Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 1,094,046	$ 1,060,595	$ 2,136,899	$ 1,992,198
Intersegment operating revenues	0	0	0	0
Earnings on common stock	53,935	46,341	110,426	102,685
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	254,689	227,442	746,231	651,565
Intersegment operating revenues	7,888	7,999	26,164	26,717
Earnings on common stock	9,118	(7,878)	51,762	36,795
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	839,357	833,153	1,390,668	1,340,633
Intersegment operating revenues	27,394	29,693	57,740	47,191
Earnings on common stock	45,649	56,275	59,848	67,946
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	65,149	56,981	138,796	121,635
Intersegment operating revenues	0	0	0	0
Earnings on common stock	7,823	4,410	18,856	14,235
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	146,077	127,584	520,311	459,337
Intersegment operating revenues	0	0	0	0
Earnings on common stock	(4,494)	(5,893)	22,768	26,624
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	43,463	42,877	87,124	70,593
Intersegment operating revenues	7,888	7,999	26,164	26,717
Earnings on common stock	5,789	(6,395)	10,138	(4,064)
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	129,309	137,053	245,976	252,415
Intersegment operating revenues	10,271	12,556	31,139	22,369
Earnings on common stock	19,180	32,995	40,120	53,279
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	434,452	418,345	598,875	580,323
Intersegment operating revenues	8,106	12,958	12,123	17,251
Earnings on common stock	10,554	10,025	(13,019)	(10,557)
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	275,109	277,259	545,002	507,065
Intersegment operating revenues	7,273	2,340	11,010	3,914
Earnings on common stock	14,307	12,915	30,875	24,579
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	487	496	815	830
Intersegment operating revenues	1,744	1,839	3,468	3,657
Earnings on common stock	1,608	340	1,872	645
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment operating revenues	(35,282)	(37,692)	(83,904)	(73,908)
Earnings on common stock	$ (832)	$ (2,056)	$ (1,184)	$ (2,056)