Segments - Summary of Certain Financial Data For Each of Segments (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net Service Revenues		$ 2,767.1	$ 2,816.0	$ 2,774.6
Adjusted EBITDA	[1]	324.7	298.7	287.9
Capital Expenditures		78.4	71.3	107.3
Net Income (Loss)		66.4	(7.7)	14.0
Interest expense, net		62.4	97.4	53.3
Income tax expense		30.1	4.6	5.6
Depreciation expense		108.4	105.2	98.9
Amortization expense		35.8	44.5	51.5
Business transformation and integration costs	[2]	$ 44.1	$ 23.7	$ 21.5
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]		Gain (Loss) on Disposition of Business	Gain (Loss) on Disposition of Business	Gain (Loss) on Disposition of Business
Gain on divestiture	[3]	$ (43.6)	$ 0.0	$ 0.1
Equity-based compensation	[4]	20.5	22.3	19.0
COVID-19 related expenses	[5]	0.0	0.4	11.4
Debt extinguishment	[6]	0.6	8.3	12.6
Severance and related costs	[7]	16.6	8.9	1.6
Business integration	[8]	(0.4)	6.2	8.2
IT infrastructure, transformation, and other	[9]	27.9	8.6	11.7
Corporate
Segment Reporting Information [Line Items]
Adjusted EBITDA		(61.3)	(62.0)	(64.6)
Capital Expenditures		4.3	7.0	11.9
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		1,964.0	2,066.5	2,082.0
Adjusted EBITDA		279.7	277.9	278.8
Capital Expenditures		61.3	56.1	82.9
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		808.8	758.0	698.8
Adjusted EBITDA		106.3	82.8	73.7
Capital Expenditures		12.8	8.2	12.5
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		$ (5.7)	$ (8.5)	$ (6.2)

[1] Presented below is a reconciliation of Net (loss) income to Adjusted EBITDA:

[2] Business transformation and integration costs consist of (i) severance and related costs; (ii) business integration costs and (iii) information technology infrastructure, transformation and other costs.

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

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

[5] 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. Additionally, fiscal year 2022 includes refunds related to employee retention credits allowed under the CARES Act.

[6] Represents losses on the extinguishment of debt related to Amendments No. 6, No. 7, and No. 8 to the Credit Agreement, in the fiscal years ended September 30, 2022, 2023, and 2024 respectively, and includes accelerated amortization of deferred financing fees and original issue discount as well as fees paid to lenders and third parties.

[7] Represents severance and related costs incurred in connection with the Company's One BrightView initiative and CEO transition.

[8] Represents isolated expenses specifically related to the integration of acquired companies such as one-time employee retention costs, employee onboarding and training costs, fleet and uniform rebranding costs, and adjustments to performance based contingent consideration. The Company excludes Business integration costs from the non-GAAP measures disclosed above since such expenses vary in amount due to the number of acquisitions and size of acquired companies as well as factors specific to each acquisition, and as a result lack predictability as to occurrence and/or timing, and create a lack of comparability between periods.

[9] Represents expenses related to distinct initiatives, typically significant enterprise-wide changes. Such expenses are excluded from the measures 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.