Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 1,091,126		$ 1,009,749		$ 2,521,224	$ 2,116,864
Operating income	82,031		62,505		200,553	170,000
Income from continuing operations	147,633		35,762		231,508	104,528
Discontinued operations, net of tax	(16,941)		34,905		(62,464)	(2,098)
Net income	130,692	$ 38,353	70,667	$ 31,763	169,044	102,430
Electric
Segment Reporting Information [Line Items]
Revenues	90,989		85,551		186,686	179,235
Operating income	21,561		8,325		42,654	23,369
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	218,995		210,510		784,590	660,998
Operating income	2,428		(638)		60,932	55,617
Pipeline
Segment Reporting Information [Line Items]
Revenues	34,209		29,818		48,683	40,938
Operating income	13,886		12,602		26,926	24,484
Construction services
Segment Reporting Information [Line Items]
Revenues	746,933		683,870		1,501,265	1,235,693
Operating income	54,310		46,141		89,527	75,639
Other
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Operating income	(10,154)		(3,925)		(19,486)	(9,109)
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(11,165)		(10,875)		(39,105)	(39,183)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(27)		(34)		(55)	(68)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(69)		(71)		(139)	(137)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(7,918)		(7,787)		(34,188)	(33,729)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	0		(1,527)		0	(2,353)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,151)		(1,456)		(4,723)	(2,896)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	11,165		10,875		39,105	39,183
Regulated operation
Segment Reporting Information [Line Items]
Revenues	340,492		323,616		1,014,423	877,233
Income from continuing operations	21,832		4,429		86,296	60,047
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	90,989		85,551		186,686	179,235
Income from continuing operations	16,338		4,601		32,945	15,880
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	218,995		210,510		784,590	660,998
Income from continuing operations	(3,157)		(7,498)		35,771	28,817
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	30,508		27,555		43,147	37,000
Income from continuing operations	8,651		7,326		17,580	15,350
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	7,795		7,509		34,152	33,543
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	27		34		55	68
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	69		71		139	137
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	7,699		7,404		33,958	33,338
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	750,634		686,133		1,506,801	1,239,631
Income from continuing operations	125,801		31,333		145,212	44,481
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,701		2,263		5,536	3,938
Income from continuing operations	286		(57)		(172)	(674)
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	746,933		683,870		1,501,265	1,235,693
Income from continuing operations	41,167		35,324		69,976	57,349
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Income from continuing operations	84,348		(3,934)		75,408	(12,194)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,370		3,366		4,953	5,640
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	219		336		230	391
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	0		1,527		0	2,353
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,151		$ 1,503		$ 4,723	$ 2,896