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Retirement Benefits - Participation in Multiemployer Defined Benefit Pension Plan (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | $ 1,252 | $ 829 | $ 1,300
Red Zone |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Multiemployer plan, collective-bargaining arrangement, percentage of contributions required for multiple collective-bargaining arrangements | 65.00% |  |
Orange Zone |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Multiemployer plan, collective-bargaining arrangement, percentage of contributions required for multiple collective-bargaining arrangements | 80.00% |  |
Yellow Zone |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Multiemployer plan, collective-bargaining arrangement, percentage of contributions required for multiple collective-bargaining arrangements | 80.00% |  |
Green Zone |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Multiemployer plan, collective-bargaining arrangement, percentage of contributions required for multiple collective-bargaining arrangements | 80.00% |  |
Multiemployer Plans, Pension |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | $ 11,977 | 11,182 | 11,347
Multiemployer Plans, Pension | Pension Trust Fund for Operating Engineers Pension Plan |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | 2,278 | 2,314 | 1,932
Multiemployer Plans, Pension | Carpenter Funds Administrative Office |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | 915 | 547 | 748
Multiemployer Plans, Pension | Laborers Pension Trust for Northern California |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | 787 | 857 | 880
Multiemployer Plans, Pension | Cement Mason Pension Trust Fund For Northern California |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | 426 | 320 | 504
Multiemployer Plans, Pension | All other funds |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Contributions | $ 7,571 | $ 7,144 | $ 7,283

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- Definition Amount of cost for employer contribution to multiemployer plan. Multiemployer plan includes, but is not limited to, pension plan determined to be individually significant and insignificant and other postretirement benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=35742348&loc=SL14450788-114948 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=29639808&loc=d3e29008-114946 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450702-114947 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450691-114947 + Details
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- Definition Percentage of employer contribution required by collective-bargaining arrangement (CBA) to total contribution from all participating employers in multiemployer pension plan determined to be individually significant. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947 + Details
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