Goodwill and Intangible Assets - Amortization Expense Associated with Intangible Assets (Detail) - 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
Finite-Lived Intangible Assets [Line Items]
Amortization expense of intangible assets	$ 1,400	$ 1,700	$ 1,700	$ 1,700	$ 1,700	$ 1,800	$ 1,800	$ 1,900	$ 12,709	[1]	$ 13,982	[1]	$ 13,673	[1]
Cost of revenues
Finite-Lived Intangible Assets [Line Items]
Amortization expense of intangible assets									6,511		7,300		7,594
Selling, general and administrative expenses
Finite-Lived Intangible Assets [Line Items]
Amortization expense of intangible assets									$ 6,198		$ 6,682		$ 6,079

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