Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business segment data [Line Items]
Operating revenues		$ 1,165,239	$ 1,272,548	$ 1,067,639	$ 937,925	$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 4,443,351	$ 4,128,828	$ 4,014,052
Intersegment operating revenues										0	0	0
Depreciation, depletion and amortization										207,486	216,318	211,747
Interest expense										82,788	87,848	91,179
Income taxes										65,041	93,132	70,664
Earnings (loss) on common stock:										284,215	232,411	175,704
Loss from discontinued operations, net of tax	[1]									(3,783)	(300,354)	(834,080)
Loss from discontinued operations attributable to noncontrolling interest										0	(131,691)	(35,256)
Total earnings (loss) on common stock										280,432	63,748	(623,120)
Capital expenditures	[2]									351,924	372,420	572,134
Total assets		6,334,666				6,284,467				6,334,666	6,284,467	6,565,154
Property, plant and equipment		6,770,829				6,510,229				6,770,829	6,510,229
Less accumulated depreciation, depletion and amortization		2,691,641				2,578,902				2,691,641	2,578,902	2,489,322
Net property, plant and equipment		4,079,188				3,931,327				4,079,188	3,931,327	3,898,380
Additional information [Abstract]
Net noncash capital expenditure-related accounts payable and AFUDC										10,500	(15,800)	35,300
Electric:
Business segment data [Line Items]
Depreciation, depletion and amortization										47,715	50,220	37,583
Interest expense										25,377	24,982	17,421
Income taxes										7,699	1,449	11,523
Capital expenditures										109,107	111,134	332,876
Total assets	[3]	1,470,922				1,406,694				1,470,922	1,406,694	1,325,858
Property, plant and equipment	[3]	1,982,264				1,888,613				1,982,264	1,888,613	1,786,148
Natural gas distribution:
Business segment data [Line Items]
Depreciation, depletion and amortization										69,381	65,426	64,756
Interest expense										31,234	30,405	29,471
Income taxes										22,756	9,181	11,377
Capital expenditures										146,981	126,272	130,793
Total assets	[3]	2,201,081				2,099,296				2,201,081	2,099,296	2,038,433
Property, plant and equipment	[3]	2,319,845				2,179,413				2,319,845	2,179,413	2,076,581
Pipeline and midstream:
Business segment data [Line Items]
Depreciation, depletion and amortization										16,788	24,885	27,981
Interest expense										4,990	7,903	9,895
Income taxes										12,281	12,408	7,505
Capital expenditures										31,054	34,467	18,315
Total assets		566,295				550,615				566,295	550,615	591,651
Property, plant and equipment		700,284				672,199				700,284	672,199	758,729
Construction materials and contracting:
Business segment data [Line Items]
Depreciation, depletion and amortization										55,862	58,413	65,937
Interest expense										14,778	15,265	15,183
Income taxes										5,405	60,625	41,619
Capital expenditures										44,302	37,845	48,126
Total assets		1,238,696				1,220,459				1,238,696	1,220,459	1,261,963
Property, plant and equipment		1,560,048				1,549,375				1,560,048	1,549,375	1,553,428
Construction services:
Business segment data [Line Items]
Depreciation, depletion and amortization										15,739	15,307	13,420
Interest expense										3,742	4,059	3,959
Income taxes										25,558	17,748	16,432
Capital expenditures										18,630	60,344	38,269
Total assets		591,382				513,093				591,382	513,093	442,845
Property, plant and equipment		177,265				171,361				177,265	171,361	163,279
Other
Business segment data [Line Items]
Depreciation, depletion and amortization										2,001	2,067	2,070
Interest expense										3,564	5,854	15,853
Income taxes										(1,809)	(2,028)	(9,834)
Capital expenditures										1,850	2,358	3,755
Total assets	[4]	261,419				283,255				261,419	283,255	287,940
Property, plant and equipment		31,123				49,268				31,123	49,268	49,537
Discontinued operations, held-for-sale
Business segment data [Line Items]
Total assets		$ 4,871				$ 211,055				4,871	211,055	616,464
Eliminations
Business segment data [Line Items]
Intersegment operating revenues										(58,060)	(57,430)	(78,786)
Interest expense										(897)	(620)	(603)
Income taxes										(6,849)	(6,251)	(7,958)
Earnings (loss) on common stock:	[1]									6,849	6,251	5,016
Regulated:
Business segment data [Line Items]
Operating revenues										1,244,759	1,141,454	1,149,038
Intersegment operating revenues										48,867	48,794	49,065
Earnings (loss) on common stock:										102,211	91,384	80,201
Regulated: | Electric:
Business segment data [Line Items]
Operating revenues										342,805	322,356	280,615
Intersegment operating revenues										0	0	0
Earnings (loss) on common stock:										49,366	42,222	35,914
Regulated: | Natural gas distribution:
Business segment data [Line Items]
Operating revenues										848,388	766,115	817,419
Intersegment operating revenues										0	0	0
Earnings (loss) on common stock:										32,225	27,102	23,607
Regulated: | Pipeline and midstream:
Business segment data [Line Items]
Operating revenues										53,566	52,983	51,004
Intersegment operating revenues										48,867	48,794	49,065
Earnings (loss) on common stock:										20,620	22,060	20,680
Nonregulated:
Business segment data [Line Items]
Operating revenues										3,198,592	2,987,374	2,865,014
Intersegment operating revenues										9,193	8,636	29,721
Earnings (loss) on common stock:										175,155	134,776	90,487
Nonregulated: | Pipeline and midstream:
Business segment data [Line Items]
Operating revenues										19,602	39,602	54,281
Intersegment operating revenues										178	223	554
Earnings (loss) on common stock:										(127)	1,375	(7,430)
Nonregulated: | Construction materials and contracting:
Business segment data [Line Items]
Operating revenues										1,811,964	1,873,696	1,901,530
Intersegment operating revenues										565	574	2,752
Earnings (loss) on common stock:										123,398	102,687	89,096
Nonregulated: | Construction services:
Business segment data [Line Items]
Operating revenues										1,366,317	1,072,663	907,767
Intersegment operating revenues										1,285	609	18,660
Earnings (loss) on common stock:										53,306	33,945	23,762
Nonregulated: | Other
Business segment data [Line Items]
Operating revenues										709	1,413	1,436
Intersegment operating revenues										7,165	7,230	7,755
Earnings (loss) on common stock:										$ (1,422)	$ (3,231)	$ (14,941)

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

[2] Capital expenditures for 2017, 2016 and 2015 include noncash capital expenditure-related accounts payable and AFUDC, totaling $10.5 million, $(15.8) million and $35.3 million, respectively.

[3] Includes allocations of common utility property

[4] Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets)