Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business segment data [Line Items]
External operating revenues		$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 1,016,826	$ 1,198,342	$ 938,039	$ 860,845	$ 4,128,828	$ 4,014,052	$ 4,115,073
Intersegment operating revenues										0	0	0
Depreciation, depletion and amortization										216,318	211,747	203,084
Interest expense										87,848	91,179	86,871
Income taxes										93,132	70,664	64,422
Earnings (loss) on common stock before income (loss) from discontinued operations										232,411	175,704	184,342
Income (loss) from discontinued operations, net of tax	[1]									(300,354)	(834,080)	109,311
Loss from discontinued operations attributable to noncontrolling interest										(131,691)	(35,256)	(3,895)
Total earnings (loss) on common stock										63,748	(623,120)	297,548
Capital expenditures	[2]									372,420	572,134	434,457
Total assets		6,284,467				6,565,154				6,284,467	6,565,154	7,805,405
Property, plant and equipment		6,510,229				6,387,702				6,510,229	6,387,702
Less accumulated depreciation, depletion and amortization		2,578,902				2,489,322				2,578,902	2,489,322	2,385,202
Net property, plant and equipment		3,931,327				3,898,380				3,931,327	3,898,380	3,520,320
Additional information [Abstract]
Net noncash capital expenditure-related accounts payable and AFUDC										(15,800)	35,300	5,100
Electric:
Business segment data [Line Items]
Depreciation, depletion and amortization										50,220	37,583	35,008
Interest expense										24,982	17,421	15,595
Income taxes										1,449	11,523	12,442
Capital expenditures										111,134	332,876	185,121
Total assets	[3]	1,406,694				1,325,858				1,406,694	1,325,858	1,028,001
Property, plant and equipment	[3]	1,888,613				1,786,148				1,888,613	1,786,148	1,457,101
Natural gas distribution:
Business segment data [Line Items]
Depreciation, depletion and amortization										65,426	64,756	54,700
Interest expense										30,405	29,471	27,217
Income taxes										9,181	11,377	11,350
Capital expenditures										126,272	130,793	120,613
Total assets	[3]	2,099,296				2,038,433				2,099,296	2,038,433	1,935,271
Property, plant and equipment	[3]	2,179,413				2,076,581				2,179,413	2,076,581	1,904,759
Pipeline and midstream:
Business segment data [Line Items]
Depreciation, depletion and amortization										24,885	27,981	29,749
Interest expense										7,903	9,895	9,946
Income taxes										12,408	7,505	12,232
Capital expenditures										34,467	18,315	61,754
Total assets		550,615				591,651				550,615	591,651	651,925
Property, plant and equipment		672,199				758,729				672,199	758,729	818,388
Construction materials and contracting:
Business segment data [Line Items]
Depreciation, depletion and amortization										58,413	65,937	68,557
Interest expense										15,265	15,183	16,368
Income taxes										60,625	41,619	18,586
Capital expenditures										37,845	48,126	37,896
Total assets		1,220,459				1,261,963				1,220,459	1,261,963	1,260,534
Property, plant and equipment		1,549,375				1,553,428				1,549,375	1,553,428	1,529,942
Construction services:
Business segment data [Line Items]
Depreciation, depletion and amortization										15,307	13,420	12,874
Interest expense										4,059	3,959	4,176
Income taxes										17,748	16,432	24,753
Capital expenditures										60,344	38,269	26,942
Total assets		513,093				442,845				513,093	442,845	437,322
Property, plant and equipment		171,361				163,279				171,361	163,279	144,395
Other
Business segment data [Line Items]
Depreciation, depletion and amortization										2,067	2,070	2,196
Interest expense										5,854	15,853	13,823
Income taxes										(2,028)	(9,834)	(11,136)
Capital expenditures										2,358	3,755	2,131
Total assets	[4]	283,255				287,940				283,255	287,940	315,495
Property, plant and equipment		49,268				49,537				49,268	49,537	50,937
Discontinued operations, held-for-sale
Business segment data [Line Items]
Total assets		$ 211,055				$ 616,464				211,055	616,464	2,176,857
Eliminations
Business segment data [Line Items]
Intersegment operating revenues										(57,430)	(78,786)	(136,302)
Interest expense										(620)	(603)	(254)
Income taxes										(6,251)	(7,958)	(3,805)
Earnings (loss) on common stock before income (loss) from discontinued operations	[1]									6,251	5,016	(6,095)
Regulated:
Business segment data [Line Items]
External operating revenues										1,141,454	1,149,038	1,246,903
Intersegment operating revenues										48,794	49,065	45,013
Earnings (loss) on common stock before income (loss) from discontinued operations										91,384	80,201	82,655
Regulated: | Electric:
Business segment data [Line Items]
External operating revenues										322,356	280,615	277,874
Intersegment operating revenues										0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations										42,222	35,914	36,731
Regulated: | Natural gas distribution:
Business segment data [Line Items]
External operating revenues										766,115	817,419	921,986
Intersegment operating revenues										0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations										27,102	23,607	30,484
Regulated: | Pipeline and midstream:
Business segment data [Line Items]
External operating revenues										52,983	51,004	47,043
Intersegment operating revenues										48,794	49,065	45,013
Earnings (loss) on common stock before income (loss) from discontinued operations										22,060	20,680	15,440
Nonregulated:
Business segment data [Line Items]
External operating revenues										2,987,374	2,865,014	2,868,170
Intersegment operating revenues										8,636	29,721	91,289
Earnings (loss) on common stock before income (loss) from discontinued operations										134,776	90,487	107,782
Nonregulated: | Pipeline and midstream:
Business segment data [Line Items]
External operating revenues										39,602	54,281	64,494
Intersegment operating revenues										223	554	742
Earnings (loss) on common stock before income (loss) from discontinued operations										1,375	(7,430)	9,226
Nonregulated: | Construction materials and contracting:
Business segment data [Line Items]
External operating revenues										1,873,696	1,901,530	1,740,089
Intersegment operating revenues										574	2,752	25,241
Earnings (loss) on common stock before income (loss) from discontinued operations										102,687	89,096	51,510
Nonregulated: | Construction services:
Business segment data [Line Items]
External operating revenues										1,072,663	907,767	1,062,055
Intersegment operating revenues										609	18,660	57,474
Earnings (loss) on common stock before income (loss) from discontinued operations										33,945	23,762	54,432
Nonregulated: | Other
Business segment data [Line Items]
External operating revenues										1,413	1,436	1,532
Intersegment operating revenues										7,230	7,755	7,832
Earnings (loss) on common stock before income (loss) from discontinued operations										$ (3,231)	$ (14,941)	$ (7,386)

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

[2] Capital expenditures for 2016, 2015 and 2014 include noncash capital expenditure-related accounts payable and AFUDC, totaling $(15.8) million, $35.3 million and $5.1 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)