Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 26, 2015
Components of net periodic benefit costs included in operating expenses [Abstract]
Settlement charge	$ (6,100)	$ (2,000)	$ (2,000)
Pension Benefits [Member]
Components of net periodic benefit costs included in operating expenses [Abstract]
Interest cost	4,164	5,041	5,128
Expected return on plan assets	(2,150)	(3,418)	(4,913)
Amortization of prior service credit	0	0	0
Settlement charge	6,100	2,000	2,000
Recognized net actuarial loss (gain)	5,201	5,218	4,522
Net periodic benefit costs	13,315	8,841	6,737
Retiree Health and Life Benefits [Member]
Components of net periodic benefit costs included in operating expenses [Abstract]
Interest cost	385	520	575
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	(399)	(490)
Settlement charge	0	0	0
Recognized net actuarial loss (gain)	(389)	(299)	(196)
Net periodic benefit costs	$ (4)	$ (178)	$ (111)