Financial Instruments and Risk Management - Reconciliation of the Beginning and Ending Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2018	Mar. 25, 2017
Reconciliation of beginning and ending balances of assets measured at fair value on recurring basis using significant unobservable inputs [Rollforward]
Beginning balance of over-the-counter options	$ 4,108	$ 809
Beginning balance realized during the period	(2,466)	(369)
Contracts purchased during the period	800	0
Change in the fair value of outstanding contracts	(1,008)	(143)
Ending balance of over-the-counter options	1,434	297
Reconciliation of beginning and ending balances of liabilities measured at fair value on recurring basis using significant unobservable inputs [Rollforward]
Beginning balance of over-the-counter options	737	0
Beginning balance realized during the period	(532)	0
Contracts purchased during the period	0	0
Change in the fair value of outstanding contracts	(205)	0
Ending balance of over-the-counter options	$ 0	$ 0