Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Significant multiemployer plans
Contributions to Plan	$ 44,824,000	$ 43,887,000	$ 39,763,000
National Electrical Benefit Fund [Member]
Significant multiemployer plans
Contributions to Plan	6,330,000	6,048,000	5,731,000
Minimum future contributions (as a percent)	3.00%
National Electrical Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	20,694,000	21,002,000	17,081,000
Eighth District Electrical Pension Fund [Member]
Significant multiemployer plans
Contributions to Plan	5,197,000	5,672,000	3,733,000
Minimum future contributions (in dollars per hour)	1.71
IBEW Local 1249 Pension Plan [Member]
Significant multiemployer plans
Contributions to Plan	2,103,000	3,147,000	3,679,000
Minimum future contributions (in dollars per hour)	9.60
Eighth District Electrical Pension Fund Annuity Plan [Member]
Significant multiemployer plans
Contributions to Plan	3,553,000	2,427,000	2,560,000
Multiemployer Plan, Individually Insignificant Multiemployer Plans [Member]
Significant multiemployer plans
Contributions to Plan	$ 6,947,000	$ 5,591,000	$ 6,979,000