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Retirement Benefits - Participation in Multiemployer Defined Benefit Pension Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	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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No definition available.

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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

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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

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