Business segment data (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Business segment data [Line Items]
Operating revenues	$ 5,680,733	$ 5,532,750	$ 5,336,776
Depreciation, depletion and amortization	299,214	285,100	256,017
Operating income (loss)	534,219	544,925	481,220
Interest expense	93,984	96,519	98,587
Income taxes	88,920	84,590	63,279
Income from continuing operations	377,731	390,527	335,166
Income (loss) from discontinued operations, net of tax	400	(322)	287
Net income	378,131	390,205	335,453
Capital expenditures	935,806	648,279	636,498
Total assets	8,910,435	8,053,372	7,683,059
Property, Plant and Equipment, Gross	8,972,849	8,300,770
Less accumulated depreciation, depletion and amortization	3,216,461	3,133,831	2,991,486
Net property, plant and equipment	5,756,388	5,166,939	4,917,142
Additional information [Abstract]
Net noncash acquisitions, capital expenditure-related accounts payable and AFUDC	38,700	(15,700)	4,800
Electric
Business segment data [Line Items]
Operating revenues	349,039	331,538	351,725
Depreciation, depletion and amortization	66,750	62,998	58,721
Operating income (loss)	66,335	63,434	64,039
Interest expense	26,712	26,699	25,334
Income taxes	(7,626)	(11,636)	(12,650)
Capital expenditures	82,427	114,676	99,449
Total assets	1,810,695	2,123,693	1,680,194
Property, Plant and Equipment, Gross	2,295,646	2,323,403	2,227,145
Natural gas distribution
Business segment data [Line Items]
Operating revenues	971,364	847,651	865,222
Depreciation, depletion and amortization	86,065	84,580	79,564
Operating income (loss)	89,173	73,082	69,188
Interest expense	37,265	36,798	35,488
Income taxes	8,366	5,746	1,405
Capital expenditures	170,411	193,048	206,799
Total assets	2,929,519	2,302,770	2,574,965
Property, Plant and Equipment, Gross	3,015,164	2,868,853	2,688,123
Pipeline
Business segment data [Line Items]
Operating revenues	82,858	85,346	84,192
Depreciation, depletion and amortization	20,569	21,669	21,220
Operating income (loss)	48,078	49,436	42,796
Interest expense	7,010	7,622	7,198
Income taxes	9,594	7,650	7,219
Capital expenditures	234,803	62,224	71,477
Total assets	913,945	703,377	677,482
Property, Plant and Equipment, Gross	1,051,868	821,697	834,215
Construction materials and contracting
Business segment data [Line Items]
Operating revenues	2,228,306	2,177,585	2,189,651
Depreciation, depletion and amortization	100,974	89,626	77,450
Operating income (loss)	191,077	214,498	179,955
Interest expense	19,218	20,577	23,792
Income taxes	43,459	47,431	37,389
Capital expenditures	417,524	191,635	190,092
Total assets	2,161,653	1,798,493	1,684,161
Property, Plant and Equipment, Gross	2,347,696	2,028,476	1,910,562
Construction services
Business segment data [Line Items]
Operating revenues	2,049,082	2,090,685	1,845,896
Depreciation, depletion and amortization	20,270	23,523	17,038
Operating income (loss)	145,754	147,644	126,426
Interest expense	3,540	4,095	5,331
Income taxes	35,426	35,797	29,973
Capital expenditures	29,140	83,651	60,500
Total assets	845,262	818,662	761,127
Property, Plant and Equipment, Gross	225,758	220,796	213,370
Other
Business segment data [Line Items]
Operating revenues	84	(55)	90
Depreciation, depletion and amortization	4,586	2,704	2,024
Operating income (loss)	(6,198)	(3,169)	(1,184)
Interest expense	342	883	1,859
Income taxes	(299)	(398)	(57)
Capital expenditures	1,501	3,045	8,181
Total assets	248,489	305,157	303,279
Property, Plant and Equipment, Gross	36,717	37,545	35,213
Assets held for sale
Business segment data [Line Items]
Total assets	872	1,220	1,851
Intersegment eliminations
Business segment data [Line Items]
Operating revenues	(77,606)	(76,969)	(77,149)
Interest expense	(103)	(155)	(415)
Intersegment eliminations | Electric
Business segment data [Line Items]
Operating revenues	(543)	(491)	0
Intersegment eliminations | Natural gas distribution
Business segment data [Line Items]
Operating revenues	(576)	(534)	0
Intersegment eliminations | Pipeline
Business segment data [Line Items]
Operating revenues	(59,678)	(58,531)	(56,252)
Intersegment eliminations | Construction materials and contracting
Business segment data [Line Items]
Operating revenues	(624)	(417)	(1,066)
Intersegment eliminations | Construction services
Business segment data [Line Items]
Operating revenues	(2,555)	(5,038)	(3,370)
Intersegment eliminations | Other
Business segment data [Line Items]
Operating revenues	(13,630)	(11,958)	(16,461)
Total intersegment operating revenues
Business segment data [Line Items]
Operating revenues	0	0	0
Regulated Operation
Business segment data [Line Items]
Operating revenues	1,390,343	1,249,146	1,279,304
Income from continuing operations	143,085	135,103	122,535
Regulated Operation | Electric
Business segment data [Line Items]
Operating revenues	349,039	331,538	351,725
Income from continuing operations	51,906	55,601	54,763
Regulated Operation | Natural gas distribution
Business segment data [Line Items]
Operating revenues	971,364	847,651	865,222
Income from continuing operations	51,596	44,049	39,517
Regulated Operation | Pipeline
Business segment data [Line Items]
Operating revenues	69,940	69,957	62,357
Income from continuing operations	39,583	35,453	28,255
Regulated Operation | Intersegment eliminations
Business segment data [Line Items]
Operating revenues	60,108	59,002	56,037
Regulated Operation | Intersegment eliminations | Electric
Business segment data [Line Items]
Operating revenues	543	491	0
Regulated Operation | Intersegment eliminations | Natural gas distribution
Business segment data [Line Items]
Operating revenues	576	534	0
Regulated Operation | Intersegment eliminations | Pipeline
Business segment data [Line Items]
Operating revenues	58,989	57,977	56,037
Nonregulated Operation
Business segment data [Line Items]
Operating revenues	4,290,390	4,283,604	4,057,472
Income from continuing operations	234,646	255,424	212,631
Nonregulated Operation | Pipeline
Business segment data [Line Items]
Operating revenues	12,918	15,389	21,835
Income from continuing operations	1,313	1,559	1,348
Nonregulated Operation | Construction materials and contracting
Business segment data [Line Items]
Operating revenues	2,228,306	2,177,585	2,189,651
Income from continuing operations	129,755	147,325	120,371
Nonregulated Operation | Construction services
Business segment data [Line Items]
Operating revenues	2,049,082	2,090,685	1,845,896
Income from continuing operations	109,402	109,721	92,998
Nonregulated Operation | Other
Business segment data [Line Items]
Operating revenues	84	(55)	90
Income from continuing operations	(5,824)	(3,181)	(2,086)
Nonregulated Operation | Intersegment eliminations
Business segment data [Line Items]
Operating revenues	17,498	17,967	21,112
Nonregulated Operation | Intersegment eliminations | Pipeline
Business segment data [Line Items]
Operating revenues	689	554	215
Nonregulated Operation | Intersegment eliminations | Construction materials and contracting
Business segment data [Line Items]
Operating revenues	624	417	1,066
Nonregulated Operation | Intersegment eliminations | Construction services
Business segment data [Line Items]
Operating revenues	2,555	5,038	3,370
Nonregulated Operation | Intersegment eliminations | Other
Business segment data [Line Items]
Operating revenues	$ 13,630	$ 11,958	$ 16,461