Segment Information - Revenues And Percentage Of Revenues By Reportable Market Segments (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
Product Concentration Risk | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, Percentage									100.00%	100.00%	100.00%
Applications
Segment Reporting Information [Line Items]
Revenues									$ 585,571	$ 553,167	$ 532,642
Applications | Product Concentration Risk | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, Percentage									57.00%	59.00%	61.00%
Scores
Segment Reporting Information [Line Items]
Revenues									$ 342,648	$ 266,354	$ 241,059
Scores | Product Concentration Risk | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, Percentage									33.00%	29.00%	27.00%
Decision Management Software
Segment Reporting Information [Line Items]
Revenues									$ 104,256	$ 112,648	$ 107,655
Decision Management Software | Product Concentration Risk | Sales Revenue
Segment Reporting Information [Line Items]
Concentration of risk, Percentage									10.00%	12.00%	12.00%