Segment Information - Revenues and Percentage of Revenues on Customers Basis (Details) - 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
Revenue, Major Customer [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
Experian
Revenue, Major Customer [Line Items]
Revenues									109,638	80,347	65,753
TransUnion & Equifax customer
Revenue, Major Customer [Line Items]
Revenues									143,922	104,475	97,427
Other customers
Revenue, Major Customer [Line Items]
Revenues									$ 778,915	$ 747,347	$ 718,176
Sales Revenue | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Concentration of risk, Percentage									100.00%	100.00%	100.00%
Sales Revenue | Customer Concentration Risk | Experian
Revenue, Major Customer [Line Items]
Concentration of risk, Percentage									11.00%	9.00%	8.00%
Sales Revenue | Customer Concentration Risk | TransUnion & Equifax customer
Revenue, Major Customer [Line Items]
Concentration of risk, Percentage									14.00%	11.00%	11.00%
Sales Revenue | Customer Concentration Risk | Other customers
Revenue, Major Customer [Line Items]
Concentration of risk, Percentage									75.00%	80.00%	81.00%