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Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
Business segment data (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 1,303,573		$ 1,064,597		$ 2,394,764	$ 2,040,890
Operating Income (Loss)	97,311		73,992		161,530	143,368
Income from continuing operations	63,145		44,075		104,234	86,036
Income (loss) from discontinued operations, net of tax	(1,320)		(273)		(1,483)	203
Net income	61,825	$ 40,926	43,802	$ 42,437	102,751	86,239
Electric
Segment Reporting Information [Line Items]
Revenues	81,011		78,499		173,578	165,904
Operating Income (Loss)	9,791		13,027		27,779	31,182
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	133,867		129,540		476,014	462,204
Operating Income (Loss)	(2,621)		(4,371)		47,696	44,169
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	28,680		24,060		37,293	32,893
Operating Income (Loss)	10,698		8,482		20,602	16,650
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	595,799		509,388		822,911	722,672
Operating Income (Loss)	46,178		37,301		4,597	10,992
Construction services
Segment Reporting Information [Line Items]
Revenues	464,192		323,020		884,925	657,071
Operating Income (Loss)	31,966		19,356		59,431	39,990
Other
Segment Reporting Information [Line Items]
Revenues	24		90		43	146
Operating Income (Loss)	1,299		197		1,425	385
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(11,281)		(9,981)		(43,285)	(34,490)
Intersegment eliminations \| Electric
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Intersegment eliminations \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Intersegment eliminations \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	(7,513)		(6,539)		(31,468)	(28,298)
Intersegment eliminations \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(168)		(235)		(264)	(336)
Intersegment eliminations \| Construction services
Segment Reporting Information [Line Items]
Revenues	(721)		(540)		(849)	(550)
Intersegment eliminations \| Other
Segment Reporting Information [Line Items]
Revenues	(2,879)		(2,667)		(10,704)	(5,306)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	236,247		226,684		675,864	651,143
Income from continuing operations	7,597		7,521		66,606	58,686
Regulated operation \| Electric
Segment Reporting Information [Line Items]
Revenues	81,011		78,499		173,578	165,904
Income from continuing operations	7,471		9,133		22,976	22,216
Regulated operation \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	133,867		129,540		476,014	462,204
Income from continuing operations	(6,252)		(6,852)		30,248	25,771
Regulated operation \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	21,369		18,645		26,272	23,035
Income from continuing operations	6,378		5,240		13,382	10,699
Regulated operation \| Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	7,426		6,446		31,349	28,182
Regulated operation \| Intersegment eliminations \| Electric
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Regulated operation \| Intersegment eliminations \| Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0		0		0	0
Regulated operation \| Intersegment eliminations \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	7,426		6,446		31,349	28,182
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,067,326		837,913		1,718,900	1,389,747
Income from continuing operations	55,548		36,554		37,628	27,350
Nonregulated operation \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	7,311		5,415		11,021	9,858
Income from continuing operations	742		467		579	288
Nonregulated operation \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	595,799		509,388		822,911	722,672
Income from continuing operations	29,166		24,336		(5,283)	815
Nonregulated operation \| Construction services
Segment Reporting Information [Line Items]
Revenues	464,192		323,020		884,925	657,071
Income from continuing operations	22,845		14,088		42,869	29,179
Nonregulated operation \| Other
Segment Reporting Information [Line Items]
Revenues	24		90		43	146
Income from continuing operations	2,795		(2,337)		(537)	(2,932)
Nonregulated operation \| Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,855		3,535		11,936	6,308
Nonregulated operation \| Intersegment eliminations \| Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	87		93		119	116
Nonregulated operation \| Intersegment eliminations \| Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	168		235		264	336
Nonregulated operation \| Intersegment eliminations \| Construction services
Segment Reporting Information [Line Items]
Revenues	721		540		849	550
Nonregulated operation \| Intersegment eliminations \| Other
Segment Reporting Information [Line Items]
Revenues	$ 2,879		$ 2,667		$ 10,704	$ 5,306