Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 29, 2018
Components of net periodic benefit costs included in operating expenses [Abstract]
Settlement charge	$ 1,051	$ 0	$ 0
Pension Benefits [Member]
Components of net periodic benefit costs included in operating expenses [Abstract]
Interest cost	2,726	3,943	3,778
Expected return on plan assets	(1,270)	(1,723)	(1,894)
Amortization of prior service credit	0	0	0
Settlement charge	1,051	0	0
Recognized net actuarial loss (gain)	3,448	3,466	3,683
Net periodic benefit costs	5,955	5,686	5,567
Retiree Health and Life Benefits [Member]
Components of net periodic benefit costs included in operating expenses [Abstract]
Interest cost	151	275	277
Expected return on plan assets	0	0	0
Amortization of prior service credit	(498)	(498)	(498)
Settlement charge	0	0	0
Recognized net actuarial loss (gain)	(786)	(761)	(654)
Net periodic benefit costs	$ (1,133)	$ (984)	$ (875)