Goodwill and Intangible Assets - Amortization Expense Associated with Intangible Assets (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
Finite-Lived Intangible Assets [Line Items]
Amortization expense of intangible assets	$ 500	$ 600	$ 600	$ 700	$ 1,400	$ 1,700	$ 1,700	$ 1,700	$ 6,594	[1]	$ 12,709	[1]	$ 13,982	[1]
Cost of revenues
Finite-Lived Intangible Assets [Line Items]
Amortization expense of intangible assets									2,380		6,511		7,300
Selling, general and administrative expenses
Finite-Lived Intangible Assets [Line Items]
Amortization expense of intangible assets									$ 4,214		$ 6,198		$ 6,682

[1] Cost of revenues and selling, general and administrative expenses exclude the amortization of intangible assets. See Note 7.