Segment Information - Revenues and Percentage of Revenues on Customers Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,160,083	$ 1,000,146	$ 934,983
Experian
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	148,037	109,097	80,096
TransUnion & Equifax customer
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	183,523	142,179	99,735
Other customers
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 828,523	$ 748,870	$ 755,152
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	13.00%	11.00%	9.00%
Revenue Benchmark | Customer Concentration Risk | TransUnion & Equifax customer
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage	16.00%	14.00%	11.00%
Revenue Benchmark | Customer Concentration Risk | Other customers
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage	71.00%	75.00%	80.00%