Receivables and Allowance for Credit Losses - Credit Quality Indicator (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Notes Receivable and Allowance for Losses [Line Items]
2021	$ 0
2020	0
2019	31,614
Prior	127,514
Total	159,128	$ 139,317
Senior Notes
Notes Receivable and Allowance for Losses [Line Items]
2021	0
2020	0
2019	28,867
Prior	94,659
Total	123,526	104,716
Subordinated
Notes Receivable and Allowance for Losses [Line Items]
2021	0
2020	0
2019	2,747
Prior	31,445
Total	34,192	33,234
Unsecured
Notes Receivable and Allowance for Losses [Line Items]
2021	0
2020	0
2019	0
Prior	1,410
Total	$ 1,410	$ 1,367