Goodwill rollforward (Details) (USD $)	6 Months Ended				12 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Dec. 31, 2011
Goodwill [Roll Forward]
Balance as of beginning of period	$ 634,931,000	[1]	$ 634,633,000	[1]	$ 634,633,000	[1]
Goodwill acquired during the year	458,000	[2]	298,000	[2]	298,000	[2]
Balance as of end of period	635,389,000	[1]	634,931,000	[1]	634,931,000	[1]
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	345,736,000		345,736,000		345,736,000
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	345,736,000		345,736,000		345,736,000
Pipeline and energy services [Member]
Goodwill [Line Items]
Goodwill, impaired, accumulated impairment loss	12,300,000		12,300,000		12,300,000
Goodwill [Roll Forward]
Balance as of beginning of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	176,290,000		176,290,000		176,290,000
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	176,290,000		176,290,000		176,290,000
Construction services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	103,168,000		102,870,000		102,870,000
Goodwill acquired during the year	458,000	[2]	298,000	[2]	298,000	[2]
Balance as of end of period	$ 103,626,000		$ 103,168,000		$ 103,168,000

[1] Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.

[2] Includes a purchase price adjustment that was not material related to an acquisition in a prior period.