Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 1,563,799			$ 1,280,787			$ 3,958,563	$ 3,321,678
Operating Income (Loss)	189,470			140,004			351,000	283,372
Income from continuing operations	136,128			107,369			240,362	193,405
Income (loss) from discontinued operations, net of tax	1,509			(118)			26	85
Net income	137,637	$ 61,825	$ 40,926	107,251	$ 43,802	$ 42,437	240,388	193,490
Electric
Segment Reporting Information [Line Items]
Revenues	89,845			86,080			263,423	251,984
Operating Income (Loss)	21,930			21,140			49,708	52,321
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	93,642			92,248			569,656	554,452
Operating Income (Loss)	(15,565)			(11,665)			32,132	32,503
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	32,533			29,071			69,827	61,965
Operating Income (Loss)	11,416			9,036			32,017	25,687
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	869,376			743,763			1,692,287	1,466,435
Operating Income (Loss)	143,024			109,598			147,622	120,591
Construction services
Segment Reporting Information [Line Items]
Revenues	478,381			329,578			1,363,305	986,649
Operating Income (Loss)	29,950			11,863			89,381	51,853
Other
Segment Reporting Information [Line Items]
Revenues	22			47			65	193
Operating Income (Loss)	(1,285)			32			140	417
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(8,043)			(7,171)			(51,328)	(41,661)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Intersegment eliminations | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	(3,831)			(3,187)			(35,298)	(31,485)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(124)			(165)			(388)	(501)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(1,226)			(782)			(2,076)	(1,332)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(2,862)			(3,037)			(13,566)	(8,343)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	209,444			200,617			885,309	851,761
Income from continuing operations	7,599			13,506			74,206	72,193
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	89,845			86,080			263,423	251,984
Income from continuing operations	16,291			15,284			39,267	37,500
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	93,642			92,248			569,656	554,452
Income from continuing operations	(15,625)			(11,887)			14,623	13,884
Regulated operation | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	25,957			22,289			52,230	45,325
Income from continuing operations	6,933			10,109			20,316	20,809
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,750			3,070			35,098	31,252
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	3,750			3,070			35,098	31,252
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,354,355			1,080,170			3,073,254	2,469,917
Income from continuing operations	128,529			93,863			166,156	121,212
Nonregulated operation | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	6,576			6,782			17,597	16,640
Income from continuing operations	801			848			1,380	1,136
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	869,376			743,763			1,692,287	1,466,435
Income from continuing operations	102,611			78,876			97,328	79,691
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	478,381			329,578			1,363,305	986,649
Income from continuing operations	21,113			9,278			63,982	38,457
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	22			47			65	193
Income from continuing operations	4,004			4,861			3,466	1,928
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,293			4,101			16,230	10,409
Nonregulated operation | Intersegment eliminations | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	81			117			200	233
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	124			165			388	501
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	1,226			782			2,076	1,332
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 2,862			$ 3,037			$ 13,566	$ 8,343