Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenues		$ 1,067,639	$ 1,043,948	$ 2,005,565	$ 1,904,163
Intersegment operating revenues		0	0	0	0
Earnings on common stock before income (loss) from discontinued operations		43,805	46,127	79,272	77,821
Loss from discontinued operations, net of tax	[1]	(3,190)	(276,102)	(1,504)	(294,138)
Loss from discontinued operations attributable to noncontrolling interest		0	(120,651)	0	(131,691)
Earnings (loss) on common stock		40,615	(109,324)	77,768	(84,626)
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		225,485	206,052	659,100	591,918
Intersegment operating revenues		6,353	6,594	27,841	27,691
Earnings on common stock before income (loss) from discontinued operations		10,554	5,809	57,303	47,457
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		842,154	837,896	1,346,465	1,312,245
Intersegment operating revenues		2,284	1,879	4,153	4,211
Earnings on common stock before income (loss) from discontinued operations		31,158	40,318	17,703	30,364
Electric [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		74,574	73,832	162,799	156,755
Intersegment operating revenues		0	0	0	0
Earnings on common stock before income (loss) from discontinued operations		7,859	8,022	22,191	19,141
Natural gas distribution [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		131,592	112,770	474,111	412,165
Intersegment operating revenues		0	0	0	0
Earnings on common stock before income (loss) from discontinued operations		(2,797)	(7,777)	25,064	17,464
Pipeline and midstream [Member] | Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		19,319	19,450	22,190	22,998
Intersegment operating revenues		6,353	6,594	27,841	27,691
Earnings on common stock before income (loss) from discontinued operations		5,492	5,564	10,048	10,852
Pipeline and midstream [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		4,520	10,268	8,163	18,966
Intersegment operating revenues		59	36	93	119
Earnings on common stock before income (loss) from discontinued operations		(238)	737	(865)	739
Construction materials and contracting [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		501,426	541,257	702,203	751,108
Intersegment operating revenues		172	97	258	215
Earnings on common stock before income (loss) from discontinued operations		21,168	33,696	1,255	19,225
Construction services [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		336,009	285,924	635,580	541,424
Intersegment operating revenues		295	77	301	539
Earnings on common stock before income (loss) from discontinued operations		12,391	6,990	19,753	12,964
Other [Member] | Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		199	447	519	747
Intersegment operating revenues		1,758	1,669	3,501	3,338
Earnings on common stock before income (loss) from discontinued operations		(2,163)	(1,105)	(2,440)	(2,564)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues		(8,637)	(8,473)	(31,994)	(31,902)
Earnings on common stock before income (loss) from discontinued operations	[1]	$ 2,093	$ 0	$ 4,266	$ 0

[1] Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.