Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 1,587,289			$ 1,563,799			$ 4,147,591	$ 3,958,563
Operating income (loss)	203,711			189,470			399,036	351,000
Income from continuing operations	153,015			136,128			278,395	240,362
Income (loss) from discontinued operations, net of tax	63			1,509			(484)	26
Net income	153,078	$ 99,703	$ 25,130	137,637	$ 61,825	$ 40,926	277,911	240,388
Electric
Segment Reporting Information [Line Items]
Revenues	87,447			89,845			249,793	263,423
Operating income (loss)	21,159			21,930			48,827	49,708
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	94,703			93,642			562,641	569,656
Operating income (loss)	(18,393)			(15,565)			32,285	32,132
Pipeline
Segment Reporting Information [Line Items]
Revenues	32,068			32,533			70,381	69,827
Operating income (loss)	11,762			11,416			35,406	32,017
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	822,439			869,376			1,705,629	1,692,287
Operating income (loss)	148,522			143,024			179,994	147,622
Construction services
Segment Reporting Information [Line Items]
Revenues	550,608			478,381			1,559,217	1,363,305
Operating income (loss)	40,507			29,950			102,185	89,381
Other
Segment Reporting Information [Line Items]
Revenues	24			22			(70)	65
Operating income (loss)	154			(1,285)			339	140
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(7,543)			(8,043)			(50,849)	(51,328)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(195)			0			(586)	0
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(185)			0			(555)	0
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(3,622)			(3,831)			(36,820)	(35,298)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(110)			(124)			(262)	(388)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(425)			(1,226)			(3,674)	(2,076)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,006)			(2,862)			(8,952)	(13,566)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	210,115			209,444			870,151	885,309
Income from continuing operations	6,985			7,599			77,991	74,206
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	87,447			89,845			249,793	263,423
Income from continuing operations	16,787			16,291			40,314	39,267
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	94,703			93,642			562,641	569,656
Income from continuing operations	(17,614)			(15,625)			13,795	14,623
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	27,965			25,957			57,717	52,230
Income from continuing operations	7,812			6,933			23,882	20,316
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,923			3,750			37,724	35,098
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	195			0			586	0
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	185			0			555	0
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,543			3,750			36,583	35,098
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,377,174			1,354,355			3,277,440	3,073,254
Income from continuing operations	146,030			128,529			200,404	166,156
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	4,103			6,576			12,664	17,597
Income from continuing operations	189			801			444	1,380
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	822,439			869,376			1,705,629	1,692,287
Income from continuing operations	107,307			102,611			122,113	97,328
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	550,608			478,381			1,559,217	1,363,305
Income from continuing operations	29,789			21,113			74,544	63,982
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	24			22			(70)	65
Income from continuing operations	8,745			4,004			3,303	3,466
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,620			4,293			13,125	16,230
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	79			81			237	200
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	110			124			262	388
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	425			1,226			3,674	2,076
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,006			$ 2,862			$ 8,952	$ 13,566