Segment Information - Revenues and Percentage of Revenues on Geographical Basis (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
Segment Reporting Information [Line Items]
Revenues	$ 279,791	$ 259,505	$ 257,858	$ 235,321	$ 253,205	$ 230,986	$ 228,378	$ 219,600	$ 1,032,475	$ 932,169	$ 881,356
United States
Segment Reporting Information [Line Items]
Revenues									679,425	598,765	567,443
United Kingdom
Segment Reporting Information [Line Items]
Revenues									77,053	71,989	86,485
Other countries
Segment Reporting Information [Line Items]
Revenues									$ 275,997	$ 261,415	$ 227,428
Geographic Concentration Risk | Sales Revenue
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									66.00%	64.00%	64.00%
Geographic Concentration Risk | United Kingdom | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, Percentage									7.00%	8.00%	10.00%
Geographic Concentration Risk | Other countries | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, Percentage									27.00%	28.00%	26.00%