Fair Value of Assets and Liabilities - Carrying Values and Estimated Fair Values of Financial Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Liabilities
Contingent consideration liability	$ 1,620	$ 744	$ 311	$ 424
Carrying Value | Fair Value, Measurements, Nonrecurring
Assets
Receivables from broker-dealers and clearing organizations	3,490	3,967
Forgivable loans receivable	1,834	1,567
Other receivables, net	5,672	4,265
Total assets	41,875	38,055
Liabilities
Accrued commissions and payroll payable	18,590	12,862
Accounts payable and accrued expenses	8,643	7,275
Total liabilities	27,233	20,137
Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring
Assets
Receivables from broker-dealers and clearing organizations	3,490	3,967
Forgivable loans receivable	1,834	1,567
Other receivables, net	5,672	4,265
Total assets	41,875	38,055
Liabilities
Accrued commissions and payroll payable	18,590	12,862
Accounts payable and accrued expenses	8,643	7,275
Total liabilities	27,233	20,137
Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Receivables from broker-dealers and clearing organizations	0	0
Forgivable loans receivable	0	0
Other receivables, net	0	0
Total assets	30,879	28,256
Liabilities
Accrued commissions and payroll payable	0	0
Accounts payable and accrued expenses	0	0
Total liabilities	0	0
Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Receivables from broker-dealers and clearing organizations	3,490	3,967
Forgivable loans receivable	1,834	1,567
Other receivables, net	5,672	4,265
Total assets	10,996	9,799
Liabilities
Accrued commissions and payroll payable	18,590	12,862
Accounts payable and accrued expenses	8,643	7,275
Total liabilities	27,233	20,137
Cash | Carrying Value | Fair Value, Measurements, Nonrecurring
Assets
Cash and cash equivalents	30,443	27,920
Cash | Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring
Assets
Cash and cash equivalents	30,443	27,920
Cash | Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Cash and cash equivalents	30,443	27,920
Cash | Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Cash and cash equivalents	0	0
Cash deposits with clearing organizations | Carrying Value | Fair Value, Measurements, Nonrecurring
Assets
Cash and cash equivalents	436	336
Cash deposits with clearing organizations | Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring
Assets
Cash and cash equivalents	436	336
Cash deposits with clearing organizations | Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Cash and cash equivalents	436	336
Cash deposits with clearing organizations | Total Estimated Fair Value | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Cash and cash equivalents	$ 0	$ 0