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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended
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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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