Goodwill rollforward (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Goodwill [Roll Forward]
Balance as of beginning of period	$ 634,931	[1]	$ 634,633	[1]	$ 634,633	[1]
Goodwill acquired during the year	458	[2]	298	[2]	298	[2]
Balance as of end of period	635,389	[1]	634,931	[1]	634,931	[1]
Electric [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	0		0		0
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	0		0		0
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	345,736		345,736		345,736
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	345,736		345,736		345,736
Pipeline and energy services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	9,737	[1]	9,737	[1]	9,737	[1]
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	9,737	[1]	9,737	[1]	9,737	[1]
Exploration and production [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	0		0		0
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	0		0		0
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	176,290		176,290		176,290
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	176,290		176,290		176,290
Construction services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	103,168		102,870		102,870
Goodwill acquired during the year	458	[2]	298	[2]	298	[2]
Balance as of end of period	103,626		103,168		103,168
All Other Segments [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	0		0		0
Goodwill acquired during the year	0	[2]	0	[2]	0	[2]
Balance as of end of period	$ 0		$ 0		$ 0

[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.