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Long-Term Debt - Additional Information (Details) - USD ($)				1 Months Ended				9 Months Ended
	Jul. 02, 2018	Apr. 28, 2017	Dec. 18, 2013	Jun. 30, 2018	May 31, 2018	Dec. 31, 2017	May 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Debt Instrument [Line Items]
Repayments of debt								$ 64,820,000	$ 164,222,000
Proceeds from lines of credit								55,000,000
Level 2 Inputs
Debt Instrument [Line Items]
Long-term debt, fair value				$ 1,690,159,000				1,690,159,000		$ 1,633,802,000
Receivables Financing Agreement \| Wholly-owned Subsidiary
Debt Instrument [Line Items]
Credit agreement, maximum borrowing capacity		$ 175,000,000
Debt repayments								53,750,000
Debt instrument, maturity date		Apr. 27, 2020
Proceeds from lines of credit							$ 150,000,000	70,000,000
First Lien Credit Agreement
Debt Instrument [Line Items]
Debt instrument, original discount			$ 3,675,000
Debt instrument, effective interest rate			4.00%
First Lien Credit Agreement \| First Lien Revolving Credit Facility One
Debt Instrument [Line Items]
Credit agreement, maximum borrowing capacity						$ 7,500,000
Debt instrument, maturity date						Dec. 18, 2018
First Lien Credit Agreement \| First Lien Credit Facility Term Loans
Debt Instrument [Line Items]
Credit agreement date			Dec. 18, 2013
Debt instrument, term			7 years
Credit agreement, maximum borrowing capacity			$ 1,460,000,000
Debt repayments								10,950,000
First Lien Credit Agreement \| First Lien Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, term			5 years
Credit agreement, maximum borrowing capacity			$ 210,000,000	235,357,000		$ 200,357,000		$ 235,357,000
Credit agreement, increased capacity				$ 35,000,000
Proceeds from lines of credit					$ 55,000,000
First Lien Credit Agreement \| First Lien Revolving Credit Facility Two
Debt Instrument [Line Items]
Credit agreement, maximum borrowing capacity						$ 192,857,000
Debt instrument, maturity date						Sep. 18, 2020
Second Lien Credit Agreement \| Second Lien Credit Facility Term Loan Due 2021
Debt Instrument [Line Items]
Debt instrument, term			8 years
Credit agreement, maximum borrowing capacity			$ 235,000,000
Debt instrument, original discount			$ 1,175,000
Debt instrument, effective interest rate			7.50%
Subsequent Event
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 501,100,000
IPO \| Subsequent Event \| Second Lien Credit Facility Term Loan Due 2021
Debt Instrument [Line Items]
Repayments of debt	110,000,000
IPO \| Subsequent Event \| Revolving Credit Facility
Debt Instrument [Line Items]
Repayments of debt	55,000,000
IPO \| Subsequent Event \| First Lien Credit Facility Term Loans
Debt Instrument [Line Items]
Repayments of debt	$ 336,100,000