Goodwill and other intangible assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Goodwill [Roll Forward]
Balance as of beginning of period	$ 636,039	[1]	$ 634,931	[1]
Goodwill acquired during the year	0		1,108
Balance as of end of period	636,039	[1]	636,039	[1]
Natural gas distribution:
Goodwill [Roll Forward]
Balance as of beginning of period	345,736		345,736
Goodwill acquired during the year	0		0
Balance as of end of period	345,736		345,736
Pipeline and energy services:
Goodwill [Roll Forward]
Balance as of beginning of period	9,737	[1]	9,737	[1]
Goodwill acquired during the year	0		0
Balance as of end of period	9,737	[1]	9,737	[1]
Accumulated impairment which occurred in prior periods	12,300		12,300
Construction materials and contracting:
Goodwill [Roll Forward]
Balance as of beginning of period	176,290		176,290
Goodwill acquired during the year	0		0
Balance as of end of period	176,290		176,290
Construction services:
Goodwill [Roll Forward]
Balance as of beginning of period	104,276		103,168
Goodwill acquired during the year	0		1,108	[2]
Balance as of end of period	$ 104,276		$ 104,276

[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 contingent consideration that was not material related to an acquisition in a prior period.