Segments - Summary of Certain Financial Data For Each of Segments (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Net Service Revenues		$ 708.3	$ 738.8	$ 1,970.0	$ 2,038.4
Capital Expenditures		103.5	23.2	195.8	46.0
Segment Adjusted EBITDA	[1]	113.2	107.9	238.8	219.5
Interest expense, net		13.4	15.1	40.3	48.2
Depreciation expense		38.9	28.1	101.2	79.8
Amortization expense		7.1	8.6	22.3	27.4
Business transformation and integration costs	[2]	4.2	17.1	21.2	33.9
Gain on divestiture	[3]		(0.1)		(44.0)
Equity-based compensation	[4]	4.7	5.1	14.1	15.3
Debt extinguishment	[5]		0.6	0.7	0.6
Income before income taxes		44.9	33.4	39.0	58.3
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		508.8	524.7	1,410.9	1,477.5
Capital Expenditures		98.3	16.8	171.3	34.7
Segment Adjusted EBITDA		81.7	80.4	172.7	166.9
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		201.3	215.0	565.0	564.8
Capital Expenditures		5.2	6.4	24.5	11.3
Segment Adjusted EBITDA		31.5	27.5	66.1	52.6
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		$ (1.8)	$ (0.9)	$ (5.9)	$ (3.9)

[1] Presented below is a reconciliation of (Loss) before income taxes to Segment Adjusted EBITDA:

[2] 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.

[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 losses on the extinguishment of debt related to Amendment No. 9 to the Credit Agreement, in the fiscal year ending September 30, 2025 and includes accelerated amortization of deferred financing fees and original issue discount as well as fees paid to lenders and third parties.