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		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net Service Revenues		$ 2,672.8	$ 2,767.1	$ 2,816.0
Segment Adjusted EBITDA		352.3	324.7	298.7
Interest expense, net		53.7	62.4	97.4
Depreciation expense		142.2	108.4	105.2
Amortization expense		29.3	35.8	44.5
Business transformation and integration costs	[1]	26.1	44.1	23.7
Equity-based compensation	[2]	19.0	$ 20.5	22.3
COVID-19 related expenses	[3]			0.4
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]			Gain (Loss) on Disposition of Business
Gain on divestiture	[4]		$ (43.6)
Debt extinguishment	[5]	0.7	0.6	8.3
Income (loss) before income taxes		81.3	96.5	(3.1)
Capital Expenditures		254.2	78.4	71.3
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		1,891.3	1,964.0	2,066.5
Direct costs	[6]	1,086.5	1,114.5	1,154.7
Other segment items	[7]	559.3	613.3	678.7
Segment Adjusted EBITDA		245.5	236.2	233.1
Capital Expenditures		205.0	64.4	61.2
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		789.1	808.8	758.0
Direct costs	[6]	581.0	613.8	586.7
Other segment items	[7]	101.3	106.5	105.7
Segment Adjusted EBITDA		106.8	88.5	65.6
Capital Expenditures		49.2	14.0	10.1
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		(7.6)	(5.7)	(8.5)
Direct costs	[6]	(7.4)	(2.7)	(8.8)
Other segment items	[7]	(0.2)	(3.0)	0.3
Segment Adjusted EBITDA		$ 0.0	$ 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] Represents expenses related to the Company’s response to the COVID-19 pandemic principally temporary and incremental salary and related expenses, personal protective equipment, cleaning and supply purchases, and other.

[4] Represents the realized gain on sale and transaction related expenses from the divestiture of U.S. Lawns on January 12, 2024.

[5] Represents losses on the extinguishment of debt related to Amendments No. 9, No. 8, and No. 7 to the Credit Agreement, in the fiscal years ended September 30, 2025, 2024, and 2023, respectively, and includes accelerated amortization of deferred financing fees and original issue discount as well as fees paid to lenders and third parties.

[6] The Company’s significant segment expense, direct costs, aligns with the segment level information that is regularly provided to our chief operating decision maker. 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.

[7] 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.