Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Revenues	$ 1,043,948	$ 938,039	$ 1,904,163	$ 1,798,884
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before loss from discontinued operations	46,127	25,890	77,821	46,259
Loss from discontinued operations, net of tax	(276,102)	(263,419)	(294,138)	(593,404)
Loss from discontinued operations attributable to noncontrolling interest	(120,651)	(7,754)	(131,691)	(11,282)
Loss on common stock	(109,324)	(229,775)	(84,626)	(535,863)
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	206,052	215,678	591,918	622,167
Intersegment operating revenues	6,594	6,564	27,691	27,625
Earnings (loss) on common stock before loss from discontinued operations	5,809	4,863	47,457	39,997
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	837,896	722,361	1,312,245	1,176,717
Intersegment operating revenues	1,879	6,650	4,211	21,270
Earnings (loss) on common stock before loss from discontinued operations	40,318	21,769	30,364	7,995
Electric [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	73,832	64,265	156,755	136,041
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before loss from discontinued operations	8,022	5,910	19,141	14,237
Natural gas distribution [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	112,770	132,965	412,165	463,538
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before loss from discontinued operations	(7,777)	(5,375)	17,464	16,075
Pipeline and midstream [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	19,450	18,448	22,998	22,588
Intersegment operating revenues	6,594	6,564	27,691	27,625
Earnings (loss) on common stock before loss from discontinued operations	5,564	4,328	10,852	9,685
Pipeline and midstream [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	10,268	14,749	18,966	27,749
Intersegment operating revenues	36	110	119	316
Earnings (loss) on common stock before loss from discontinued operations	737	(966)	739	89
Construction materials and contracting [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	541,257	495,640	751,108	701,298
Intersegment operating revenues	97	1,257	215	2,205
Earnings (loss) on common stock before loss from discontinued operations	33,696	20,136	19,225	5,501
Construction services [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	285,924	211,515	541,424	446,918
Intersegment operating revenues	77	3,491	539	15,186
Earnings (loss) on common stock before loss from discontinued operations	6,990	7,003	12,964	11,763
Other [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	447	457	747	752
Intersegment operating revenues	1,669	1,792	3,338	3,563
Earnings (loss) on common stock before loss from discontinued operations	(1,105)	(4,404)	(2,564)	(9,358)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues	(8,473)	(13,214)	(31,902)	(48,895)
Earnings (loss) on common stock before loss from discontinued operations	$ 0	$ (742)	$ 0	$ (1,733)