Reportable Segments and Major Concentration Data - Consolidated Net Revenues (Detail) - 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
Entity Wide Portfolio Carrying Amount, Major Customer [Line Items]
Revenues	$ 53,100	$ 51,737	$ 56,451	$ 52,283	$ 49,697	$ 50,140	$ 54,125	$ 46,809	$ 213,571	$ 200,771	$ 196,082
Customer A [Member] | Customer Concentration Risk [Member] | Revenues [Member]
Entity Wide Portfolio Carrying Amount, Major Customer [Line Items]
Revenues									$ 22,490	$ 22,397	$ 20,246
Concentration percentage									11.00%	11.00%	10.00%
Customer B [Member] | Customer Concentration Risk [Member] | Revenues [Member]
Entity Wide Portfolio Carrying Amount, Major Customer [Line Items]
Revenues									$ 22,040	$ 17,825	$ 19,585
Concentration percentage									10.00%	9.00%	10.00%