Revenues and Accounts Receivable - Schedule of Accounts Receivable, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues And Accounts Receivable [Abstract]
Accounts receivable, invoiced	$ 68,974	$ 57,228
Accounts receivable, other	1,876	1,139
Allowance for doubtful accounts	(4,379)	(4,265)	$ (1,327)
Accounts receivable—net	$ 66,471	$ 54,102