Segment Information - Revenues and Long-Lived Assets by Geographic Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 182,069	$ 183,633	$ 189,628	$ 192,624	$ 184,585	$ 170,150	$ 188,213	$ 187,557	$ 747,954	$ 730,505	$ 720,000
Long-lived assets	345,139				343,112				345,139	343,112
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									638,737	626,805	606,134
Long-lived assets	326,736				326,347				326,736	326,347
Mexico
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									101,231	94,905	100,689
Long-lived assets	17,033				15,893				17,033	15,893
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									7,986	8,795	$ 13,177
Long-lived assets	$ 1,370				$ 872				$ 1,370	$ 872