Segment and Geographic Information - Net Revenues based upon Source of Order by Geographic Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 780,848	$ 631,560	$ 527,499
North America
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	327,250	233,243	174,432
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	325,300	232,700	172,900
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	48,764	51,690	37,283
Rest of Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 92,597	$ 83,921	$ 59,959