Earnings Per Share - Basic (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	Sep. 30, 2018	Jun. 25, 2018	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2018
Numerator
Net (loss) income									$ 2,673		$ (979)		$ 563		$ (4,961)
Less: Net (loss) income attributable to non-controlling interests											(560)		3,608		1,937
Net loss attributable to i3 Verticals, Inc.	$ (651)	$ (356)	$ 737	$ (149)	$ (930)	$ (1,191)	$ (1,102)	$ 178		$ (7,634)	$ (419)		$ (3,045)		$ (6,898)
Denominator
Basic net (loss) income per share (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]
Class A Common Stock
Numerator
Net (loss) income											$ (979)		$ 563		$ 2,673
Less: Net (loss) income attributable to non-controlling interests											(560)		3,608		1,937
Net loss attributable to i3 Verticals, Inc.											$ (419)		$ (3,045)		$ 736
Denominator
Weighted average shares of Class A common stock outstanding (shares)											14,833,378		10,490,981		8,812,630

[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.