Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Total external operating revenues	$ 852,807	$ 901,805
Total intersegment operating revenues	0	0
Earnings on common stock	35,619	42,806
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	395,081	477,481
Total intersegment operating revenues	20,409	24,741
Earnings on common stock	35,827	42,960
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	457,726	424,324
Total intersegment operating revenues	11,831	29,046
Earnings on common stock	(208)	(154)
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	57,963	57,845
Total intersegment operating revenues	0	0
Earnings on common stock	7,559	8,524
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	307,891	370,385
Total intersegment operating revenues	0	0
Earnings on common stock	25,508	27,516
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	29,227	49,251
Total intersegment operating revenues	20,409	24,741
Earnings on common stock	2,760	6,920
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	88,494	78,410
Total intersegment operating revenues	11,328	25,541
Earnings on common stock	12,930	16,269
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	149,268	143,533
Total intersegment operating revenues	151	0
Earnings on common stock	(24,932)	(21,402)
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	218,151	202,180
Total intersegment operating revenues	25	1,217
Earnings on common stock	11,403	4,632
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	1,813	201
Total intersegment operating revenues	327	2,288
Earnings on common stock	391	347
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	0	0
Total intersegment operating revenues	(32,240)	(53,787)
Earnings on common stock	$ 0	$ 0