Business segment data - Schedule of Segment Reporting Information, by Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenues	$ 1,047,545		$ 1,091,126		$ 2,261,370	$ 2,521,224
Operating income	89,746		82,031		225,693	200,553
Income from continuing operations	60,574		147,633		161,472	231,508
Discontinued operations, net of tax	(138)		(16,941)		(138)	(62,464)
Net income	60,436	$ 100,898	130,692	$ 38,353	161,334	169,044
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,056,960		1,099,140		2,301,252	2,555,606
Operating Segments | Electric
Segment Reporting Information [Line Items]
Revenues	99,157		90,989		206,883	186,686
Operating income	18,631		21,561		41,419	42,654
Operating Segments | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	201,431		218,995		660,901	784,590
Operating income	1,204		2,428		59,442	60,932
Operating Segments | Pipeline
Segment Reporting Information [Line Items]
Revenues	43,831		34,209		64,843	48,683
Operating income	23,272		13,886		45,922	26,926
Operating Segments | Construction services
Segment Reporting Information [Line Items]
Revenues	703,124		746,933		1,328,726	1,501,265
Operating income	51,309		54,310		90,193	89,527
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	2		0		17	0
Operating income	(4,670)		(10,154)		(11,283)	(19,486)
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(10,833)		(11,165)		(42,732)	(39,105)
Regulated operation
Segment Reporting Information [Line Items]
Revenues	340,544		340,492		926,561	1,014,423
Income from continuing operations	27,360		21,832		100,703	86,296
Regulated operation | Operating Segments
Segment Reporting Information [Line Items]
Revenues	340,544		340,492		926,561	1,014,423
Regulated operation | Operating Segments | Electric
Segment Reporting Information [Line Items]
Revenues	99,157		90,989		206,883	186,686
Income from continuing operations	15,527		16,338		33,396	32,945
Regulated operation | Operating Segments | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	201,431		218,995		660,901	784,590
Income from continuing operations	(5,007)		(3,157)		35,063	35,771
Regulated operation | Operating Segments | Pipeline
Segment Reporting Information [Line Items]
Revenues	39,956		30,508		58,777	43,147
Income from continuing operations	16,840		8,651		32,244	17,580
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(8,967)		(7,795)		(39,327)	(34,152)
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(64)		(27)		(93)	(55)
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(49)		(69)		(105)	(139)
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(8,854)		(7,699)		(39,129)	(33,958)
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	707,001		750,634		1,334,809	1,506,801
Income from continuing operations	33,214		125,801		60,769	145,212
Nonregulated operation | Operating Segments
Segment Reporting Information [Line Items]
Revenues	707,001		750,634		1,334,809	1,506,801
Nonregulated operation | Operating Segments | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,875		3,701		6,066	5,536
Income from continuing operations	419		286		73	(172)
Nonregulated operation | Operating Segments | Construction services
Segment Reporting Information [Line Items]
Revenues	703,124		746,933		1,328,726	1,501,265
Income from continuing operations	38,971		41,167		67,186	69,976
Nonregulated operation | Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	2		0		17	0
Income from continuing operations	(6,176)		84,348		(6,490)	75,408
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(1,866)		(3,370)		(3,405)	(4,953)
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(201)		(219)		(236)	(230)
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(249)		0		(336)	0
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ (1,416)		$ (3,151)		$ (2,833)	$ (4,723)