Segment Information - Revenues and Long-Lived Assets by Geographic Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 206,331	$ 199,942	$ 202,734	$ 204,508	$ 182,069	$ 183,633	$ 189,628	$ 192,624	$ 813,515	$ 747,954	$ 730,505
Long-lived assets	73,649				57,959				73,649	57,959
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									636,155	638,737	626,805
Long-lived assets	52,310				47,523				52,310	47,523
Latin America
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									169,240	101,231	94,905
Long-lived assets	20,986				9,960				20,986	9,960
Canada and other
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									8,120	7,986	$ 8,795
Long-lived assets	$ 353				$ 476				$ 353	$ 476