Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business segment data [Line Items]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Depreciation, depletion and amortization									285,100	256,017	220,205
Operating income (loss)	145,890	203,711	138,254	57,070	130,221	189,470	97,311	64,218	544,925	481,220	401,723
Interest expense									96,519	98,587	84,614
Income taxes									84,590	63,279	47,485
Income from continuing operations	112,131	153,015	99,842	25,539	94,804	136,128	63,145	41,089	390,527	335,166	269,386
Income (loss) from discontinued operations, net of tax	163	63	(139)	(409)	261	1,509	(1,320)	(163)	(322)	287	2,932
Net income	112,294	$ 153,078	$ 99,703	$ 25,130	95,065	$ 137,637	$ 61,825	$ 40,926	390,205	335,453	272,318
Capital expenditures									648,279	636,498	769,303
Total assets	8,053,372				7,683,059				8,053,372	7,683,059	6,988,110
Property, Plant and Equipment, Gross	8,300,770				7,908,628				8,300,770	7,908,628
Less accumulated depreciation, depletion and amortization	3,133,831				2,991,486				3,133,831	2,991,486	2,818,644
Net property, plant and equipment	5,166,939				4,917,142				5,166,939	4,917,142	4,578,677
Additional information [Abstract]
Net noncash acquisitions, capital expenditure-related accounts payable and AFUDC									(15,700)	4,800	33,400
Electric
Business segment data [Line Items]
Operating revenues									331,538	351,725	335,123
Depreciation, depletion and amortization									62,998	58,721	50,982
Operating income (loss)									63,434	64,039	65,148
Interest expense									26,699	25,334	25,860
Income taxes									(11,636)	(12,650)	(6,482)
Capital expenditures									114,676	99,449	186,105
Total assets	2,123,693				1,680,194				2,123,693	1,680,194	1,613,822
Property, Plant and Equipment, Gross	2,323,403				2,227,145				2,323,403	2,227,145	2,148,569
Natural gas distribution
Business segment data [Line Items]
Operating revenues									847,651	865,222	823,247
Depreciation, depletion and amortization									84,580	79,564	72,486
Operating income (loss)									73,082	69,188	72,336
Interest expense									36,798	35,488	30,768
Income taxes									5,746	1,405	4,075
Capital expenditures									193,048	206,799	205,896
Total assets	2,302,770				2,574,965				2,302,770	2,574,965	2,375,871
Property, Plant and Equipment, Gross	2,868,853				2,688,123				2,868,853	2,688,123	2,499,093
Pipeline
Business segment data [Line Items]
Operating revenues									85,346	84,192	78,018
Depreciation, depletion and amortization									21,669	21,220	17,896
Operating income (loss)									49,436	42,796	36,128
Interest expense									7,622	7,198	5,964
Income taxes									7,650	7,219	2,677
Capital expenditures									62,224	71,477	70,057
Total assets	703,377				677,482				703,377	677,482	616,959
Property, Plant and Equipment, Gross	821,697				834,215				821,697	834,215	764,959
Construction materials and contracting
Business segment data [Line Items]
Operating revenues									2,177,585	2,189,651	1,925,185
Depreciation, depletion and amortization									89,626	77,450	61,158
Operating income (loss)									214,498	179,955	141,426
Interest expense									20,577	23,792	17,290
Income taxes									47,431	37,389	28,357
Capital expenditures									191,635	190,092	280,396
Total assets	1,798,493				1,684,161				1,798,493	1,684,161	1,508,032
Property, Plant and Equipment, Gross	2,028,476				1,910,562				2,028,476	1,910,562	1,768,006
Construction services
Business segment data [Line Items]
Operating revenues									2,090,685	1,845,896	1,369,772
Depreciation, depletion and amortization									23,523	17,038	15,728
Operating income (loss)									147,644	126,426	86,764
Interest expense									4,095	5,331	3,551
Income taxes									35,797	29,973	20,000
Capital expenditures									83,651	60,500	25,081
Total assets	818,662				761,127				818,662	761,127	604,798
Property, Plant and Equipment, Gross	220,796				213,370				220,796	213,370	188,586
Other
Business segment data [Line Items]
Operating revenues									(55)	90	207
Depreciation, depletion and amortization									2,704	2,024	1,955
Operating income (loss)									(3,169)	(1,184)	(79)
Interest expense									883	1,859	2,762
Income taxes									(398)	(57)	(1,142)
Capital expenditures									3,045	8,181	1,768
Total assets	305,157				303,279				305,157	303,279	266,111
Property, Plant and Equipment, Gross	37,545				35,213				37,545	35,213	28,108
Assets held for sale
Business segment data [Line Items]
Total assets	$ 1,220				$ 1,851				1,220	1,851	2,517
Intersegment eliminations
Business segment data [Line Items]
Operating revenues									(76,969)	(77,149)	(64,307)
Interest expense									(155)	(415)	(1,581)
Intersegment eliminations | Electric
Business segment data [Line Items]
Operating revenues									(491)	0	0
Intersegment eliminations | Natural gas distribution
Business segment data [Line Items]
Operating revenues									(534)	0	0
Intersegment eliminations | Pipeline
Business segment data [Line Items]
Operating revenues									(58,531)	(56,252)	(50,905)
Intersegment eliminations | Construction materials and contracting
Business segment data [Line Items]
Operating revenues									(417)	(1,066)	(669)
Intersegment eliminations | Construction services
Business segment data [Line Items]
Operating revenues									(5,038)	(3,370)	(1,681)
Intersegment eliminations | Other
Business segment data [Line Items]
Operating revenues									(11,958)	(16,461)	(11,052)
Total intersegment operating revenues
Business segment data [Line Items]
Operating revenues									0	0	0
Regulated Operation
Business segment data [Line Items]
Operating revenues									1,249,146	1,279,304	1,213,227
Income from continuing operations									135,103	122,535	111,637
Regulated Operation | Electric
Business segment data [Line Items]
Operating revenues									331,538	351,725	335,123
Income from continuing operations									55,601	54,763	47,000
Regulated Operation | Natural gas distribution
Business segment data [Line Items]
Operating revenues									847,651	865,222	823,247
Income from continuing operations									44,049	39,517	37,732
Regulated Operation | Pipeline
Business segment data [Line Items]
Operating revenues									69,957	62,357	54,857
Income from continuing operations									35,453	28,255	26,905
Regulated Operation | Intersegment eliminations
Business segment data [Line Items]
Operating revenues									59,002	56,037	50,580
Regulated Operation | Intersegment eliminations | Electric
Business segment data [Line Items]
Operating revenues									491	0	0
Regulated Operation | Intersegment eliminations | Natural gas distribution
Business segment data [Line Items]
Operating revenues									534	0	0
Regulated Operation | Intersegment eliminations | Pipeline
Business segment data [Line Items]
Operating revenues									57,977	56,037	50,580
Unregulated Operation
Business segment data [Line Items]
Operating revenues									4,283,604	4,057,472	3,318,325
Income from continuing operations									255,424	212,631	157,749
Unregulated Operation | Pipeline
Business segment data [Line Items]
Operating revenues									15,389	21,835	23,161
Income from continuing operations									1,559	1,348	1,554
Unregulated Operation | Construction materials and contracting
Business segment data [Line Items]
Operating revenues									2,177,585	2,189,651	1,925,185
Income from continuing operations									147,325	120,371	92,647
Unregulated Operation | Construction services
Business segment data [Line Items]
Operating revenues									2,090,685	1,845,896	1,369,772
Income from continuing operations									109,721	92,998	64,309
Unregulated Operation | Other
Business segment data [Line Items]
Operating revenues									(55)	90	207
Income from continuing operations									(3,181)	(2,086)	(761)
Unregulated Operation | Intersegment eliminations
Business segment data [Line Items]
Operating revenues									17,967	21,112	13,727
Unregulated Operation | Intersegment eliminations | Pipeline
Business segment data [Line Items]
Operating revenues									554	215	325
Unregulated Operation | Intersegment eliminations | Construction materials and contracting
Business segment data [Line Items]
Operating revenues									417	1,066	669
Unregulated Operation | Intersegment eliminations | Construction services
Business segment data [Line Items]
Operating revenues									5,038	3,370	1,681
Unregulated Operation | Intersegment eliminations | Other
Business segment data [Line Items]
Operating revenues									$ 11,958	$ 16,461	$ 11,052