Condensed Consolidated Statements of Changes in Equity (Unaudited) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Class A	Class B	Common Stock Class A	Common Stock Class B	Additional Paid-In Capital	Retained Earnings (Deficit)	Retained Earnings (Deficit) Cumulative Effect, Period of Adoption, Adjustment	Non-Controlling Interest	Non-Controlling Interest Cumulative Effect, Period of Adoption, Adjustment
Common Stock, Shares, Outstanding, beginning at Sep. 30, 2019					14,444,115	12,921,637
Stockholders' Equity, beginning at Sep. 30, 2019	$ 142,441	$ 1,345			$ 1	$ 1	$ 82,380	$ (2,309)	$ 705	$ 62,368	$ 640
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity-based compensation	2,124						2,124
Net (loss) income								(149)
Net (loss) income attributable to non-controlling interests										2,083
Net (loss) income	1,934
Exercise of equity-based awards (shares)					53,662
Exercise of equity-based awards	351						351
Common Stock, Shares, Outstanding, ending at Dec. 31, 2019					14,497,777	12,921,637
Stockholders' Equity, ending at Dec. 31, 2019	148,195				$ 1	$ 1	84,855	(1,753)		65,091
Common Stock, Shares, Outstanding, beginning at Sep. 30, 2019					14,444,115	12,921,637
Stockholders' Equity, beginning at Sep. 30, 2019	142,441	$ 1,345			$ 1	$ 1	82,380	(2,309)	$ 705	62,368	$ 640
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	588		$ 588
Net (loss) income attributable to non-controlling interests	(3,265)		(3,265)
Net (loss) income	3,853		3,853
Exercise of equity-based awards (shares)					31,125
Exercise of equity-based awards	2
Equity component of exchangeable notes, net of issuance costs and deferred taxes	27,569
Purchases of exchangeable note hedges	(28,676)
Issuance of warrants	14,669
Common Stock, Shares, Outstanding, ending at Mar. 31, 2020					15,038,918	12,411,621
Stockholders' Equity, ending at Mar. 31, 2020	166,781				$ 1	$ 1	104,122	(1,016)		63,673
Common Stock, Shares, Outstanding, beginning at Dec. 31, 2019					14,497,777	12,921,637
Stockholders' Equity, beginning at Dec. 31, 2019	148,195				$ 1	$ 1	84,855	(1,753)		65,091
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity-based compensation	2,510						2,510
Net (loss) income	737		737					737
Net (loss) income attributable to non-controlling interests	(1,182)		(1,182)							1,182
Net (loss) income	1,919		$ 1,919
Distributions to non-controlling interest holders	(3)									(3)
Redemption of common units in i3 Verticals, LLC (shares)					510,016	(510,016)
Redemption of common units in i3 Verticals, LLC	0						2,597			(2,597)
Establishment of liabilities under a tax receivable agreement and related changes to deferred tax assets associated with increases in tax basis	596						596
Exercise of equity-based awards							2
Equity component of exchangeable notes, net of issuance costs and deferred taxes							27,569
Purchases of exchangeable note hedges							(28,676)
Issuance of warrants							14,669
Common Stock, Shares, Outstanding, ending at Mar. 31, 2020					15,038,918	12,411,621
Stockholders' Equity, ending at Mar. 31, 2020	166,781				$ 1	$ 1	104,122	(1,016)		63,673
Common Stock, Shares, Outstanding, beginning at Sep. 30, 2020			18,864,143	11,900,621	18,864,143	11,900,621
Stockholders' Equity, beginning at Sep. 30, 2020	240,168				$ 2	$ 1	157,598	(2,023)		84,590
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity-based compensation	3,441						3,441
Net (loss) income								(2,572)
Net (loss) income attributable to non-controlling interests										(1,549)
Net (loss) income	(4,121)
Redemption of common units in i3 Verticals, LLC (shares)					1,019,609	(1,019,609)
Redemption of common units in i3 Verticals, LLC	0						7,185			(7,185)
Establishment of liabilities under a tax receivable agreement and related changes to deferred tax assets associated with increases in tax basis	1,257						1,257
Exercise of equity-based awards (shares)					121,019
Exercise of equity-based awards	688						688
Allocation of equity to non-controlling interests	0						(1,072)			1,072
Common Stock, Shares, Outstanding, ending at Dec. 31, 2020					20,004,771	10,881,012
Stockholders' Equity, ending at Dec. 31, 2020	241,433				$ 2	$ 1	169,097	(4,595)		76,928
Common Stock, Shares, Outstanding, beginning at Sep. 30, 2020			18,864,143	11,900,621	18,864,143	11,900,621
Stockholders' Equity, beginning at Sep. 30, 2020	240,168				$ 2	$ 1	157,598	(2,023)		84,590
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(2,132)		$ (2,132)
Net (loss) income attributable to non-controlling interests	2,042		2,042
Net (loss) income	$ (4,174)		$ (4,174)
Redemption of common units in i3 Verticals, LLC (shares)					1,671,479
Exercise of equity-based awards (shares)	360,252
Common Stock, Shares, Outstanding, ending at Mar. 31, 2021			21,919,300	10,229,142	21,919,300	10,229,142
Stockholders' Equity, ending at Mar. 31, 2021	$ 281,465				$ 2	$ 1	203,803	(4,155)		81,814
Common Stock, Shares, Outstanding, beginning at Dec. 31, 2020					20,004,771	10,881,012
Stockholders' Equity, beginning at Dec. 31, 2020	241,433				$ 2	$ 1	169,097	(4,595)		76,928
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity-based compensation	4,142						4,142
Net (loss) income	440		$ 440					440
Net (loss) income attributable to non-controlling interests	493		493							(493)
Net (loss) income	(53)		$ (53)
Redemption of common units in i3 Verticals, LLC (shares)					651,870	(651,870)
Redemption of common units in i3 Verticals, LLC	0						4,529			(4,529)
Establishment of liabilities under a tax receivable agreement and related changes to deferred tax assets associated with increases in tax basis	897						897
Exercise of equity-based awards (shares)					59,745
Exercise of equity-based awards	(199)						(199)
Allocation of equity to non-controlling interests	0						(9,908)			9,908
Issuance of Class A common stock under the 2020 Inducement Plan (in shares)					1,202,914
Issuance of Class A common stock under the 2020 Inducement Plan	35,245						35,245
Common Stock, Shares, Outstanding, ending at Mar. 31, 2021			21,919,300	10,229,142	21,919,300	10,229,142
Stockholders' Equity, ending at Mar. 31, 2021	$ 281,465				$ 2	$ 1	$ 203,803	$ (4,155)		$ 81,814