Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business segment data [Line Items]
Operating revenues	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 1,165,239	$ 1,272,548	$ 1,067,639	$ 937,925	$ 4,531,552	$ 4,443,351	$ 4,128,828
Intersegment operating revenues									0	0	0
Depreciation, depletion and amortization									220,205	207,486	216,318
Operating income (loss)	118,351	$ 140,004	$ 73,992	$ 69,376	124,282	$ 155,320	$ 78,660	$ 65,786	401,723	424,048	408,909
Interest expense									84,614	82,788	87,848
Income taxes									47,485	65,041	93,132
Earnings on common stock									269,386	284,215	232,411
Income (loss) from discontinued operations, net of tax									2,932	(3,783)	(300,354)
Loss from discontinued operations attributable to noncontrolling interest									0	0	(131,691)
Earnings on common stock									272,318	280,432	63,748
Capital expenditures									769,303	351,924	372,420
Total assets	6,988,110				6,334,666				6,988,110	6,334,666	6,284,467
Property, plant and equipment	7,397,321				6,770,829				7,397,321	6,770,829
Less accumulated depreciation, depletion and amortization	2,818,644				2,691,641				2,818,644	2,691,641	2,578,902
Net property, plant and equipment	4,578,677				4,079,188				4,578,677	4,079,188	3,931,327
Additional information [Abstract]
Net noncash acquisitions, capital expenditure-related accounts payable and AFUDC									33,400	10,500	(15,800)
Electric
Business segment data [Line Items]
Operating revenues									335,123
Depreciation, depletion and amortization									50,982	47,715	50,220
Operating income (loss)									65,148	79,902	67,929
Interest expense									25,860	25,377	24,982
Income taxes									(6,482)	7,699	1,449
Capital expenditures									186,105	109,107	111,134
Total assets	1,613,822				1,470,922				1,613,822	1,470,922	1,406,694
Property, plant and equipment	2,148,569				1,982,264				2,148,569	1,982,264	1,888,613
Natural gas distribution
Business segment data [Line Items]
Operating revenues									823,247
Depreciation, depletion and amortization									72,486	69,381	65,426
Operating income (loss)									72,336	84,239	66,166
Interest expense									30,768	31,234	30,405
Income taxes									4,075	22,756	9,181
Capital expenditures									205,896	146,981	126,272
Total assets	2,375,871				2,201,081				2,375,871	2,201,081	2,099,296
Property, plant and equipment	2,499,093				2,319,845				2,499,093	2,319,845	2,179,413
Pipeline and midstream
Business segment data [Line Items]
Operating revenues									78,018
Depreciation, depletion and amortization									17,896	16,788	24,885
Operating income (loss)									36,128	36,004	42,864
Interest expense									5,964	4,990	7,903
Income taxes									2,677	12,281	12,408
Capital expenditures									70,057	31,054	34,467
Total assets	616,959				566,295				616,959	566,295	550,615
Property, plant and equipment	764,959				700,284				764,959	700,284	672,199
Construction materials and contracting
Business segment data [Line Items]
Operating revenues									1,925,185
Depreciation, depletion and amortization									61,158	55,862	58,413
Operating income (loss)									141,426	143,230	178,753
Interest expense									17,290	14,778	15,265
Income taxes									28,357	5,405	60,625
Capital expenditures									280,396	44,302	37,845
Total assets	1,508,032				1,238,696				1,508,032	1,238,696	1,220,459
Property, plant and equipment	1,768,006				1,560,048				1,768,006	1,560,048	1,549,375
Construction services
Business segment data [Line Items]
Operating revenues									1,369,772
Depreciation, depletion and amortization									15,728	15,739	15,307
Operating income (loss)									86,764	81,292	53,546
Interest expense									3,551	3,742	4,059
Income taxes									20,000	25,558	17,748
Capital expenditures									25,081	18,630	60,344
Total assets	604,798				591,382				604,798	591,382	513,093
Property, plant and equipment	188,586				177,265				188,586	177,265	171,361
Other
Business segment data [Line Items]
Operating revenues									207
Depreciation, depletion and amortization									1,955	2,001	2,067
Operating income (loss)									(79)	(619)	(349)
Interest expense									2,762	3,564	5,854
Income taxes									(1,142)	(1,809)	(2,028)
Capital expenditures									1,768	1,850	2,358
Total assets	266,111				261,419				266,111	261,419	283,255
Property, plant and equipment	28,108				31,123				28,108	31,123	49,268
Discontinued operations, held-for-sale
Business segment data [Line Items]
Total assets	$ 2,517				$ 4,871				2,517	4,871	211,055
Intersegment eliminations
Business segment data [Line Items]
Intersegment operating revenues									(64,307)	(58,060)	(57,430)
Interest expense									(1,581)	(897)	(620)
Income taxes									0	(6,849)	(6,251)
Earnings on common stock									0	6,849	6,251
Regulated:
Business segment data [Line Items]
Operating revenues									1,213,227	1,244,759	1,141,454
Intersegment operating revenues									50,580	48,867	48,794
Earnings on common stock									111,637	102,211	91,384
Regulated: | Electric
Business segment data [Line Items]
Operating revenues									335,123	342,805	322,356
Intersegment operating revenues									0	0	0
Earnings on common stock									47,000	49,366	42,222
Regulated: | Natural gas distribution
Business segment data [Line Items]
Operating revenues									823,247	848,388	766,115
Intersegment operating revenues									0	0	0
Earnings on common stock									37,732	32,225	27,102
Regulated: | Pipeline and midstream
Business segment data [Line Items]
Operating revenues									54,857	53,566	52,983
Intersegment operating revenues									50,580	48,867	48,794
Earnings on common stock									26,905	20,620	22,060
Nonregulated:
Business segment data [Line Items]
Operating revenues									3,318,325	3,198,592	2,987,374
Intersegment operating revenues									13,727	9,193	8,636
Earnings on common stock									157,749	175,155	134,776
Nonregulated: | Pipeline and midstream
Business segment data [Line Items]
Operating revenues									23,161	19,602	39,602
Intersegment operating revenues									325	178	223
Earnings on common stock									1,554	(127)	1,375
Nonregulated: | Construction materials and contracting
Business segment data [Line Items]
Operating revenues									1,925,185	1,811,964	1,873,696
Intersegment operating revenues									669	565	574
Earnings on common stock									92,647	123,398	102,687
Nonregulated: | Construction services
Business segment data [Line Items]
Operating revenues									1,369,772	1,366,317	1,072,663
Intersegment operating revenues									1,681	1,285	609
Earnings on common stock									64,309	53,306	33,945
Nonregulated: | Other
Business segment data [Line Items]
Operating revenues									207	709	1,413
Intersegment operating revenues									11,052	7,165	7,230
Earnings on common stock									$ (761)	$ (1,422)	$ (3,231)