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Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Business segment data (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 1,280,787	$ 1,272,548	$ 3,321,678	$ 3,278,112
Earnings on common stock before income (loss) from discontinued operations	107,369	89,549	193,405	168,822
Income (loss) from discontinued operations, net of tax	(118)	(2,198)	85	(3,702)
Earnings on common stock	107,251	87,351	193,490	165,120
Electric
Segment Reporting Information [Line Items]
Revenues	86,080		251,984
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	92,248		554,452
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	29,071		61,965
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	743,763		1,466,435
Construction services
Segment Reporting Information [Line Items]
Revenues	329,578		986,649
Other
Segment Reporting Information [Line Items]
Revenues	47		193
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(7,171)	(5,586)	(41,661)	(37,581)
Earnings on common stock before income (loss) from discontinued operations	0	1,855	0	6,121
Intersegment eliminations \| Electric
Segment Reporting Information [Line Items]
Revenues	0		0
Intersegment eliminations \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0		0
Intersegment eliminations \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	(3,187)		(31,485)
Intersegment eliminations \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(165)		(501)
Intersegment eliminations \| Construction services
Segment Reporting Information [Line Items]
Revenues	(782)		(1,332)
Intersegment eliminations \| Other
Segment Reporting Information [Line Items]
Revenues	(3,037)		(8,343)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	200,617	206,936	851,761	866,035
Earnings on common stock before income (loss) from discontinued operations	13,506	10,682	72,193	67,986
Regulated operation \| Electric
Segment Reporting Information [Line Items]
Revenues	86,080	91,531	251,984	254,330
Earnings on common stock before income (loss) from discontinued operations	15,284	15,712	37,500	37,904
Regulated operation \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	92,248	92,253	554,452	566,364
Earnings on common stock before income (loss) from discontinued operations	(11,887)	(10,883)	13,884	14,181
Regulated operation \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	22,289	23,152	45,325	45,341
Earnings on common stock before income (loss) from discontinued operations	10,109	5,853	20,809	15,901
Regulated operation \| Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,070	3,081	31,252	30,923
Regulated operation \| Intersegment eliminations \| Electric
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Regulated operation \| Intersegment eliminations \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Regulated operation \| Intersegment eliminations \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	3,070	3,081	31,252	30,923
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,080,170	1,065,612	2,469,917	2,412,077
Earnings on common stock before income (loss) from discontinued operations	93,863	77,012	121,212	94,715
Nonregulated operation \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	6,782	5,356	16,640	13,518
Earnings on common stock before income (loss) from discontinued operations	848	95	1,136	(770)
Nonregulated operation \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	743,763	686,010	1,466,435	1,388,212
Earnings on common stock before income (loss) from discontinued operations	78,876	63,221	79,691	64,477
Nonregulated operation \| Construction services
Segment Reporting Information [Line Items]
Revenues	329,578	374,111	986,649	1,009,693
Earnings on common stock before income (loss) from discontinued operations	9,278	13,144	38,457	32,896
Nonregulated operation \| Other
Segment Reporting Information [Line Items]
Revenues	47	135	193	654
Earnings on common stock before income (loss) from discontinued operations	4,861	552	1,928	(1,888)
Nonregulated operation \| Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,101	2,505	10,409	6,658
Nonregulated operation \| Intersegment eliminations \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	117	38	233	132
Nonregulated operation \| Intersegment eliminations \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	165	142	501	400
Nonregulated operation \| Intersegment eliminations \| Construction services
Segment Reporting Information [Line Items]
Revenues	782	415	1,332	715
Nonregulated operation \| Intersegment eliminations \| Other
Segment Reporting Information [Line Items]
Revenues	$ 3,037	$ 1,910	$ 8,343	$ 5,411