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Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,423,657		$ 1,362,928		$ 2,651,595	$ 2,560,302
Operating income	137,689		138,254		222,853	195,325
Income from continuing operations	100,171		99,842		152,287	125,381
Income (loss) from discontinued operations, net of tax	19		(139)		34	(548)
Net income	100,190	$ 52,131	99,703	$ 25,130	152,321	124,833
Electric
Segment Reporting Information [Line Items]
Revenues	83,650		76,438		168,324	162,346
Operating income	12,806		12,810		26,672	27,669
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	153,788		141,494		503,994	467,937
Operating income	3,601		678		57,173	50,677
Pipeline
Segment Reporting Information [Line Items]
Revenues	27,773		27,658		37,685	38,313
Operating income	11,076		12,225		23,612	23,644
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	633,674		621,045		899,333	883,190
Operating income	72,452		74,741		37,563	31,472
Construction services
Segment Reporting Information [Line Items]
Revenues	524,744		496,405		1,042,214	1,008,610
Operating income	37,774		37,882		78,051	61,678
Other
Segment Reporting Information [Line Items]
Revenues	28		(112)		45	(94)
Operating income	(20)		(82)		(218)	185
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(12,440)		(12,222)		(43,375)	(43,306)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(136)		(196)		(271)	(391)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(142)		(185)		(284)	(370)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(7,862)		(7,999)		(34,092)	(33,198)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(142)		(90)		(204)	(152)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(797)		(779)		(1,839)	(3,249)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,361)		(2,973)		(6,685)	(5,946)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	260,568		241,353		702,951	660,035
Income from continuing operations	17,686		19,878		73,808	71,007
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	83,650		76,438		168,324	162,346
Income from continuing operations	10,304		12,153		21,054	23,527
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	153,788		141,494		503,994	467,937
Income from continuing operations	(707)		(959)		35,471	31,410
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	23,130		23,421		30,633	29,752
Income from continuing operations	8,089		8,684		17,283	16,070
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	7,973		8,238		34,240	33,801
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	136		196		271	391
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	142		185		284	370
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	7,695		7,857		33,685	33,040
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,163,089		1,121,575		1,948,644	1,900,267
Income from continuing operations	82,485		79,964		78,479	54,374
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	4,643		4,237		7,052	8,561
Income from continuing operations	1,106		268		810	255
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	633,674		621,045		899,333	883,190
Income from continuing operations	51,396		53,020		20,584	14,806
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	524,744		496,405		1,042,214	1,008,610
Income from continuing operations	28,885		27,932		58,709	44,755
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	28		(112)		45	(94)
Income from continuing operations	1,098		(1,256)		(1,624)	(5,442)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,467		3,984		9,135	9,505
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	167		142		407	158
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	142		90		204	152
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	797		779		1,839	3,249
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,361		$ 2,973		$ 6,685	$ 5,946

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-Subparagraph (SX 210.9-05(b)(2))
-URI http://asc.fasb.org/extlink&oid=120399901&loc=d3e537907-122884

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

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