Disaggregation of Revenue - Schedule of Revenue Disaggregation for Customer Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 29, 2018
Disaggregation Of Revenue [Line Items]
Total revenues	$ 1,107,897	$ 1,267,705	$ 1,344,413
Retail [Member] | Residential [Member]
Disaggregation Of Revenue [Line Items]
Total revenues	654,265	731,468	759,468
Retail [Member] | Commercial [Member]
Disaggregation Of Revenue [Line Items]
Total revenues	283,021	333,078	354,493
Retail [Member] | Industrial [Member]
Disaggregation Of Revenue [Line Items]
Total revenues	92,969	104,992	109,740
Retail [Member] | Agricultural [Member]
Disaggregation Of Revenue [Line Items]
Total revenues	33,718	39,937	44,581
Retail [Member] | Government [Member]
Disaggregation Of Revenue [Line Items]
Total revenues	43,441	53,764	56,598
Wholesale [Member]
Disaggregation Of Revenue [Line Items]
Total revenues	$ 483	$ 4,466	$ 19,533