Segments - Summary of Certain Financial Data For Each of Segments (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net Service Revenues		$ 2,346.0	$ 2,404.6	$ 2,353.6
Adjusted EBITDA	[1]	271.6	305.1	300.1
Capital Expenditures		52.7	89.9	86.4
Net (loss) income		(41.6)	44.4	(15.1)
Interest expense		64.6	72.5	97.8
Income tax (benefit) provision		(9.6)	12.8	(66.2)
Depreciation expense		80.5	80.1	75.3
Amortization expense		55.8	56.3	104.9
Establish public company financial reporting compliance	[2]	0.9	4.8	4.1
Business transformation and integration costs	[3]	32.5	17.5	25.5
Offering-related expenses	[4]	4.4	1.0	6.8
Debt extinguishment	[5]			25.1
Equity-based compensation	[6]	24.0	15.7	28.8
Management fees	[7]			13.1
COVID-19 related expenses	[8]	13.8
Changes in self-insured liability estimates	[9]	24.1
Sale of tree company	[10]	22.2
Corporate
Segment Reporting Information [Line Items]
Adjusted EBITDA	[1]	(58.7)	(58.6)	(68.4)
Capital Expenditures		2.7	13.9	36.0
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		1,739.1	1,813.4	1,774.8
Adjusted EBITDA	[1]	250.1	282.0	289.8
Capital Expenditures		40.6	65.4	45.5
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		610.6	595.4	583.3
Adjusted EBITDA	[1]	80.2	81.7	78.7
Capital Expenditures		9.4	10.6	4.9
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		$ (3.7)	$ (4.2)	$ (4.5)

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

[2] Represents costs incurred to establish public company financial reporting compliance, including costs to comply with the requirements of Sarbanes-Oxley and the accelerated adoption of the revenue recognition standard (ASC 606 – Revenue from Contracts with Customers) and other miscellaneous costs.

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

[4] Represents transaction related expenses incurred in connection with the IPO, subsequent registration statements, and IPO related litigation.

[5] Represents losses on the extinguishment of debt.

[6] Represents equity-based compensation expense and related taxes recognized for equity incentive plans outstanding. Fiscal year ended September 30, 2020 includes $23.6 million of equity-based compensation expense and $0.4 million of related taxes. Fiscal year ended September 30, 2018 includes $19.6 million of equity-based compensation expense related to the IPO.

[7] Represents fees paid pursuant to a monitoring agreement terminated on July 2, 2018 in connection with the completion of the IPO.

[8] Represents expenses related to the Company’s response to the COVID-19 pandemic, principally temporary and incremental salary and related expenses, personal protective equipment and cleaning and supply purchases, and other.

[9] Represents expenses related to changes in estimates and actuarial assumptions associated with the Company’s self-insured liability amounts for workers’ compensation, general liability, auto liability, and employee health care insurance programs, to reflect uncertainties associated with the current environment, including the COVID-19 pandemic.

[10] Represents the goodwill impairment charge, realized loss on sale, and transaction related expenses related to the sale of BrightView Tree Company on September 30, 2020.