Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 and Adoption of ASU 2021-08 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Assets
Goodwill	$ 292,243				$ 292,243	$ 187,005	$ 168,284
Deferred tax asset	49,992				49,992	36,755
Current liabilities
Accrued expenses and other current liabilities	50,815				50,815	24,064
Deferred revenue	29,862				29,862	10,986
Long-term tax receivable agreement obligations	39,122				39,122	27,565
Stockholders' equity
Additional paid-in-capital	211,237				211,237	157,598
Accumulated deficit	(6,480)				(6,480)	(2,023)
Non-controlling interest	84,831				84,831	84,590
Revenue					224,124	150,134	376,307
Provision for income taxes					623	(2,795)	(177)
Net loss					(7,839)	(979)	563
Net loss attributable to non-controlling interest					(3,382)	(560)	3,608
Net (loss) income attributable to i3 Verticals, Inc.	$ (482)	$ (3,280)	$ 1,303	$ (1,998)	$ (4,457)	$ (419)	$ (3,045)
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)	$ (0.02)	$ (0.15)	$ 0.06	$ (0.10)	$ (0.21)	$ (0.03)	$ (0.29)
Diluted (in USD per share)	$ (0.05)	$ (0.15)	$ 0.04	$ (0.10)	$ (0.22)	$ (0.03)	$ (0.29)
As reported
Assets
Goodwill	$ 287,448				$ 287,448
Deferred tax asset	50,619				50,619
Current liabilities
Accrued expenses and other current liabilities	50,779				50,779
Deferred revenue	29,567				29,567
Long-term tax receivable agreement obligations	39,131				39,131
Stockholders' equity
Additional paid-in-capital	211,044				211,044
Accumulated deficit	(8,813)				(8,813)
Non-controlling interest	83,511				83,511
Revenue		$ 61,964	$ 47,863	$ 43,313	219,624
Provision for income taxes		(110)	(87)	(219)	71
Net loss		(4,594)	(53)	(4,121)	(11,787)
Net loss attributable to non-controlling interest		(1,286)	(493)	(1,549)	(4,997)
Net (loss) income attributable to i3 Verticals, Inc.		$ (3,308)	$ 440	$ (2,572)	$ (6,790)
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)		$ (0.15)	$ 0.02	$ (0.13)	$ (0.32)
Diluted (in USD per share)		$ (0.15)	$ 0.00	$ (0.13)	$ (0.33)
Accounting Standards Update 2021-08
Assets
Goodwill	292,243				$ 292,243
Deferred tax asset	49,992				49,992
Current liabilities
Accrued expenses and other current liabilities	50,815				50,815
Deferred revenue	29,862				29,862
Long-term tax receivable agreement obligations	39,122				39,122
Stockholders' equity
Additional paid-in-capital	211,237				211,237
Accumulated deficit	(6,480)				(6,480)
Non-controlling interest	84,831				84,831
Revenue		$ 63,129	$ 49,197	$ 44,621	224,124
Provision for income taxes		662	(136)	(10)	623
Net loss		(4,201)	1,330	(3,022)	(7,839)
Net loss attributable to non-controlling interest		(921)	27	(1,024)	(3,382)
Net (loss) income attributable to i3 Verticals, Inc.		$ (3,280)	$ 1,303	$ (1,998)	$ (4,457)
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)		$ (0.15)	$ 0.06	$ (0.10)	$ (0.21)
Diluted (in USD per share)		$ (0.15)	$ 0.04	$ (0.10)	$ (0.22)
Accounting Standards Update 2021-08 | Adjustment
Assets
Goodwill	4,795				$ 4,795
Deferred tax asset	(627)				(627)
Current liabilities
Accrued expenses and other current liabilities	36				36
Deferred revenue	295				295
Long-term tax receivable agreement obligations	(9)				(9)
Stockholders' equity
Additional paid-in-capital	193				193
Accumulated deficit	2,333				2,333
Non-controlling interest	$ 1,320				1,320
Revenue		$ 1,165	$ 1,334	$ 1,308	4,500
Provision for income taxes		772	(49)	209	552
Net loss		393	1,383	1,099	3,948
Net loss attributable to non-controlling interest		365	520	525	1,615
Net (loss) income attributable to i3 Verticals, Inc.		$ 28	$ 863	$ 574	$ 2,333
Net loss per share attributable to Class A common stockholders:
Basic (in USD per share)		$ 0.00	$ 0.04	$ 0.03	$ 0.11
Diluted (in USD per share)		$ 0.00	$ 0.04	$ 0.03	$ 0.11