Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,227,938	$ 1,197,373
Operating income (loss)	85,164	57,070
Income from continuing operations	52,116	25,539
Income (loss) from discontinued operations, net of tax	15	(409)
Net income	52,131	25,130
Electric
Segment Reporting Information [Line Items]
Revenues	84,673	85,908
Operating income (loss)	13,865	14,859
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	350,206	326,443
Operating income (loss)	53,573	49,998
Pipeline
Segment Reporting Information [Line Items]
Revenues	9,913	10,655
Operating income (loss)	12,536	11,419
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	265,659	262,144
Operating income (loss)	(34,889)	(43,269)
Construction services
Segment Reporting Information [Line Items]
Revenues	517,470	512,205
Operating income (loss)	40,277	23,796
Other
Segment Reporting Information [Line Items]
Revenues	17	18
Operating income (loss)	(198)	267
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(30,935)	(31,085)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(136)	(195)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(142)	(185)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(26,229)	(25,199)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(62)	(63)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(1,042)	(2,470)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,324)	(2,973)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	442,381	418,682
Income from continuing operations	56,121	51,129
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	84,673	85,908
Income from continuing operations	10,749	11,374
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	350,206	326,443
Income from continuing operations	36,178	32,369
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	7,502	6,331
Income from continuing operations	9,194	7,386
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	26,268	25,563
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	136	195
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	142	185
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	25,990	25,183
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	785,557	778,691
Income from continuing operations	(4,005)	(25,590)
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	2,411	4,324
Income from continuing operations	(296)	(13)
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	265,659	262,144
Income from continuing operations	(30,813)	(38,215)
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	517,470	512,205
Income from continuing operations	29,825	16,823
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	17	18
Income from continuing operations	(2,721)	(4,185)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,667	5,522
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	239	16
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	62	63
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	1,042	2,470
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,324	$ 2,973