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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 26, 2020 | Dec. 27, 2019 | Jun. 28, 2019 | Mar. 29, 2019 | Dec. 28, 2018 | Jun. 26, 2020 | Jun. 28, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Settlement gain (loss) | $ 0 |  | $ 0 |  |  | $ (2,651) | $ 34,621
Corporate | CH2M HILL Companies, Ltd. | Other Expense | United States |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Gain (loss) related to settlement |  | $ 2,700 | $ 0 |  |  | $ (2,100) | $ 34,600
CH2M Hill and OMI Retiree Medical Plans |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Settlement gain (loss) |  |  |  | $ 2,200 | $ 32,400 |  |

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