Acquisitions (Details)					12 Months Ended
	May 31, 2019 USD ($)	Oct. 31, 2017 USD ($)	Aug. 01, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) business	Sep. 30, 2018 USD ($) business	Sep. 30, 2017 USD ($) reportable_unit
Business Acquisition [Line Items]
Number of Businesses Acquired | reportable_unit							4
Common units issued to seller					$ 0	$ 104,000	$ 275,000
Cash and cash equivalents					4,561,000
Accounts receivable					5,277,000
Settlement assets					18,000
Related party receivable					0
Inventories					106,000
Prepaid expenses and other current assets					564,000
Property and equipment					2,456,000
Other assets					1,691,000
Goodwill					168,284,000	83,954,000	58,517,000
Total assets acquired					161,942,000
Accounts payable					1,091,000
Accrued expenses and other current liabilities					2,164,000
Settlement obligations					18,000
Deferred revenue, current					2,722,000
Other long-term liabilities					1,124,000
Net assets acquired					154,823,000
Equity issued as part of acquisitions' purchase consideration					$ 0	104,000	$ 275,000
Axia Payments, LLC
Business Acquisition [Line Items]
Estimated amortization period							14 years
Other 2016 Business Combinations
Business Acquisition [Line Items]
Total purchase consideration							$ 3
Estimated amortization period							14 years
2017 Business Combinations
Business Acquisition [Line Items]
Proceeds from issuance of long-term debt used to fund the acquisition							$ 2
Accounts receivable							30,000
Inventories							15,000
Prepaid expenses and other current assets							226,000
Property and equipment							62,000
Goodwill							23,461,000
Total assets acquired							45,734,000
Deferred rent							5,000
Deferred revenue, current							58,000
Net assets acquired							45,671,000
2018 Business Combinations
Business Acquisition [Line Items]
Cash and cash equivalents						1,338,000
Accounts receivable						1,008,000
Settlement assets						350,000
Related party receivable						773,000
Inventories						1,318,000
Property and equipment						127,000
Other assets						4,000
Goodwill						25,437,000
Total assets acquired						45,459,000
Accounts payable						1,342,000
Accrued expenses and other current liabilities						3,554,000
Settlement obligations						350,000
Deferred revenue, current						2,219,000
Other long-term liabilities						1,757,000
Net assets acquired						36,237,000
Merchant Relationships Portfolio Purchase
Business Acquisition [Line Items]
Total purchase consideration				$ 1,156,000
Estimated amortization period				15 years
Acquisition-related costs				$ 56,000
Other 2018 Business Combinations
Business Acquisition [Line Items]
Number of Businesses Acquired | business					4
Common units issued to seller						550,000
Proceeds from issuance of long-term debt used to fund the acquisition						13,700,000
Contingent cash consideration						1,354,000
Total purchase consideration						15,604,000
Cash and cash equivalents						0
Accounts receivable						0
Settlement assets						350,000
Related party receivable						0
Inventories						0
Property and equipment						58,000
Other assets						4,000
Goodwill						8,914,000
Total assets acquired						16,374,000
Accounts payable						0
Accrued expenses and other current liabilities						431,000
Settlement obligations						350,000
Deferred revenue, current						190,000
Other long-term liabilities						0
Net assets acquired						$ 15,403,000
Estimated amortization period						12 years
Equity issued as part of acquisitions' purchase consideration						$ 550,000
Fairway Payments, LLC
Business Acquisition [Line Items]
Common units issued to seller			$ 275,000
Total purchase consideration			$ 39,275,000
Accounts receivable							0
Inventories							0
Prepaid expenses and other current assets							226,000
Property and equipment							5,000
Goodwill							19,309,000
Total assets acquired							39,280,000
Deferred rent							5,000
Deferred revenue, current							0
Net assets acquired							39,275,000
Estimated amortization period			15 years
Acquisition-related costs							284,000
Equity issued as part of acquisitions' purchase consideration			$ 275,000
Other 2017 Business Combinations
Business Acquisition [Line Items]
Number of Businesses Acquired | business						3
Proceeds from issuance of long-term debt used to fund the acquisition							5,175,000
Contingent cash consideration							1,221,000
Total purchase consideration							6,396,000
Accounts receivable							30,000
Inventories							15,000
Prepaid expenses and other current assets							0
Property and equipment							57,000
Goodwill							4,152,000
Total assets acquired							6,454,000
Deferred rent							0
Deferred revenue, current							58,000
Net assets acquired							6,396,000
Estimated amortization period						15 years
Acquisition-related costs						$ 233,000	184,000
Potential additional consideration						11,800,000	$ 4,700,000
San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Proceeds from issuance of long-term debt used to fund the acquisition		$ 20,000,000
Contingent cash consideration		730,000				730,000
Total purchase consideration		$ 20,834,000
Cash and cash equivalents						1,338,000
Accounts receivable						1,008,000
Settlement assets						0
Related party receivable						773,000
Inventories						1,318,000
Property and equipment						69,000
Other assets						0
Goodwill						16,523,000
Total assets acquired						29,085,000
Accounts payable						1,342,000
Accrued expenses and other current liabilities						3,123,000
Settlement obligations						0
Deferred revenue, current						2,029,000
Other long-term liabilities						1,757,000
Net assets acquired						20,834,000
Estimated amortization period		11 years
Acquisition-related costs		$ 293,000
Potential additional consideration		2,400,000
Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Common units issued to seller	$ 56,053,000
Proceeds from issuance of long-term debt used to fund the acquisition	52,492,000
Contingent cash consideration	$ 3,336,000
Cash and cash equivalents					$ 108,000
Accounts receivable					545,000
Settlement assets					0
Related party receivable					0
Inventories					45,000
Prepaid expenses and other current assets					59,000
Property and equipment					527,000
Other assets					1,689,000
Goodwill					36,522,000
Total assets acquired					56,855,000
Accounts payable					722,000
Accrued expenses and other current liabilities					56,000
Settlement obligations					0
Deferred revenue, current					24,000
Other long-term liabilities					0
Net assets acquired					56,053,000
Estimated amortization period	15 years
Acquisition-related costs	$ 444,000
Potential additional consideration	20,000,000
Equity issued as part of acquisitions' purchase consideration	56,053,000
Other 2019 Business Combinations
Business Acquisition [Line Items]
Proceeds from issuance of long-term debt used to fund the acquisition					89,074,000
Contingent cash consideration					9,696,000
Total purchase consideration					98,770,000
Cash and cash equivalents					4,453,000
Accounts receivable					4,732,000
Settlement assets					18,000
Related party receivable					0
Inventories					61,000
Prepaid expenses and other current assets					505,000
Property and equipment					1,929,000
Other assets					2,000
Goodwill					47,777,000
Total assets acquired					105,087,000
Accounts payable					369,000
Accrued expenses and other current liabilities					2,108,000
Settlement obligations					18,000
Deferred revenue, current					2,698,000
Other long-term liabilities					1,124,000
Net assets acquired					98,770,000
Acquisition-related costs					1,179,000
Potential additional consideration					34,900,000
2019 Business Combinations
Business Acquisition [Line Items]
Goodwill					84,299,000
Class A Common Stock | Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Common units issued to seller	225,000
Equity issued as part of acquisitions' purchase consideration	$ 225,000
i3Verticals, LLC | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Common units issued to seller		104,000
Equity issued as part of acquisitions' purchase consideration		$ 104,000
Exclusivity agreements
Business Acquisition [Line Items]
Intangible assets					0
Exclusivity agreements | 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						100,000
Exclusivity agreements | Other 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						$ 100,000
Estimated amortization period						10 years
Exclusivity agreements | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Intangible assets						$ 0
Exclusivity agreements | Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Intangible assets					0
Exclusivity agreements | Other 2019 Business Combinations
Business Acquisition [Line Items]
Intangible assets					0
Capitalized software
Business Acquisition [Line Items]
Intangible assets					12,840,000
Capitalized software | 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						200,000
Capitalized software | Other 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						200,000
Capitalized software | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Intangible assets						0
Capitalized software | Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Intangible assets					3,400,000
Estimated amortization period	7 years
Capitalized software | Other 2019 Business Combinations
Business Acquisition [Line Items]
Intangible assets					9,440,000
Acquired merchant relationships
Business Acquisition [Line Items]
Intangible assets					47,880,000
Acquired merchant relationships | Axia Payments, LLC
Business Acquisition [Line Items]
Estimated amortization period							15 years
Acquired merchant relationships | 2017 Business Combinations
Business Acquisition [Line Items]
Intangible assets							$ 21,360,000
Acquired merchant relationships | 2018 Business Combinations
Business Acquisition [Line Items]
Prepaid expenses and other current assets						1,184,000
Intangible assets						10,600,000
Acquired merchant relationships | Other 2018 Business Combinations
Business Acquisition [Line Items]
Prepaid expenses and other current assets						8,000
Intangible assets						5,100,000
Acquired merchant relationships | Fairway Payments, LLC
Business Acquisition [Line Items]
Intangible assets							19,200,000
Estimated amortization period			15 years
Acquired merchant relationships | Other 2017 Business Combinations
Business Acquisition [Line Items]
Intangible assets							$ 2,160,000
Acquired merchant relationships | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Prepaid expenses and other current assets						1,176,000
Intangible assets						5,500,000
Estimated amortization period		12 years
Acquired merchant relationships | Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Intangible assets					13,400,000
Estimated amortization period	15 years
Acquired merchant relationships | Other 2019 Business Combinations
Business Acquisition [Line Items]
Intangible assets					34,480,000
Non-compete agreements
Business Acquisition [Line Items]
Intangible assets					210,000
Non-compete agreements | Axia Payments, LLC
Business Acquisition [Line Items]
Estimated amortization period							3 years
Non-compete agreements | 2017 Business Combinations
Business Acquisition [Line Items]
Intangible assets							$ 80,000
Non-compete agreements | 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						1,480,000
Non-compete agreements | Other 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						$ 1,440,000
Estimated amortization period						5 years
Non-compete agreements | Fairway Payments, LLC
Business Acquisition [Line Items]
Intangible assets							40,000
Estimated amortization period			3 years
Non-compete agreements | Other 2017 Business Combinations
Business Acquisition [Line Items]
Intangible assets							$ 40,000
Non-compete agreements | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Intangible assets						$ 40,000
Estimated amortization period		2 years
Non-compete agreements | Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Intangible assets					60,000
Estimated amortization period	3 years
Non-compete agreements | Other 2019 Business Combinations
Business Acquisition [Line Items]
Intangible assets					150,000
Trade name
Business Acquisition [Line Items]
Intangible assets					2,040,000
Trade name | Axia Payments, LLC
Business Acquisition [Line Items]
Estimated amortization period							3 years
Trade name | 2017 Business Combinations
Business Acquisition [Line Items]
Intangible assets							$ 500,000
Trade name | 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						1,540,000
Trade name | Other 2018 Business Combinations
Business Acquisition [Line Items]
Intangible assets						$ 200,000
Estimated amortization period						5 years
Trade name | Fairway Payments, LLC
Business Acquisition [Line Items]
Intangible assets							500,000
Estimated amortization period			3 years
Trade name | Other 2017 Business Combinations
Business Acquisition [Line Items]
Intangible assets							$ 0
Trade name | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Intangible assets						$ 1,340,000
Estimated amortization period		5 years
Trade name | Pace Payment Systems, Inc.
Business Acquisition [Line Items]
Intangible assets					500,000
Estimated amortization period	5 years
Trade name | Other 2019 Business Combinations
Business Acquisition [Line Items]
Intangible assets					$ 1,540,000
Minimum | Acquired merchant relationships | Other 2018 Business Combinations
Business Acquisition [Line Items]
Estimated amortization period						12 years
Minimum | Acquired merchant relationships | San Diego Cash Register Company, Inc.
Business Acquisition [Line Items]
Estimated amortization period					12 years
Maximum | Acquired merchant relationships | Other 2018 Business Combinations
Business Acquisition [Line Items]
Estimated amortization period						15 years