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Pension and Other Benefit Programs - Key Assumptions (Detail)
12 Months Ended
Sep. 28, 2019
Sep. 29, 2018
Sep. 30, 2017
Pension Plans      
Schedule of Net Periodic Benefit Costs Weighted Average Assumptions [Line Items]      
Discount rate used to determine the benefit obligation 3.22% 4.31% 3.88%
Discount rate used to determine the interest cost component of net periodic benefit cost 4.09% 3.46% 3.18%
Rate of return on plan assets 7.25% 7.50% 7.50%
Weighted average rate of compensation increase to determine the benefit obligation 3.20% 3.20% 2.90%
Postretirement Medical Plans      
Schedule of Net Periodic Benefit Costs Weighted Average Assumptions [Line Items]      
Discount rate used to determine the benefit obligation 3.22% 4.31% 3.88%
Discount rate used to determine the interest cost component of net periodic benefit cost 4.10% 3.49% 3.18%
Rate of return on plan assets 7.25% 7.50% 7.50%
Year 1 increase in cost of benefits 7.00% 7.00% 7.00%
Rate of increase to which the cost of benefits is assumed to decline (the ultimate trend rate) 4.25% 4.25% 4.25%
Year that the rate reaches the ultimate trend rate 2033 2032 2031