Debt (Term Facilities and Revolving Credit Facility) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 15, 2014	Sep. 30, 2018	Sep. 30, 2019	Jul. 26, 2018	Dec. 20, 2017
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 45,000	$ 45,000
Letters of credit outstanding, amount		11,777	12,956
Unamortized discount (premium) and debt issuance costs, net		14,129	12,138
Debt instrument, unused borrowing capacity, amount		33,223	32,044
Long-term debt, gross		953,759	977,155
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity		125,000	125,000		$ 125,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 35,000
Letters of credit outstanding, amount		11,777
Debt instrument, unused borrowing capacity, amount		113,223	$ 112,044
Minimum [Member]
Line of Credit Facility [Line Items]
Debt instrument, interest rate, stated percentage			6.53%
First Lien Term Loan, due December 20, 2024
Line of Credit Facility [Line Items]
Debt instrument, term	7 years
Unamortized discount (premium) and debt issuance costs, net		14,129	$ 12,138
First Lien Term Loan, due December 20, 2024 | Term Loan [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 505,000
Debt instrument, periodic payment, principal		2,369
Line of credit facility, remaining borrowing capacity				$ 150,000
Debt instrument, interest rate, stated percentage			5.11%
First Lien Term Loan, due December 20, 2024 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 75,000
Second Lien Term Facility
Line of Credit Facility [Line Items]
Debt instrument, term	8 years
Second Lien Term Facility | Term Loan [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 75,000
London Interbank Offered Rate (LIBOR) [Member] | First Lien Term Loan, due December 20, 2024 | Term Loan [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate			3.00%
Debt instrument, interest rate, stated percentage			2.11%
Line of Credit [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity					$ 45,000
Letters of credit outstanding, amount		64	$ 204
Revolving Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross		$ 0	$ 0
Other than Ratings Condition Period [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Term Loan [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		3.00%