Segment Information - Property and Equipment Net on Geographical Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Property and equipment, net	$ 46,419	$ 53,027
United States
Segment Reporting Information [Line Items]
Property and equipment, net	29,375	38,058
United Kingdom
Segment Reporting Information [Line Items]
Property and equipment, net	8,776	7,801
Other countries
Segment Reporting Information [Line Items]
Property and equipment, net	$ 8,268	$ 7,168
Geographic Concentration Risk | Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	100.00%	100.00%
Geographic Concentration Risk | United States | Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	63.00%	72.00%
Geographic Concentration Risk | United Kingdom | Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	19.00%	15.00%
Geographic Concentration Risk | Other countries | Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	18.00%	13.00%