Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Total external operating revenues	$ 967,962	$ 930,757	$ 1,820,769	$ 1,832,562
Total intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock	53,942	44,896	89,561	87,702
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	204,455	274,538	599,533	752,018
Total intersegment operating revenues	8,937	12,504	29,347	37,245
Earnings (loss) on common stock	13,859	11,481	49,686	54,440
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	763,507	656,219	1,221,236	1,080,544
Total intersegment operating revenues	11,055	33,072	22,885	62,118
Earnings (loss) on common stock	40,083	33,415	39,875	33,262
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	52,955	49,986	110,918	107,831
Total intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock	4,419	4,807	11,978	13,331
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	116,844	164,626	424,733	535,010
Total intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock	(6,411)	1,902	19,097	29,418
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	34,656	59,926	63,882	109,177
Total intersegment operating revenues	8,937	12,504	29,347	37,245
Earnings (loss) on common stock	15,851	4,772	18,611	11,691
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	100,232	87,390	188,727	165,801
Total intersegment operating revenues	5,711	25,392	17,038	50,933
Earnings (loss) on common stock	17,957	21,326	30,887	37,596
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	438,963	375,613	588,232	519,146
Total intersegment operating revenues	3,097	0	3,248	0
Earnings (loss) on common stock	7,791	4,980	(17,141)	(16,423)
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	223,858	192,697	442,010	394,877
Total intersegment operating revenues	219	5,379	244	6,596
Earnings (loss) on common stock	8,684	6,138	20,087	10,771
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	454	519	2,267	720
Total intersegment operating revenues	2,028	2,301	2,355	4,589
Earnings (loss) on common stock	5,651	971	6,042	1,318
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Total external operating revenues	0	0	0	0
Total intersegment operating revenues	(19,992)	(45,576)	(52,232)	(99,363)
Earnings (loss) on common stock	$ 0	$ 0	$ 0	$ 0