Business - Additional Information (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Jul. 02, 2018 USD ($) $ / shares shares	Jun. 08, 2018	Sep. 30, 2020 USD ($) segment $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)
Business [Line Items]
Number of reportable segments | segment			2
Repayments of debt			$ 10.4	$ 13.0	$ 1,493.9
Extinguishment of debt, amount	$ 501.1
Common stock, authorized | shares			500,000,000	500,000,000
Preferred Stock, Shares Authorized | shares			50,000,000	50,000,000
Preferred stock, par value | $ / shares			$ 0.01	$ 0.01
Reverse stock split ratio		0.4276
Reverse stock split description			2.33839-for-one reverse stock split of the Company’s common stock.
Reverse stock split effective date		Jun. 08, 2018
IPO
Business [Line Items]
Common stock, shares issued | shares	24,495,000
Initial offering price | $ / shares	$ 22.00
Net proceeds from the issuance of common stock	$ 501.2
Underwriting discounts and estimated offering expenses	$ 37.7
Payments of stock issuance costs					5.4
IPO | Monitoring Agreement | Affiliate of KKR
Business [Line Items]
Ownership interest in percentage	55.90%
IPO | Third Amended and Restated Certificate of Incorporation
Business [Line Items]
Common stock, authorized | shares	500,000,000
Preferred Stock, Shares Authorized | shares	50,000,000
Preferred stock, par value | $ / shares	$ 0.01
IPO | Bright View Holdings | Monitoring Agreement
Business [Line Items]
Ownership interest in percentage	13.00%
IPO | Selling, General and Administrative Expense
Business [Line Items]
Payments of stock issuance costs					$ 6.8
IPO | Second Lien Credit Facility Term Loan Due 2021
Business [Line Items]
Repayments of debt	$ 110.0
IPO | Revolving Credit Facility
Business [Line Items]
Repayments of debt	55.0
IPO | First Lien Credit Facility Term Loans
Business [Line Items]
Repayments of debt	$ 336.1