Segments - Summary of Certain Financial Data For Each of Segments and Reconciliation of Segment Adjusted EBITDA to Income (Loss) Before Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Net Service Revenues		$ 614.7	$ 599.2
Segment Adjusted EBITDA		53.5	52.1
Interest expense, net		13.5	14.2
Depreciation expense		42.9	30.4
Amortization expense		6.2	8.1
Business transformation and integration costs	[1]	7.0	9.2
Equity-based compensation	[2]	4.3	4.8
(Loss) before income taxes		(20.4)	(14.6)
Capital Expenditures		54.7	58.7
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		436.4	409.3
Direct costs	[3]	252.9	230.1
Other segment items	[4]	148.1	144.6
Segment Adjusted EBITDA		35.4	34.6
Capital Expenditures		48.7	41.7
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		179.2	191.8
Direct costs	[3]	132.9	146.0
Other segment items	[4]	28.2	28.3
Segment Adjusted EBITDA		18.1	17.5
Capital Expenditures		6.0	17.0
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		(0.9)	(1.9)
Direct costs	[3]	(0.9)	(1.8)
Other segment items	[4]	0.0	(0.1)
Segment Adjusted EBITDA		$ 0.0	$ 0.0

[1] Business transformation and integration costs consist of severance and related costs, information technology, infrastructure, transformation, and other costs. These costs represent expenses related to distinct initiatives, typically significant enterprise-wide changes, including actions taken as part of the Company's One BrightView initiative. Such expenses are excluded from Segment Adjusted EBITDA disclosed above since such expenses vary in amount based on occurrence as well as factors specific to each of the activities, are outside of the normal operations of the business, and create a lack of comparability between periods.

[2] Represents equity-based compensation expense and related taxes recognized for equity incentive plans outstanding

[3] The Company's significant segment expense, direct costs, aligns with the segment level information that is regularly provided to our CODM. Direct costs include direct labor, materials, and other costs that are directly incurred as a result of the delivery of services. Direct costs do not include costs of sales that are allocated to services including fuel and depreciation. Intersegment expenses are included within the amounts shown.

[4] Other segment items for both segments primarily include indirect compensation costs, auto and equipment costs, selling, general, and administrative costs and allocation of corporate expenses.