Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,586,013			$ 1,587,289			$ 4,237,608	$ 4,147,591
Operating income	189,169			203,711			412,023	399,036
Income from continuing operations	138,962			153,015			291,249	278,395
Income (loss) from discontinued operations, net of tax	314			63			348	(484)
Net income	139,276	$ 100,190	$ 52,131	153,078	$ 99,703	$ 25,130	291,597	277,911
Electric
Segment Reporting Information [Line Items]
Revenues	98,930			87,447			267,254	249,793
Operating income	27,310			21,159			53,982	48,827
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	110,378			94,703			614,373	562,641
Operating income	(14,249)			(18,393)			42,925	32,285
Pipeline
Segment Reporting Information [Line Items]
Revenues	30,968			32,068			68,653	70,381
Operating income	12,284			11,762			35,896	35,406
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	831,189			822,439			1,730,521	1,705,629
Operating income	133,712			148,522			171,274	179,994
Construction services
Segment Reporting Information [Line Items]
Revenues	514,523			550,608			1,556,737	1,559,217
Operating income	30,120			40,507			108,172	102,185
Other
Segment Reporting Information [Line Items]
Revenues	25			24			70	(70)
Operating income	(8)			154			(226)	339
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(7,983)			(7,543)			(51,356)	(50,849)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(136)			(195)			(407)	(586)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(142)			(185)			(425)	(555)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(3,908)			(3,622)			(38,000)	(36,820)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(110)			(110)			(314)	(262)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(289)			(425)			(2,128)	(3,674)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,398)			(3,006)			(10,082)	(8,952)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	237,065			210,115			940,017	870,151
Income from continuing operations	15,399			6,985			89,207	77,991
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	98,930			87,447			267,254	249,793
Income from continuing operations	20,564			16,787			41,618	40,314
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	110,378			94,703			614,373	562,641
Income from continuing operations	(15,390)			(17,614)			20,081	13,795
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	27,757			27,965			58,390	57,717
Income from continuing operations	10,225			7,812			27,508	23,882
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,123			3,923			38,362	37,724
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	136			195			407	586
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	142			185			425	555
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,845			3,543			37,530	36,583
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,348,948			1,377,174			3,297,591	3,277,440
Income from continuing operations	123,563			146,030			202,042	200,404
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,211			4,103			10,263	12,664
Income from continuing operations	356			189			1,166	444
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	831,189			822,439			1,730,521	1,705,629
Income from continuing operations	96,282			107,307			116,865	122,113
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	514,523			550,608			1,556,737	1,559,217
Income from continuing operations	23,130			29,789			81,839	74,544
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	25			24			70	(70)
Income from continuing operations	3,795			8,745			2,172	3,303
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,860			3,620			12,994	13,125
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	63			79			470	237
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	110			110			314	262
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	289			425			2,128	3,674
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,398			$ 3,006			$ 10,082	$ 8,952