Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business segment data [Line Items]
Operating revenues	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 5,336,776	$ 4,531,552	$ 4,443,351
Intersegment operating revenues:									0	0	0
Depreciation, depletion and amortization									256,017	220,205	207,486
Operating income (loss):	130,221	189,470	97,311	64,218	118,351	140,004	73,992	69,376	481,220	401,723	424,048
Interest expense									98,587	84,614	82,788
Income taxes									63,279	47,485	65,041
Earnings on common stock before income (loss) from discontinued operations									335,166	269,386	284,215
Income (loss) from discontinued operations, net of tax (a)	261	$ 1,509	$ (1,320)	$ (163)	2,846	$ (118)	$ (273)	$ 477	287	2,932	(3,783)
Earnings on common stock									335,453	272,318	280,432
Capital expenditures:									636,498	769,303	351,924
Total assets	7,683,059				6,988,110				7,683,059	6,988,110	6,334,666
Property, plant and equipment	7,908,628				7,397,321				7,908,628	7,397,321
Less accumulated depreciation, depletion and amortization	2,991,486				2,818,644				2,991,486	2,818,644	2,691,641
Net property, plant and equipment	4,917,142				4,578,677				4,917,142	4,578,677	4,079,188
Additional information [Abstract]
Net noncash acquisitions, capital expenditure-related accounts payable and AFUDC									4,800	33,400	10,500
Electric
Business segment data [Line Items]
Operating revenues									351,725	335,123
Depreciation, depletion and amortization									58,721	50,982	47,715
Operating income (loss):									64,039	65,148	79,902
Interest expense									25,334	25,860	25,377
Income taxes									(12,650)	(6,482)	7,699
Capital expenditures:									99,449	186,105	109,107
Total assets	1,680,194				1,613,822				1,680,194	1,613,822	1,470,922
Property, plant and equipment	2,227,145				2,148,569				2,227,145	2,148,569	1,982,264
Natural gas distribution
Business segment data [Line Items]
Operating revenues									865,222	823,247
Depreciation, depletion and amortization									79,564	72,486	69,381
Operating income (loss):									69,188	72,336	84,239
Interest expense									35,488	30,768	31,234
Income taxes									1,405	4,075	22,756
Capital expenditures:									206,799	205,896	146,981
Total assets	2,574,965				2,375,871				2,574,965	2,375,871	2,201,081
Property, plant and equipment	2,688,123				2,499,093				2,688,123	2,499,093	2,319,845
Pipeline and midstream
Business segment data [Line Items]
Operating revenues									84,192	78,018
Depreciation, depletion and amortization									21,220	17,896	16,788
Operating income (loss):									42,796	36,128	36,004
Interest expense									7,198	5,964	4,990
Income taxes									7,219	2,677	12,281
Capital expenditures:									71,477	70,057	31,054
Total assets	677,482				616,959				677,482	616,959	566,295
Property, plant and equipment	834,215				764,959				834,215	764,959	700,284
Construction materials and contracting
Business segment data [Line Items]
Operating revenues									2,189,651	1,925,185
Depreciation, depletion and amortization									77,450	61,158	55,862
Operating income (loss):									179,955	141,426	143,230
Interest expense									23,792	17,290	14,778
Income taxes									37,389	28,357	5,405
Capital expenditures:									190,092	280,396	44,302
Total assets	1,684,161				1,508,032				1,684,161	1,508,032	1,238,696
Property, plant and equipment	1,910,562				1,768,006				1,910,562	1,768,006	1,560,048
Construction services
Business segment data [Line Items]
Operating revenues									1,845,896	1,369,772
Depreciation, depletion and amortization									17,038	15,728	15,739
Operating income (loss):									126,426	86,764	81,292
Interest expense									5,331	3,551	3,742
Income taxes									29,973	20,000	25,558
Capital expenditures:									60,500	25,081	18,630
Total assets	761,127				604,798				761,127	604,798	591,382
Property, plant and equipment	213,370				188,586				213,370	188,586	177,265
Other
Business segment data [Line Items]
Operating revenues									90	207
Depreciation, depletion and amortization									2,024	1,955	2,001
Operating income (loss):									(1,184)	(79)	(619)
Interest expense									1,859	2,762	3,564
Income taxes									(57)	(1,142)	(1,809)
Capital expenditures:									8,181	1,768	1,850
Total assets	303,279				266,111				303,279	266,111	261,419
Property, plant and equipment	35,213				28,108				35,213	28,108	31,123
Assets held for sale
Business segment data [Line Items]
Total assets	$ 1,851				$ 2,517				1,851	2,517	4,871
Intersegment eliminations
Business segment data [Line Items]
Intersegment operating revenues:									(77,149)	(64,307)	(58,060)
Interest expense									(415)	(1,581)	(897)
Income taxes									0	0	(6,849)
Earnings on common stock before income (loss) from discontinued operations									0	0	6,849
Regulated:
Business segment data [Line Items]
Operating revenues									1,279,304	1,213,227	1,244,759
Intersegment operating revenues:									56,037	50,580	48,867
Earnings on common stock before income (loss) from discontinued operations									122,535	111,637	102,211
Regulated: | Electric
Business segment data [Line Items]
Operating revenues									351,725	335,123	342,805
Intersegment operating revenues:									0	0	0
Earnings on common stock before income (loss) from discontinued operations									54,763	47,000	49,366
Regulated: | Natural gas distribution
Business segment data [Line Items]
Operating revenues									865,222	823,247	848,388
Intersegment operating revenues:									0	0	0
Earnings on common stock before income (loss) from discontinued operations									39,517	37,732	32,225
Regulated: | Pipeline and midstream
Business segment data [Line Items]
Operating revenues									62,357	54,857	53,566
Intersegment operating revenues:									56,037	50,580	48,867
Earnings on common stock before income (loss) from discontinued operations									28,255	26,905	20,620
Nonregulated:
Business segment data [Line Items]
Operating revenues									4,057,472	3,318,325	3,198,592
Intersegment operating revenues:									21,112	13,727	9,193
Earnings on common stock before income (loss) from discontinued operations									212,631	157,749	175,155
Nonregulated: | Pipeline and midstream
Business segment data [Line Items]
Operating revenues									21,835	23,161	19,602
Intersegment operating revenues:									215	325	178
Earnings on common stock before income (loss) from discontinued operations									1,348	1,554	(127)
Nonregulated: | Construction materials and contracting
Business segment data [Line Items]
Operating revenues									2,189,651	1,925,185	1,811,964
Intersegment operating revenues:									1,066	669	565
Earnings on common stock before income (loss) from discontinued operations									120,371	92,647	123,398
Nonregulated: | Construction services
Business segment data [Line Items]
Operating revenues									1,845,896	1,369,772	1,366,317
Intersegment operating revenues:									3,370	1,681	1,285
Earnings on common stock before income (loss) from discontinued operations									92,998	64,309	53,306
Nonregulated: | Other
Business segment data [Line Items]
Operating revenues									90	207	709
Intersegment operating revenues:									16,461	11,052	7,165
Earnings on common stock before income (loss) from discontinued operations									$ (2,086)	$ (761)	$ (1,422)