Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Jun. 25, 2018
Class of Stock [Line Items]
Benefit from deferred income taxes		$ (3,207,000)	$ (586,000)	$ (682,000)
Valuation allowance		20,230,000	16,609,000
Investment in partnership		47,897,000	40,880,000
Accrued interest related to uncertain tax positions		7,000	0
Accrued penalties related to uncertain tax positions		0	0
Unrecognized tax benefits		184,000	0
Percent of tax benefits payable to continuing equity owners					85.00%
Tax benefits retained (percent)					15.00%
Purchase of common units in i3 Verticals, LLC from selling unitholder	$ (4,635,000)	(5,080,000)	(12,077,000)	0
Deferred tax asset recognized		$ 64,231,000	50,773,000
Increase in net deferred tax assets			25,776,000
Increase tax receivable agreement liability			22,413,000
Period of payment to continuing equity owners		25 years
i3 Vericals, LLC
Class of Stock [Line Items]
Valuation allowance		$ 16,271,000
Change in valuation allowance for deferred tax assets		2,143,000
Investment in partnership		47,897,000
Minimum
Class of Stock [Line Items]
Expected payments for repurchase of redeemable noncontrolling interest		0
Maximum
Class of Stock [Line Items]
Expected payments for repurchase of redeemable noncontrolling interest		2,460,000
Continuing Equity Owner | Class B Common Stock
Class of Stock [Line Items]
Deferred tax asset recognized	960,000	31,626,000	26,736,000	960,000
Tax benefits due to continuing equity owners	816,000	27,565,000	$ 23,229,000	$ 816,000
Increase in deferred tax assets		6,307,000
Increase tax receivable agreement liability		$ 5,361,000
Non-Controlling Interest
Class of Stock [Line Items]
Purchase of common units in i3 Verticals, LLC from selling unitholder	$ (4,635,000)
Common Stock | Class B Common Stock
Class of Stock [Line Items]
Redemption of common units in i3 Verticals, LLC (shares)		(1,021,016)	(4,292,169)
Common Stock | Class A Common Stock
Class of Stock [Line Items]
Redemption of common units in i3 Verticals, LLC (shares)		1,021,016	4,292,169
Federal
Class of Stock [Line Items]
Net operating loss carryforwards		$ 26,984,000
Tax credit carryforward		901,000
Benefit from deferred income taxes		6,568,000
Valuation allowance		388,000
Change in valuation allowance for deferred tax assets		383,000
State
Class of Stock [Line Items]
Net operating loss carryforwards		81,827,000
Benefit from deferred income taxes		5,302,000
Valuation allowance		3,571,000
Change in valuation allowance for deferred tax assets		$ 1,094,000