Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 1,060,595	$ 967,962	$ 1,992,198	$ 1,820,769
Intersegment operating revenues	0	0	0	0
Earnings on common stock	46,341	53,942	102,685	89,561
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	227,442	204,455	651,565	599,533
Intersegment operating revenues	7,999	8,937	26,717	29,347
Earnings on common stock	(7,878)	13,859	36,795	49,686
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	833,153	763,507	1,340,633	1,221,236
Intersegment operating revenues	29,693	11,055	47,191	22,885
Earnings on common stock	56,275	40,083	67,946	39,875
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	56,981	52,955	121,635	110,918
Intersegment operating revenues	0	0	0	0
Earnings on common stock	4,410	4,419	14,235	11,978
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	127,584	116,844	459,337	424,733
Intersegment operating revenues	0	0	0	0
Earnings on common stock	(5,893)	(6,411)	26,624	19,097
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	42,877	34,656	70,593	63,882
Intersegment operating revenues	7,999	8,937	26,717	29,347
Earnings on common stock	(6,395)	15,851	(4,064)	18,611
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	137,053	100,232	252,415	188,727
Intersegment operating revenues	12,556	5,711	22,369	17,038
Earnings on common stock	32,995	17,957	53,279	30,887
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	418,345	438,963	580,323	588,232
Intersegment operating revenues	12,958	3,097	17,251	3,248
Earnings on common stock	10,025	7,791	(10,557)	(17,141)
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	277,259	223,858	507,065	442,010
Intersegment operating revenues	2,340	219	3,914	244
Earnings on common stock	12,915	8,684	24,579	20,087
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	496	454	830	2,267
Intersegment operating revenues	1,839	2,028	3,657	2,355
Earnings on common stock	340	5,651	645	6,042
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment operating revenues	(37,692)	(19,992)	(73,908)	(52,232)
Earnings on common stock	$ (2,056)	$ 0	$ (2,056)	$ 0