Debt (Summary of Debt) (Details) - USD ($) $ in Millions		Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021	Dec. 18, 2019
Debt Instrument [Line Items]
Revolving secured line of credit balance outstanding		$ 102.1	$ 30.9
Secured Debt		4,034.2	3,756.4
Senior notes balance outstanding		796.0	794.5
Restricted cash and cash equivalents pledged as collateral		435.1	410.0	$ 384.7	$ 410.9
Mortgage note	[1]	8.5	8.9
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Revolving secured line of credit balance outstanding		102.1	30.9
Amount available for borrowing	[2]	$ 307.9	$ 379.1
Interest rate		7.26%	6.25%
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		244.9	0.0
Restricted cash and cash equivalents pledged as collateral		$ 9.7	$ 1.0
Interest rate		7.60%	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		$ 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		$ 0.0	$ 0.0
Amount available for borrowing	[2]	75.0	75.0
Loans pledged as collateral		0.0	0.0
Restricted cash and cash equivalents pledged as collateral		$ 0.0	$ 0.0
Interest rate		0.00%	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		$ 0.0	$ 0.0
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		554.8	627.5
Restricted cash and cash equivalents pledged as collateral		$ 45.7	$ 51.0
Interest rate		5.15%	5.15%
Term ABS 2019-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 0.0	$ 64.4
Loans pledged as collateral		0.0	200.9
Restricted cash and cash equivalents pledged as collateral		$ 0.0	$ 24.5
Interest rate		0.00%	3.00%
Term ABS 2020-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 0.0	$ 144.6
Loans pledged as collateral		0.0	362.5
Restricted cash and cash equivalents pledged as collateral		$ 0.0	$ 38.8
Interest rate		0.00%	2.51%
Term ABS 2020-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 62.9	$ 307.0
Loans pledged as collateral		289.1	452.0
Restricted cash and cash equivalents pledged as collateral		$ 34.2	$ 43.9
Interest rate		2.70%	1.81%
Term ABS 2020-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 196.1	$ 520.7
Loans pledged as collateral		467.1	655.1
Restricted cash and cash equivalents pledged as collateral		$ 45.4	$ 53.9
Interest rate		1.86%	1.47%
Term ABS 2021-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 100.0	$ 100.0
Loans pledged as collateral		112.7	115.0
Restricted cash and cash equivalents pledged as collateral		$ 8.9	$ 8.5
Interest rate		7.53%	6.52%
Term ABS 2021-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 269.0	$ 500.0
Loans pledged as collateral		460.3	572.9
Restricted cash and cash equivalents pledged as collateral		$ 39.7	$ 44.5
Interest rate		1.25%	1.12%
Term ABS 2021-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 338.9	$ 450.0
Loans pledged as collateral		438.0	519.9
Restricted cash and cash equivalents pledged as collateral		$ 36.4	$ 38.8
Interest rate		1.19%	1.14%
Term ABS 2021-4 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 250.1	$ 250.1
Loans pledged as collateral		267.9	278.5
Restricted cash and cash equivalents pledged as collateral		$ 22.0	$ 21.8
Interest rate		1.44%	1.44%
Term ABS 2022-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 350.0	$ 350.0
Loans pledged as collateral		379.1	434.7
Restricted cash and cash equivalents pledged as collateral		$ 26.6	$ 27.7
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		214.4	229.3
Restricted cash and cash equivalents pledged as collateral		$ 15.0	$ 25.6
Interest rate		7.76%	6.65%
Term ABS 2022-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 389.9	$ 389.9
Loans pledged as collateral		423.9	429.2
Restricted cash and cash equivalents pledged as collateral		$ 29.3	$ 27.6
Interest rate		7.68%	7.68%
Term ABS 2023-1 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 400.0
Loans pledged as collateral		629.9	$ 0.0
Restricted cash and cash equivalents pledged as collateral		$ 39.4
Interest rate		6.92%
Term ABS 2023-2 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 400.0
Loans pledged as collateral		694.6	0.0
Restricted cash and cash equivalents pledged as collateral		$ 41.9
Interest rate		6.39%
Term ABS 2023-3 [Member]
Debt Instrument [Line Items]
Secured Debt		$ 400.0
Loans pledged as collateral		604.8	0.0
Restricted cash and cash equivalents pledged as collateral		$ 37.2
Interest rate		6.86%
2024 Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes balance outstanding		$ 400.0	$ 400.0			$ 400.0
Interest rate		5.125%	5.125%			5.125%
2026 Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes balance outstanding		$ 400.0	$ 400.0
Interest rate		6.625%	6.625%
Mortgages [Member]
Debt Instrument [Line Items]
Interest rate		6.87%	5.46%
Mortgage note		$ 8.5	$ 8.9

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

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