Quarterly Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 25, 2018	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Revenue	$ 38,272	$ 31,573	$ 39,178	$ 41,111	$ 108,562	$ 97,483	$ 85,394	$ 84,868		$ 150,134		$ 376,307		$ 323,508
Income (loss) from operations	1,198	437	2,041	4,097	1,869	1,194	(203)	3,530		7,773		6,390		12,361
Income (loss) before income taxes	(2,899)	(2,815)	(143)	2,083	(148)	(724)	(1,358)	2,616
Net income (loss) attributable to i3 Verticals, Inc.	$ (651)	$ (356)	$ 737	$ (149)	$ (930)	$ (1,191)	$ (1,102)	$ 178	$ (7,634)	$ (419)		$ (3,045)		$ (6,898)
Basic earnings (loss) per share attributable to i3 Verticals, Inc. (in USD per share)	$ (0.04)	$ (0.02)	$ 0.05	$ (0.01)	$ (0.07)	$ (0.12)	$ (0.12)	$ 0.02		$ (0.03)	[1]	$ (0.29)	[1]	$ 0.08	[1]
Diluted earnings (loss) per share attributable to i3 Verticals, Inc. (in USD per share)	$ (0.06)	$ (0.02)	$ 0.05	$ (0.01)	$ (0.07)	$ (0.12)	$ (0.12)	$ 0.02		$ (0.03)	[1]	$ (0.29)	[1]	$ 0.08	[1]
Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to i3 Verticals, Inc.										$ (419)		$ (3,045)		$ 736
Diluted earnings (loss) per share attributable to i3 Verticals, Inc. (in USD per share)														$ 0.08
Restricted Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	94,577	193,709	215,564	232,828	271,881	285,433	277,758	295,405		204,969		282,801		299,412
Class B Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)		12,404,368	12,769,568	12,921,637	12,921,637	16,184,026	17,112,164	17,213,806				15,856,855
Out-of-the-money Options | Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	1,297,500	1,498,000	959,000	689,500	446,000	443,000	30,500	754,750		1,327,500		626,500
Stock options | Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	1,225,697	1,127,509		976,594	1,131,760	1,188,987	1,012,916			1,179,538		1,009,858

[1] Basic and diluted net income per share of Class A common stock are presented only for the period after the Company’s Reorganization Transactions. See Note 1 for a description of the Reorganization Transactions. See Note 18 for the calculation of income per common share.