Segment Information - Revenues and Percentage of Revenues on Geographical Basis (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 235,824	$ 238,778	$ 206,678	$ 200,076	$ 232,756	$ 209,366	$ 207,109	$ 189,550	$ 881,356	$ 838,781	$ 788,985
United States
Segment Reporting Information [Line Items]
Revenues									567,443	505,109	457,270
United Kingdom
Segment Reporting Information [Line Items]
Revenues									86,485	93,855	98,784
Other countries
Segment Reporting Information [Line Items]
Revenues									$ 227,428	$ 239,817	$ 232,931
Geographic Concentration Risk
Segment Reporting Information [Line Items]
Concentration of risk, percentage									100.00%	100.00%	100.00%
Geographic Concentration Risk | United States | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, percentage									64.00%	60.00%	58.00%
Geographic Concentration Risk | United Kingdom | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, percentage									10.00%	11.00%	12.00%
Geographic Concentration Risk | Other countries | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, percentage									26.00%	29.00%	30.00%