Receivables and Allowance for Credit Losses - Schedule Of Notes Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Notes Receivable and Allowance for Losses [Line Items]
Notes receivable	$ 159,128	$ 139,317
Total allowance for notes receivable credit losses	(20,039)	(19,484)	$ (4,556)
Total notes receivable, net of allowance	139,089	119,833
Current portion, net of allowance	71,334	24,048
Long-term portion, net of allowance	67,755	95,785
Senior Notes
Notes Receivable and Allowance for Losses [Line Items]
Notes receivable	123,526	104,716
Subordinated
Notes Receivable and Allowance for Losses [Line Items]
Notes receivable	34,192	33,234
Unsecured
Notes Receivable and Allowance for Losses [Line Items]
Notes receivable	$ 1,410	$ 1,367