Significant Accounting Policies - Concentration of Risks (Detail) - Revenues - Customer Concentration Risk	3 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Department of Transportation
Concentration Risk [Line Items]
Concentration risk percentage	3100.00%	3730.00%
Alabama Department of Transportation
Concentration Risk [Line Items]
Concentration risk percentage	10.80%	9.60%
North Carolina Department of Transportation
Concentration Risk [Line Items]
Concentration risk percentage	8.50%	14.20%

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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