Debt (Schedule of Line of Credit Facilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity	$ 45,000	$ 45,000
Total debt	887,552	885,470
Debt instrument, unused borrowing capacity, amount	33,176	32,037
Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	11,824	12,963
Debt instrument, unused borrowing capacity, amount	113,176	112,037
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity	125,000	125,000
Line of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	33	52
Revolving Credit Facility
Line of Credit Facility [Line Items]
Total debt	$ 0	$ 0