Segment Information - Revenues and Percentage of Revenues on Customers Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenue, Major Customer [Line Items]
Total revenues	$ 1,294,562	$ 1,160,083	$ 1,000,146
Experian
Revenue, Major Customer [Line Items]
Total revenues	181,036	148,037	109,097
TransUnion & Equifax customer
Revenue, Major Customer [Line Items]
Total revenues	239,166	183,523	142,179
Other customers
Revenue, Major Customer [Line Items]
Total revenues	$ 874,360	$ 828,523	$ 748,870
Revenue Benchmark | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage	100.00%	100.00%	100.00%
Revenue Benchmark | Customer Concentration Risk | Experian
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage	14.00%	13.00%	11.00%
Revenue Benchmark | Customer Concentration Risk | TransUnion & Equifax customer
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage	18.00%	16.00%	14.00%
Revenue Benchmark | Customer Concentration Risk | Other customers
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage	68.00%	71.00%	75.00%