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

Related Parties - Schedule Of Related Party Transactions (Detail) - Affiliated Entity - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Purchaser of subsidiary
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	$ 0	$ 0
Receivable (Payable)	621	756
Disposed entity
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	0	0
Receivable (Payable)	396	846
Land Development Project
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	0	0
Receivable (Payable)	774	774
Subcontracting Services
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	(11,110)	(19,491)
Receivable (Payable)	(654)	(1,238)
Construction Services
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	824	5,936
Receivable (Payable)	123	2,434
Island Pond
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	(320)	(320)
Receivable (Payable)	0	0
Vehicles - Purchases
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	(869)	(441)
Receivable (Payable)	0	0
Vehicles - Rent expense
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	(677)	(1,050)
Receivable (Payable)	0	0
Consulting Services
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	(271)	(265)
Receivable (Payable)	0	0
SunTx
Related Party Transaction [Line Items]
Revenue Earned (Expense Incurred)	(1,403)	(1,252)
Receivable (Payable)	$ 0	$ 0