Segment Information - Revenues and Long-Lived Assets by Geographic Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 214,339	$ 202,465	$ 214,730	$ 215,695	$ 205,976	$ 199,612	$ 202,398	$ 204,170	$ 847,229	$ 812,156	$ 747,951
Long-lived assets	67,357				73,649				67,357	73,649
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									650,413	635,780	638,945
Long-lived assets	43,274				52,310				43,274	52,310
Mexico
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									138,897	122,702	101,020
Long-lived assets	18,566				18,497				18,566	18,497
Other Latin America
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									52,288	45,554	0
Long-lived assets	5,432				2,489				5,432	2,489
Canada and other
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									5,631	8,120	$ 7,986
Long-lived assets	$ 85				$ 353				$ 85	$ 353