Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 1,362,928		$ 1,303,573		$ 2,560,302	$ 2,394,764
Operating income (loss)	138,254		97,311		195,325	161,530
Income from continuing operations	99,842		63,145		125,381	104,234
Loss from discontinued operations, net of tax	(139)		(1,320)		(548)	(1,483)
Net income	99,703	$ 25,130	61,825	$ 40,926	124,833	102,751
Electric
Segment Reporting Information [Line Items]
Revenues	76,438		81,011		162,346	173,578
Operating income (loss)	12,810		9,791		27,669	27,779
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	141,494		133,867		467,937	476,014
Operating income (loss)	678		(2,621)		50,677	47,696
Pipeline
Segment Reporting Information [Line Items]
Revenues	27,658		28,680		38,313	37,293
Operating income (loss)	12,225		10,698		23,644	20,602
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	621,045		595,799		883,190	822,911
Operating income (loss)	74,741		46,178		31,472	4,597
Construction services
Segment Reporting Information [Line Items]
Revenues	496,405		464,192		1,008,610	884,925
Operating income (loss)	37,882		31,966		61,678	59,431
Other
Segment Reporting Information [Line Items]
Revenues	(112)		24		(94)	43
Operating income (loss)	(82)		1,299		185	1,425
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(12,222)		(11,281)		(43,306)	(43,285)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(196)		0		(391)	0
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(185)		0		(370)	0
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(7,999)		(7,513)		(33,198)	(31,468)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(90)		(168)		(152)	(264)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(779)		(721)		(3,249)	(849)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(2,973)		(2,879)		(5,946)	(10,704)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	241,353		236,247		660,035	675,864
Income from continuing operations	19,878		7,597		71,007	66,606
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	76,438		81,011		162,346	173,578
Income from continuing operations	12,153		7,471		23,527	22,976
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	141,494		133,867		467,937	476,014
Income from continuing operations	(959)		(6,252)		31,410	30,248
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	23,421		21,369		29,752	26,272
Income from continuing operations	8,684		6,378		16,070	13,382
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	8,238		7,426		33,801	31,349
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	196		0		391	0
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	185		0		370	0
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	7,857		7,426		33,040	31,349
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,121,575		1,067,326		1,900,267	1,718,900
Income from continuing operations	79,964		55,548		54,374	37,628
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	4,237		7,311		8,561	11,021
Income from continuing operations	268		742		255	579
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	621,045		595,799		883,190	822,911
Income from continuing operations	53,020		29,166		14,806	(5,283)
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	496,405		464,192		1,008,610	884,925
Income from continuing operations	27,932		22,845		44,755	42,869
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	(112)		24		(94)	43
Income from continuing operations	(1,256)		2,795		(5,442)	(537)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,984		3,855		9,505	11,936
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	142		87		158	119
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	90		168		152	264
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	779		721		3,249	849
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 2,973		$ 2,879		$ 5,946	$ 10,704