Summary of Significant Accounting Policies (Details 4)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounts receivable | Concentrations of credit risk | One customer
Concentrations
Number of customers individually exceeding 10.0% of consolidated accounts receivable		1
Percentage of concentration risk		12.20%
Accounts receivable | Concentrations of credit risk | Two customers
Concentrations
Number of customers individually exceeding 10.0% of consolidated accounts receivable	2
Percentage of concentration risk	24.60%
Revenues | Customer concentration risk | One customer
Concentrations
Number of customers individually exceeding 10.0% of consolidated contract revenue	1
Percentage of concentration risk	15.10%
Revenues | Customer concentration risk | Two customers
Concentrations
Number of customers individually exceeding 10.0% of consolidated contract revenue			2
Revenues | Customer concentration risk | Ten customers
Concentrations
Percentage of concentration risk	59.60%	58.10%	61.60%
Revenues | Customer concentration risk | Dominion Resources, Inc.
Concentrations
Percentage of concentration risk			19.30%
Revenues | Customer concentration risk | National Grid
Concentrations
Percentage of concentration risk			10.60%
Labour employees covered by collective bargaining agreements | Field labor employees
Concentrations
Percentage of concentration risk	88.00%