Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 1,064,597	$ 1,067,639		$ 2,040,890	$ 2,005,565
Intersegment operating revenues	0	0		0	0
Earnings on common stock before income (loss) from discontinued operations	44,075	43,805		86,036	79,272
Income (loss) from discontinued operations, net of tax	(273)	(3,190)	[1]	203	(1,504)	[1]
Earnings on common stock	43,802	40,615		86,239	77,768
Electric
Segment Reporting Information [Line Items]
Revenues	78,499			165,904
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	129,540			462,204
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	24,060			32,893
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	509,388			722,672
Construction services
Segment Reporting Information [Line Items]
Revenues	323,020			657,071
Other
Segment Reporting Information [Line Items]
Revenues	90			146
Intersegment eliminations
Segment Reporting Information [Line Items]
Intersegment operating revenues	(9,981)	(8,637)		(34,490)	(31,994)
Earnings on common stock before income (loss) from discontinued operations	0	2,093	[1]	0	4,266	[1]
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	226,684	225,485		651,143	659,100
Intersegment operating revenues	6,446	6,353		28,182	27,841
Earnings on common stock before income (loss) from discontinued operations	7,521	10,554		58,686	57,303
Regulated operation [Member] | Electric
Segment Reporting Information [Line Items]
Revenues	78,499	74,574		165,904	162,799
Intersegment operating revenues	0	0		0	0
Earnings on common stock before income (loss) from discontinued operations	9,133	7,859		22,216	22,191
Regulated operation [Member] | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	129,540	131,592		462,204	474,111
Intersegment operating revenues	0	0		0	0
Earnings on common stock before income (loss) from discontinued operations	(6,852)	(2,797)		25,771	25,064
Regulated operation [Member] | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	18,645	19,319		23,035	22,190
Intersegment operating revenues	6,446	6,353		28,182	27,841
Earnings on common stock before income (loss) from discontinued operations	5,240	5,492		10,699	10,048
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	837,913	842,154		1,389,747	1,346,465
Intersegment operating revenues	3,535	2,284		6,308	4,153
Earnings on common stock before income (loss) from discontinued operations	36,554	31,158		27,350	17,703
Nonregulated operation [Member] | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	5,415	4,520		9,858	8,163
Intersegment operating revenues	93	59		116	93
Earnings on common stock before income (loss) from discontinued operations	467	(238)		288	(865)
Nonregulated operation [Member] | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	509,388	501,426		722,672	702,203
Intersegment operating revenues	235	172		336	258
Earnings on common stock before income (loss) from discontinued operations	24,336	21,168		815	1,255
Nonregulated operation [Member] | Construction services
Segment Reporting Information [Line Items]
Revenues	323,020	336,009		657,071	635,580
Intersegment operating revenues	540	295		550	301
Earnings on common stock before income (loss) from discontinued operations	14,088	12,391		29,179	19,753
Nonregulated operation [Member] | Other
Segment Reporting Information [Line Items]
Revenues	90	199		146	519
Intersegment operating revenues	2,667	1,758		5,306	3,501
Earnings on common stock before income (loss) from discontinued operations	$ (2,337)	$ (2,163)		$ (2,932)	$ (2,440)

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