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Goodwill and other intangible assets (Details) (USD $)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 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 5,191,000 5,170,000
Balance as of end of period 634,931,000 634,654,000 634,931,000 634,654,000 634,633,000
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 23,337,000 26,199,000 23,337,000 26,199,000 25,271,000
Amortization expense for amortizable intangible assets 1,000,000 1,100,000 1,900,000 2,100,000  
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 2,764,000 2,743,000
Balance as of end of period 103,168,000 102,891,000 103,168,000 102,891,000 102,870,000
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 (9,395,000) (10,688,000) (9,395,000) (10,688,000) (11,625,000)
Other amortizable intangible assets, net 12,307,000 14,254,000 12,307,000 14,254,000 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 (5,062,000) (6,033,000) (5,062,000) (6,033,000) (6,425,000)
Other amortizable intangible assets, net 2,623,000 3,372,000 2,623,000 3,372,000 2,980,000
Other [Member}
          
Other amortizable intangible assets [Abstract]          
Other amortizable intangible assets 12,899,000 12,063,000 12,899,000 12,063,000 13,217,000
Accumulated amortization (4,492,000) (3,490,000) (4,492,000) (3,490,000) (4,243,000)
Other amortizable intangible assets, net $ 8,407,000 $ 8,573,000 $ 8,407,000 $ 8,573,000 $ 8,974,000