		1.33%	1.24%
Term ABS 2021-4 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 178.1	$ 221.6
Loans pledged as collateral		235.4	255.2
Restricted cash and cash equivalents pledged as collateral		$ 21.9	$ 21.0
Interest rate		1.51%	1.46%
Term ABS 2022-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 350.0	$ 350.0
Loans pledged as collateral		376.7	378.2
Restricted cash and cash equivalents pledged as collateral		$ 30.2	$ 27.4
Interest rate		5.03%	5.03%
Term ABS 2022-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 200.0	$ 200.0
Loans pledged as collateral		209.7	212.1
Restricted cash and cash equivalents pledged as collateral		$ 16.5	$ 14.7
Interest rate		7.66%	7.66%
Term ABS 2022-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 389.9	$ 389.9
Loans pledged as collateral		421.0	418.9
Restricted cash and cash equivalents pledged as collateral		$ 32.1	$ 28.9
Interest rate		7.68%	7.68%
Term ABS 2023-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 400.0	$ 400.0
Loans pledged as collateral		583.3	611.6
Restricted cash and cash equivalents pledged as collateral		$ 40.9	$ 38.5
Interest rate		6.92%	6.92%
Term ABS 2023-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 400.0	$ 400.0
Loans pledged as collateral		684.4	701.7
Restricted cash and cash equivalents pledged as collateral		$ 46.1	$ 42.0
Interest rate		6.39%	6.39%
Term ABS 2023-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 400.0	$ 400.0
Loans pledged as collateral		648.4	643.8
Restricted cash and cash equivalents pledged as collateral		$ 45.0	$ 40.3
Interest rate		6.86%	6.86%
Term ABS 2023-A [Member]
Debt Instrument [Line Items]
Secured Debt		$ 200.0	$ 200.0
Loans pledged as collateral		301.4	273.4
Restricted cash and cash equivalents pledged as collateral		$ 20.0	$ 17.2
Interest rate		7.51%	7.51%
Term ABS 2023-5 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 294.0	$ 294.0
Loans pledged as collateral		477.1	433.9
Restricted cash and cash equivalents pledged as collateral		$ 37.3	$ 52.2
Interest rate		6.54%	6.54%
Term ABS 2024-A [Member]
Debt Instrument [Line Items]
Secured Debt		$ 200.0
Loans pledged as collateral		285.4	$ 0.0
Restricted cash and cash equivalents pledged as collateral		$ 19.9
Interest rate		7.45%
Term ABS 2024-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 500.0
Loans pledged as collateral		655.8	0.0
Restricted cash and cash equivalents pledged as collateral		$ 105.5
Interest rate		6.01%
2026 Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes balance outstanding		$ 400.0	$ 400.0
Interest rate		6.625%	6.625%
2028 Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes balance outstanding		$ 600.0	$ 600.0
Interest rate		9.25%	9.25%
Mortgages [Member]
Debt Instrument [Line Items]
Interest rate		6.87%	6.88%
Mortgage note		$ 8.3	$ 8.4

[1] Measured at amortized cost with fair value disclosed.

[2] Availability may be limited by the amount of assets pledged as collateral.