Segments - Summary of Certain Financial Data For Each of Segments (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2021	Sep. 30, 2020		Sep. 30, 2019
Segment Reporting Information [Line Items]
Net Service Revenues		$ 2,553.6	$ 2,346.0		$ 2,404.6
Adjusted EBITDA	[1]	302.3	271.6		305.1
Capital Expenditures		61.2	52.7		89.9
Net income (loss)		46.3	(41.6)		44.4
Interest expense		42.3	64.6		72.5
Income tax (benefit) provision		4.6	(9.6)		12.8
Depreciation expense		84.7	80.5		80.1
Amortization expense		52.3	55.8		56.3
Establish public company financial reporting compliance	[2]		0.9		4.8
Business transformation and integration costs	[3]	28.5	32.5		17.5
Offering-related expenses	[4]	0.6	4.4		1.0
Equity-based compensation	[5]	20.0	24.0		15.7
COVID-19 related expenses	[6]	23.0	13.8		0.0
Changes in self-insured liability estimates		24.1	24.1	[7]	0.0	[7]
Sale of tree company	[8]		22.2		0.0
Corporate
Segment Reporting Information [Line Items]
Adjusted EBITDA		(62.5)	(58.7)		(58.6)
Capital Expenditures		2.6	2.7		13.9
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net Service Revenues		1,982.9	1,729.4		1,813.4
Adjusted EBITDA		299.6	248.7		282.0
Capital Expenditures		52.4	40.6		65.4
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net Service Revenues		574.9	620.3		595.4
Adjusted EBITDA		65.2	81.6		81.7
Capital Expenditures		6.2	9.4		10.6
Eliminations
Segment Reporting Information [Line Items]
Net Service Revenues		$ (4.2)	$ (3.7)		$ (4.2)

[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) vehicle fleet rebranding costs; (iii) business integration costs and (iv) information technology infrastructure, transformation costs, and other.

[4] Represents transaction related expenses incurred for IPO related litigation and completed or contemplated subsequent registration statements.

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

[6] 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.

[7] 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.

[8] 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.