Segment Information - Revenues and Percentage of Revenues on Geographical Basis (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 232,756	$ 209,366	$ 207,109	$ 189,550	$ 221,570	$ 197,610	$ 185,462	$ 184,343	$ 838,781	$ 788,985	$ 743,444
United States
Segment Reporting Information [Line Items]
Revenues									505,109	457,270	449,437
United Kingdom
Segment Reporting Information [Line Items]
Revenues									93,855	98,784	79,238
Other countries
Segment Reporting Information [Line Items]
Revenues									$ 239,817	$ 232,931	$ 214,769
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									60.00%	58.00%	60.00%
Geographic Concentration Risk | United Kingdom | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, percentage									11.00%	12.00%	11.00%
Geographic Concentration Risk | Other countries | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, percentage									29.00%	30.00%	29.00%