Summary of Significant Accounting Policies - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Concentration Risk [Line Items]
Revenue	$ 31,573	$ 97,483	$ 111,862	$ 267,745
Revenue earned over time
Concentration Risk [Line Items]
Revenue	22,889	87,241	79,005	237,789
Revenue earned at a point in time
Concentration Risk [Line Items]
Revenue	8,684	10,242	32,857	29,956
Merchant Services
Concentration Risk [Line Items]
Revenue	21,531	87,264	74,014	241,841
Merchant Services | Revenue earned over time
Concentration Risk [Line Items]
Revenue	15,972	79,682	52,639	217,232
Merchant Services | Revenue earned at a point in time
Concentration Risk [Line Items]
Revenue	5,559	7,582	21,375	24,609
Proprietary Software and Payments
Concentration Risk [Line Items]
Revenue	10,458	10,219	39,205	25,904
Proprietary Software and Payments | Revenue earned over time
Concentration Risk [Line Items]
Revenue	7,319	7,559	27,702	20,557
Proprietary Software and Payments | Revenue earned at a point in time
Concentration Risk [Line Items]
Revenue	3,139	2,660	11,503	5,347
Other
Concentration Risk [Line Items]
Revenue	(416)	0	(1,357)	0
Other | Revenue earned over time
Concentration Risk [Line Items]
Revenue	(402)	0	(1,336)	0
Other | Revenue earned at a point in time
Concentration Risk [Line Items]
Revenue	(14)	0	(21)	0
Payments revenue
Concentration Risk [Line Items]
Revenue	20,647	87,406	75,404	241,877
Payments revenue | Merchant Services
Concentration Risk [Line Items]
Revenue	18,060	82,027	61,376	225,488
Payments revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Revenue	2,999	5,379	15,375	16,389
Payments revenue | Other
Concentration Risk [Line Items]
Revenue	(412)	0	(1,347)	0
Other revenue
Concentration Risk [Line Items]
Revenue	10,926	10,077	36,458	25,868
Other revenue | Merchant Services
Concentration Risk [Line Items]
Revenue	3,471	5,237	12,638	16,353
Other revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Revenue	7,459	4,840	23,830	9,515
Other revenue | Other
Concentration Risk [Line Items]
Revenue	$ (4)	$ 0	$ (10)	$ 0