Supporting Financial Information - Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 164,542	$ 164,542
Total accumulated amortization	(43,879)	(39,765)
Total amortization expense	120,663	124,777
Customer contracts and related customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	113,622	113,622
Total accumulated amortization	(32,771)	(29,929)
Covenants not to compete
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	637	637
Total accumulated amortization	(394)	(378)
Trade name
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	13,034	13,034
Total accumulated amortization	(2,510)	(2,185)
Backlog
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	37,249	37,249
Total accumulated amortization	$ (8,204)	$ (7,273)