Summary of Significant Accounting Policies - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Concentration Risk [Line Items]
Total revenue	$ 39,178	$ 85,394	$ 80,289	$ 170,262
Revenue earned over time
Concentration Risk [Line Items]
Total revenue	28,445	75,608	56,116	150,548
Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	10,733	9,786	24,173	19,714
Merchant Services
Concentration Risk [Line Items]
Total revenue	25,018	76,875	52,483	154,577
Merchant Services | Revenue earned over time
Concentration Risk [Line Items]
Total revenue	18,537	68,602	36,667	137,550
Merchant Services | Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	6,481	8,273	15,816	17,027
Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	14,691	8,519	28,747	15,685
Proprietary Software and Payments | Revenue earned over time
Concentration Risk [Line Items]
Total revenue	10,434	7,006	20,383	12,998
Proprietary Software and Payments | Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	4,257	1,513	8,364	2,687
Other
Concentration Risk [Line Items]
Total revenue	(531)	0	(941)	0
Other | Revenue earned over time
Concentration Risk [Line Items]
Total revenue	(526)	0	(934)	0
Other | Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	(5)	0	(7)	0
Payments revenue
Concentration Risk [Line Items]
Total revenue	26,386	76,732	54,757	154,471
Payments revenue | Merchant Services
Concentration Risk [Line Items]
Total revenue	20,863	70,924	43,316	143,461
Payments revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	6,050	5,808	12,376	11,010
Payments revenue | Other
Concentration Risk [Line Items]
Total revenue	(527)	0	(935)	0
Other revenue
Concentration Risk [Line Items]
Total revenue	12,792	8,662	25,532	15,791
Other revenue | Merchant Services
Concentration Risk [Line Items]
Total revenue	4,155	5,951	9,167	11,116
Other revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	8,641	2,711	16,371	4,675
Other revenue | Other
Concentration Risk [Line Items]
Total revenue	$ (4)	$ 0	$ (6)	$ 0