Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 1,000,795			$ 1,002,487			$ 3,522,020	$ 3,119,351
Operating income	84,604			61,002			285,157	231,003
Income from continuing operations	78,218			42,369			309,726	146,897
Discontinued operations, net of tax	(3,289)			105,602			(65,752)	103,504
Net income	74,929	$ 130,692	$ 38,353	147,971	$ 70,667	$ 31,763	243,974	250,401
Electric
Segment Reporting Information [Line Items]
Revenues	108,089			99,283			294,775	278,518
Operating income	28,718			30,395			71,372	53,764
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	134,896			132,145			919,487	793,142
Operating income	(15,258)			(16,688)			45,674	38,929
Pipeline
Segment Reporting Information [Line Items]
Revenues	40,404			36,058			89,086	76,997
Operating income	17,767			14,963			44,693	39,448
Construction services
Segment Reporting Information [Line Items]
Revenues	717,406			735,001			2,218,672	1,970,694
Operating income	50,091			34,949			139,619	110,588
Other
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Operating income	3,286			(2,617)			(16,201)	(11,726)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(27)			(34)			(82)	(102)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(69)			(69)			(208)	(206)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(3,701)			(3,581)			(37,889)	(37,309)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	0			(2,037)			0	(4,391)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(1,689)			(1,472)			(6,412)	(4,368)
Intersegment eliminations | Consolidated Operations
Segment Reporting Information [Line Items]
Revenues	(5,486)			(7,193)			(44,591)	(46,376)
Total intersegment operating revenues | Consolidated Operations
Segment Reporting Information [Line Items]
Revenues	5,486			7,193			44,591	46,376
Regulated operation
Segment Reporting Information [Line Items]
Revenues	279,534			263,774			1,293,959	1,141,007
Income from continuing operations	14,628			12,919			100,923	73,249
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	108,089			99,283			294,775	278,518
Income from continuing operations	20,942			21,596			53,887	37,476
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	134,896			132,145			919,487	793,142
Income from continuing operations	(17,764)			(18,058)			18,007	10,758
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	36,549			32,346			79,697	69,347
Income from continuing operations	11,450			9,381			29,029	25,015
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	27			34			82	102
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	69			69			208	206
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,666			3,508			37,624	36,845
Regulated operation | Intersegment eliminations | Regulated Operations
Segment Reporting Information [Line Items]
Revenues	3,762			3,611			37,914	37,153
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	721,261			738,713			2,228,061	1,978,344
Income from continuing operations	63,590			29,450			208,803	73,648
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,855			3,712			9,389	7,650
Income from continuing operations	508			779			337	(179)
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	717,406			735,001			2,218,672	1,970,694
Income from continuing operations	35,975			29,248			105,951	86,596
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Income from continuing operations	27,107			(577)			102,515	(12,769)
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	35			73			265	464
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	0			2,037			0	4,391
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	1,689			1,472			6,412	4,368
Nonregulated operation | Intersegment eliminations | Non-regulated Operations
Segment Reporting Information [Line Items]
Revenues	$ 1,724			$ 3,582			$ 6,677	$ 9,223