Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 1,977,206			$ 1,586,013			$ 5,114,686	$ 4,237,608
Operating income	209,851			189,169			397,763	412,023
Income from continuing operations	147,933			138,962			250,298	291,249
Discontinued operations, net of tax	38			314			103	348
Net income	147,971	$ 70,667	$ 31,763	139,276	$ 100,190	$ 52,131	250,401	291,597
Electric
Segment Reporting Information [Line Items]
Revenues	99,193			98,930			278,249	267,254
Operating income	30,395			27,310			53,764	53,982
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	132,074			110,378			792,932	614,373
Operating income	(16,688)			(14,249)			38,929	42,925
Pipeline
Segment Reporting Information [Line Items]
Revenues	36,045			30,968			76,977	68,653
Operating income	14,963			12,284			39,447	35,896
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	975,085			831,189			1,996,533	1,730,521
Operating income	147,367			133,712			157,116	171,274
Construction services
Segment Reporting Information [Line Items]
Revenues	734,809			514,523			1,969,995	1,556,737
Operating income	34,949			30,120			110,588	108,172
Other
Segment Reporting Information [Line Items]
Revenues	0			25			0	70
Operating income	(1,135)			(8)			(2,081)	(226)
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(10,868)			(7,983)			(57,047)	(51,356)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(124)			(136)			(371)	(407)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(139)			(142)			(416)	(425)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(3,595)			(3,908)			(37,328)	(38,000)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(343)			(110)			(674)	(314)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(2,229)			(289)			(5,090)	(2,128)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(4,438)			(3,398)			(13,168)	(10,082)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	263,614			237,065			1,140,528	940,017
Income from continuing operations	12,917			15,399			72,761	89,207
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	99,193			98,930			278,249	267,254
Income from continuing operations	21,596			20,564			37,476	41,618
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	132,074			110,378			792,932	614,373
Income from continuing operations	(18,058)			(15,390)			10,758	20,081
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	32,347			27,757			69,347	58,390
Income from continuing operations	9,379			10,225			24,527	27,508
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,771			4,123			37,632	38,362
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	124			136			371	407
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	139			142			416	425
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,508			3,845			36,845	37,530
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,713,592			1,348,948			3,974,158	3,297,591
Income from continuing operations	135,016			123,563			177,537	202,042
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,698			3,211			7,630	10,263
Income from continuing operations	492			356			(187)	1,166
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	975,085			831,189			1,996,533	1,730,521
Income from continuing operations	102,764			96,282			95,337	116,865
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	734,809			514,523			1,969,995	1,556,737
Income from continuing operations	27,977			23,130			83,755	81,839
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	0			25			0	70
Income from continuing operations	3,783			3,795			(1,368)	2,172
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	7,097			3,860			19,415	12,994
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	87			63			483	470
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	343			110			674	314
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	2,229			289			5,090	2,128
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 4,438			$ 3,398			$ 13,168	$ 10,082