SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									9 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021		Mar. 31, 2021	[2]	Dec. 31, 2020	[2]	Jun. 30, 2022	Jun. 30, 2021		Sep. 30, 2021	Sep. 30, 2020	[2]
Assets
Goodwill	$ 353,882			$ 289,046						$ 353,882	$ 289,046		$ 292,243
Deferred tax asset	51,698			50,101						51,698	50,101		49,992
Current liabilities
Deferred revenue	22,641			21,107						22,641	21,107		29,862
Long-term tax receivable agreement obligations	39,866			39,617						39,866	39,617		39,122
Stockholders' equity
Additional paid-in capital	226,269			207,460						226,269	207,460		211,237
Accumulated deficit	(20,109)			(5,998)						(20,109)	(5,998)		(6,480)
Non-controlling interest	84,329			83,332						84,329	83,332		84,831
Revenue	80,553			63,129	[1]					232,612	156,947	[1]
Benefit from income taxes	(1,810)			662	[1]					(1,154)	516	[1]
Net loss	(4,688)	$ (10,438)	$ (3,681)	(4,201)	[1],[2]	$ 1,330		$ (3,022)		(18,807)	(5,894)	[1],[3],[4]
Net loss attributable to non-controlling interest	(960)			(921)	[1]					(5,178)	(1,918)	[1]
Net loss attributable to i3 Verticals, Inc.	$ (3,728)			$ (3,280)	[1]					$ (13,629)	$ (3,976)	[1]
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)	$ (0.17)			$ (0.15)	[1]					$ (0.62)	$ (0.19)	[1]
Basic net loss per share (in USD per share)	$ (0.17)			$ (0.15)	[1]					$ (0.62)	$ (0.19)	[1]
Statement of Stockholders' Equity [Abstract]
Net loss	$ (4,688)	(10,438)	(3,681)	$ (4,201)	[1],[2]	1,330		(3,022)		$ (18,807)	$ (5,894)	[1],[3],[4]
Establishment of liabilities under a tax receivable agreement and related changes to deferred tax assets associated with increases in tax basis	748	(1,288)	345	28	[2]	617		1,162
Balance	290,492	287,495	293,053	284,797	[2]	283,572		242,437		290,492	284,797	[2]	$ 289,591	$ 240,168
Cash flows from operating activities:
Net loss	$ (4,688)	$ (10,438)	$ (3,681)	(4,201)	[1],[2]	$ 1,330		$ (3,022)		(18,807)	(5,894)	[1],[3],[4]
Adjustments to reconcile net loss to net cash provided by operating activities:
(Benefit from) provision for deferred income taxes										(1,154)	517	[3],[4]
Changes in operating liabilities:
Deferred revenue										(7,325)	(3,768)	[3],[4]
Other long-term liabilities										$ 0	(698)	[3],[4]
As reported
Assets
Goodwill				284,251							284,251
Deferred tax asset				51,247							51,247
Current liabilities
Deferred revenue				20,118							20,118
Long-term tax receivable agreement obligations				39,626							39,626
Stockholders' equity
Additional paid-in capital				207,697							207,697
Accumulated deficit				(7,463)							(7,463)
Non-controlling interest				81,891							81,891
Revenue				61,964							153,140
Benefit from income taxes				(110)							(416)
Net loss				(4,594)							(8,769)
Net loss attributable to non-controlling interest				(1,286)							(3,328)
Net loss attributable to i3 Verticals, Inc.				$ (3,308)							$ (5,441)
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)				$ (0.15)							$ (0.26)
Basic net loss per share (in USD per share)				$ (0.15)							$ (0.26)
Statement of Stockholders' Equity [Abstract]
Net loss				$ (4,594)							$ (8,769)
Establishment of liabilities under a tax receivable agreement and related changes to deferred tax assets associated with increases in tax basis				(141)
Balance				282,128							282,128
Cash flows from operating activities:
Net loss				(4,594)							(8,769)
Adjustments to reconcile net loss to net cash provided by operating activities:
(Benefit from) provision for deferred income taxes											(416)
Changes in operating liabilities:
Deferred revenue											39
Other long-term liabilities											(697)
Adjustment
Assets
Goodwill				4,795							4,795
Deferred tax asset				(1,146)							(1,146)
Current liabilities
Deferred revenue				989							989
Long-term tax receivable agreement obligations				(9)							(9)
Stockholders' equity
Additional paid-in capital				(237)							(237)
Accumulated deficit				1,465							1,465
Non-controlling interest				1,441							1,441
Revenue				1,165							3,807
Benefit from income taxes				772							932
Net loss				393							2,875
Net loss attributable to non-controlling interest				365							1,410
Net loss attributable to i3 Verticals, Inc.				$ 28							$ 1,465
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)				$ 0.00							$ 0.07
Basic net loss per share (in USD per share)				$ 0.00							$ 0.07
Statement of Stockholders' Equity [Abstract]
Net loss				$ 393							$ 2,875
Establishment of liabilities under a tax receivable agreement and related changes to deferred tax assets associated with increases in tax basis				169
Balance				2,669							2,669
Cash flows from operating activities:
Net loss				$ 393							2,875
Adjustments to reconcile net loss to net cash provided by operating activities:
(Benefit from) provision for deferred income taxes											933
Changes in operating liabilities:
Deferred revenue											(3,807)
Other long-term liabilities											$ (1)

[1] Effective October 1, 2020, the Company's financial statements are presented in accordance with ASU 2021-08, Accounting Standards Codification Topic 805, Accounting for Contract Assets and Contract Liabilities from Contracts with Customers. See Note 2 to the interim consolidated financial statements for a description of the recently adopted accounting pronouncement and the impacts of adoption on the condensed consolidated statements of operations.

[2] Effective October 1, 2020, the Company's financial statements are presented in accordance with ASU 2021-08, Accounting Standards Codification Topic 805, Accounting for Contract Assets and Contract Liabilities from Contracts with Customers. See Note 2 to the interim consolidated financial statements for a description of the recently adopted accounting pronouncement and the impacts of adoption on the condensed consolidated statement of changes in equity.

[3] Effective October 1, 2020, the Company's financial statements are presented in accordance with ASU 2021-08, Accounting Standards Codification Topic 805, Accounting for Contract Assets and Contract Liabilities from Contracts with Customers. See Note 2 to the interim consolidated financial statements for a description of the recently adopted accounting pronouncement and the impacts of adoption on the condensed consolidated statements of cash flows.

[4] The prior period amounts included in the statement of cash flows have been updated to correct settlement assets as restricted cash, which were previously reported as cash flows used in operating and investing activities. These adjustments reflect a decrease in prior year cash flows provided by operating activities of $2,046 and a decrease of $7,009 in cash outflows for acquisitions of businesses, net of cash and restricted cash acquired within cash flows used in investing activities, resulting in a combined increase of $4,963 in cash, cash equivalents, and restricted cash at the end of the period.