Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Revenues	$ 1,737,337	$ 1,416,559
Operating income	74,472	67,343
Income from continuing operations	38,343	31,733
Discontinued operations, net of tax	10	30
Net income	38,353	31,763
Electric
Segment Reporting Information [Line Items]
Revenues	95,602	93,594
Operating income	21,091	15,044
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	565,517	450,418
Operating income	58,505	56,256
Pipeline
Segment Reporting Information [Line Items]
Revenues	14,331	11,120
Operating income	13,040	11,882
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	307,768	309,836
Operating income	(44,563)	(44,605)
Construction services
Segment Reporting Information [Line Items]
Revenues	754,119	551,591
Operating income	35,217	29,498
Other
Segment Reporting Information [Line Items]
Revenues	0	0
Operating income	(8,818)	(732)
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(31,767)	(31,730)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(122)	(124)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(150)	(136)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(26,410)	(25,940)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(132)	(130)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(214)	(1,059)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(4,739)	(4,341)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	31,767	31,730
Regulated operation
Segment Reporting Information [Line Items]
Revenues	673,757	553,456
Income from continuing operations	64,294	55,549
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	95,602	93,594
Income from continuing operations	16,607	11,278
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	565,517	450,418
Income from continuing operations	38,928	36,315
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	12,638	9,444
Income from continuing operations	8,759	7,956
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	26,531	26,193
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	122	124
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	150	136
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	26,259	25,933
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,063,580	863,103
Income from continuing operations	(25,951)	(23,816)
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	1,693	1,676
Income from continuing operations	(460)	(619)
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	307,768	309,836
Income from continuing operations	(41,320)	(40,010)
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	754,119	551,591
Income from continuing operations	26,074	21,324
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	0	0
Income from continuing operations	(10,245)	(4,511)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	5,236	5,537
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	151	7
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	132	130
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	214	1,059
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 4,739	$ 4,341