Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues		$ 1,272,548	$ 1,208,567	$ 3,278,112	$ 3,112,730
Intersegment operating revenues		0	0	0	0
Earnings on common stock before loss from discontinued operations		89,549	88,215	168,822	166,036
Loss from discontinued operations, net of tax	[1]	(2,198)	(5,400)	(3,702)	(299,538)
Loss from discontinued operations attributable to noncontrolling interest		0	0	0	(131,691)
Earnings (loss) on common stock		87,351	82,815	165,120	(1,811)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues		(5,586)	(5,681)	(37,581)	(37,583)
Earnings on common stock before loss from discontinued operations	[1]	1,855	5,599	6,121	5,599
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		206,936	192,079	866,035	783,997
Intersegment operating revenues		3,081	3,278	30,923	30,969
Earnings on common stock before loss from discontinued operations		10,682	5,564	67,986	53,021
Regulated operation [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Revenues		91,531	82,156	254,330	238,911
Intersegment operating revenues		0	0	0	0
Earnings on common stock before loss from discontinued operations		15,712	12,699	37,904	31,840
Regulated operation [Member] | Natural gas distribution [Member]
Segment Reporting Information [Line Items]
Revenues		92,253	87,941	566,364	500,106
Intersegment operating revenues		0	0	0	0
Earnings on common stock before loss from discontinued operations		(10,883)	(12,524)	14,181	4,940
Regulated operation [Member] | Pipeline and midstream [Member]
Segment Reporting Information [Line Items]
Revenues		23,152	21,982	45,341	44,980
Intersegment operating revenues		3,081	3,278	30,923	30,969
Earnings on common stock before loss from discontinued operations		5,853	5,389	15,901	16,241
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		1,065,612	1,016,488	2,412,077	2,328,733
Intersegment operating revenues		2,505	2,403	6,658	6,614
Earnings on common stock before loss from discontinued operations		77,012	77,052	94,715	107,416
Nonregulated operation [Member] | Pipeline and midstream [Member]
Segment Reporting Information [Line Items]
Revenues		5,356	10,732	13,518	29,697
Intersegment operating revenues		38	41	132	161
Earnings on common stock before loss from discontinued operations		95	1,304	(770)	2,043
Nonregulated operation [Member] | Construction materials and contracting [Member]
Segment Reporting Information [Line Items]
Revenues		686,010	724,535	1,388,212	1,475,643
Intersegment operating revenues		142	155	400	370
Earnings on common stock before loss from discontinued operations		63,221	69,523	64,477	88,747
Nonregulated operation [Member] | Construction services [Member]
Segment Reporting Information [Line Items]
Revenues		374,111	280,801	1,009,693	822,226
Intersegment operating revenues		415	3	715	541
Earnings on common stock before loss from discontinued operations		13,144	7,234	32,896	20,198
Nonregulated operation [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenues		135	420	654	1,167
Intersegment operating revenues		1,910	2,204	5,411	5,542
Earnings on common stock before loss from discontinued operations		$ 552	$ (1,009)	$ (1,888)	$ (3,572)

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