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Employee Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant multiemployer plans
Contributions to Plan	$ 64,888	$ 53,796	$ 52,686
National Electrical Benefit Fund [Member]
Significant multiemployer plans
Contributions to Plan	9,542	9,040	6,930
Eighth District Electrical Management Pension Fund [Member]
Significant multiemployer plans
Contributions to Plan	7,908	7,519	5,598
Indiana,Kentucky,Ohio Regional Council of Carpenters Pension Fund [Member]
Significant multiemployer plans
Contributions to Plan	2,515	696	116
IBEW Local 1249 Pension Plan [Member]
Significant multiemployer plans
Contributions to Plan	1,315	630	2,042
National Electrical Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	27,633	22,840	24,226
IBEW Local 769 Management Pension Plan [Member]
Significant multiemployer plans
Contributions to Plan	2,115	1,709	2,120
Eighth District Electrical Pension Fund Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	4,109	4,883	3,700
All other plans [Member]
Significant multiemployer plans
Contributions to Plan	$ 9,751	$ 6,479	$ 7,954

X

- Definition

Amount of contributions made to multiemployer plan by employer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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 2: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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- References

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