Organization, Business and Basis of Presentation (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 item	Jun. 30, 2013
Organization, Business and Basis of Presentation
Number of business segments			2
Use of Estimates
Income from operations	$ 12,395	$ 15,362	$ 22,527	$ 26,752
Net income	7,741	9,462	14,013	16,422
Income per common share, diluted (in dollars per share)	$ 0.36	$ 0.44	$ 0.64	$ 0.76
Contracts in process | Adjustment
Use of Estimates
Gross margin increased as a result of change in estimates pertaining to large transmission projects (as a percent)	1.90%	1.30%	2.20%	1.00%
Income from operations	4,300	2,700	9,700	4,000
Net income	$ 2,700	$ 1,700	$ 6,100	$ 2,500
Income per common share, diluted (in dollars per share)	$ 0.13	$ 0.08	$ 0.28	$ 0.12