Segments - Summary of Certain Financial Data For Each of Segments (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Net service revenues		$ 673.6	$ 608.1	$ 1,879.9	$ 1,737.9
Adjusted EBITDA	[1]	93.6	91.0	212.8	181.6
Capital expenditures		16.8	10.8	44.7	45.9
Net income (loss)		25.2	(2.4)	19.5	(35.5)
Interest expense		9.4	15.4	32.5	49.9
Income tax expense (benefit)		9.5	(0.3)	8.1	(11.9)
Depreciation expense related to property and equipment		21.3	20.9	63.6	60.4
Amortization expense		12.6	13.6	39.0	40.6
Establish public company financial reporting compliance	[2]				0.9
Business transformation and integration costs	[3]	7.2	8.2	20.0	25.8
Offering-related expenses	[4]	0.3	2.6	0.5	4.1
Equity-based compensation	[5]	5.3	4.9	15.5	18.2
COVID-19 related expenses	[6]	2.8	4.0	14.1	5.0
Changes in self-insured liability estimates	[7]		24.1		24.1
Severance and related costs		0.1	2.7	0.3	3.3
Business integration	[8]	3.4	1.8	9.2	10.6
IT infrastructure, transformation, and other	[9]	3.7	3.7	10.5	11.9
Corporate
Segment Reporting Information [Line Items]
Adjusted EBITDA	[1]	(15.7)	(14.1)	(46.3)	(45.2)
Capital expenditures		0.8	1.2	2.2	2.4
Operating Segments | Maintenance Services
Segment Reporting Information [Line Items]
Net service revenues		524.6	456.3	1,478.4	1,288.7
Adjusted EBITDA	[1]	90.6	83.3	212.5	171.7
Capital expenditures		13.3	8.0	37.5	34.0
Operating Segments | Development Services
Segment Reporting Information [Line Items]
Net service revenues		150.3	152.8	404.7	451.9
Adjusted EBITDA	[1]	18.7	21.8	46.6	55.1
Capital expenditures		2.7	1.6	5.0	9.5
Eliminations
Segment Reporting Information [Line Items]
Net service revenues		$ (1.3)	$ (1.0)	$ (3.2)	$ (2.7)

[1] Presented below is a reconciliation of Net income (loss) 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 isolated expenses specifically related to the integration of acquired companies such as one-time employee retention costs, employee onboarding and training costs, and fleet and uniform rebranding costs. The Company excludes Business integration costs from the 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.