Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 1,285,782	$ 1,173,518	$ 3,277,980	$ 2,994,288
Intersegment operating revenues	0	0	0	0
Earnings on common stock	84,285	(29,842)	186,970	59,719
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	192,103	184,863	843,670	784,399
Intersegment operating revenues	4,906	7,046	31,623	36,393
Earnings on common stock	5,523	5,491	42,319	55,175
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	1,093,679	988,655	2,434,310	2,209,889
Intersegment operating revenues	23,092	19,366	70,283	42,251
Earnings on common stock	79,964	(35,333)	147,910	4,544
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	68,314	63,492	189,949	174,410
Intersegment operating revenues	0	0	0	0
Earnings on common stock	11,417	11,000	25,652	22,977
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	77,417	80,069	536,756	504,805
Intersegment operating revenues	0	0	0	0
Earnings on common stock	(11,204)	(8,782)	15,420	10,314
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	46,372	41,302	116,965	105,184
Intersegment operating revenues	4,906	7,046	31,623	36,393
Earnings on common stock	5,310	3,273	1,247	21,884
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	119,234	100,380	371,648	289,106
Intersegment operating revenues	10,714	8,076	33,083	25,114
Earnings on common stock	17,434	(87,748)	70,713	(56,860)
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	706,982	641,500	1,287,305	1,229,731
Intersegment operating revenues	7,422	8,508	24,673	11,756
Earnings on common stock	49,159	41,889	38,602	24,748
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	267,038	246,358	774,103	688,368
Intersegment operating revenues	3,097	834	7,011	1,078
Earnings on common stock	12,154	9,863	36,733	29,951
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	425	417	1,254	2,684
Intersegment operating revenues	1,859	1,948	5,516	4,303
Earnings on common stock	1,217	663	1,862	6,705
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment operating revenues	(27,998)	(26,412)	(101,906)	(78,644)
Earnings on common stock	$ (1,202)	$ 0	$ (3,259)	$ 0