Revenues and Percentage of Revenues on Geographical Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 221,570	$ 197,610	$ 185,462	$ 184,343	$ 190,327	$ 183,772	$ 179,325	$ 190,020	$ 788,985	$ 743,444	$ 676,423
Concentration of risk, percentage									100.00%	100.00%	100.00%
United States
Segment Reporting Information [Line Items]
Revenues									457,270	449,437	410,178
United States | Sales Revenue, Services, Net
Segment Reporting Information [Line Items]
Concentration of risk, percentage									58.00%	60.00%	61.00%
United Kingdom
Segment Reporting Information [Line Items]
Revenues									98,784	79,238	66,964
United Kingdom | Sales Revenue, Services, Net
Segment Reporting Information [Line Items]
Concentration of risk, percentage									12.00%	11.00%	10.00%
Other countries
Segment Reporting Information [Line Items]
Revenues									$ 232,931	$ 214,769	$ 199,281
Other countries | Sales Revenue, Services, Net
Segment Reporting Information [Line Items]
Concentration of risk, percentage									30.00%	29.00%	29.00%