Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Federal statutory rate effect of:	21.00%	21.00%	24.50%
Effective income tax rate reconciliation, Change in enacted tax rate, amount			$ 8.6
Remeasurement of deferred tax	0.00%	0.00%	8.50%
Effective income tax rate reconciliation, Repatriation of foreign earnings, amount			$ 11.2
Repatriation transition tax	0.00%	0.00%	11.00%
Operating loss carryforwards	$ 6.9	$ 7.1
State and local net operating loss carryforwards of net of valuation allowance	4.7
Federal operating loss carryforwards, net of valuation allowances	0.6
Foreign tax credit carryforwards	2.4	5.0	$ 5.1
Valuation allowance	(12.4)	(8.5)
Net change in total valuation allowance	3.9
Foreign earnings repatriated	30.0	$ 13.0
Gain Capital Holdings Inc [Member]
Operating Loss Carryforwards [Line Items]
Foreign tax credit carryforwards	1.1
Foreign tax credits, net	1.3
Gain Capital U.K. Ltd [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 0.9