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 Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting Information [Line Items]
Net Service Revenues		$ 717,600	$ 708,300	$ 2,035,300	$ 1,970,000
Direct costs	[1]	440,300	435,600	1,275,800	1,230,100
Other segment items	[2]	181,200	159,500	530,900	501,100
Segment Adjusted EBITDA		96,100	113,200	228,600	238,800
Interest expense, net		14,900	13,400	42,100	40,300
Depreciation expense		46,700	38,900	137,030	101,200
Amortization expense		5,100	7,100	16,700	22,300
Business transformation and integration costs	[3]	6,300	4,200	19,000	21,200
Equity-based compensation	[4]	4,900	4,700	13,700	14,100
Debt extinguishment	[5]	9,800	0	9,800	700
Income (loss) before income taxes		8,400	44,900	(9,700)	39,000
Capital Expenditures		65,200	103,500	178,800	195,800
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		517,900	508,800	1,508,900	1,410,900
Direct costs	[1]	301,100	291,600	893,500	815,100
Other segment items	[2]	153,800	135,500	447,400	423,100
Segment Adjusted EBITDA		63,000	81,700	168,000	172,700
Capital Expenditures		55,400	98,300	157,400	171,300
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		201,900	201,300	530,700	565,000
Direct costs	[1]	140,700	145,900	385,900	420,900
Other segment items	[2]	28,100	23,900	84,200	78,000
Segment Adjusted EBITDA		33,100	31,500	60,600	66,100
Capital Expenditures		9,800	5,200	21,400	24,500
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		(2,200)	(1,800)	(4,300)	(5,900)
Direct costs	[1]	(1,500)	(1,900)	(3,600)	(5,900)
Other segment items	[2]	(700)	(100)	(700)	(0)
Segment Adjusted EBITDA		$ 0	$ 0	$ 0	$ 0

[1] 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.

[2] 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.

[3] 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, and other non-operating expenses. 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.

[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 the Tenth Credit Agreement Amendment, the Eleventh Credit Agreement Amendment, and the Sixth Amendment to the Receivables Financing Agreement, in the nine months ended June 30, 2026, and the Ninth Credit Agreement Amendment, in the nine months ended June 30, 2025 and includes accelerated amortization of deferred financing fees and original issue discount as well as fees paid to lenders and third parties.