Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues		$ 976,293	$ 937,925
Intersegment operating revenues		0	0
Earnings on common stock before income from discontinued operations		41,960	35,467
Income from discontinued operations, net of tax		477	1,687	[1]
Earnings on common stock		42,437	37,154
Electric
Segment Reporting Information [Line Items]
Revenues		87,404
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues		332,664
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues		8,833
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues		213,284
Construction services
Segment Reporting Information [Line Items]
Revenues		334,050
Other
Segment Reporting Information [Line Items]
Revenues		58
Intersegment eliminations
Segment Reporting Information [Line Items]
Intersegment operating revenues		(24,509)	(23,358)
Earnings on common stock before income from discontinued operations	[1]	0	2,173
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		424,459	433,614
Intersegment operating revenues		21,735	21,489
Earnings on common stock before income from discontinued operations		51,166	46,751
Regulated operation [Member] | Electric
Segment Reporting Information [Line Items]
Revenues		87,404	88,225
Intersegment operating revenues		0	0
Earnings on common stock before income from discontinued operations		13,084	14,333
Regulated operation [Member] | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues		332,664	342,519
Intersegment operating revenues		0	0
Earnings on common stock before income from discontinued operations		32,623	27,861
Regulated operation [Member] | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues		4,391	2,870
Intersegment operating revenues		21,735	21,489
Earnings on common stock before income from discontinued operations		5,459	4,557
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		551,834	504,311
Intersegment operating revenues		2,774	1,869
Earnings on common stock before income from discontinued operations		(9,206)	(13,457)
Nonregulated operation [Member] | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues		4,442	3,643
Intersegment operating revenues		24	34
Earnings on common stock before income from discontinued operations		(179)	(628)
Nonregulated operation [Member] | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues		213,284	200,776
Intersegment operating revenues		101	86
Earnings on common stock before income from discontinued operations		(23,521)	(19,912)
Nonregulated operation [Member] | Construction services
Segment Reporting Information [Line Items]
Revenues		334,050	299,572
Intersegment operating revenues		11	6
Earnings on common stock before income from discontinued operations		15,090	7,362
Nonregulated operation [Member] | Other
Segment Reporting Information [Line Items]
Revenues		58	320
Intersegment operating revenues		2,638	1,743
Earnings on common stock before income from discontinued operations		$ (596)	$ (279)

[1] Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.