Goodwill and other intangible assets (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Goodwill [Roll Forward]
Balance as of beginning of period			$ 634,633,000	$ 629,463,000	$ 629,463,000
Goodwill aquired during the year			298,000	5,170,000	5,170,000
Balance as of end of period	634,931,000	634,633,000	634,931,000	634,633,000	634,633,000
Accumulated impairment which occurred in prior periods	12,300,000	12,300,000	12,300,000	12,300,000	12,300,000
Other amortizable intangible assets [Abstract]
Other amortizable intangible assets, net	22,248,000	26,112,000	22,248,000	26,112,000	25,271,000
Amortization expense for amortizable intangible assets	1,100,000	1,300,000	3,000,000	3,400,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2011			4,000,000
2012			3,900,000
2013			3,700,000
2014			3,400,000
2015			2,700,000
Thereafter			7,500,000
Electric [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			0	0	0
Goodwill aquired during the year			0	0	0
Balance as of end of period	0	0	0	0	0
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			345,736,000	345,736,000	345,736,000
Goodwill aquired during the year			0	0	0
Balance as of end of period	345,736,000	345,736,000	345,736,000	345,736,000	345,736,000
Construction services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			102,870,000	100,127,000	100,127,000
Goodwill aquired during the year			298,000	2,743,000	2,743,000
Balance as of end of period	103,168,000	102,870,000	103,168,000	102,870,000	102,870,000
Pipeline and energy services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			9,737,000	7,857,000	7,857,000
Goodwill aquired during the year			0	1,880,000	1,880,000
Balance as of end of period	9,737,000	9,737,000	9,737,000	9,737,000	9,737,000
Natural gas and oil production [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			0	0	0
Goodwill aquired during the year			0	0	0
Balance as of end of period	0	0	0	0	0
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			176,290,000	175,743,000	175,743,000
Goodwill aquired during the year			0	547,000	547,000
Balance as of end of period	176,290,000	176,290,000	176,290,000	176,290,000	176,290,000
Other [Member]
Goodwill [Roll Forward]
Balance as of beginning of period			0	0	0
Goodwill aquired during the year			0	0	0
Balance as of end of period	0	0	0	0	0
Customer Relationships [Member]
Other amortizable intangible assets [Abstract]
Other amortizable intangible assets	21,702,000	24,942,000	21,702,000	24,942,000	24,942,000
Accumulated amortization	(9,896,000)	(11,273,000)	(9,896,000)	(11,273,000)	(11,625,000)
Other amortizable intangible assets, net	11,806,000	13,669,000	11,806,000	13,669,000	13,317,000
Noncompete Agreements [Member]
Other amortizable intangible assets [Abstract]
Other amortizable intangible assets	7,685,000	9,405,000	7,685,000	9,405,000	9,405,000
Accumulated amortization	(5,222,000)	(6,231,000)	(5,222,000)	(6,231,000)	(6,425,000)
Other amortizable intangible assets, net	2,463,000	3,174,000	2,463,000	3,174,000	2,980,000
Other [Member}
Other amortizable intangible assets [Abstract]
Other amortizable intangible assets	12,901,000	13,217,000	12,901,000	13,217,000	13,217,000
Accumulated amortization	(4,922,000)	(3,948,000)	(4,922,000)	(3,948,000)	(4,243,000)
Other amortizable intangible assets, net	$ 7,979,000	$ 9,269,000	$ 7,979,000	$ 9,269,000	$ 8,974,000