Employee Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant multiemployer plans
Contributions to Plan	$ 53,796	$ 52,686	$ 44,824
National Electrical Benefit Fund [Member]
Significant multiemployer plans
Contributions to Plan	9,040	6,930	6,330
Eighth District Electrical Management Pension Fund [Member]
Significant multiemployer plans
Contributions to Plan	7,519	5,598	5,197
IBEW Local 1249 Pension Plan [Member]
Significant multiemployer plans
Contributions to Plan	630	2,042	2,103
National Electrical Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	22,840	24,226	20,694
Eighth District Electrical Pension Fund Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	4,883	3,700	3,553
IBEW Local 769 Management Pension Plan [Member]
Significant multiemployer plans
Contributions to Plan	1,709	2,120	1,833
All other plans [Member]
Significant multiemployer plans
Contributions to Plan	$ 7,175	$ 8,070	$ 5,114