Significant Accounting Policies - Concentration of Risks (Details) - Revenues - Customer Concentration Risk	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Private
Concentration Risk [Line Items]
Concentration risk percentage	35.10%	35.60%	39.00%	39.20%
Public
Concentration Risk [Line Items]
Concentration risk percentage	64.90%	64.40%	61.00%	60.80%
North Carolina Department of Transportation
Concentration Risk [Line Items]
Concentration risk percentage		12.50%		10.30%
Florida Department of Transportation
Concentration Risk [Line Items]
Concentration risk percentage		13.40%		13.90%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 275
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-Name Accounting Standards Codification
-Section 50
-Paragraph 20
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-20

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
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-Name Accounting Standards Codification
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
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-Name Accounting Standards Codification
-Section 50
-Paragraph 18
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
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-Name Accounting Standards Codification
-Section 50
-Paragraph 18
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482836/275-10-55-18

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 275
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-Name Accounting Standards Codification
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-Paragraph 20
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-URI https://asc.fasb.org/1943274/2147482861/275-10-50-20

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478785/954-310-50-2

+ Details

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References + Details

Construction Partners, Inc. 10-Q