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Employee Benefit Plans - Summary of multi-employer benefit plans (Details) - USD ($) $ in Thousands | 12 Months Ended
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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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