Revenue Recognition - Reimbursable Payroll Related and Other Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenues	$ 131,564	$ 140,861	$ 288,510	$ 300,753
Equity based compensation expense and related reimbursements	1,206	(290)	4,209	658
Reimbursable compensation and benefits
Disaggregation of Revenue [Line Items]
Total revenues	13,159	12,823	26,384	25,670
Other reimbursable expenses
Disaggregation of Revenue [Line Items]
Total revenues	$ 75,208	$ 84,227	$ 174,593	$ 182,202