Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Total external operating revenues	$ 931,604	$ 852,807
Total intersegment operating revenues	0	0
Earnings (loss) on common stock	56,344	35,619
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	424,124	395,081
Total intersegment operating revenues	18,718	20,409
Earnings (loss) on common stock	44,673	35,827
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	507,480	457,726
Total intersegment operating revenues	17,498	11,831
Earnings (loss) on common stock	11,671	(208)
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	64,654	57,963
Total intersegment operating revenues	0	0
Earnings (loss) on common stock	9,825	7,559
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	331,754	307,891
Total intersegment operating revenues	0	0
Earnings (loss) on common stock	32,518	25,508
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	27,716	29,227
Total intersegment operating revenues	18,718	20,409
Earnings (loss) on common stock	2,330	2,760
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	115,363	88,494
Total intersegment operating revenues	9,812	11,328
Earnings (loss) on common stock	20,284	12,930
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	161,977	149,268
Total intersegment operating revenues	4,294	151
Earnings (loss) on common stock	(20,582)	(24,932)
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	229,806	218,151
Total intersegment operating revenues	1,574	25
Earnings (loss) on common stock	11,664	11,403
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	334	1,813
Total intersegment operating revenues	1,818	327
Earnings (loss) on common stock	305	391
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	0	0
Total intersegment operating revenues	(36,216)	(32,240)
Earnings (loss) on common stock	$ 0	$ 0