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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 27, 2020 | Mar. 29, 2019 | Dec. 28, 2018 | Mar. 27, 2020 | Mar. 29, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Settlement gain (loss) | $ 0 | $ 32,449 |  | $ (2,651) | $ 34,621
CH2M Hill and OMI Retiree Medical Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Settlement gain (loss) |  | $ 32,400 | $ 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 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(7) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 + Details
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