Construction Partners, Inc. Annual report pursuant to Section 13 and 15(d)

Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Employee Benefit Plans (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan Disclosure [Line Items]
Age of retirement plan eligible participants	18 years	18 years
Minimum eligibility period of employment for participation in retirement plan	6 months	6 months
Defined contribution costs		$ 6.8	$ 5.5	$ 3.9
Employee Stock Purchase Plan
Defined Benefit Plan Disclosure [Line Items]
ESPP compensation expense		$ 0.1	$ 0.0	$ 0.0
Employee Stock Purchase Plan \| Employee Stock
Defined Benefit Plan Disclosure [Line Items]
ESPP purchase price of common stock, percent of market price	85.00%