Quarterly Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 25, 2018	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Revenue	$ 108,562	$ 97,483	$ 85,394	$ 84,868	$ 84,053		$ 84,536	$ 77,698	$ 77,221		$ 376,307		$ 323,508		$ 262,571
Income from operations	1,869	1,194	(203)	3,530	3,495		2,923	2,596	3,347		6,390		12,361		7,600
(Loss) income before income taxes	(148)	(724)	(1,358)	2,616	2,646		37	(6,586)	(721)
Net (loss) income attributable to i3 Verticals, Inc.	$ (930)	$ (1,191)	$ (1,102)	$ 178	$ 834	$ 736	$ (564)	$ (6,836)	$ (332)	$ (7,634)	$ (3,045)		$ (6,898)		$ 902
Basic {loss} earnings per share attributble to i3 Verticals, Inc. (in USD per share)	$ (0.07)	$ (0.12)	$ (0.12)	$ 0.02	$ 0.09		$ (0.01)				$ (0.29)	[1]	$ 0.08	[1]
Diluted (loss) earnings per share attributable to i3 Verticals, Inc. (in USD per share)	$ (0.07)	$ (0.12)	$ (0.12)	$ 0.02	$ 0.09	$ 0.08	$ (0.01)				$ (0.29)	[1]	$ 0.08	[1]
Antidilutive securities excluded from computation of earnings per share (shares)						299,412					282,801
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)	271,881	285,433	277,758	295,405	299,412		279,273
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)	12,921,637	16,184,026	17,112,164	17,213,806	17,213,806		17,213,806				15,856,855
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	1,131,760	1,188,987	1,012,916		565,840		377,537				1,009,858
Restricted Class A Common Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	271,881	285,433	277,758		299,412		279,273
Out-of-the-money Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	446,000	443,000	30,500	754,750							626,500

[1] Basic and diluted net income per 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.