Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 1,280,500	$ 1,213,203	$ 3,129,064	$ 3,066,529
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations	63,007	64,556	103,164	125,989
Income (loss) from discontinued operations, net of tax	(202,626)	38,482	(778,647)	87,475
Earnings (loss) on common stock	(139,619)	103,038	(675,483)	213,464
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	185,224	184,793	806,986	863,569
Intersegment operating revenues	3,740	3,728	31,365	27,938
Earnings (loss) on common stock before income (loss) from discontinued operations	5,699	797	45,696	49,033
Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	1,095,276	1,028,410	2,322,078	2,202,960
Intersegment operating revenues	4,880	25,517	26,151	52,489
Earnings (loss) on common stock before income (loss) from discontinued operations	57,282	65,403	59,117	80,026
Electric [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	74,604	68,936	210,646	207,732
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations	12,605	9,162	26,842	28,018
Natural gas distribution [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	89,520	96,185	553,058	616,496
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations	(12,298)	(12,252)	3,777	10,516
Pipeline and energy services [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	21,100	19,672	43,282	39,341
Intersegment operating revenues	3,740	3,728	31,365	27,938
Earnings (loss) on common stock before income (loss) from discontinued operations	5,392	3,887	15,077	10,499
Pipeline and energy services [Member] | Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	96,896	17,168	174,732	47,028
Intersegment operating revenues	145	174	460	544
Earnings (loss) on common stock before income (loss) from discontinued operations	(14,903)	1,173	(22,175)	4,699
Construction materials and contracting [Member] | Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	774,288	740,496	1,475,585	1,339,371
Intersegment operating revenues	244	6,322	2,450	18,445
Earnings (loss) on common stock before income (loss) from discontinued operations	68,823	55,218	74,324	42,199
Construction services [Member] | Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	223,676	270,313	670,594	815,313
Intersegment operating revenues	2,112	16,420	17,298	27,431
Earnings (loss) on common stock before income (loss) from discontinued operations	4,742	9,876	16,505	40,751
Other [Member] | Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	416	433	1,167	1,248
Intersegment operating revenues	2,379	2,601	5,943	6,069
Earnings (loss) on common stock before income (loss) from discontinued operations	(1,380)	(864)	(9,537)	(7,623)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues	(8,620)	(29,245)	(57,516)	(80,427)
Earnings (loss) on common stock before income (loss) from discontinued operations	$ 26	$ (1,644)	$ (1,649)	$ (3,070)
Oil and natural gas properties [Member]
Segment Reporting Information [Line Items]
Percent of oil and natural gas production to be sold			90.00%