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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 27, 2019	Dec. 28, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,465	$ 2,489
Plan Amendment and settlement gain	(2,651)	(1,363)
CH2M Hill and OMI Retiree Medical Plans
Defined Benefit Plan Disclosure [Line Items]
Plan Amendment and settlement gain	$ 2,700	$ 2,200

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- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(7)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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-Publisher FASB
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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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-Publisher FASB
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