Debt (Schedule of Line of Credit Facilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021		Sep. 30, 2020
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity	$ 60,000		$ 45,000
Total debt	801,861		885,470
Debt instrument, unused borrowing capacity, amount	48,496		32,037
Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	11,504		12,963
Debt instrument, unused borrowing capacity, amount	243,496		112,037
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity	350,000		125,000
Line of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	0		52
Revolving Credit Facility
Line of Credit Facility [Line Items]
Total debt	$ 0	[1]	$ 0

[1] On April 1, 2021, the Company paid off the outstanding balance of the 2014 Term Loan (as defined below) and entered into the 2021 Credit Agreement (as defined below).