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Retirement Benefits - Participation in Multiemployer Defined Benefit Pension Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan Disclosure [Line Items]
Contributions	$ 829	$ 1,300	$ 1,500
Multiemployer Plans, Pension
Defined Contribution Plan Disclosure [Line Items]
Contributions	11,182	11,347	12,586
Multiemployer Plans, Pension | Pension Trust Fund for Operating Engineers Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions	2,314	1,932	2,477
Multiemployer Plans, Pension | Laborers Pension Trust for Northern California
Defined Contribution Plan Disclosure [Line Items]
Contributions	857	880	953
Multiemployer Plans, Pension | Carpenter Funds Administrative Office
Defined Contribution Plan Disclosure [Line Items]
Contributions	547	748	727
Multiemployer Plans, Pension | Cement Mason Pension Trust Fund For Northern California
Defined Contribution Plan Disclosure [Line Items]
Contributions	320	504	423
Multiemployer Plans, Pension | All other funds
Defined Contribution Plan Disclosure [Line Items]
Contributions	$ 7,144	$ 7,283	$ 8,006

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Amount of contributions made to multiemployer plan by employer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450691-114947

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

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