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Employee Benefit Plans - Summary of multi-employer benefit plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant multiemployer plans
Contributions to Plan	$ 70,420	$ 64,888	$ 53,796
Multiemployer Plans, Pension [Member] | National Electrical Benefit Fund [Member]
Significant multiemployer plans
Contributions to Plan	9,840	9,542	9,040
Multiemployer Plans, Pension [Member] | Eighth District Electrical Pension Fund [Member]
Significant multiemployer plans
Contributions to Plan	9,707	7,908	7,519
Multiemployer Plans, Pension [Member] | Indiana,Kentucky,Ohio Regional Council of Carpenters Pension Fund [Member]
Significant multiemployer plans
Contributions to Plan	1,157	2,515	696
Multiemployer Plans, Pension [Member] | IBEW Local 769 Management Pension Plan A [Member]
Significant multiemployer plans
Contributions to Plan	2,587	2,115	1,709
Multiemployer Plans, Pension [Member] | Alaska Electrical Pension Plan [Member]
Significant multiemployer plans
Contributions to Plan	2,723	1,951	1,873
Multiemployer Plans, Postretirement Benefit [Member] | National Electrical Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	26,559	27,633	22,840
Multiemployer Plans, Postretirement Benefit [Member] | Eighth District Electrical Pension Fund Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	4,785	4,109	4,883
Multiemployer Plans Other Pension Post Retirement And Supplemental Plans [Member] | All other plans [Member]
Significant multiemployer plans
Contributions to Plan	$ 13,062	$ 9,115	$ 5,236

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- Definition

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
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-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

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