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Significant Accounting Policies (Details) $ in Thousands |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Apr. 23, 2018 | Jun. 30, 2019 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) | Jun. 30, 2018 USD ($) | Sep. 30, 2018
Concentration Risk [Line Items] |  |  |  |  |  |
Revenues |  | $ 227,290 | $ 195,075 | $ 545,921 | $ 464,395 |
Class B Common Stock |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |
Stock split, conversion ratio | 25.2 |  |  |  |  | 25.2
Revenues | Customer Concentration Risk | Private |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |
Revenues |  | 66,207 | 56,293 | 166,193 | 148,525 |
Revenues | Customer Concentration Risk | Public |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |
Revenues |  | $ 161,083 | $ 138,782 | $ 379,728 | $ 315,870 |
Revenues | Department of Transportation | Customer Concentration Risk |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |
Concentration risk percentage |  | 41.80% | 45.60% | 39.40% | 40.90% |
Revenues | Alabama Department of Transportation | Customer Concentration Risk |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |
Concentration risk percentage |  | 15.40% | 17.00% | 12.90% | 14.80% |
Revenues | North Carolina Department of Transportation | Customer Concentration Risk |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |
Concentration risk percentage |  | 13.90% | 14.80% | 13.40% | 13.50% |

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