Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 1,197,373	$ 1,091,191
Operating income (loss)	57,070	64,218
Income (loss) from continuing operations	25,539	41,089
Loss from discontinued operations, net of tax	(409)	(163)
Net income	25,130	40,926
Electric
Segment Reporting Information [Line Items]
Revenues	85,908	92,566
Operating income (loss)	14,859	17,987
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	326,443	342,147
Operating income (loss)	49,998	50,317
Pipeline
Segment Reporting Information [Line Items]
Revenues	10,655	8,613
Operating income (loss)	11,419	9,904
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	262,144	227,113
Operating income (loss)	(43,269)	(41,580)
Construction services
Segment Reporting Information [Line Items]
Revenues	512,205	420,733
Operating income (loss)	23,796	27,465
Other
Segment Reporting Information [Line Items]
Revenues	18	19
Operating income (loss)	267	125
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(31,085)	(32,003)
Intersegment eliminations \| Electric
Segment Reporting Information [Line Items]
Revenues	(195)	0
Intersegment eliminations \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(185)	0
Intersegment eliminations \| Pipeline
Segment Reporting Information [Line Items]
Revenues	(25,199)	(23,955)
Intersegment eliminations \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(63)	(95)
Intersegment eliminations \| Construction services
Segment Reporting Information [Line Items]
Revenues	(2,470)	(129)
Intersegment eliminations \| Other
Segment Reporting Information [Line Items]
Revenues	(2,973)	(7,824)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	418,682	439,617
Income (loss) from continuing operations	51,129	59,009
Regulated operation \| Electric
Segment Reporting Information [Line Items]
Revenues	85,908	92,566
Income (loss) from continuing operations	11,374	15,505
Regulated operation \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	326,443	342,147
Income (loss) from continuing operations	32,369	36,500
Regulated operation \| Pipeline
Segment Reporting Information [Line Items]
Revenues	6,331	4,904
Income (loss) from continuing operations	7,386	7,004
Regulated operation \| Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	25,563	23,922
Regulated operation \| Intersegment eliminations \| Electric
Segment Reporting Information [Line Items]
Revenues	195	0
Regulated operation \| Intersegment eliminations \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	185	0
Regulated operation \| Intersegment eliminations \| Pipeline
Segment Reporting Information [Line Items]
Revenues	25,183	23,922
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	778,691	651,574
Income (loss) from continuing operations	(25,590)	(17,920)
Nonregulated operation \| Pipeline
Segment Reporting Information [Line Items]
Revenues	4,324	3,709
Income (loss) from continuing operations	(13)	(163)
Nonregulated operation \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	262,144	227,113
Income (loss) from continuing operations	(38,215)	(34,449)
Nonregulated operation \| Construction services
Segment Reporting Information [Line Items]
Revenues	512,205	420,733
Income (loss) from continuing operations	16,823	20,024
Nonregulated operation \| Other
Segment Reporting Information [Line Items]
Revenues	18	19
Income (loss) from continuing operations	(4,185)	(3,332)
Nonregulated operation \| Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	5,522	8,081
Nonregulated operation \| Intersegment eliminations \| Pipeline
Segment Reporting Information [Line Items]
Revenues	16	33
Nonregulated operation \| Intersegment eliminations \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	63	95
Nonregulated operation \| Intersegment eliminations \| Construction services
Segment Reporting Information [Line Items]
Revenues	2,470	129
Nonregulated operation \| Intersegment eliminations \| Other
Segment Reporting Information [Line Items]
Revenues	$ 2,973	$ 7,824

Amount after tax of income (loss) from continuing operations attributable to the parent.

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