Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenues	$ 1,416,559	$ 1,227,938
Operating income	67,343	85,164
Income from continuing operations	31,733	52,116
Income from discontinued operations, net of tax	30	15
Net income	31,763	52,131
Electric
Segment Reporting Information [Line Items]
Revenues	93,594	84,673
Operating income	15,044	13,865
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	450,418	350,206
Operating income	56,256	53,573
Pipeline
Segment Reporting Information [Line Items]
Revenues	11,120	9,913
Operating income	11,882	12,536
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	309,836	265,659
Operating income	(44,605)	(34,889)
Construction services
Segment Reporting Information [Line Items]
Revenues	551,591	517,470
Operating income	29,498	40,277
Other
Segment Reporting Information [Line Items]
Revenues	0	17
Operating income	(732)	(198)
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(31,730)	(30,935)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(124)	(136)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(136)	(142)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(25,940)	(26,229)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(130)	(62)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(1,059)	(1,042)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(4,341)	(3,324)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	553,456	442,381
Income from continuing operations	55,549	56,121
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	93,594	84,673
Income from continuing operations	11,278	10,749
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	450,418	350,206
Income from continuing operations	36,315	36,178
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	9,444	7,502
Income from continuing operations	7,956	9,194
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	26,193	26,268
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	124	136
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	136	142
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	25,933	25,990
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	863,103	785,557
Income from continuing operations	(23,816)	(4,005)
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	1,676	2,411
Income from continuing operations	(619)	(296)
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	309,836	265,659
Income from continuing operations	(40,010)	(30,813)
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	551,591	517,470
Income from continuing operations	21,324	29,825
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	0	17
Income from continuing operations	(4,511)	(2,721)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	5,537	4,667
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	7	239
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	130	62
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	1,059	1,042
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 4,341	$ 3,324