Auto Loans Receivable - Financing Receivable Credit Quality Indicators (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Auto Loans Receivable
Auto loans receivable, Originated in current year	$ 174.2		$ 285.4
Auto loans receivable, Originated in year before current year	250.3		84.6
Auto loans receivable, Originated two years before current year	74.5		51.7
Auto loans receivable, Originated three years before current year	44.3		17.3
Auto loans receivable, Originated four years before current year	14.3		10.6
Auto loans receivable, Originated five years or more before current year	8.9		1.6
Total auto loans receivable	566.5		451.2
Financing Receivable, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Current-period gross write-offs, 2024 origination	0.1
Current-period gross write-offs, 2023 origination	5.9
Current-period gross write-offs, 2022 origination	4.3
Current-period gross write-offs, 2021 origination	2.7
Current-period gross write-offs, 2020 origination	0.8
Current-period gross write-offs, Prior to 2020 origination	0.2
Current-period gross write-offs	14.0	$ 16.0
Palladium
Auto Loans Receivable
Auto loans receivable, Originated in current year	15.8
Auto loans receivable, Originated in year before current year	0.0
Auto loans receivable, Originated two years before current year	0.0
Auto loans receivable, Originated three years before current year	0.0
Auto loans receivable, Originated four years before current year	0.0
Auto loans receivable, Originated five years or more before current year	0.0
Total auto loans receivable	15.8
Rhodium
Auto Loans Receivable
Auto loans receivable, Originated in current year	27.6		27.6
Auto loans receivable, Originated in year before current year	23.1		0.1
Auto loans receivable, Originated two years before current year	0.1		0.0
Auto loans receivable, Originated three years before current year	0.0		0.0
Auto loans receivable, Originated four years before current year	0.0		0.0
Auto loans receivable, Originated five years or more before current year	0.0		0.0
Total auto loans receivable	50.8		27.7
Platinum
Auto Loans Receivable
Auto loans receivable, Originated in current year	112.7		136.3
Auto loans receivable, Originated in year before current year	118.5		14.7
Auto loans receivable, Originated two years before current year	13.2		8.2
Auto loans receivable, Originated three years before current year	7.2		3.3
Auto loans receivable, Originated four years before current year	2.8		3.1
Auto loans receivable, Originated five years or more before current year	2.6		0.5
Total auto loans receivable	257.0		166.1
Gold
Auto Loans Receivable
Auto loans receivable, Originated in current year	17.9		64.3
Auto loans receivable, Originated in year before current year	57.6		35.9
Auto loans receivable, Originated two years before current year	31.8		18.5
Auto loans receivable, Originated three years before current year	16.1		6.8
Auto loans receivable, Originated four years before current year	5.6		4.5
Auto loans receivable, Originated five years or more before current year	4.0		0.8
Total auto loans receivable	133.0		130.8
Silver
Auto Loans Receivable
Auto loans receivable, Originated in current year	0.2		50.2
Auto loans receivable, Originated in year before current year	45.0		33.0
Auto loans receivable, Originated two years before current year	28.7		16.2
Auto loans receivable, Originated three years before current year	13.7		5.2
Auto loans receivable, Originated four years before current year	4.2		2.8
Auto loans receivable, Originated five years or more before current year	2.1		0.3
Total auto loans receivable	93.9		107.7
Bronze
Auto Loans Receivable
Auto loans receivable, Originated in current year	0.0		6.7
Auto loans receivable, Originated in year before current year	5.9		0.7
Auto loans receivable, Originated two years before current year	0.6		6.0
Auto loans receivable, Originated three years before current year	5.0		1.5
Auto loans receivable, Originated four years before current year	1.3		0.1
Auto loans receivable, Originated five years or more before current year	0.1		0.0
Total auto loans receivable	12.9		15.0
Copper
Auto Loans Receivable
Auto loans receivable, Originated in current year	0.0		0.3
Auto loans receivable, Originated in year before current year	0.2		0.2
Auto loans receivable, Originated two years before current year	0.1		2.8
Auto loans receivable, Originated three years before current year	2.3		0.5
Auto loans receivable, Originated four years before current year	0.4		0.1
Auto loans receivable, Originated five years or more before current year	0.1		0.0
Total auto loans receivable	$ 3.1		$ 3.9