MDU Resources (MDU) 10-Q 4 Nov 21 2021 Q3 Quarterly report Financial data

Cover page

Cover page - shares	9 Months Ended
	Sep. 30, 2021	Oct. 28, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	1-03480
Entity Registrant Name	MDU RESOURCES GROUP INC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	30-1133956
Entity Address, Address Line One	1200 West Century Avenue
Entity Address, Address Line Two	P.O. Box 5650
Entity Address, City or Town	Bismarck
Entity Address, State or Province	ND
Entity Address, Postal Zip Code	58506-5650
City Area Code	701
Local Phone Number	530-1000
Title of 12(b) Security	Common Stock, par value $1.00 per share
Trading Symbol	MDU
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity common stock, shares outstanding		203,350,740
Entity Central Index Key	0000067716
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating revenues:
Operating revenues:	$ 1,586,013	$ 1,587,289	$ 4,237,608	$ 4,147,591
Operating expenses:
Operation and maintenance:	1,212,898	1,214,019	3,102,353	3,064,189
Purchased natural gas sold	41,135	31,524	280,584	253,780
Depreciation, depletion and amortization	75,229	72,084	222,613	212,832
Taxes, other than income	48,096	50,501	163,819	167,197
Electric fuel and purchased power	19,486	15,450	56,216	50,557
Total operating expenses	1,396,844	1,383,578	3,825,585	3,748,555
Operating income	189,169	203,711	412,023	399,036
Other income	5,930	4,612	18,310	13,669
Interest expense	23,389	23,761	70,224	73,132
Income before income taxes	171,710	184,562	360,109	339,573
Income taxes	32,748	31,547	68,860	61,178
Income from continuing operations	138,962	153,015	291,249	278,395
Income (loss) from discontinued operations, net of tax	314	63	348	(484)
Net income	$ 139,276	$ 153,078	$ 291,597	$ 277,911
Earnings per share - basic:
Income from continuing operations	$ 0.68	$ 0.76	$ 1.44	$ 1.39
Discontinued operations, net of tax	0	0	0	0
Earnings per share - basic	0.68	0.76	1.44	1.39
Earnings per share - diluted:
Income from continuing operations	0.68	0.76	1.44	1.39
Discontinued operations, net of tax	0	0	0	0
Earnings per share - diluted	$ 0.68	$ 0.76	$ 1.44	$ 1.39
Weighted average common shares outstanding - basic	202,863	200,522	201,647	200,495
Weighted average common shares outstanding - diluted	203,190	200,619	201,955	200,515
Regulated operation
Operating revenues:
Operating revenues:	$ 237,065	$ 210,115	$ 940,017	$ 870,151
Operating expenses:
Operation and maintenance:	90,058	89,080	273,796	259,791
Income from continuing operations	15,399	6,985	89,207	77,991
Nonregulated operation
Operating revenues:
Operating revenues:	1,348,948	1,377,174	3,297,591	3,277,440
Operating expenses:
Operation and maintenance:	1,122,840	1,124,939	2,828,557	2,804,398
Income from continuing operations	$ 123,563	$ 146,030	$ 202,042	$ 200,404

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net income	$ 139,276	$ 153,078	$ 291,597	$ 277,911
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	36	37	109	109
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	111	111	334	334
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, tax	151	152	462	456
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	466	471	1,389	1,413
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(15)	(13)	(40)	19
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(54)	(50)	(150)	72
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	6	7	21	9
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	20	28	80	34
Net unrealized gain (loss) on available-for-sale investments	(34)	(22)	(70)	106
Other comprehensive income	543	560	1,653	1,853
Comprehensive income attributable to common stockholders	$ 139,819	$ 153,638	$ 293,250	$ 279,764

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Current assets:
Cash and cash equivalents	$ 57,241	$ 59,547	$ 66,070
Receivables, net	979,862	873,986	1,009,433
Inventories	312,285	291,167	286,223
Current regulatory assets	133,135	68,527	79,125
Prepayments and other current assets	83,364	44,120	61,706
Total current assets	1,565,887	1,337,347	1,502,557
Noncurrent assets:
Property, plant and equipment	8,634,737	8,300,770	8,203,751
Less accumulated depreciation, depletion and amortization	3,175,756	3,133,831	3,115,805
Net property, plant and equipment	5,458,981	5,166,939	5,087,946
Goodwill	717,646	714,963	712,677
Other intangible assets, net	22,241	25,496	26,376
Regulatory assets	379,245	379,381	369,764
Investments	171,859	165,022	158,440
Operating lease right-of-use assets	109,439	120,113	120,534
Other	143,565	144,111	147,205
Total noncurrent assets	7,002,976	6,716,025	6,622,942
Total assets	8,568,863	8,053,372	8,125,499
Current liabilities:
Short-term borrowings	50,000	50,000	75,000
Long-term debt due within one year	1,548	1,555	1,558
Accounts payable	525,704	426,264	436,718
Taxes payable	87,455	88,844	96,708
Dividends payable	43,212	42,611	41,608
Accrued compensation	105,867	90,629	110,730
Operating lease liabilities due within one year	30,502	33,655	33,770
Regulatory liabilities due within one year	16,491	31,450	39,837
Other accrued liabilities	212,140	198,514	231,061
Total current liabilities	1,072,919	963,522	1,066,990
Noncurrent liabilities:
Long-term debt	2,326,718	2,211,575	2,268,732
Deferred income taxes	557,582	516,098	533,524
Asset retirement obligations	455,522	440,356	425,567
Regulatory liabilities	426,141	428,075	434,936
Operating lease liabilities	79,397	86,868	87,140
Other	318,306	327,773	295,845
Total noncurrent liabilities	4,163,666	4,010,745	4,045,744
Commitments and contingencies
Stockholders' equity:
Common stock	203,889	201,061	201,061
Other paid-in capital	1,458,208	1,371,385	1,366,494
Retained earnings	1,720,232	1,558,363	1,489,085
Accumulated other comprehensive loss	(46,425)	(48,078)	(40,249)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)	(3,626)
Total stockholders' equity	3,332,278	3,079,105	3,012,765
Total liabilities and stockholders' equity	$ 8,568,863	$ 8,053,372	$ 8,125,499
Common Stock, Shares Authorized	500,000,000	500,000,000	500,000,000
Common Stock, Par or Stated Value Per Share	$ 1	$ 1	$ 1
Common Stock, Shares, Issued	203,889,661	201,061,198	201,061,198
Treasury Stock, Common, Shares	538,921	538,921	538,921

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common stock	Other paid-in capital	Retained earnings	Accumulated other comprehensive loss	Treasury stock
Common stock balance (in shares) at Dec. 31, 2019		200,922,790
Treasury stock (in shares) at Dec. 31, 2019						(538,921)
Balance at Dec. 31, 2019	$ 2,847,246	$ 200,923	$ 1,355,404	$ 1,336,647	$ (42,102)	$ (3,626)
Net income	25,130			25,130
Other comprehensive income	707				707
Dividends declared on common stock	(41,789)			(41,789)
Stock-based compensation	2,250		2,250
Issuance of common stock upon vesting of stock-based compensation, tax withholding shares		26,406
Issuance of common stock upon vesting of stock-based compensation, tax withholding shares value		$ 26
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings			(388)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(362)
Issuance of common stock (shares)		112,002
Issuance of common stock	3,410	$ 112	3,298
Common stock balance (in shares) at Mar. 31, 2020		201,061,198
Treasury stock (in shares) at Mar. 31, 2020						(538,921)
Balance at Mar. 31, 2020	2,836,592	$ 201,061	1,360,564	1,319,988	(41,395)	$ (3,626)
Common stock balance (in shares) at Dec. 31, 2019		200,922,790
Treasury stock (in shares) at Dec. 31, 2019						(538,921)
Balance at Dec. 31, 2019	2,847,246	$ 200,923	1,355,404	1,336,647	(42,102)	$ (3,626)
Net income	277,911
Other comprehensive income	1,853
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	$ (362)
Common stock balance (in shares) at Sep. 30, 2020	201,061,198	201,061,198
Treasury stock (in shares) at Sep. 30, 2020						(538,921)
Balance at Sep. 30, 2020	$ 3,012,765	$ 201,061	1,366,494	1,489,085	(40,249)	$ (3,626)
Common stock balance (in shares) at Mar. 31, 2020		201,061,198
Treasury stock (in shares) at Mar. 31, 2020						(538,921)
Balance at Mar. 31, 2020	2,836,592	$ 201,061	1,360,564	1,319,988	(41,395)	$ (3,626)
Net income	99,703			99,703
Other comprehensive income	586				586
Dividends declared on common stock	(41,812)			(41,812)
Stock-based compensation	2,618		2,618
Common stock balance (in shares) at Jun. 30, 2020		201,061,198
Treasury stock (in shares) at Jun. 30, 2020						(538,921)
Balance at Jun. 30, 2020	2,897,687	$ 201,061	1,363,182	1,377,879	(40,809)	$ (3,626)
Net income	153,078			153,078
Other comprehensive income	560				560
Dividends declared on common stock	(41,872)			(41,872)
Stock-based compensation	$ 3,312		3,312
Common stock balance (in shares) at Sep. 30, 2020	201,061,198	201,061,198
Treasury stock (in shares) at Sep. 30, 2020						(538,921)
Balance at Sep. 30, 2020	$ 3,012,765	$ 201,061	1,366,494	1,489,085	(40,249)	$ (3,626)
Common stock balance (in shares) at Dec. 31, 2020	201,061,198	201,061,198
Treasury stock (in shares) at Dec. 31, 2020						(538,921)
Balance at Dec. 31, 2020	$ 3,079,105	$ 201,061	1,371,385	1,558,363	(48,078)	$ (3,626)
Net income	52,131			52,131
Other comprehensive income	568				568
Dividends declared on common stock	(42,943)			(42,943)
Stock-based compensation	2,574		2,574
Repurchase of common stock						(392,294)
Repurchase of common stock	(6,701)					$ (6,701)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings			(10,828)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings						392,294
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings						$ 6,701
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(4,127)
Issuance of common stock (shares)		672,260
Issuance of common stock	19,699	$ 672	19,027
Common stock balance (in shares) at Mar. 31, 2021		201,733,458
Treasury stock (in shares) at Mar. 31, 2021						(538,921)
Balance at Mar. 31, 2021	$ 3,100,306	$ 201,733	1,382,158	1,567,551	(47,510)	$ (3,626)
Common stock balance (in shares) at Dec. 31, 2020	201,061,198	201,061,198
Treasury stock (in shares) at Dec. 31, 2020						(538,921)
Balance at Dec. 31, 2020	$ 3,079,105	$ 201,061	1,371,385	1,558,363	(48,078)	$ (3,626)
Net income	291,597
Other comprehensive income	1,653
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	$ (4,127)
Common stock balance (in shares) at Sep. 30, 2021	203,889,661	203,889,661
Treasury stock (in shares) at Sep. 30, 2021						(538,921)
Balance at Sep. 30, 2021	$ 3,332,278	$ 203,889	1,458,208	1,720,232	(46,425)	$ (3,626)
Common stock balance (in shares) at Mar. 31, 2021		201,733,458
Treasury stock (in shares) at Mar. 31, 2021						(538,921)
Balance at Mar. 31, 2021	3,100,306	$ 201,733	1,382,158	1,567,551	(47,510)	$ (3,626)
Net income	100,190			100,190
Other comprehensive income	542				542
Dividends declared on common stock	(43,336)			(43,336)
Stock-based compensation	6,150		6,150
Issuance of common stock (shares)		1,088,843
Issuance of common stock	34,950	$ 1,089	33,861
Common stock balance (in shares) at Jun. 30, 2021		202,822,301
Treasury stock (in shares) at Jun. 30, 2021						(538,921)
Balance at Jun. 30, 2021	3,198,802	$ 202,822	1,422,169	1,624,405	(46,968)	$ (3,626)
Net income	139,276			139,276
Other comprehensive income	543				543
Dividends declared on common stock	(43,449)			(43,449)
Stock-based compensation	2,925		2,925
Issuance of common stock (shares)		1,067,360
Issuance of common stock	$ 34,181	$ 1,067	33,114
Common stock balance (in shares) at Sep. 30, 2021	203,889,661	203,889,661
Treasury stock (in shares) at Sep. 30, 2021						(538,921)
Balance at Sep. 30, 2021	$ 3,332,278	$ 203,889	$ 1,458,208	$ 1,720,232	$ (46,425)	$ (3,626)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Operating activities:
Net income	$ 291,597	$ 277,911
Income (loss) from discontinued operations, net of tax	348	(484)
Income from continuing operations	291,249	278,395
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	222,613	212,832
Deferred income taxes	30,353	16,409
Changes in current assets and liabilities, net of acquisitions:
Receivables	(104,438)	(100,141)
Inventories	(21,402)	(9,581)
Other current assets	(74,413)	1,269
Accounts payable	33,173	19,125
Other current liabilities	14,175	74,286
Other noncurrent changes	(25,425)	(10,139)
Net cash provided by continuing operations	365,885	482,455
Net cash used in discontinued operations	(75)	(688)
Net cash provided by operating activities	365,810	481,767
Investing activities:
Capital expenditures	(428,114)	(413,842)
Acquisitions, net of cash acquired	(13,721)	(71,479)
Net proceeds from sale or disposition of property and other	12,954	23,463
Investments	(3,777)	24
Net cash used in investing activities	(432,658)	(461,834)
Financing activities:
Issuance of short-term borrowings	50,000	75,000
Repayment of short-term borrowings	(50,000)	0
Issuance of long-term debt	178,517	100,125
Repayment of long-term debt	(63,835)	(73,699)
Proceeds from issuance of common stock	88,830	3,410
Dividends paid	(128,142)	(124,796)
Repurchase of common stock	(6,701)	0
Tax withholding on stock-based compensation	(4,127)	(362)
Net cash provided by (used in) financing activities	64,542	(20,322)
Decrease in cash and cash equivalents	(2,306)	(389)
Cash and cash equivalents -- beginning of year	59,547	66,459
Cash and cash equivalents -- end of period	$ 57,241	$ 66,070

Basis of presentation

Basis of presentation	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of presentation The accompanying consolidated interim financial statements were prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Interim financial statements do not include all disclosures provided in annual financial statements and, accordingly, these financial statements should be read in conjunction with those appearing in the 2020 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses. Beginning in March 2020, governmental restrictions and guidelines implemented to control the spread of COVID-19 reduced commercial and interpersonal activity throughout the Company's areas of operation. Most of the Company's products and services are considered essential to the United States and its communities and, as a result, operations have generally continued throughout the COVID-19 pandemic and reopening of the country's economy. The Company has assessed the impacts of the COVID-19 pandemic on its results of operations for the nine months ended September 30, 2021 and 2020, and determined there were no material adverse impacts. The assets and liabilities of the Company's discontinued operations have been classified as held for sale and are included in prepayments and other current assets, noncurrent assets - other and other accrued liabilities on the Consolidated Balance Sheets. The results and supporting activities are shown in income (loss) from discontinued operations on the Consolidated Statements of Income. Unless otherwise indicated, the amounts presented in the accompanying notes to the consolidated financial statements relate to the Company's continuing operations.

New accounting standards

New accounting standards	9 Months Ended
	Sep. 30, 2021
Accounting Standards Update and Change in Accounting Principle [Abstract]
New accounting standards	New accounting standards Recently adopted accounting standards ASU 2018-14 - Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued guidance on modifying the disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans as part of the disclosure framework project. The guidance removed disclosures that are no longer considered cost beneficial, clarified the specific requirements of disclosures and added disclosure requirements identified as relevant. The guidance added, among other things, the requirement to include an explanation for significant gains and losses related to changes in benefit obligations for the period. The guidance removed, among other things, the disclosure requirement to disclose the amount of net periodic benefit costs to be amortized over the next fiscal year from accumulated other comprehensive income (loss) and the effect a one percentage point change in assumed health care cost trend rates would have on certain benefit components. The Company adopted the guidance on January 1, 2021, on a retrospective basis. The Company determined the new guidance will not materially impact its consolidated financial statement disclosures. ASU 2019-12 - Simplifying the Accounting for Income Taxes In December 2019, the FASB issued guidance on simplifying the accounting for income taxes by removing certain exceptions in ASC 740 and providing simplification amendments. The guidance removed exceptions on intraperiod tax allocations and reporting and provided simplification on accounting for franchise taxes, tax basis goodwill and tax law changes. The Company adopted the guidance on January 1, 2021, and determined it did not have a material impact on its results of operations, financial position, cash flows or disclosures. Recently issued accounting standards not yet adopted ASU 2020-04 - Reference Rate Reform In March 2020, the FASB issued optional guidance to ease the facilitation of the effects of reference rate reform on financial reporting. The guidance applied to certain contract modifications, hedging relationships and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. LIBOR is expected to be retired with a full phase-out by the end of 2021 and replaced by a new reference rate, which includes SOFR. The guidance can be applied beginning in the interim period that includes March 12, 2020, and cannot be applied to contract modifications or hedging relationships entered into or evaluated after December 31, 2022. The Company has updated its credit agreements to include language regarding the successor or alternate rate to LIBOR, and a review of other contracts and agreements is on-going. The Company does not expect the guidance to have a material impact on its results of operations, financial position, cash flows or disclosures.

Seasonality of operations

Seasonality of operations	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Seasonality of operations	Seasonality of operations Some of the Company's operations are highly seasonal and revenues from, and certain expenses for, such operations may fluctuate significantly among quarterly periods. Accordingly, the interim results for particular businesses, and for the Company as a whole, may not be indicative of results for the full fiscal year.

Receivables and allowance for e

Receivables and allowance for expected credit losses	9 Months Ended
	Sep. 30, 2021
Credit Loss [Abstract]
Receivables and allowance for expected credit loss	Receivables and allowance for expected credit losses Receivables consists primarily of trade receivables from the sale of goods and services, which are recorded at the invoiced amount, and contract assets, net of expected credit losses. For more information on contract assets, see Note 9. The Company's trade receivables are all due in 12 months or less. The total balance of receivables past due 90 days or more was $37.9 million, $48.9 million and $43.9 million at September 30, 2021 and 2020, and December 31, 2020, respectively. The Company's expected credit losses are determined through a review using historical credit loss experience; changes in asset specific characteristics; current conditions; and reasonable and supportable future forecasts, among other specific account data, and is performed at least quarterly. The Company develops and documents its methodology to determine its allowance for expected credit losses at each of its reportable business segments. Risk characteristics used by the business segments may include customer mix, knowledge of customers, general economic conditions of the various local economies and impacts of COVID-19, among others. Specific account balances are written off when management determines the amounts to be uncollectible. Management has reviewed the balance reserved through the allowance for expected credit losses and believes it is reasonable. Details of the Company's expected credit losses were as follows: Electric Natural gas Pipeline Construction Construction Total (In thousands) At December 31, 2020 $ 899 $ 2,571 $ 2 $ 6,164 $ 5,722 $ 15,358 Current expected credit loss provision 538 1,273 — (1,049) (1,079) (317) Less write-offs charged against the allowance 888 1,107 — 273 401 2,669 Credit loss recoveries collected 129 213 — — — 342 At March 31, 2021 $ 678 $ 2,950 $ 2 $ 4,842 $ 4,242 $ 12,714 Current expected credit loss provision (110) (103) — 11 (639) (841) Less write-offs charged against the allowance 341 787 — 232 64 1,424 Credit loss recoveries collected 100 199 — — — 299 At June 30, 2021 $ 327 $ 2,259 $ 2 $ 4,621 $ 3,539 $ 10,748 Current expected credit loss provision 388 411 — 233 (675) 357 Less write-offs charged against the allowance 525 1,178 — 184 265 2,152 Credit loss recoveries collected 92 168 — — 93 353 At September 30, 2021 $ 282 $ 1,660 $ 2 $ 4,670 $ 2,692 $ 9,306 Electric Natural gas Pipeline Construction Construction Total (In thousands) At January 1, 2020 $ 328 $ 1,056 $ — $ 5,357 $ 1,756 $ 8,497 Current expected credit loss provision 555 1,156 — 694 1,150 3,555 Less write-offs charged against the allowance 500 624 — 68 73 1,265 Credit loss recoveries collected 109 229 — — — 338 At March 31, 2020 $ 492 $ 1,817 $ — $ 5,983 $ 2,833 $ 11,125 Current expected credit loss provision 303 190 — (314) 896 1,075 Less write-offs charged against the allowance 224 677 — 44 454 1,399 Credit loss recoveries collected 88 201 — — — 289 At June 30, 2020 $ 659 $ 1,531 $ — $ 5,625 $ 3,275 $ 11,090 Current expected credit loss provision 435 811 2 728 1,635 3,611 Less write-offs charged against the allowance 269 692 — 229 117 1,307 Credit loss recoveries collected 75 203 — — — 278 At September 30, 2020 $ 900 $ 1,853 $ 2 $ 6,124 $ 4,793 $ 13,672

Inventories and natural gas in

Inventories and natural gas in storage	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventories and natural gas in storage	Inventories and natural gas in storage Natural gas in storage for the Company's regulated operations is generally valued at lower of cost or market using the last-in, first-out method or lower of cost or net realizable value using the average cost or first-in, first-out method. The majority of all other inventories are valued at the lower of cost or net realizable value using the average cost method. The portion of the cost of natural gas in storage expected to be used within 12 months was included in inventories. Inventories on the Consolidated Balance Sheets were as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Aggregates held for resale $ 183,693 $ 168,132 $ 175,782 Asphalt oil 32,964 27,587 28,238 Natural gas in storage (current) 28,683 27,135 21,919 Materials and supplies 27,253 26,609 25,142 Merchandise for resale 26,639 21,525 21,087 Other 13,053 15,235 18,999 Total $ 312,285 $ 286,223 $ 291,167 The remainder of natural gas in storage, which largely represents the cost of gas required to maintain pressure levels for normal operating purposes, was included in noncurrent assets - other and was $47.5 million at September 30, 2021 and December 31, 2020, and $48.3 million at September 30, 2020.

Earnings per share

Earnings per share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Earnings per share	Earnings per share Basic earnings per share is computed by dividing net income by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings per share is computed by dividing net income by the total of the weighted average number of shares of common stock outstanding during the applicable period, plus the effect of non-vested performance share awards and restricted stock units. Common stock outstanding includes issued shares less shares held in treasury. Net income was the same for both the basic and diluted earnings per share calculations. A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations follows: Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In thousands, except per share amounts) Weighted average common shares outstanding - basic 202,863 200,522 201,647 200,495 Effect of dilutive performance share awards and restricted stock units 327 97 308 20 Weighted average common shares outstanding - diluted 203,190 200,619 201,955 200,515 Shares excluded from the calculation of diluted earnings per share — 87 — 139 Dividends declared per common share $ .2125 $ .2075 $ .6375 $ .6225

Equity

Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Equity	Equity The Company currently has a shelf registration statement on file with the SEC, under which the Company may issue and sell any combination of common stock and debt securities. The Company may sell such securities if warranted by market conditions and the Company's capital requirements. In August 2020, the Company amended the Distribution Agreement dated February 22, 2019, with J.P. Morgan Securities LLC and MUFG Securities Americas Inc., as sales agents. This agreement, as amended, allows the offering, issuance and sale of up to 6.4 million shares of the Company's common stock in connection with an "at-the-market" offering. The common stock may be offered for sale, from time to time, in accordance with the terms and conditions of this agreement. As of September 30, 2021, the Company had capacity to issue up to 3.6 million additional shares of common stock under the "at-the-market" offering program. Details of the Company's "at-the-market" offering activity was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In millions) Shares issued 1.0 — 2.8 — Net proceeds * $ 34.2 $ — $ 88.8 $ — Issuance costs $ .5 $ — $ 1.2 $ — * Net proceeds were used for capital expenditures.

Accumulated other comprehensive

Accumulated other comprehensive loss	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Accumulated other comprehensive loss	Accumulated other comprehensive loss The after-tax changes in the components of accumulated other comprehensive loss were as follows: Net Unrealized Postretirement Net Unrealized Total (In thousands) At December 31, 2020 $ (984) $ (47,207) $ 113 $ (48,078) Other comprehensive loss before reclassifications — — (44) (44) Amounts reclassified from accumulated other comprehensive loss 111 466 35 612 Net current-period other comprehensive income (loss) 111 466 (9) 568 At March 31, 2021 $ (873) $ (46,741) $ 104 $ (47,510) Other comprehensive loss before reclassifications — — (52) (52) Amounts reclassified from accumulated other comprehensive loss 112 457 25 594 Net current-period other comprehensive income (loss) 112 457 (27) 542 At June 30, 2021 $ (761) $ (46,284) $ 77 $ (46,968) Other comprehensive loss before reclassifications — — (54) (54) Amounts reclassified from accumulated other comprehensive loss 111 466 20 597 Net current-period other comprehensive income (loss) 111 466 (34) 543 At September 30, 2021 $ (650) $ (45,818) $ 43 $ (46,425) Net Unrealized Postretirement Net Unrealized Total (In thousands) At December 31, 2019 $ (1,430) $ (40,734) $ 62 $ (42,102) Other comprehensive income before reclassifications — — 135 135 Amounts reclassified from (to) accumulated other comprehensive loss 111 462 (1) 572 Net current-period other comprehensive income 111 462 134 707 At March 31, 2020 $ (1,319) $ (40,272) $ 196 $ (41,395) Other comprehensive loss before reclassifications — — (13) (13) Amounts reclassified from accumulated other comprehensive loss 112 480 7 599 Net current-period other comprehensive income (loss) 112 480 (6) 586 At June 30, 2020 $ (1,207) $ (39,792) $ 190 $ (40,809) Other comprehensive loss before reclassifications — — (50) (50) Amounts reclassified from accumulated other comprehensive loss 111 471 28 610 Net current-period other comprehensive income (loss) 111 471 (22) 560 At September 30, 2020 $ (1,096) $ (39,321) $ 168 $ (40,249) The following amounts were reclassified out of accumulated other comprehensive loss into net income. The amounts presented in parenthesis indicate a decrease to net income on the Consolidated Statements of Income. The reclassifications were as follows: Three Months Ended Nine Months Ended Location on Consolidated September 30, September 30, 2021 2020 2021 2020 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (147) $ (148) $ (443) $ (443) Interest expense 36 37 109 109 Income taxes (111) (111) (334) (334) Amortization of postretirement liability losses included in net periodic benefit cost (617) (623) (1,851) (1,869) Other income 151 152 462 456 Income taxes (466) (471) (1,389) (1,413) Reclassification adjustment on available-for-sale investments included in net income (26) (35) (101) (43) Other income 6 7 21 9 Income taxes (20) (28) (80) (34) Total reclassifications $ (597) $ (610) $ (1,803) $ (1,781)

Revenue from contracts with cus

Revenue from contracts with customers	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customer [Abstract]
Revenue from contracts with customers	Revenue from contracts with customers Revenue is recognized when a performance obligation is satisfied by transferring control over a product or service to a customer. Revenue is measured based on consideration specified in a contract with a customer and excludes any sales incentives and amounts collected on behalf of third parties. The Company is considered an agent for certain taxes collected from customers. As such, the Company presents revenues net of these taxes at the time of sale to be remitted to governmental authorities, including sales and use taxes. Disaggregation In the following tables, revenue is disaggregated by the type of customer or service provided. The Company believes this level of disaggregation best depicts how the nature, amount, timing and uncertainty of revenue and cash flows are affected by economic factors. The table also includes a reconciliation of the disaggregated revenue by reportable segments. For more information on the Company's business segments, see Note 17. Three Months Ended September 30, 2021 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 36,412 $ 55,284 $ — $ — $ — $ — $ 91,696 Commercial utility sales 39,033 35,338 — — — — 74,371 Industrial utility sales 10,495 5,395 — — — — 15,890 Other utility sales 1,993 — — — — — 1,993 Natural gas transportation — 11,812 28,058 — — — 39,870 Natural gas storage — — 3,483 — — — 3,483 Contracting services — — — 415,106 — — 415,106 Construction materials — — — 617,823 — — 617,823 Intrasegment eliminations — — — (201,630) — — (201,630) Inside specialty contracting — — — — 317,238 — 317,238 Outside specialty contracting — — — — 185,842 — 185,842 Other 12,589 2,110 3,290 — 75 3,423 21,487 Intersegment eliminations (136) (142) (3,908) (110) (289) (3,398) (7,983) Revenues from contracts with customers 100,386 109,797 30,923 831,189 502,866 25 1,575,186 Revenues out of scope (1,456) 581 45 — 11,657 — 10,827 Total external operating revenues $ 98,930 $ 110,378 $ 30,968 $ 831,189 $ 514,523 $ 25 $ 1,586,013 Three Months Ended September 30, 2020 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 33,086 $ 48,399 $ — $ — $ — $ — $ 81,485 Commercial utility sales 35,688 28,731 — — — — 64,419 Industrial utility sales 8,428 4,454 — — — — 12,882 Other utility sales 1,779 — — — — — 1,779 Natural gas transportation — 11,159 27,583 — — — 38,742 Natural gas gathering — — 973 — — — 973 Natural gas storage — — 3,885 — — — 3,885 Contracting services — — — 448,569 — — 448,569 Construction materials — — — 608,673 — — 608,673 Intrasegment eliminations — — — (234,693) — — (234,693) Inside specialty contracting — — — — 352,845 — 352,845 Outside specialty contracting — — — — 187,202 — 187,202 Other 7,663 2,590 3,203 — 486 3,030 16,972 Intersegment eliminations (195) (185) (3,622) (110) (425) (3,006) (7,543) Revenues from contracts with customers 86,449 95,148 32,022 822,439 540,108 24 1,576,190 Revenues out of scope 998 (445) 46 — 10,500 — 11,099 Total external operating revenues $ 87,447 $ 94,703 $ 32,068 $ 822,439 $ 550,608 $ 24 $ 1,587,289 Nine Months Ended September 30, 2021 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 99,106 $ 341,901 $ — $ — $ — $ — $ 441,007 Commercial utility sales 105,795 203,002 — — — — 308,797 Industrial utility sales 30,878 20,387 — — — — 51,265 Other utility sales 5,384 — — — — — 5,384 Natural gas transportation — 35,715 85,160 — — — 120,875 Natural gas storage — — 10,606 — — — 10,606 Contracting services — — — 791,964 — — 791,964 Construction materials — — — 1,332,997 — — 1,332,997 Intrasegment eliminations — — — (394,126) — — (394,126) Inside specialty contracting — — — — 1,020,130 — 1,020,130 Outside specialty contracting — — — — 502,328 — 502,328 Other 31,827 7,913 10,760 — 168 10,152 60,820 Intersegment eliminations (407) (425) (38,000) (314) (2,128) (10,082) (51,356) Revenues from contracts with customers 272,583 608,493 68,526 1,730,521 1,520,498 70 4,200,691 Revenues out of scope (5,329) 5,880 127 — 36,239 — 36,917 Total external operating revenues $ 267,254 $ 614,373 $ 68,653 $ 1,730,521 $ 1,556,737 $ 70 $ 4,237,608 Nine Months Ended September 30, 2020 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 93,389 $ 313,753 $ — $ — $ — $ — $ 407,142 Commercial utility sales 99,152 184,754 — — — — 283,906 Industrial utility sales 26,867 18,633 — — — — 45,500 Other utility sales 5,018 — — — — — 5,018 Natural gas transportation — 33,307 82,980 — — — 116,287 Natural gas gathering — — 4,244 — — — 4,244 Natural gas storage — — 10,035 — — — 10,035 Contracting services — — — 850,326 — — 850,326 Construction materials — — — 1,299,081 — — 1,299,081 Intrasegment eliminations — — — (443,516) — — (443,516) Inside specialty contracting — — — — 1,049,975 — 1,049,975 Outside specialty contracting — — — — 478,047 — 478,047 Other 23,830 7,800 9,807 — 1,249 8,882 51,568 Intersegment eliminations (586) (555) (36,820) (262) (3,674) (8,952) (50,849) Revenues from contracts with customers 247,670 557,692 70,246 1,705,629 1,525,597 (70) 4,106,764 Revenues out of scope 2,123 4,949 135 — 33,620 — 40,827 Total external operating revenues $ 249,793 $ 562,641 $ 70,381 $ 1,705,629 $ 1,559,217 $ (70) $ 4,147,591 Presented in the previous tables are intrasegment revenues within the construction materials and contracting segment to highlight the focus on vertical integration as this segment sells materials to both third parties and internal customers. Due to consolidation requirements, these revenues must be eliminated against construction materials to arrive at the external operating revenue total for the segment. Contract balances The timing of revenue recognition may differ from the timing of invoicing to customers. The timing of invoicing to customers does not necessarily correlate with the timing of revenues being recognized under the cost-to-cost method of accounting. Contracts from contracting services are billed as work progresses in accordance with agreed upon contractual terms. Generally, billing to the customer occurs contemporaneous to revenue recognition. A variance in timing of the billings may result in a contract asset or a contract liability. A contract asset occurs when revenues are recognized under the cost-to-cost measure of progress, which exceeds amounts billed on uncompleted contracts. Such amounts will be billed as standard contract terms allow, usually based on various measures of performance or achievement. A contract liability occurs when there are billings in excess of revenues recognized under the cost-to-cost measure of progress on uncompleted contracts. Contract liabilities decrease as revenue is recognized from the satisfaction of the related performance obligation. The changes in contract assets and liabilities were as follows: September 30, 2021 December 31, 2020 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 185,086 $ 104,345 $ 80,741 Receivables, net Contract liabilities - current (153,408) (158,603) 5,195 Accounts payable Contract liabilities - noncurrent (157) (52) (105) Noncurrent liabilities - other Net contract assets (liabilities) $ 31,521 $ (54,310) $ 85,831 The Company recognized $7.1 million and $152.4 million in revenue for the three and nine months ended September 30, 2021, respectively, which was previously included in contract liabilities at December 31, 2020. The Company recognized $15.6 million and $137.3 million in revenue for the three and nine months ended September 30, 2020, respectively, which was previously included in contract liabilities at December 31, 2019. The Company recognized a net increase in revenues of $19.2 million and $63.3 million for the three and nine months ended September 30, 2021, respectively, from performance obligations satisfied in prior periods. The Company recognized a net increase in revenues of $34.7 million and $58.8 million for the three and nine months ended September 30, 2020, respectively, from performance obligations satisfied in prior periods. Remaining performance obligations The remaining performance obligations, also referred to as backlog, at the construction materials and contracting and construction services segments include unrecognized revenues that the Company reasonably expects to be realized. These unrecognized revenues can include: projects that have a written award, a letter of intent, a notice to proceed, an agreed upon work order to perform work on mutually accepted terms and conditions and change orders or claims to the extent management believes additional contract revenues will be earned and are deemed probable of collection. Excluded from remaining performance obligations are potential orders under master service agreements. The majority of the Company's construction contracts have an original duration of less than two years. The remaining performance obligations at the pipeline segment include firm transportation and storage contracts with fixed pricing and fixed volumes. The Company has applied the practical expedient, which does not require additional disclosures for contracts with an original duration of less than 12 months, to certain firm transportation and non-regulated contracts. The Company's firm transportation and firm storage contracts included in the remaining performance obligations have weighted average remaining durations of approximately four and one years, respectively. At September 30, 2021, the Company's remaining performance obligations were $2.4 billion. The Company expects to recognize the following revenue amounts in future periods related to these remaining performance obligations: $1.6 billion within the next 12 months or less; $300.0 million within the next 13 to 24 months; and $478.7 million in 25 months or more.

Business Combinations

Business Combinations	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Business Combinations	Business combinations The following acquisitions were accounted for as business combinations in accordance with ASC 805 - Business Combinations. The results of the acquired businesses have been included in the Consolidated Financial Statements beginning on the acquisition date. Pro forma financial amounts reflecting the effects of the business combinations are not presented, as none of these business combinations, individually or in the aggregate, were material to the Company's financial position or results of operations. The acquisitions are also subject to customary adjustments based on, among other things, the amount of cash, debt and working capital in the business as of the closing date. The amounts included in the Consolidated Balance Sheets for these adjustments are considered provisional until final settlement has occurred. In 2021 and 2020, the construction materials and contracting segment's acquisitions included: • Mt. Hood Rock, a construction aggregates business in Oregon, acquired in April 2021. At September 30, 2021, the purchase price allocation was preliminary and will be finalized within 12 months of the acquisition date. • The assets of McMurry Ready-Mix Co., an aggregates and concrete supplier in Wyoming, acquired in December 2020. In the third quarter of 2021, the Company finalized the provisional accounting and recorded an immaterial measurement period adjustment. • The assets of Oldcastle Infrastructure Spokane, a prestressed-concrete business in Washington, acquired in February 2020. As of December 31, 2020, the purchase price adjustments had been settled with no material adjustments to the provisional accounting. In February 2020, the construction services segment acquired PerLectric, Inc., an electrical construction company in Virginia. As of March 31, 2021, the purchase price adjustments had been settled with no material adjustments to the provisional accounting. In 2021, the total purchase price for acquisitions was $13.8 million, subject to certain adjustments, with cash acquired totaling $100,000. The purchase price includes consideration paid of $13.7 million. The amounts allocated to the aggregated assets acquired and liabilities assumed during 2021 were as follows: $700,000 to current assets; $13.0 million to property, plant and equipment; $2.9 million to goodwill; $600,000 to other intangible assets; $200,000 to current liabilities; $100,000 to noncurrent liabilities - other and $3.2 million to deferred tax liabilities. The Company issued debt to finance the acquisitions. In 2020, the total purchase price for acquisitions was $110.2 million, subject to certain adjustments, with cash acquired totaling $1.7 million. The purchase price includes consideration paid of $106.0 million and $2.5 million of indemnity holdback liabilities. The amounts allocated to the aggregated assets acquired and liabilities assumed during 2020 were as follows: $54.8 million to current assets; $27.1 million to property, plant and equipment; $33.6 million to goodwill; $19.0 million to other intangible assets; $22.6 million to current liabilities; $300,000 to noncurrent liabilities - other and $1.4 million to asset retirement obligations. The Company issued debt to finance the acquisitions. Costs incurred for acquisitions are included in operation and maintenance expense on the Consolidated Statements of Income and were not material for the three or nine months ended September 30, 2021 and 2020.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Lessee, Operating Leases	Leases The Company's leases primarily include operating leases for equipment, buildings, easements and vehicles. The Company leases certain equipment to third parties through its utility and construction services segments, which are considered short-term operating leases with terms of less than 12 months. The Company recognized revenue from operating leases of $11.8 million and $36.6 million for the three and nine months ended September 30, 2021, respectively. The Company recognized revenue from operating leases of $10.7 million and $34.0 million for the three and nine months ended September 30, 2020, respectively. At September 30, 2021, the Company had $8.3 million of lease receivables with a majority due within 12 months.

Goodwill and other intangible a

Goodwill and other intangible assets	9 Months Ended
	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and other intangible assets	Goodwill and other intangible assets The changes in the carrying amount of goodwill were as follows: Balance at January 1, 2021 Goodwill Measurement Balance at September 30, 2021 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 226,003 2,900 (217) 228,686 Construction services 143,224 — — 143,224 Total $ 714,963 $ 2,900 $ (217) $ 717,646 Balance at January 1, 2020 Goodwill Measurement Balance at September 30, 2020 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 217,234 6,483 — 223,717 Construction services 118,388 24,436 400 143,224 Total $ 681,358 $ 30,919 $ 400 $ 712,677 Balance at January 1, 2020 Goodwill Measurement Balance at December 31, 2020 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 217,234 8,778 (9) 226,003 Construction services 118,388 24,436 400 143,224 Total $ 681,358 $ 33,214 $ 391 $ 714,963 Other amortizable intangible assets were as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Customer relationships $ 29,423 $ 27,551 $ 28,836 Less accumulated amortization 9,705 5,958 6,887 19,718 21,593 21,949 Noncompete agreements 3,991 3,941 3,941 Less accumulated amortization 2,710 2,176 2,309 1,281 1,765 1,632 Other 11,957 12,853 12,927 Less accumulated amortization 10,715 9,835 11,012 1,242 3,018 1,915 Total $ 22,241 $ 26,376 $ 25,496 The previous tables include goodwill and intangible assets associated with the business combinations completed during 2021 and 2020. For more information related to these business combinations, see Note 10. Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2021, was $1.2 million and $3.9 million, respectively. Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2020, was $2.1 million and $6.8 million, respectively. Estimated amortization expense for identifiable intangible assets as of September 30, 2021, was: Remainder of 2021 2022 2023 2024 2025 Thereafter (In thousands) Amortization expense $ 1,220 $ 4,678 $ 4,329 $ 3,957 $ 2,060 $ 5,997

Regulatory assets and liabiliti

Regulatory assets and liabilities	9 Months Ended
	Sep. 30, 2021
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets and Liabilities	Regulatory assets and liabilities The following table summarizes the individual components of unamortized regulatory assets and liabilities: Estimated Recovery or Refund Period as of September 30, 2021 * September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Regulatory assets: Current: Natural gas costs recoverable through rate adjustments Up to 1 year $ 98,211 $ 48,478 $ 42,481 Conservation programs Up to 1 year 8,495 7,857 7,117 Cost recovery mechanisms Up to 1 year 4,693 8,177 10,645 Other Up to 1 year 21,736 14,613 8,284 133,135 79,125 68,527 Noncurrent: Pension and postretirement benefits ** 155,888 157,015 155,942 Plant costs/asset retirement obligations Over plant lives 73,528 68,815 71,740 Plant retirement - 48,544 57,499 65,919 Cost recovery mechanisms Up to 10 years 43,955 14,281 16,245 Manufactured gas plant site remediation - 26,000 25,964 26,429 Taxes recoverable from customers Over plant lives 11,438 10,847 10,785 Natural gas costs recoverable through rate adjustments Up to 3 years 6,084 24,677 21,539 Long-term debt refinancing costs Up to 39 years 3,952 3,826 4,426 Other Up to 18 years 9,856 6,840 6,356 379,245 369,764 379,381 Total regulatory assets $ 512,380 $ 448,889 $ 447,908 Regulatory liabilities: Current: Taxes refundable to customers Up to 1 year $ 3,867 $ 4,223 $ 3,557 Electric fuel and purchased power deferral Up to 1 year 3,205 6,171 3,667 Natural gas costs refundable through rate adjustments - — 20,556 18,565 Other Up to 1 year 9,419 8,887 5,661 16,491 39,837 31,450 Noncurrent: Taxes refundable to customers Over plant lives 218,566 232,186 227,850 Plant removal and decommissioning costs Over plant lives 172,683 173,367 167,171 Pension and postretirement benefits ** 16,915 17,991 16,989 Other Up to 21 years 17,977 11,392 16,065 426,141 434,936 428,075 Total regulatory liabilities $ 442,632 $ 474,773 $ 459,525 Net regulatory position $ 69,748 $ (25,884) $ (11,617) * Estimated recovery or refund period for amounts currently being recovered or refunded in rates charged to customers. ** Recovered as expense is incurred or cash contributions are made. Regulatory assets not earning a rate of return were approximately $318.2 million at September 30, 2021 and 2020, and $332.5 million at December 31, 2020; however, these regulatory assets are expected to be recovered from customers in future rates. These assets are largely comprised of the unfunded portion of pension and postretirement benefits, asset retirement obligations, accelerated depreciation on plant retirement and the estimated future cost of manufactured gas plant site remediation. In February 2021, a prolonged period of unseasonably cold temperatures in the central United States significantly increased the demand for electric and natural gas services and contributed to increased market prices. Overall, Montana-Dakota and Great Plains incurred approximately $44.0 million in increased natural gas costs in order to maintain services for its customers. These extraordinary gas costs were recorded as regulatory assets as they are expected to be recovered from customers. Montana-Dakota and Great Plains have received approval for the recovery of purchased gas adjustments related to the cold-weather event in all jurisdictions impacted, including out-of-cycle purchased gas adjustment requests in most jurisdictions. For a discussion of the Company's most recent cases by jurisdiction, see Note 19. In 2019, the Company experienced increased natural gas costs in Washington from the rupture of the Enbridge pipeline in Canada in late 2018. As a result, the Company requested, and the WUTC approved, recovery of the balance of natural gas costs recoverable related to this period of time over three years rather than its normal one-year recovery period. In February 2019, the Company announced the retirement of three aging coal-fired electric generating units. The Company accelerated the depreciation related to these facilities in property, plant and equipment and recorded the difference between the accelerated depreciation, in accordance with GAAP, and the depreciation approved for rate-making purposes as regulatory assets. The first unit ceased operations on March 31, 2021, and in the second quarter of 2021, the Company began amortizing plant retirement and closure costs related to this facility. Requests have been filed with the NDPSC and SDPUC to offset the savings associated with the cessation of operations of this unit with the amortization of the deferred regulatory assets. In the second quarter of 2021, the Company moved the costs being recovered for this facility from plant retirement to cost recovery mechanisms in the previous table. The remaining two units are expected to be retired in early 2022. The Company expects to recover the regulatory assets related to the plant retirements in future rates. If, for any reason, the Company's regulated businesses cease to meet the criteria for application of regulatory accounting for all or part of their operations, the regulatory assets and liabilities relating to those portions ceasing to meet such criteria would be removed from the balance sheet and included in the statement of income or accumulated other comprehensive loss in the period in which the discontinuance of regulatory accounting occurs.

Fair value measurements

Fair value measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair value measurements	Fair value measurements The Company measures its investments in certain fixed-income and equity securities at fair value with changes in fair value recognized in income. The Company anticipates using these investments, which consist of insurance contracts, to satisfy its obligations under its unfunded, nonqualified defined benefit and defined contribution plans for executive officers and certain key management employees, and invests in these fixed-income and equity securities for the purpose of earning investment returns and capital appreciation. These investments, which totaled $106.4 million, $95.7 million and $100.1 million, at September 30, 2021 and 2020, and December 31, 2020, respectively, are classified as investments on the Consolidated Balance Sheets. The net unrealized gains on these investments were $373,000 and $4.2 million for the three and nine months ended September 30, 2021, respectively. The net unrealized gains on these investments were $3.3 million and $8.7 million for the three and nine months ended September 30, 2020, respectively. The change in fair value, which is considered part of the cost of the plan, is classified in other income on the Consolidated Statements of Income. The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities, which include mortgage-backed securities and U.S. Treasury securities. These available-for-sale securities are recorded at fair value and are classified as investments on the Consolidated Balance Sheets. Unrealized gains or losses are recorded in accumulated other comprehensive income (loss). Details of available-for-sale securities were as follows: September 30, 2021 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 8,597 $ 93 $ 11 $ 8,679 U.S. Treasury securities 2,681 — 28 2,653 Total $ 11,278 $ 93 $ 39 $ 11,332 September 30, 2020 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 9,813 $ 213 $ 4 $ 10,022 U.S. Treasury securities 1,166 3 — 1,169 Total $ 10,979 $ 216 $ 4 $ 11,191 December 31, 2020 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 9,799 $ 156 $ 9 $ 9,946 U.S. Treasury securities 1,386 — 5 1,381 Total $ 11,185 $ 156 $ 14 $ 11,327 Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the measurement date. The fair value ASC establishes a hierarchy for grouping assets and liabilities, based on the significance of inputs. The estimated fair values of the Company's assets and liabilities measured on a recurring basis are determined using the market approach. The Company's money market funds are valued at the net asset value of shares held at the end of the quarter, based on published market quotations on active markets, or using other known sources including pricing from outside sources. The estimated fair value of the Company's mortgage-backed securities and U.S. Treasury securities are based on comparable market transactions, other observable inputs or other sources, including pricing from outside sources. The estimated fair value of the Company's insurance contracts are based on contractual cash surrender values that are determined primarily by investments in managed separate accounts of the insurer. These amounts approximate fair value. The managed separate accounts are valued based on other observable inputs or corroborated market data. Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value. The Company's assets measured at fair value on a recurring basis were as follows: Fair Value Measurements at September 30, 2021, Using Quoted Prices in Significant Significant Balance at September 30, 2021 (In thousands) Assets: Money market funds $ — $ 9,907 $ — $ 9,907 Insurance contracts* — 106,371 — 106,371 Available-for-sale securities: Mortgage-backed securities — 8,679 — 8,679 U.S. Treasury securities — 2,653 — 2,653 Total assets measured at fair value $ — $ 127,610 $ — $ 127,610 * The insurance contracts invest approximately 63 percent in fixed-income investments, 16 percent in common stock of large-cap companies, 8 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 5 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at September 30, 2020, Using Quoted Prices in Significant Significant Balance at September 30, 2020 (In thousands) Assets: Money market funds $ — $ 8,478 $ — $ 8,478 Insurance contract* — 95,687 — 95,687 Available-for-sale securities: Mortgage-backed securities — 10,022 — 10,022 U.S. Treasury securities — 1,169 — 1,169 Total assets measured at fair value $ — $ 115,356 $ — $ 115,356 * The insurance contract invests approximately 68 percent in fixed-income investments, 15 percent in common stock of large-cap companies, 6 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 4 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at December 31, 2020, Using Quoted Prices in Significant Significant Balance at December 31, 2020 (In thousands) Assets: Money market funds $ — $ 8,917 $ — $ 8,917 Insurance contract* — 100,104 — 100,104 Available-for-sale securities: Mortgage-backed securities — 9,946 — 9,946 U.S. Treasury securities — 1,381 — 1,381 Total assets measured at fair value $ — $ 120,348 $ — $ 120,348 * The insurance contract invests approximately 57 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 9 percent in common stock of small-cap companies, 5 percent in target date investments and 2 percent in cash equivalents. The Company applies the provisions of the fair value measurement standard to its nonrecurring, non-financial measurements, including long-lived asset impairments. These assets are not measured at fair value on an ongoing basis but are subject to fair value adjustments only in certain circumstances. The Company reviews the carrying value of its long-lived assets, excluding goodwill, whenever events or changes in circumstances indicate that such carrying amounts may not be recoverable. The Company performed fair value assessments of the assets acquired and liabilities assumed in the business combinations that occurred during 2021 and 2020. The fair value of these assets and liabilities were determined based on Level 2 and Level 3 inputs. The Company's long-term debt is not measured at fair value on the Consolidated Balance Sheets and the fair value is being provided for disclosure purposes only. The fair value was categorized as Level 2 in the fair value hierarchy and was based on discounted future cash flows using current market interest rates. The estimated fair value of the Company's Level 2 long-term debt was as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Carrying amount $ 2,328,266 $ 2,270,290 $ 2,213,130 Fair value $ 2,599,907 $ 2,593,743 $ 2,537,289 The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values .

Debt

Debt	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt	Debt Certain debt instruments of the Company's subsidiaries contain restrictive and financial covenants and cross-default provisions. In order to borrow under the debt agreements, the subsidiary companies must be in compliance with the applicable covenants and certain other conditions, all of which the subsidiaries, as applicable, were in compliance with at September 30, 2021. In the event the subsidiaries do not comply with the applicable covenants and other conditions, alternative sources of funding may need to be pursued. Montana-Dakota's and Centennial's respective commercial paper programs are supported by revolving credit agreements. While the amount of commercial paper outstanding does not reduce available capacity under the respective revolving credit agreements, Montana-Dakota and Centennial do not issue commercial paper in an aggregate amount exceeding the available capacity under the credit agreements. The commercial paper borrowings may vary during the period, largely the result of fluctuations in working capital requirements due to the seasonality of certain operations of the Company's subsidiaries. Long-term debt Long-term Debt Outstanding Long-term debt outstanding was as follows: Weighted Average Interest Rate at September 30, 2021 September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Senior Notes due on dates ranging from October 22, 2022 to September 15, 2061 4.36 % $ 2,025,000 $ 1,900,000 $ 1,950,000 Commercial paper supported by revolving credit agreements .35 % 168,800 214,150 125,600 Credit agreements due on June 7, 2024 2.54 % 94,300 91,000 95,900 Medium-Term Notes due on dates ranging from September 15, 2027 to March 16, 2029 7.32 % 35,000 35,000 35,000 Term Loan Agreement due on September 3, 2032 2.00 % 7,700 8,400 8,400 Other notes due on dates ranging from January 2, 2022 to January 1, 2061 .83 % 3,198 28,284 4,034 Less unamortized debt issuance costs 5,709 6,526 5,803 Less discount 23 18 1 Total long-term debt 2,328,266 2,270,290 2,213,130 Less current maturities 1,548 1,558 1,555 Net long-term debt $ 2,326,718 $ 2,268,732 $ 2,211,575 Schedule of Debt Maturities Long-term debt maturities, which excludes unamortized debt issuance costs and discount, at September 30, 2021, were as follows: Remainder of 2021 2022 2023 2024 2025 Thereafter (In thousands) Long-term debt maturities $ 77 $ 148,021 $ 77,921 $ 324,521 $ 177,802 $ 1,605,656

Cash flow information

Cash flow information	9 Months Ended
	Sep. 30, 2021
Supplemental Cash Flow Information [Abstract]
Cash flow information	Cash flow information Cash expenditures for interest and income taxes were as follows: Nine Months Ended September 30, 2021 2020 (In thousands) Interest, net* $ 59,876 $ 63,086 Income taxes paid, net $ 61,250 $ 43,448 * AFUDC - borrowed was $1.6 million and $2.0 million for the nine months ended September 30, 2021 and 2020, respectively. Noncash investing and financing transactions were as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Right-of-use assets obtained in exchange for new operating lease liabilities $ 26,120 $ 41,315 $ 54,356 Property, plant and equipment additions in accounts payable $ 88,355 $ 33,240 $ 26,082 Accrual for holdback payment related to a business combination $ — $ 5,000 $ 2,500

Business segment data

Business segment data	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Business segment data	Business segment data The Company's reportable segments are those that are based on the Company's method of internal reporting, which generally segregates the strategic business units due to differences in products, services and regulation. The internal reporting of these operating segments is defined based on the reporting and review process used by the Company's chief executive officer. The Company's operations are located within the United States. The electric segment generates, transmits and distributes electricity in Montana, North Dakota, South Dakota and Wyoming. The natural gas distribution segment distributes natural gas in those states, as well as in Idaho, Minnesota, Oregon and Washington. These operations also supply related value-added services. The pipeline segment provides natural gas transportation and underground storage services through a regulated pipeline system primarily in the Rocky Mountain and northern Great Plains regions of the United States. This segment also provides non-regulated cathodic protection and other energy-related services. In 2020, the pipeline segment divested its regulated and non-regulated natural gas gathering assets. With the completion of these sales, the segment exited the natural gas gathering business. The construction materials and contracting segment mines, processes and sells construction aggregates (crushed stone, sand and gravel); produces and sells asphalt mix; and supplies ready-mixed concrete. This segment focuses on vertical integration of its contracting services with its construction materials to support the aggregate-based product lines including aggregate placement, asphalt and concrete paving, and site development and grading. Although not common to all locations, other products include the sale of cement, liquid asphalt for various commercial and roadway applications, various finished concrete products and other building materials and related contracting services. This segment operates in the central, southern and western United States, as well as Alaska and Hawaii. The construction services segment provides inside and outside specialty contracting services in 43 states plus Washington D.C. Its inside services include design, construction and maintenance of electrical and communication wiring and infrastructure, fire suppression systems, and mechanical piping and services. Its outside services include design, construction and maintenance of overhead and underground electrical distribution and transmission lines, substations, external lighting, traffic signalization, and gas pipelines, as well as utility excavation and the manufacture and distribution of transmission line construction equipment. This segment also constructs and maintains renewable energy projects. These specialty contracting services are provided to utilities and large manufacturing, commercial, industrial, institutional and governmental customers. The Other category includes the activities of Centennial Capital, which, through its subsidiary InterSource Insurance Company, insures various types of risks as a captive insurer for certain of the Company's subsidiaries. The function of the captive insurer is to fund the self-insured layers of the insured Company's general liability, automobile liability, pollution liability and other coverages. Centennial Capital also owns certain real and personal property. In addition, the Other category includes certain assets, liabilities and tax adjustments of the holding company primarily associated with corporate functions and certain general and administrative costs (reflected in operation and maintenance expense) and interest expense, which were previously allocated to the refining business and Fidelity and do not meet the criteria for income (loss) from discontinued operations. The Other category also includes Centennial Resources' former investment in Brazil. Discontinued operations include the results and supporting activities of Fidelity other than certain general and administrative costs and interest expense as described above. The information below follows the same accounting policies as described in Note 2 of the Notes to Consolidated Financial Statements in the 2020 Annual Report. Information on the Company's segments was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In thousands) External operating revenues: Regulated operations: Electric $ 98,930 $ 87,447 $ 267,254 $ 249,793 Natural gas distribution 110,378 94,703 614,373 562,641 Pipeline 27,757 27,965 58,390 57,717 237,065 210,115 940,017 870,151 Non-regulated operations: Pipeline 3,211 4,103 10,263 12,664 Construction materials and contracting 831,189 822,439 1,730,521 1,705,629 Construction services 514,523 550,608 1,556,737 1,559,217 Other 25 24 70 (70) 1,348,948 1,377,174 3,297,591 3,277,440 Total external operating revenues $ 1,586,013 $ 1,587,289 $ 4,237,608 $ 4,147,591 Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In thousands) Intersegment operating revenues: Regulated operations: Electric $ 136 $ 195 $ 407 $ 586 Natural gas distribution 142 185 425 555 Pipeline 3,845 3,543 37,530 36,583 4,123 3,923 38,362 37,724 Non-regulated operations: Pipeline 63 79 470 237 Construction materials and contracting 110 110 314 262 Construction services 289 425 2,128 3,674 Other 3,398 3,006 10,082 8,952 3,860 3,620 12,994 13,125 Intersegment eliminations (7,983) (7,543) (51,356) (50,849) Total intersegment operating revenues $ — $ — $ — $ — Operating income (loss): Electric $ 27,310 $ 21,159 $ 53,982 $ 48,827 Natural gas distribution (14,249) (18,393) 42,925 32,285 Pipeline 12,284 11,762 35,896 35,406 Construction materials and contracting 133,712 148,522 171,274 179,994 Construction services 30,120 40,507 108,172 102,185 Other (8) 154 (226) 339 Total operating income $ 189,169 $ 203,711 $ 412,023 $ 399,036 Net income (loss): Regulated operations: Electric $ 20,564 $ 16,787 $ 41,618 $ 40,314 Natural gas distribution (15,390) (17,614) 20,081 13,795 Pipeline 10,225 7,812 27,508 23,882 15,399 6,985 89,207 77,991 Non-regulated operations: Pipeline 356 189 1,166 444 Construction materials and contracting 96,282 107,307 116,865 122,113 Construction services 23,130 29,789 81,839 74,544 Other 3,795 8,745 2,172 3,303 123,563 146,030 202,042 200,404 Income from continuing operations 138,962 153,015 291,249 278,395 Income (loss) from discontinued operations, net of tax 314 63 348 (484) Net income $ 139,276 $ 153,078 $ 291,597 $ 277,911

Employee benefit plans

Employee benefit plans	9 Months Ended
	Sep. 30, 2021
Retirement Benefits [Abstract]
Employee benefit plans	Employee benefit plans Pension and other postretirement plans The Company has noncontributory qualified defined benefit pension plans and other postretirement benefit plans for certain eligible employees. Components of net periodic benefit credit for the Company's pension and other postretirement benefit plans were as follows: Pension Benefits Other Three Months Ended September 30, 2021 2020 2021 2020 (In thousands) Components of net periodic benefit credit: Service cost $ — $ — $ 400 $ 383 Interest cost 2,455 3,023 466 609 Expected return on assets (4,894) (4,987) (1,275) (1,115) Amortization of prior service credit — — (349) (349) Amortization of net actuarial loss 2,004 1,793 6 71 Net periodic benefit credit, including amount capitalized (435) (171) (752) (401) Less amount capitalized — — 46 39 Net periodic benefit credit $ (435) $ (171) $ (798) $ (440) Pension Benefits Other Nine Months Ended September 30, 2021 2020 2021 2020 (In thousands) Components of net periodic benefit credit: Service cost $ — $ — $ 1,200 $ 1,149 Interest cost 7,365 9,070 1,398 1,827 Expected return on assets (14,682) (14,962) (3,825) (3,764) Amortization of prior service credit — — (1,047) (1,048) Amortization of net actuarial loss 6,012 5,379 18 215 Net periodic benefit credit, including amount capitalized (1,305) (513) (2,256) (1,621) Less amount capitalized — — 129 106 Net periodic benefit credit $ (1,305) $ (513) $ (2,385) $ (1,727) The components of net periodic benefit credit, other than the service cost component, are included in other income on the Consolidated Statements of Income. The service cost component is included in operation and maintenance expense on the Consolidated Statements of Income. Nonqualified defined benefit plans In addition to the qualified defined benefit pension plans reflected in the table at the beginning of this note, the Company also has unfunded, nonqualified defined benefit plans for executive officers and certain key management employees. The Company's net periodic benefit cost for these plans was $769,000 and $2.3 million for the three and nine months ended September 30, 2021, respectively. The Company's net periodic benefit cost for these plans for the three and nine months ended September 30, 2020, was $1.0 million and $2.9 million, respectively. The components of net periodic benefit cost for these plans are included in other income on the Consolidated Statements of Income.

Regulatory matters

Regulatory matters	9 Months Ended
	Sep. 30, 2021
Regulated Operations [Abstract]
Regulatory Matters	Regulatory matters The Company regularly reviews the need for electric and natural gas rate changes in each of the jurisdictions in which service is provided. The Company files for rate adjustments to seek recovery of operating costs and capital investments, as well as reasonable returns as allowed by regulators. Certain regulatory proceedings and cases may also contain recurring mechanisms that can have an annual true-up. Examples of these recurring mechanisms include: infrastructure riders, transmission trackers, renewable resource cost adjustment riders, as well as weather normalization and decoupling mechanisms. The following paragraphs summarize the Company's significant open regulatory proceedings and cases by jurisdiction including updates to those reported in the 2020 Annual Report. The Company is unable to predict the ultimate outcome of these matters, the timing of final decisions of the various regulators and courts, or the effect on the Company's results of operations, financial position or cash flows. IPUC On January 12, 2021, Intermountain filed an application with the IPUC for a decrease in its depreciation and amortization rates of approximately $2.9 million annually or a decrease from a combined rate of 3.0 percent to 2.6 percent. On June 3, 2021, Intermountain filed a joint settlement agreement with the IPUC Staff reflecting a revised annual decrease of approximately $3.8 million or approximately 2.4 percent. On August 18, 2021, the settlement agreement was approved with rates retroactive to January 1, 2021. MNPUC Great Plains defers the difference between the actual cost of gas spent to serve customers and that recovered from customers on a monthly basis. Annually, Great Plains prepares a true-up pursuant to the purchased gas adjustment tariff. On August 30, 2021, the MNPUC issued an order to allow Great Plains recovery of an out-of-cycle cost of gas adjustment of $8.8 million over a period of 27 months. The order was effective September 1, 2021, and is subject to a prudence review by the MNPUC. The requested increase was for the February 2021 extreme cold weather, primarily in the central United States, and market conditions surrounding the natural gas commodity market. The MNPUC prudence review is pending with an order to be issued on or before August 29, 2022. NDPSC On July 15, 2021, Montana-Dakota filed an annual update to its transmission cost adjustment rider with the NDPSC requesting to recover revenues of approximately $14.5 million, which includes a true-up of the prior period adjustment, resulting in a decrease of approximately $1.1 million from current rates. This filing includes approximately $5.1 million related to transmission capital projects. On September 22, 2021, the NDPSC approved the decrease with rates effective November 1, 2021. SDPUC On August 19, 2020, the SDPUC approved the use of deferred accounting by Montana-Dakota to track expenses and revenues related to the COVID-19 pandemic. Montana-Dakota has determined the deferred accounting order was not necessary as no costs were recorded as regulatory assets. The filing was withdrawn by Montana-Dakota on July 30, 2021. On March 11, 2021, Montana-Dakota filed an informational update to the infrastructure rider rate tariff with the SDPUC related to the retirement of Unit 1 at Lewis & Clark Station. The filing includes the annual revenue requirement offset by the related amortization of the accelerated depreciation on the plant, net of excess deferred income taxes, and the decommissioning costs projected to be incurred in 2021 resulting in no impact to customers. This matter is pending before the SDPUC. WUTC On June 1, 2021, Cascade filed its annual pipeline cost recovery mechanism requesting an increase in annual revenue of approximately $2.1 million or approximately 0.8 percent. On October 15, 2021, Cascade filed an update reflecting a revised increase in annual revenue of approximately $1.7 million, or approximately 0.6 percent, which includes actual costs as of September 30, 2021. On October 28, 2021, the filing was approved with rates effective November 1, 2021. On September 30, 2021, Cascade filed an application with the WUTC for a natural gas rate increase of approximately $13.7 million annually or approximately 5.1 percent above current rates. The requested increase was primarily to recover investments made in infrastructure upgrades, as well as to recover 2021 wage increases. The WUTC has 11 months to render a final decision on the rate case. This matter is pending before the WUTC. FERC On September 1, 2021, Montana-Dakota filed an update to its transmission formula rate under the MISO tariff for its multi-value project for $13.4 million, which is effective January 1, 2022.

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries, which may include, but are not limited to, matters involving property damage, personal injury, and environmental, contractual, statutory and regulatory obligations. The Company accrues a liability for those contingencies when the incurrence of a loss is probable and the amount can be reasonably estimated. If a range of amounts can be reasonably estimated and no amount within the range is a better estimate than any other amount, then the minimum of the range is accrued. The Company does not accrue liabilities when the likelihood that the liability has been incurred is probable but the amount cannot be reasonably estimated or when the liability is believed to be only reasonably possible or remote. For contingencies where an unfavorable outcome is probable or reasonably possible and which are material, the Company discloses the nature of the contingency and, in some circumstances, an estimate of the possible loss. Accruals are based on the best information available, but in certain situations management is unable to estimate an amount or range of a reasonably possible loss including, but not limited to when: (1) the damages are unsubstantiated or indeterminate, (2) the proceedings are in the early stages, (3) numerous parties are involved, or (4) the matter involves novel or unsettled legal theories. At September 30, 2021 and 2020, and December 31, 2020, the Company accrued liabilities which have not been discounted, including liabilities held for sale, of $34.7 million, $72.7 million and $41.5 million, respectively. At September 30, 2021 and 2020, and December 31, 2020, the Company also recorded corresponding insurance receivables of $11.7 million, $49.1 million and $17.5 million, respectively, and regulatory assets of $21.1 million, $20.9 million and $21.3 million, respectively, related to the accrued liabilities. The accruals are for contingencies resulting from litigation, production taxes, royalty claims and environmental matters. This includes amounts that have been accrued for matters discussed in Environmental matters within this note. The Company will continue to monitor each matter and adjust accruals as might be warranted based on new information and further developments. Management believes that the outcomes with respect to probable and reasonably possible losses in excess of the amounts accrued, net of insurance recoveries, while uncertain, either cannot be estimated or will not have a material effect upon the Company's financial position, results of operations or cash flows. Unless otherwise required by GAAP, legal costs are expensed as they are incurred. Environmental matters The Company is a party to claims for the cleanup of environmental contamination at certain manufactured gas plant sites, as well as a superfund site. There were no material changes to the Company's environmental matters that were previously reported in the 2020 Annual Report. Guarantees In 2009, multiple sale agreements were signed to sell the Company's ownership interests in the Brazilian Transmission Lines. In connection with the sale, Centennial agreed to guarantee payment of any indemnity obligations of certain of the Company's indirect wholly owned subsidiaries. The remaining guarantee is expected to expire in 2021. The guarantees were required by the buyers as a condition to the sale of the Brazilian Transmission Lines. Certain subsidiaries of the Company have outstanding guarantees to third parties that guarantee the performance of other subsidiaries of the Company. These guarantees are related to construction contracts, insurance deductibles and loss limits, and certain other guarantees. At September 30, 2021, the fixed maximum amounts guaranteed under these agreements aggregate $147.6 million. Certain of the guarantees also have no fixed maximum amounts specified. At September 30, 2021, the amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate to $900,000 in 2021; $39.3 million in 2022; $50.3 million in 2023; $46.6 million in 2024; $600,000 in 2025; $900,000 thereafter; and $9.0 million, which has no scheduled maturity date. There were no amounts outstanding under the previously mentioned guarantees at September 30, 2021. In the event of default under these guarantee obligations, the subsidiary issuing the guarantee for that particular obligation would be required to make payments under its guarantee. Certain subsidiaries have outstanding letters of credit to third parties related to insurance policies and other agreements, some of which are guaranteed by other subsidiaries of the Company. At September 30, 2021, the fixed maximum amounts guaranteed under these letters of credit aggregated $25.5 million. At September 30, 2021, the amounts of scheduled expiration of the maximum amounts guaranteed under these letters of credit aggregate to $21.2 million in 2021 and $4.3 million in 2022. There were no amounts outstanding under the previously mentioned letters of credit at September 30, 2021. In the event of default under these letter of credit obligations, the subsidiary guaranteeing the letter of credit would be obligated for reimbursement of payments made under the letter of credit. In addition, Centennial, Knife River and MDU Construction Services have issued guarantees to third parties related to the routine purchase of maintenance items, materials and lease obligations for which no fixed maximum amounts have been specified. These guarantees have no scheduled maturity date. In the event a subsidiary of the Company defaults under these obligations, Centennial, Knife River or MDU Construction Services would be required to make payments under these guarantees. Any amounts outstanding by subsidiaries of the Company were reflected on the Consolidated Balance Sheet at September 30, 2021. In the normal course of business, Centennial has surety bonds related to construction contracts and reclamation obligations of its subsidiaries. In the event a subsidiary of Centennial does not fulfill a bonded obligation, Centennial would be responsible to the surety bond company for completion of the bonded contract or obligation. A large portion of the surety bonds is expected to expire within the next 12 months; however, Centennial will likely continue to enter into surety bonds for its subsidiaries in the future. At September 30, 2021, approximately $897.2 million of surety bonds were outstanding, which were not reflected on the Consolidated Balance Sheet. Variable interest entities The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary. Fuel Contract Coyote Station entered into a coal supply agreement with Coyote Creek that provides for the purchase of coal necessary to supply the coal requirements of the Coyote Station for the period May 2016 through December 2040. Coal purchased under the coal supply agreement is reflected in inventories on the Consolidated Balance Sheets and is recovered from customers as a component of electric fuel and purchased power. The coal supply agreement creates a variable interest in Coyote Creek due to the transfer of all operating and economic risk to the Coyote Station owners, as the agreement is structured so that the price of the coal will cover all costs of operations, as well as future reclamation costs. The Coyote Station owners are also providing a guarantee of the value of the assets of Coyote Creek as they would be required to buy the assets at book value should they terminate the contract prior to the end of the contract term and are providing a guarantee of the value of the equity of Coyote Creek in that they are required to buy the entity at the end of the contract term at equity value. Although the Company has determined that Coyote Creek is a VIE, the Company has concluded that it is not the primary beneficiary of Coyote Creek because the authority to direct the activities of the entity is shared by the four unrelated owners of the Coyote Station, with no primary beneficiary existing. As a result, Coyote Creek is not required to be consolidated in the Company's financial statements. At September 30, 2021, the Company's exposure to loss as a result of the Company's involvement with the VIE, based on the Company's ownership percentage, was $32.1 million.

Basis of presentation (Policies

Basis of presentation (Policies)	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	The accompanying consolidated interim financial statements were prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Interim financial statements do not include all disclosures provided in annual financial statements and, accordingly, these financial statements should be read in conjunction with those appearing in the 2020 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses.

New accounting standards (Polic

New accounting standards (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Standards Update and Change in Accounting Principle [Abstract]
New accounting standards	Recently adopted accounting standards ASU 2018-14 - Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued guidance on modifying the disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans as part of the disclosure framework project. The guidance removed disclosures that are no longer considered cost beneficial, clarified the specific requirements of disclosures and added disclosure requirements identified as relevant. The guidance added, among other things, the requirement to include an explanation for significant gains and losses related to changes in benefit obligations for the period. The guidance removed, among other things, the disclosure requirement to disclose the amount of net periodic benefit costs to be amortized over the next fiscal year from accumulated other comprehensive income (loss) and the effect a one percentage point change in assumed health care cost trend rates would have on certain benefit components. The Company adopted the guidance on January 1, 2021, on a retrospective basis. The Company determined the new guidance will not materially impact its consolidated financial statement disclosures. ASU 2019-12 - Simplifying the Accounting for Income Taxes In December 2019, the FASB issued guidance on simplifying the accounting for income taxes by removing certain exceptions in ASC 740 and providing simplification amendments. The guidance removed exceptions on intraperiod tax allocations and reporting and provided simplification on accounting for franchise taxes, tax basis goodwill and tax law changes. The Company adopted the guidance on January 1, 2021, and determined it did not have a material impact on its results of operations, financial position, cash flows or disclosures. Recently issued accounting standards not yet adopted ASU 2020-04 - Reference Rate Reform In March 2020, the FASB issued optional guidance to ease the facilitation of the effects of reference rate reform on financial reporting. The guidance applied to certain contract modifications, hedging relationships and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. LIBOR is expected to be retired with a full phase-out by the end of 2021 and replaced by a new reference rate, which includes SOFR. The guidance can be applied beginning in the interim period that includes March 12, 2020, and cannot be applied to contract modifications or hedging relationships entered into or evaluated after December 31, 2022. The Company has updated its credit agreements to include language regarding the successor or alternate rate to LIBOR, and a review of other contracts and agreements is on-going. The Company does not expect the guidance to have a material impact on its results of operations, financial position, cash flows or disclosures.

Receivables and allowance for_2

Receivables and allowance for expected credit losses (Policies)	9 Months Ended
	Sep. 30, 2021
Credit Loss [Abstract]
Accounts receivable and allowance for doubtful accounts	Receivables consists primarily of trade receivables from the sale of goods and services, which are recorded at the invoiced amount, and contract assets, net of expected credit losses. For more information on contract assets, see Note 9. The Company's trade receivables are all due in 12 months or less.
Expected credit loss	The Company's expected credit losses are determined through a review using historical credit loss experience; changes in asset specific characteristics; current conditions; and reasonable and supportable future forecasts, among other specific account data, and is performed at least quarterly. The Company develops and documents its methodology to determine its allowance for expected credit losses at each of its reportable business segments. Risk characteristics used by the business segments may include customer mix, knowledge of customers, general economic conditions of the various local economies and impacts of COVID-19, among others. Specific account balances are written off when management determines the amounts to be uncollectible.

Inventories and natural gas i_2

Inventories and natural gas in storage (Policies)	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventories and natural gas in storage	Natural gas in storage for the Company's regulated operations is generally valued at lower of cost or market using the last-in, first-out method or lower of cost or net realizable value using the average cost or first-in, first-out method. The majority of all other inventories are valued at the lower of cost or net realizable value using the average cost method.

Earnings per share (Policies)

Earnings per share (Policies)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Earnings per share	Basic earnings per share is computed by dividing net income by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings per share is computed by dividing net income by the total of the weighted average number of shares of common stock outstanding during the applicable period, plus the effect of non-vested performance share awards and restricted stock units. Common stock outstanding includes issued shares less shares held in treasury.

Revenue from contracts with c_2

Revenue from contracts with customers (Policies)	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customer [Abstract]
Revenue from contracts with customers	Revenue is recognized when a performance obligation is satisfied by transferring control over a product or service to a customer. Revenue is measured based on consideration specified in a contract with a customer and excludes any sales incentives and amounts collected on behalf of third parties. The Company is considered an agent for certain taxes collected from customers. As such, the Company presents revenues net of these taxes at the time of sale to be remitted to governmental authorities, including sales and use taxes.

Business Combinations (Policies

Business Combinations (Policies)	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Business Combinations Policy	The following acquisitions were accounted for as business combinations in accordance with ASC 805 - Business Combinations. The results of the acquired businesses have been included in the Consolidated Financial Statements beginning on the acquisition date. Pro forma financial amounts reflecting the effects of the business combinations are not presented, as none of these business combinations, individually or in the aggregate, were material to the Company's financial position or results of operations.

Fair value disclosures (Policie

Fair value disclosures (Policies)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair value measurements	The Company measures its investments in certain fixed-income and equity securities at fair value with changes in fair value recognized in income.
Investments	The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities, which include mortgage-backed securities and U.S. Treasury securities. These available-for-sale securities are recorded at fair value and are classified as investments on the Consolidated Balance Sheets

Business segment data (Policies

Business segment data (Policies)	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Business segment data	The Company's reportable segments are those that are based on the Company's method of internal reporting, which generally segregates the strategic business units due to differences in products, services and regulation. The internal reporting of these operating segments is defined based on the reporting and review process used by the Company's chief executive officer.

Contingencies (Policies)

Contingencies (Policies)	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Contingencies	The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries, which may include, but are not limited to, matters involving property damage, personal injury, and environmental, contractual, statutory and regulatory obligations. The Company accrues a liability for those contingencies when the incurrence of a loss is probable and the amount can be reasonably estimated. If a range of amounts can be reasonably estimated and no amount within the range is a better estimate than any other amount, then the minimum of the range is accrued. The Company does not accrue liabilities when the likelihood that the liability has been incurred is probable but the amount cannot be reasonably estimated or when the liability is believed to be only reasonably possible or remote. For contingencies where an unfavorable outcome is probable or reasonably possible and which are material, the Company discloses the nature of the contingency and, in some circumstances, an estimate of the possible loss. Accruals are based on the best information available, but in certain situations management is unable to estimate an amount or range of a reasonably possible loss including, but not limited to when: (1) the damages are unsubstantiated or indeterminate, (2) the proceedings are in the early stages, (3) numerous parties are involved, or (4) the matter involves novel or unsettled legal theories.
Variable interest entity	The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary.

Receivables and allowance for_3

Receivables and allowance for expected credit losses (Tables)	9 Months Ended
	Sep. 30, 2021
Credit Loss [Abstract]
Accounts Receivable, Allowance for Credit Loss [Table Text Block]	Details of the Company's expected credit losses were as follows: Electric Natural gas Pipeline Construction Construction Total (In thousands) At December 31, 2020 $ 899 $ 2,571 $ 2 $ 6,164 $ 5,722 $ 15,358 Current expected credit loss provision 538 1,273 — (1,049) (1,079) (317) Less write-offs charged against the allowance 888 1,107 — 273 401 2,669 Credit loss recoveries collected 129 213 — — — 342 At March 31, 2021 $ 678 $ 2,950 $ 2 $ 4,842 $ 4,242 $ 12,714 Current expected credit loss provision (110) (103) — 11 (639) (841) Less write-offs charged against the allowance 341 787 — 232 64 1,424 Credit loss recoveries collected 100 199 — — — 299 At June 30, 2021 $ 327 $ 2,259 $ 2 $ 4,621 $ 3,539 $ 10,748 Current expected credit loss provision 388 411 — 233 (675) 357 Less write-offs charged against the allowance 525 1,178 — 184 265 2,152 Credit loss recoveries collected 92 168 — — 93 353 At September 30, 2021 $ 282 $ 1,660 $ 2 $ 4,670 $ 2,692 $ 9,306 Electric Natural gas Pipeline Construction Construction Total (In thousands) At January 1, 2020 $ 328 $ 1,056 $ — $ 5,357 $ 1,756 $ 8,497 Current expected credit loss provision 555 1,156 — 694 1,150 3,555 Less write-offs charged against the allowance 500 624 — 68 73 1,265 Credit loss recoveries collected 109 229 — — — 338 At March 31, 2020 $ 492 $ 1,817 $ — $ 5,983 $ 2,833 $ 11,125 Current expected credit loss provision 303 190 — (314) 896 1,075 Less write-offs charged against the allowance 224 677 — 44 454 1,399 Credit loss recoveries collected 88 201 — — — 289 At June 30, 2020 $ 659 $ 1,531 $ — $ 5,625 $ 3,275 $ 11,090 Current expected credit loss provision 435 811 2 728 1,635 3,611 Less write-offs charged against the allowance 269 692 — 229 117 1,307 Credit loss recoveries collected 75 203 — — — 278 At September 30, 2020 $ 900 $ 1,853 $ 2 $ 6,124 $ 4,793 $ 13,672

Inventories and natural gas i_3

Inventories and natural gas in storage (Tables)	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventories	Inventories on the Consolidated Balance Sheets were as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Aggregates held for resale $ 183,693 $ 168,132 $ 175,782 Asphalt oil 32,964 27,587 28,238 Natural gas in storage (current) 28,683 27,135 21,919 Materials and supplies 27,253 26,609 25,142 Merchandise for resale 26,639 21,525 21,087 Other 13,053 15,235 18,999 Total $ 312,285 $ 286,223 $ 291,167

Earnings per share (Tables)

Earnings per share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Weighted average common shares outstanding	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations follows: Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In thousands, except per share amounts) Weighted average common shares outstanding - basic 202,863 200,522 201,647 200,495 Effect of dilutive performance share awards and restricted stock units 327 97 308 20 Weighted average common shares outstanding - diluted 203,190 200,619 201,955 200,515 Shares excluded from the calculation of diluted earnings per share — 87 — 139 Dividends declared per common share $ .2125 $ .2075 $ .6375 $ .6225

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Schedule of Stock by Class	Details of the Company's "at-the-market" offering activity was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In millions) Shares issued 1.0 — 2.8 — Net proceeds * $ 34.2 $ — $ 88.8 $ — Issuance costs $ .5 $ — $ 1.2 $ — * Net proceeds were used for capital expenditures.

Accumulated other comprehensi_2

Accumulated other comprehensive loss (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Accumulated comprehensive loss	The after-tax changes in the components of accumulated other comprehensive loss were as follows: Net Unrealized Postretirement Net Unrealized Total (In thousands) At December 31, 2020 $ (984) $ (47,207) $ 113 $ (48,078) Other comprehensive loss before reclassifications — — (44) (44) Amounts reclassified from accumulated other comprehensive loss 111 466 35 612 Net current-period other comprehensive income (loss) 111 466 (9) 568 At March 31, 2021 $ (873) $ (46,741) $ 104 $ (47,510) Other comprehensive loss before reclassifications — — (52) (52) Amounts reclassified from accumulated other comprehensive loss 112 457 25 594 Net current-period other comprehensive income (loss) 112 457 (27) 542 At June 30, 2021 $ (761) $ (46,284) $ 77 $ (46,968) Other comprehensive loss before reclassifications — — (54) (54) Amounts reclassified from accumulated other comprehensive loss 111 466 20 597 Net current-period other comprehensive income (loss) 111 466 (34) 543 At September 30, 2021 $ (650) $ (45,818) $ 43 $ (46,425) Net Unrealized Postretirement Net Unrealized Total (In thousands) At December 31, 2019 $ (1,430) $ (40,734) $ 62 $ (42,102) Other comprehensive income before reclassifications — — 135 135 Amounts reclassified from (to) accumulated other comprehensive loss 111 462 (1) 572 Net current-period other comprehensive income 111 462 134 707 At March 31, 2020 $ (1,319) $ (40,272) $ 196 $ (41,395) Other comprehensive loss before reclassifications — — (13) (13) Amounts reclassified from accumulated other comprehensive loss 112 480 7 599 Net current-period other comprehensive income (loss) 112 480 (6) 586 At June 30, 2020 $ (1,207) $ (39,792) $ 190 $ (40,809) Other comprehensive loss before reclassifications — — (50) (50) Amounts reclassified from accumulated other comprehensive loss 111 471 28 610 Net current-period other comprehensive income (loss) 111 471 (22) 560 At September 30, 2020 $ (1,096) $ (39,321) $ 168 $ (40,249)
Reclassification out of accumulated other comprehensive loss	The following amounts were reclassified out of accumulated other comprehensive loss into net income. The amounts presented in parenthesis indicate a decrease to net income on the Consolidated Statements of Income. The reclassifications were as follows: Three Months Ended Nine Months Ended Location on Consolidated September 30, September 30, 2021 2020 2021 2020 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (147) $ (148) $ (443) $ (443) Interest expense 36 37 109 109 Income taxes (111) (111) (334) (334) Amortization of postretirement liability losses included in net periodic benefit cost (617) (623) (1,851) (1,869) Other income 151 152 462 456 Income taxes (466) (471) (1,389) (1,413) Reclassification adjustment on available-for-sale investments included in net income (26) (35) (101) (43) Other income 6 7 21 9 Income taxes (20) (28) (80) (34) Total reclassifications $ (597) $ (610) $ (1,803) $ (1,781)

Revenue from contracts with c_3

Revenue from contracts with customers (Tables)	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customer [Abstract]
Disaggregation of revenue	In the following tables, revenue is disaggregated by the type of customer or service provided. The Company believes this level of disaggregation best depicts how the nature, amount, timing and uncertainty of revenue and cash flows are affected by economic factors. The table also includes a reconciliation of the disaggregated revenue by reportable segments. For more information on the Company's business segments, see Note 17. Three Months Ended September 30, 2021 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 36,412 $ 55,284 $ — $ — $ — $ — $ 91,696 Commercial utility sales 39,033 35,338 — — — — 74,371 Industrial utility sales 10,495 5,395 — — — — 15,890 Other utility sales 1,993 — — — — — 1,993 Natural gas transportation — 11,812 28,058 — — — 39,870 Natural gas storage — — 3,483 — — — 3,483 Contracting services — — — 415,106 — — 415,106 Construction materials — — — 617,823 — — 617,823 Intrasegment eliminations — — — (201,630) — — (201,630) Inside specialty contracting — — — — 317,238 — 317,238 Outside specialty contracting — — — — 185,842 — 185,842 Other 12,589 2,110 3,290 — 75 3,423 21,487 Intersegment eliminations (136) (142) (3,908) (110) (289) (3,398) (7,983) Revenues from contracts with customers 100,386 109,797 30,923 831,189 502,866 25 1,575,186 Revenues out of scope (1,456) 581 45 — 11,657 — 10,827 Total external operating revenues $ 98,930 $ 110,378 $ 30,968 $ 831,189 $ 514,523 $ 25 $ 1,586,013 Three Months Ended September 30, 2020 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 33,086 $ 48,399 $ — $ — $ — $ — $ 81,485 Commercial utility sales 35,688 28,731 — — — — 64,419 Industrial utility sales 8,428 4,454 — — — — 12,882 Other utility sales 1,779 — — — — — 1,779 Natural gas transportation — 11,159 27,583 — — — 38,742 Natural gas gathering — — 973 — — — 973 Natural gas storage — — 3,885 — — — 3,885 Contracting services — — — 448,569 — — 448,569 Construction materials — — — 608,673 — — 608,673 Intrasegment eliminations — — — (234,693) — — (234,693) Inside specialty contracting — — — — 352,845 — 352,845 Outside specialty contracting — — — — 187,202 — 187,202 Other 7,663 2,590 3,203 — 486 3,030 16,972 Intersegment eliminations (195) (185) (3,622) (110) (425) (3,006) (7,543) Revenues from contracts with customers 86,449 95,148 32,022 822,439 540,108 24 1,576,190 Revenues out of scope 998 (445) 46 — 10,500 — 11,099 Total external operating revenues $ 87,447 $ 94,703 $ 32,068 $ 822,439 $ 550,608 $ 24 $ 1,587,289 Nine Months Ended September 30, 2021 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 99,106 $ 341,901 $ — $ — $ — $ — $ 441,007 Commercial utility sales 105,795 203,002 — — — — 308,797 Industrial utility sales 30,878 20,387 — — — — 51,265 Other utility sales 5,384 — — — — — 5,384 Natural gas transportation — 35,715 85,160 — — — 120,875 Natural gas storage — — 10,606 — — — 10,606 Contracting services — — — 791,964 — — 791,964 Construction materials — — — 1,332,997 — — 1,332,997 Intrasegment eliminations — — — (394,126) — — (394,126) Inside specialty contracting — — — — 1,020,130 — 1,020,130 Outside specialty contracting — — — — 502,328 — 502,328 Other 31,827 7,913 10,760 — 168 10,152 60,820 Intersegment eliminations (407) (425) (38,000) (314) (2,128) (10,082) (51,356) Revenues from contracts with customers 272,583 608,493 68,526 1,730,521 1,520,498 70 4,200,691 Revenues out of scope (5,329) 5,880 127 — 36,239 — 36,917 Total external operating revenues $ 267,254 $ 614,373 $ 68,653 $ 1,730,521 $ 1,556,737 $ 70 $ 4,237,608 Nine Months Ended September 30, 2020 Electric Natural gas Pipeline Construction Construction Other Total (In thousands) Residential utility sales $ 93,389 $ 313,753 $ — $ — $ — $ — $ 407,142 Commercial utility sales 99,152 184,754 — — — — 283,906 Industrial utility sales 26,867 18,633 — — — — 45,500 Other utility sales 5,018 — — — — — 5,018 Natural gas transportation — 33,307 82,980 — — — 116,287 Natural gas gathering — — 4,244 — — — 4,244 Natural gas storage — — 10,035 — — — 10,035 Contracting services — — — 850,326 — — 850,326 Construction materials — — — 1,299,081 — — 1,299,081 Intrasegment eliminations — — — (443,516) — — (443,516) Inside specialty contracting — — — — 1,049,975 — 1,049,975 Outside specialty contracting — — — — 478,047 — 478,047 Other 23,830 7,800 9,807 — 1,249 8,882 51,568 Intersegment eliminations (586) (555) (36,820) (262) (3,674) (8,952) (50,849) Revenues from contracts with customers 247,670 557,692 70,246 1,705,629 1,525,597 (70) 4,106,764 Revenues out of scope 2,123 4,949 135 — 33,620 — 40,827 Total external operating revenues $ 249,793 $ 562,641 $ 70,381 $ 1,705,629 $ 1,559,217 $ (70) $ 4,147,591
Contract balances	The changes in contract assets and liabilities were as follows: September 30, 2021 December 31, 2020 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 185,086 $ 104,345 $ 80,741 Receivables, net Contract liabilities - current (153,408) (158,603) 5,195 Accounts payable Contract liabilities - noncurrent (157) (52) (105) Noncurrent liabilities - other Net contract assets (liabilities) $ 31,521 $ (54,310) $ 85,831

Goodwill and other intangible_2

Goodwill and other intangible assets (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying amount of goodwill	The changes in the carrying amount of goodwill were as follows: Balance at January 1, 2021 Goodwill Measurement Balance at September 30, 2021 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 226,003 2,900 (217) 228,686 Construction services 143,224 — — 143,224 Total $ 714,963 $ 2,900 $ (217) $ 717,646 Balance at January 1, 2020 Goodwill Measurement Balance at September 30, 2020 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 217,234 6,483 — 223,717 Construction services 118,388 24,436 400 143,224 Total $ 681,358 $ 30,919 $ 400 $ 712,677 Balance at January 1, 2020 Goodwill Measurement Balance at December 31, 2020 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 217,234 8,778 (9) 226,003 Construction services 118,388 24,436 400 143,224 Total $ 681,358 $ 33,214 $ 391 $ 714,963
Other amortizable intangible assets	Other amortizable intangible assets were as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Customer relationships $ 29,423 $ 27,551 $ 28,836 Less accumulated amortization 9,705 5,958 6,887 19,718 21,593 21,949 Noncompete agreements 3,991 3,941 3,941 Less accumulated amortization 2,710 2,176 2,309 1,281 1,765 1,632 Other 11,957 12,853 12,927 Less accumulated amortization 10,715 9,835 11,012 1,242 3,018 1,915 Total $ 22,241 $ 26,376 $ 25,496
Estimated amortization expense	Estimated amortization expense for identifiable intangible assets as of September 30, 2021, was: Remainder of 2021 2022 2023 2024 2025 Thereafter (In thousands) Amortization expense $ 1,220 $ 4,678 $ 4,329 $ 3,957 $ 2,060 $ 5,997

Regulatory assets and liabili_2

Regulatory assets and liabilities (Tables)	9 Months Ended
	Sep. 30, 2021
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	The following table summarizes the individual components of unamortized regulatory assets and liabilities: Estimated Recovery or Refund Period as of September 30, 2021 * September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Regulatory assets: Current: Natural gas costs recoverable through rate adjustments Up to 1 year $ 98,211 $ 48,478 $ 42,481 Conservation programs Up to 1 year 8,495 7,857 7,117 Cost recovery mechanisms Up to 1 year 4,693 8,177 10,645 Other Up to 1 year 21,736 14,613 8,284 133,135 79,125 68,527 Noncurrent: Pension and postretirement benefits ** 155,888 157,015 155,942 Plant costs/asset retirement obligations Over plant lives 73,528 68,815 71,740 Plant retirement - 48,544 57,499 65,919 Cost recovery mechanisms Up to 10 years 43,955 14,281 16,245 Manufactured gas plant site remediation - 26,000 25,964 26,429 Taxes recoverable from customers Over plant lives 11,438 10,847 10,785 Natural gas costs recoverable through rate adjustments Up to 3 years 6,084 24,677 21,539 Long-term debt refinancing costs Up to 39 years 3,952 3,826 4,426 Other Up to 18 years 9,856 6,840 6,356 379,245 369,764 379,381 Total regulatory assets $ 512,380 $ 448,889 $ 447,908 Regulatory liabilities: Current: Taxes refundable to customers Up to 1 year $ 3,867 $ 4,223 $ 3,557 Electric fuel and purchased power deferral Up to 1 year 3,205 6,171 3,667 Natural gas costs refundable through rate adjustments - — 20,556 18,565 Other Up to 1 year 9,419 8,887 5,661 16,491 39,837 31,450 Noncurrent: Taxes refundable to customers Over plant lives 218,566 232,186 227,850 Plant removal and decommissioning costs Over plant lives 172,683 173,367 167,171 Pension and postretirement benefits ** 16,915 17,991 16,989 Other Up to 21 years 17,977 11,392 16,065 426,141 434,936 428,075 Total regulatory liabilities $ 442,632 $ 474,773 $ 459,525 Net regulatory position $ 69,748 $ (25,884) $ (11,617)
Regulatory Liabilities	The following table summarizes the individual components of unamortized regulatory assets and liabilities: Estimated Recovery or Refund Period as of September 30, 2021 * September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Regulatory assets: Current: Natural gas costs recoverable through rate adjustments Up to 1 year $ 98,211 $ 48,478 $ 42,481 Conservation programs Up to 1 year 8,495 7,857 7,117 Cost recovery mechanisms Up to 1 year 4,693 8,177 10,645 Other Up to 1 year 21,736 14,613 8,284 133,135 79,125 68,527 Noncurrent: Pension and postretirement benefits ** 155,888 157,015 155,942 Plant costs/asset retirement obligations Over plant lives 73,528 68,815 71,740 Plant retirement - 48,544 57,499 65,919 Cost recovery mechanisms Up to 10 years 43,955 14,281 16,245 Manufactured gas plant site remediation - 26,000 25,964 26,429 Taxes recoverable from customers Over plant lives 11,438 10,847 10,785 Natural gas costs recoverable through rate adjustments Up to 3 years 6,084 24,677 21,539 Long-term debt refinancing costs Up to 39 years 3,952 3,826 4,426 Other Up to 18 years 9,856 6,840 6,356 379,245 369,764 379,381 Total regulatory assets $ 512,380 $ 448,889 $ 447,908 Regulatory liabilities: Current: Taxes refundable to customers Up to 1 year $ 3,867 $ 4,223 $ 3,557 Electric fuel and purchased power deferral Up to 1 year 3,205 6,171 3,667 Natural gas costs refundable through rate adjustments - — 20,556 18,565 Other Up to 1 year 9,419 8,887 5,661 16,491 39,837 31,450 Noncurrent: Taxes refundable to customers Over plant lives 218,566 232,186 227,850 Plant removal and decommissioning costs Over plant lives 172,683 173,367 167,171 Pension and postretirement benefits ** 16,915 17,991 16,989 Other Up to 21 years 17,977 11,392 16,065 426,141 434,936 428,075 Total regulatory liabilities $ 442,632 $ 474,773 $ 459,525 Net regulatory position $ 69,748 $ (25,884) $ (11,617)

Fair value measurements (Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: September 30, 2021 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 8,597 $ 93 $ 11 $ 8,679 U.S. Treasury securities 2,681 — 28 2,653 Total $ 11,278 $ 93 $ 39 $ 11,332 September 30, 2020 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 9,813 $ 213 $ 4 $ 10,022 U.S. Treasury securities 1,166 3 — 1,169 Total $ 10,979 $ 216 $ 4 $ 11,191 December 31, 2020 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 9,799 $ 156 $ 9 $ 9,946 U.S. Treasury securities 1,386 — 5 1,381 Total $ 11,185 $ 156 $ 14 $ 11,327
Assets and liabilities measured at fair value on a recurring basis	The Company's assets measured at fair value on a recurring basis were as follows: Fair Value Measurements at September 30, 2021, Using Quoted Prices in Significant Significant Balance at September 30, 2021 (In thousands) Assets: Money market funds $ — $ 9,907 $ — $ 9,907 Insurance contracts* — 106,371 — 106,371 Available-for-sale securities: Mortgage-backed securities — 8,679 — 8,679 U.S. Treasury securities — 2,653 — 2,653 Total assets measured at fair value $ — $ 127,610 $ — $ 127,610 * The insurance contracts invest approximately 63 percent in fixed-income investments, 16 percent in common stock of large-cap companies, 8 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 5 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at September 30, 2020, Using Quoted Prices in Significant Significant Balance at September 30, 2020 (In thousands) Assets: Money market funds $ — $ 8,478 $ — $ 8,478 Insurance contract* — 95,687 — 95,687 Available-for-sale securities: Mortgage-backed securities — 10,022 — 10,022 U.S. Treasury securities — 1,169 — 1,169 Total assets measured at fair value $ — $ 115,356 $ — $ 115,356 * The insurance contract invests approximately 68 percent in fixed-income investments, 15 percent in common stock of large-cap companies, 6 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 4 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at December 31, 2020, Using Quoted Prices in Significant Significant Balance at December 31, 2020 (In thousands) Assets: Money market funds $ — $ 8,917 $ — $ 8,917 Insurance contract* — 100,104 — 100,104 Available-for-sale securities: Mortgage-backed securities — 9,946 — 9,946 U.S. Treasury securities — 1,381 — 1,381 Total assets measured at fair value $ — $ 120,348 $ — $ 120,348 * The insurance contract invests approximately 57 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 9 percent in common stock of small-cap companies, 5 percent in target date investments and 2 percent in cash equivalents.
Fair value of long term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt was as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Carrying amount $ 2,328,266 $ 2,270,290 $ 2,213,130 Fair value $ 2,599,907 $ 2,593,743 $ 2,537,289

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Long-term debt outstanding	Long-term debt outstanding was as follows: Weighted Average Interest Rate at September 30, 2021 September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Senior Notes due on dates ranging from October 22, 2022 to September 15, 2061 4.36 % $ 2,025,000 $ 1,900,000 $ 1,950,000 Commercial paper supported by revolving credit agreements .35 % 168,800 214,150 125,600 Credit agreements due on June 7, 2024 2.54 % 94,300 91,000 95,900 Medium-Term Notes due on dates ranging from September 15, 2027 to March 16, 2029 7.32 % 35,000 35,000 35,000 Term Loan Agreement due on September 3, 2032 2.00 % 7,700 8,400 8,400 Other notes due on dates ranging from January 2, 2022 to January 1, 2061 .83 % 3,198 28,284 4,034 Less unamortized debt issuance costs 5,709 6,526 5,803 Less discount 23 18 1 Total long-term debt 2,328,266 2,270,290 2,213,130 Less current maturities 1,548 1,558 1,555 Net long-term debt $ 2,326,718 $ 2,268,732 $ 2,211,575
Schedule of debt maturities	Long-term debt maturities, which excludes unamortized debt issuance costs and discount, at September 30, 2021, were as follows: Remainder of 2021 2022 2023 2024 2025 Thereafter (In thousands) Long-term debt maturities $ 77 $ 148,021 $ 77,921 $ 324,521 $ 177,802 $ 1,605,656

Cash flow information (Tables)

Cash flow information (Tables)	9 Months Ended
	Sep. 30, 2021
Supplemental Cash Flow Information [Abstract]
Cash expenditures for interest and income taxes and noncash investing and financing transactions	Cash expenditures for interest and income taxes were as follows: Nine Months Ended September 30, 2021 2020 (In thousands) Interest, net* $ 59,876 $ 63,086 Income taxes paid, net $ 61,250 $ 43,448 * AFUDC - borrowed was $1.6 million and $2.0 million for the nine months ended September 30, 2021 and 2020, respectively. Noncash investing and financing transactions were as follows: September 30, 2021 September 30, 2020 December 31, 2020 (In thousands) Right-of-use assets obtained in exchange for new operating lease liabilities $ 26,120 $ 41,315 $ 54,356 Property, plant and equipment additions in accounts payable $ 88,355 $ 33,240 $ 26,082 Accrual for holdback payment related to a business combination $ — $ 5,000 $ 2,500

Business segment data (Tables)

Business segment data (Tables)	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Information on the Company's businesses	Information on the Company's segments was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In thousands) External operating revenues: Regulated operations: Electric $ 98,930 $ 87,447 $ 267,254 $ 249,793 Natural gas distribution 110,378 94,703 614,373 562,641 Pipeline 27,757 27,965 58,390 57,717 237,065 210,115 940,017 870,151 Non-regulated operations: Pipeline 3,211 4,103 10,263 12,664 Construction materials and contracting 831,189 822,439 1,730,521 1,705,629 Construction services 514,523 550,608 1,556,737 1,559,217 Other 25 24 70 (70) 1,348,948 1,377,174 3,297,591 3,277,440 Total external operating revenues $ 1,586,013 $ 1,587,289 $ 4,237,608 $ 4,147,591 Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020 (In thousands) Intersegment operating revenues: Regulated operations: Electric $ 136 $ 195 $ 407 $ 586 Natural gas distribution 142 185 425 555 Pipeline 3,845 3,543 37,530 36,583 4,123 3,923 38,362 37,724 Non-regulated operations: Pipeline 63 79 470 237 Construction materials and contracting 110 110 314 262 Construction services 289 425 2,128 3,674 Other 3,398 3,006 10,082 8,952 3,860 3,620 12,994 13,125 Intersegment eliminations (7,983) (7,543) (51,356) (50,849) Total intersegment operating revenues $ — $ — $ — $ — Operating income (loss): Electric $ 27,310 $ 21,159 $ 53,982 $ 48,827 Natural gas distribution (14,249) (18,393) 42,925 32,285 Pipeline 12,284 11,762 35,896 35,406 Construction materials and contracting 133,712 148,522 171,274 179,994 Construction services 30,120 40,507 108,172 102,185 Other (8) 154 (226) 339 Total operating income $ 189,169 $ 203,711 $ 412,023 $ 399,036 Net income (loss): Regulated operations: Electric $ 20,564 $ 16,787 $ 41,618 $ 40,314 Natural gas distribution (15,390) (17,614) 20,081 13,795 Pipeline 10,225 7,812 27,508 23,882 15,399 6,985 89,207 77,991 Non-regulated operations: Pipeline 356 189 1,166 444 Construction materials and contracting 96,282 107,307 116,865 122,113 Construction services 23,130 29,789 81,839 74,544 Other 3,795 8,745 2,172 3,303 123,563 146,030 202,042 200,404 Income from continuing operations 138,962 153,015 291,249 278,395 Income (loss) from discontinued operations, net of tax 314 63 348 (484) Net income $ 139,276 $ 153,078 $ 291,597 $ 277,911

Employee benefit plans (Tables)

Employee benefit plans (Tables)	9 Months Ended
	Sep. 30, 2021
Retirement Benefits [Abstract]
Schedule of net benefit costs	Components of net periodic benefit credit for the Company's pension and other postretirement benefit plans were as follows: Pension Benefits Other Three Months Ended September 30, 2021 2020 2021 2020 (In thousands) Components of net periodic benefit credit: Service cost $ — $ — $ 400 $ 383 Interest cost 2,455 3,023 466 609 Expected return on assets (4,894) (4,987) (1,275) (1,115) Amortization of prior service credit — — (349) (349) Amortization of net actuarial loss 2,004 1,793 6 71 Net periodic benefit credit, including amount capitalized (435) (171) (752) (401) Less amount capitalized — — 46 39 Net periodic benefit credit $ (435) $ (171) $ (798) $ (440) Pension Benefits Other Nine Months Ended September 30, 2021 2020 2021 2020 (In thousands) Components of net periodic benefit credit: Service cost $ — $ — $ 1,200 $ 1,149 Interest cost 7,365 9,070 1,398 1,827 Expected return on assets (14,682) (14,962) (3,825) (3,764) Amortization of prior service credit — — (1,047) (1,048) Amortization of net actuarial loss 6,012 5,379 18 215 Net periodic benefit credit, including amount capitalized (1,305) (513) (2,256) (1,621) Less amount capitalized — — 129 106 Net periodic benefit credit $ (1,305) $ (513) $ (2,385) $ (1,727)

Receivables and allowance for_4

Receivables and allowance for expected credit losses (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Credit Loss [Abstract]
Accounts Receivable, Noncurrent, 90 Days or More Past Due, Still Accruing	$ 37.9	$ 43.9	$ 48.9

Receivables and allowance for_5

Receivables and allowance for expected credit losses (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	$ 10,748	$ 12,714	$ 15,358	$ 11,090	$ 11,125	$ 8,497
Current expected credit loss provision	357	(841)	(317)	3,611	1,075	3,555
Less write-offs charged against the allowance	2,152	1,424	2,669	1,307	1,399	1,265
Credit loss recoveries collected	353	299	342	278	289	338
Balance	9,306	10,748	12,714	13,672	11,090	11,125
Electric
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	327	678	899	659	492	328
Current expected credit loss provision	388	(110)	538	435	303	555
Less write-offs charged against the allowance	525	341	888	269	224	500
Credit loss recoveries collected	92	100	129	75	88	109
Balance	282	327	678	900	659	492
Natural gas distribution
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	2,259	2,950	2,571	1,531	1,817	1,056
Current expected credit loss provision	411	(103)	1,273	811	190	1,156
Less write-offs charged against the allowance	1,178	787	1,107	692	677	624
Credit loss recoveries collected	168	199	213	203	201	229
Balance	1,660	2,259	2,950	1,853	1,531	1,817
Pipeline
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	2	2	2	0	0	0
Current expected credit loss provision	0	0	0	2	0	0
Less write-offs charged against the allowance	0	0	0	0	0	0
Credit loss recoveries collected	0	0	0	0	0	0
Balance	2	2	2	2	0	0
Construction materials and contracting
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	4,621	4,842	6,164	5,625	5,983	5,357
Current expected credit loss provision	233	11	(1,049)	728	(314)	694
Less write-offs charged against the allowance	184	232	273	229	44	68
Credit loss recoveries collected	0	0	0	0	0	0
Balance	4,670	4,621	4,842	6,124	5,625	5,983
Construction services
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance	3,539	4,242	5,722	3,275	2,833	1,756
Current expected credit loss provision	(675)	(639)	(1,079)	1,635	896	1,150
Less write-offs charged against the allowance	265	64	401	117	454	73
Credit loss recoveries collected	93	0	0	0	0	0
Balance	$ 2,692	$ 3,539	$ 4,242	$ 4,793	$ 3,275	$ 2,833

Inventories and natural gas i_4

Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 183,693	$ 175,782	$ 168,132
Asphalt oil	32,964	28,238	27,587
Natural gas in storage (current)	28,683	21,919	27,135
Materials and supplies	27,253	25,142	26,609
Merchandise for resale	26,639	21,087	21,525
Other	13,053	18,999	15,235
Total	312,285	291,167	286,223
Natural gas in storage noncurrent	$ 47,500	$ 47,500	$ 48,300

Earnings per share (Details)

Earnings per share (Details) - $ / shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Weighted average common shares outstanding - basic	202,863	200,522	201,647	200,495
Effect of dilutive performance share awards and restricted stock units	327	97	308	20
Weighted average common shares outstanding - diluted	203,190	200,619	201,955	200,515
Shares excluded from the calculation of diluted earnings per share	0	87	0	139
Dividends declared per common share	$ 0.2125	$ 0.2075	$ 0.6375	$ 0.6225

Equity (Details)

Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Common stock shares authorized under equity program	500,000,000	500,000,000	500,000,000	500,000,000	500,000,000
Net proceeds *			$ 88,830	$ 3,410
At-the-market offering
Class of Stock [Line Items]
Common stock shares authorized under equity program	6,400,000		6,400,000
Common stock shares reserved for future issuance	3,600,000		3,600,000
Shares issued	1,000,000	0	2,800,000	0
Net proceeds *	$ 34,200	$ 0	$ 88,800	$ 0
Issuance costs	$ 500	$ 0	$ 1,200	$ 0

Accumulated other comprehensi_3

Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,198,802	$ 3,100,306	$ 3,079,105	$ 2,897,687	$ 2,836,592	$ 2,847,246	$ 3,079,105	$ 2,847,246
Other comprehensive income (loss)	543	542	568	560	586	707	1,653	1,853
Balance	3,332,278	3,198,802	3,100,306	3,012,765	2,897,687	2,836,592	3,332,278	3,012,765
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(761)	(873)	(984)	(1,207)	(1,319)	(1,430)	(984)	(1,430)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	111	112	111	111	112	111
Other comprehensive income (loss)	111	112	111	111	112	111
Balance	(650)	(761)	(873)	(1,096)	(1,207)	(1,319)	(650)	(1,096)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(46,284)	(46,741)	(47,207)	(39,792)	(40,272)	(40,734)	(47,207)	(40,734)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	466	457	466	471	480	462
Other comprehensive income (loss)	466	457	466	471	480	462
Balance	(45,818)	(46,284)	(46,741)	(39,321)	(39,792)	(40,272)	(45,818)	(39,321)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	77	104	113	190	196	62	113	62
Other comprehensive income (loss) before reclassifications	(54)	(52)	(44)	(50)	(13)	135
Amounts reclassified from (to) accumulated other comprehensive loss	20	25	35	28	7	(1)
Other comprehensive income (loss)	(34)	(27)	(9)	(22)	(6)	134
Balance	43	77	104	168	190	196	43	168
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(46,968)	(47,510)	(48,078)	(40,809)	(41,395)	(42,102)	(48,078)	(42,102)
Other comprehensive income (loss) before reclassifications	(54)	(52)	(44)	(50)	(13)	135
Amounts reclassified from (to) accumulated other comprehensive loss	597	594	612	610	599	572
Other comprehensive income (loss)	543	542	568	560	586	707
Balance	$ (46,425)	$ (46,968)	$ (47,510)	$ (40,249)	$ (40,809)	$ (41,395)	$ (46,425)	$ (40,249)

Reclassification out of accumul

Reclassification out of accumulated other comprehensive loss (Details 2) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	$ (23,389)			$ (23,761)			$ (70,224)	$ (73,132)
Income taxes	(32,748)			(31,547)			(68,860)	(61,178)
Other income	5,930			4,612			18,310	13,669
Net income	139,276	$ 100,190	$ 52,131	153,078	$ 99,703	$ 25,130	291,597	277,911
Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Net income	(597)			(610)			(1,803)	(1,781)
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive loss | Interest rate contract
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	(147)			(148)			(443)	(443)
Income taxes	36			37			109	109
Net income	(111)			(111)			(334)	(334)
Amortization of postretirement liability losses included in net periodic benefit cost | Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Income taxes	151			152			462	456
Other income	(617)			(623)			(1,851)	(1,869)
Net income	(466)			(471)			(1,389)	(1,413)
Reclassification adjustment on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Income taxes	6			7			21	9
Other income	(26)			(35)			(101)	(43)
Net income	$ (20)			$ (28)			$ (80)	$ (34)

Disaggregation of revenue (Deta

Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 1,586,013	$ 1,587,289	$ 4,237,608	$ 4,147,591
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(7,983)	(7,543)	(51,356)	(50,849)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(201,630)	(234,693)	(394,126)	(443,516)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	39,870	38,742	120,875	116,287
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues:		973		4,244
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,483	3,885	10,606	10,035
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	415,106	448,569	791,964	850,326
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	617,823	608,673	1,332,997	1,299,081
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	317,238	352,845	1,020,130	1,049,975
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	185,842	187,202	502,328	478,047
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	21,487	16,972	60,820	51,568
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	1,575,186	1,576,190	4,200,691	4,106,764
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	10,827	11,099	36,917	40,827
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	91,696	81,485	441,007	407,142
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	74,371	64,419	308,797	283,906
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	15,890	12,882	51,265	45,500
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,993	1,779	5,384	5,018
Electric
Disaggregation of Revenue [Line Items]
Operating revenues:	98,930	87,447	267,254	249,793
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(136)	(195)	(407)	(586)
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	12,589	7,663	31,827	23,830
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	100,386	86,449	272,583	247,670
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	(1,456)	998	(5,329)	2,123
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	36,412	33,086	99,106	93,389
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	39,033	35,688	105,795	99,152
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	10,495	8,428	30,878	26,867
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,993	1,779	5,384	5,018
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues:	110,378	94,703	614,373	562,641
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(142)	(185)	(425)	(555)
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	11,812	11,159	35,715	33,307
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	2,110	2,590	7,913	7,800
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	109,797	95,148	608,493	557,692
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	581	(445)	5,880	4,949
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	55,284	48,399	341,901	313,753
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	35,338	28,731	203,002	184,754
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	5,395	4,454	20,387	18,633
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Operating revenues:	30,968	32,068	68,653	70,381
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(3,908)	(3,622)	(38,000)	(36,820)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	28,058	27,583	85,160	82,980
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues:		973		4,244
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,483	3,885	10,606	10,035
Pipeline | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	3,290	3,203	10,760	9,807
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	30,923	32,022	68,526	70,246
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	45	46	127	135
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	831,189	822,439	1,730,521	1,705,629
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(110)	(110)	(314)	(262)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(201,630)	(234,693)	(394,126)	(443,516)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	415,106	448,569	791,964	850,326
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	617,823	608,673	1,332,997	1,299,081
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	831,189	822,439	1,730,521	1,705,629
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0	0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues:	514,523	550,608	1,556,737	1,559,217
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(289)	(425)	(2,128)	(3,674)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	317,238	352,845	1,020,130	1,049,975
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	185,842	187,202	502,328	478,047
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	75	486	168	1,249
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	502,866	540,108	1,520,498	1,525,597
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	11,657	10,500	36,239	33,620
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	25	24	70	(70)
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(3,398)	(3,006)	(10,082)	(8,952)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	3,423	3,030	10,152	8,882
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	25	24	70	(70)
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 0	$ 0	$ 0	$ 0

Contract balances (Details 2)

Contract balances (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Contract assets	$ 185,086		$ 185,086		$ 104,345
Change in contract assets			80,741
Contract liabilities - current	(153,408)		(153,408)		(158,603)
Change in contract liabilities - current			5,195
Contract liabilities - noncurrent	(157)		(157)		(52)
Change in contract liabilities - noncurrent			(105)
Net contract assets (liabilities)	31,521		31,521		$ (54,310)
Change in net contract assets (liabilities)			85,831
Amounts included in contract liability at the beginning of the period	7,100	$ 15,600	152,400	$ 137,300
Amounts from performance obligations satisfied in prior periods	$ 19,200	$ 34,700	$ 63,300	$ 58,800

Revenue from contracts with c_4

Revenue from contracts with customers Remaining performance obligations (Details 3) $ in Millions	Sep. 30, 2021USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 2,400
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,600
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 300
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	13 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 478.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	25 months

Business Combinations (Details)

Business Combinations (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill	$ 717,646,000	$ 714,963,000	$ 712,677,000	$ 681,358,000
2021 Acquisition
Business Acquisition [Line Items]
Gross Aggregate Consideration	13,800,000
Cash Assumed	100,000
Business Combination, Consideration Transferred	13,700,000
Current Assets	700,000
Property, Plant, and Equipment	13,000,000
Goodwill	2,900,000
Other Intangibles	600,000
Current Liabilities	200,000
Deferred Credits and Other Liabilities	100,000
Deferred Tax Liabilities	$ 3,200,000

Business Combinations (Details

Business Combinations (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill	$ 714,963,000	$ 717,646,000	$ 712,677,000	$ 681,358,000
2020 Acquisition
Business Acquisition [Line Items]
Gross Aggregate Consideration	110,200,000
Cash Assumed	1,700,000
Business Combination, Consideration Transferred	106,000,000
Business Combination, Consideration Transferred, Liabilities Incurred	2,500,000
Current Assets	54,800,000
Property, Plant, and Equipment	27,100,000
Goodwill	33,600,000
Other Intangibles	19,000,000
Current Liabilities	22,600,000
Deferred Credits and Other Liabilities	300,000
Asset Retirement Obligation	$ 1,400,000

Lessor accounting (Details)

Lessor accounting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Operating Lease, Lease Income	$ 11.8	$ 10.7	$ 36.6	$ 34
Lessor, Operating Lease, Payments to be Received, Next Twelve Months	$ 8.3		$ 8.3

Goodwill rollforward (Details)

Goodwill rollforward (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Goodwill [Roll Forward]
Balance at beginning of period	$ 714,963	$ 681,358	$ 681,358
Goodwill acquired during the year	2,900	30,919	33,214
Measurement period adjustments	(217)	400	391
Balance at end of period	717,646	712,677	714,963
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736	345,736	345,736
Goodwill acquired during the year	0	0	0
Measurement period adjustments	0	0	0
Balance at end of period	345,736	345,736	345,736
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	226,003	217,234	217,234
Goodwill acquired during the year	2,900	6,483	8,778
Measurement period adjustments	(217)	0	(9)
Balance at end of period	228,686	223,717	226,003
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	143,224	118,388	118,388
Goodwill acquired during the year	0	24,436	24,436
Measurement period adjustments	0	400	400
Balance at end of period	$ 143,224	$ 143,224	$ 143,224

Other intangible assets (Detail

Other intangible assets (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 22,241	$ 26,376	$ 22,241	$ 26,376	$ 25,496
Amortization of intangible assets	1,200	2,100	3,900	6,800
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	29,423	27,551	29,423	27,551	28,836
Intangible assets, less accumulated amortization	9,705	5,958	9,705	5,958	6,887
Intangible assets, net (excluding goodwill)	19,718	21,593	19,718	21,593	21,949
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	3,991	3,941	3,991	3,941	3,941
Intangible assets, less accumulated amortization	2,710	2,176	2,710	2,176	2,309
Intangible assets, net (excluding goodwill)	1,281	1,765	1,281	1,765	1,632
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	11,957	12,853	11,957	12,853	12,927
Intangible assets, less accumulated amortization	10,715	9,835	10,715	9,835	11,012
Intangible assets, net (excluding goodwill)	$ 1,242	$ 3,018	$ 1,242	$ 3,018	$ 1,915

Future amortization expense (De

Future amortization expense (Details 3) $ in Thousands	Sep. 30, 2021USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Remainder of 2021	$ 1,220
2022	4,678
2023	4,329
2024	3,957
2025	2,060
Thereafter	$ 5,997

Regulatory assets (Details)

Regulatory assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Feb. 28, 2021	Dec. 31, 2020	Sep. 30, 2020
Regulatory Assets
Regulatory Assets, Current	$ 133,135		$ 68,527	$ 79,125
Regulatory Assets, Noncurrent	379,245		379,381	369,764
Total regulatory assets	512,380		447,908	448,889
Regulatory assets not earning a rate of return	318,200		332,500	318,200
Natural gas costs recoverable through rate adjustments
Regulatory Assets
Regulatory Assets, Current	98,211		42,481	48,478
Regulatory Assets, Noncurrent	$ 6,084		21,539	24,677
Total regulatory assets		$ 44,000
Natural gas costs recoverable through rate adjustments | Minimum [Member]
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Natural gas costs recoverable through rate adjustments | Maximum [Member]
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 3 years
Conservation programs
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Regulatory Assets, Current	$ 8,495		7,117	7,857
Cost recovery mechanisms
Regulatory Assets
Regulatory Assets, Current	4,693		10,645	8,177
Regulatory Assets, Noncurrent	$ 43,955		16,245	14,281
Cost recovery mechanisms | Minimum [Member]
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Cost recovery mechanisms | Maximum [Member]
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 10 years
Other regulatory assets
Regulatory Assets
Regulatory Assets, Current	$ 21,736		8,284	14,613
Regulatory Assets, Noncurrent	$ 9,856		6,356	6,840
Other regulatory assets | Minimum [Member]
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Other regulatory assets | Maximum [Member]
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 18 years
Pension and postretirement benefits
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	**
Regulatory Assets, Noncurrent	$ 155,888		155,942	157,015
Plant costs/asset retirement obligations
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Over plant lives
Regulatory Assets, Noncurrent	$ 73,528		71,740	68,815
Plant retirement
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	-
Regulatory Assets, Noncurrent	$ 48,544		65,919	57,499
Manufactured gas plant site remediation
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	-
Regulatory Assets, Noncurrent	$ 26,000		26,429	25,964
Taxes recoverable from customers
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Over plant lives
Regulatory Assets, Noncurrent	$ 11,438		10,785	10,847
Long-term debt refinancing costs
Regulatory Assets
Estimated Recovery or Refund Period as of September 30, 2021	Up to 39 years
Regulatory Assets, Noncurrent	$ 3,952		$ 4,426	$ 3,826

Regulatory liabilities (Details

Regulatory liabilities (Details 2) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 16,491	$ 31,450	$ 39,837
Regulatory liabilities	426,141	428,075	434,936
Total regulatory liabilities	442,632	459,525	474,773
Net regulatory position	69,748	(11,617)	(25,884)
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	3,867	3,557	4,223
Regulatory liabilities	$ 218,566	227,850	232,186
Taxes refundable to customers | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Taxes refundable to customers | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Over plant lives
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Regulatory liabilities due within one year	$ 3,205	3,667	6,171
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	-
Regulatory liabilities due within one year	$ 0	18,565	20,556
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	9,419	5,661	8,887
Regulatory liabilities	$ 17,977	16,065	11,392
Other regulatory liabilities | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Other regulatory liabilities | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 21 years
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Over plant lives
Regulatory liabilities	$ 172,683	167,171	173,367
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	**
Regulatory liabilities	$ 16,915	$ 16,989	$ 17,991

Fair value measurements Insuran

Fair value measurements Insurance contracts (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 106,400,000	$ 95,700,000	$ 106,400,000	$ 95,700,000	$ 100,100,000
Net unrealized gain on investments used to satisfy obligations under nonqualified benefit plans	$ 373,000	$ 3,300,000	$ 4,200,000	$ 8,700,000

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Available-for-sale securities [Abstract]
Cost	$ 11,278	$ 11,185	$ 10,979
Gross Unrealized Gains	93	156	216
Gross Unrealized Losses	39	14	4
Fair Value	11,332	11,327	11,191
Mortgage-backed securities
Available-for-sale securities [Abstract]
Cost	8,597	9,799	9,813
Gross Unrealized Gains	93	156	213
Gross Unrealized Losses	11	9	4
Fair Value	8,679	9,946	10,022
U.S. Treasury securities
Available-for-sale securities [Abstract]
Cost	2,681	1,386	1,166
Gross Unrealized Gains	0	0	3
Gross Unrealized Losses	28	5	0
Fair Value	$ 2,653	$ 1,381	$ 1,169

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	63.00%	57.00%	68.00%
Percentage investment in common stock of large-cap companies	16.00%	18.00%	15.00%
Percentage investment in common stock of mid-cap companies	8.00%	9.00%	6.00%
Percentage investment in common stock of small-cap companies	7.00%	9.00%	6.00%
Percentage investment in target date investments	5.00%	5.00%	4.00%
Percentage investment in cash and cash equivalents	1.00%	2.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 127,610	$ 120,348	$ 115,356
Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	127,610	120,348	115,356
Money market funds | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	9,907	8,917	8,478
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	9,907	8,917	8,478
Insurance contracts* | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	106,371	100,104	95,687
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	106,371	100,104	95,687
Mortgage-backed securities | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,679	9,946	10,022
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,679	9,946	10,022
U.S. Treasury securities | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	2,653	1,381	1,169
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 2,653	$ 1,381	$ 1,169

Fair value measurements (Deta_2

Fair value measurements (Details 4) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 2,328,266	$ 2,213,130	$ 2,270,290
Carrying amount
Fair value, balance sheet grouping [Line Items]
Long-term debt	2,328,266	2,213,130	2,270,290
Fair value
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 2,599,907	$ 2,537,289	$ 2,593,743

Long-term debt outstanding (Det

Long-term debt outstanding (Details 1) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,328,266	$ 2,213,130	$ 2,270,290
Long-term debt due within one year	1,548	1,555	1,558
Long-term debt	$ 2,326,718	2,211,575	2,268,732
Senior Notes
Long-term debt outstanding [Line Items]
Weighted Average Interest Rate	4.36%
Long-term debt	$ 2,025,000	1,950,000	1,900,000
Commercial Paper
Long-term debt outstanding [Line Items]
Weighted Average Interest Rate	0.35%
Long-term debt	$ 168,800	125,600	214,150
Credit Agreements
Long-term debt outstanding [Line Items]
Weighted Average Interest Rate	2.54%
Long-term debt	$ 94,300	95,900	91,000
Medium-term Notes
Long-term debt outstanding [Line Items]
Weighted Average Interest Rate	7.32%
Long-term debt	$ 35,000	35,000	35,000
Term Loan Agreements
Long-term debt outstanding [Line Items]
Weighted Average Interest Rate	2.00%
Long-term debt	$ 7,700	8,400	8,400
Other Notes
Long-term debt outstanding [Line Items]
Weighted Average Interest Rate	0.83%
Long-term debt	$ 3,198	4,034	28,284
Long-term Debt
Long-term debt outstanding [Line Items]
Unamortized Debt Issuance Costs	5,709	5,803	6,526
Discount	$ 23	$ 1	$ 18

Schedule of debt maturities (De

Schedule of debt maturities (Details 2) $ in Thousands	Sep. 30, 2021USD ($)
Long-term debt maturities [Line Items]
Remainder of 2021	$ 77
2022	148,021
2023	77,921
2024	324,521
2025	177,802
Thereafter	$ 1,605,656

Cash flow information (Details)

Cash flow information (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Supplemental Cash Flow Information [Abstract]
Interest, net*	$ 59,876	$ 63,086
Income taxes paid, net	61,250	43,448
AFUDC borrowed	1,600	2,000
Right-of-use assets obtained in exchange for new operating lease liabilities	26,120	41,315	$ 54,356
Property, plant and equipment additions in accounts payable	88,355	33,240	26,082
Accrual for holdback payment related to a business combination	$ 0	$ 5,000	$ 2,500

Business segment data (Details)

Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,586,013			$ 1,587,289			$ 4,237,608	$ 4,147,591
Operating income	189,169			203,711			412,023	399,036
Income from continuing operations	138,962			153,015			291,249	278,395
Income (loss) from discontinued operations, net of tax	314			63			348	(484)
Net income	139,276	$ 100,190	$ 52,131	153,078	$ 99,703	$ 25,130	291,597	277,911
Electric
Segment Reporting Information [Line Items]
Revenues	98,930			87,447			267,254	249,793
Operating income	27,310			21,159			53,982	48,827
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	110,378			94,703			614,373	562,641
Operating income	(14,249)			(18,393)			42,925	32,285
Pipeline
Segment Reporting Information [Line Items]
Revenues	30,968			32,068			68,653	70,381
Operating income	12,284			11,762			35,896	35,406
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	831,189			822,439			1,730,521	1,705,629
Operating income	133,712			148,522			171,274	179,994
Construction services
Segment Reporting Information [Line Items]
Revenues	514,523			550,608			1,556,737	1,559,217
Operating income	30,120			40,507			108,172	102,185
Other
Segment Reporting Information [Line Items]
Revenues	25			24			70	(70)
Operating income	(8)			154			(226)	339
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(7,983)			(7,543)			(51,356)	(50,849)
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	(136)			(195)			(407)	(586)
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	(142)			(185)			(425)	(555)
Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	(3,908)			(3,622)			(38,000)	(36,820)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(110)			(110)			(314)	(262)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(289)			(425)			(2,128)	(3,674)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,398)			(3,006)			(10,082)	(8,952)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0			0			0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	237,065			210,115			940,017	870,151
Income from continuing operations	15,399			6,985			89,207	77,991
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	98,930			87,447			267,254	249,793
Income from continuing operations	20,564			16,787			41,618	40,314
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	110,378			94,703			614,373	562,641
Income from continuing operations	(15,390)			(17,614)			20,081	13,795
Regulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	27,757			27,965			58,390	57,717
Income from continuing operations	10,225			7,812			27,508	23,882
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,123			3,923			38,362	37,724
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	136			195			407	586
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	142			185			425	555
Regulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,845			3,543			37,530	36,583
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,348,948			1,377,174			3,297,591	3,277,440
Income from continuing operations	123,563			146,030			202,042	200,404
Nonregulated operation | Pipeline
Segment Reporting Information [Line Items]
Revenues	3,211			4,103			10,263	12,664
Income from continuing operations	356			189			1,166	444
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	831,189			822,439			1,730,521	1,705,629
Income from continuing operations	96,282			107,307			116,865	122,113
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	514,523			550,608			1,556,737	1,559,217
Income from continuing operations	23,130			29,789			81,839	74,544
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	25			24			70	(70)
Income from continuing operations	3,795			8,745			2,172	3,303
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,860			3,620			12,994	13,125
Nonregulated operation | Intersegment eliminations | Pipeline
Segment Reporting Information [Line Items]
Revenues	63			79			470	237
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	110			110			314	262
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	289			425			2,128	3,674
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,398			$ 3,006			$ 10,082	$ 8,952

Employee benefit plans (Details

Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	2,455	3,023	7,365	9,070
Expected return on assets	(4,894)	(4,987)	(14,682)	(14,962)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial loss	2,004	1,793	6,012	5,379
Net periodic benefit credit, including amount capitalized	(435)	(171)	(1,305)	(513)
Less amount capitalized	0	0	0	0
Net periodic benefit cost (credit)	(435)	(171)	(1,305)	(513)
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	400	383	1,200	1,149
Interest cost	466	609	1,398	1,827
Expected return on assets	(1,275)	(1,115)	(3,825)	(3,764)
Amortization of prior service credit	(349)	(349)	(1,047)	(1,048)
Amortization of net actuarial loss	6	71	18	215
Net periodic benefit credit, including amount capitalized	(752)	(401)	(2,256)	(1,621)
Less amount capitalized	46	39	129	106
Net periodic benefit cost (credit)	(798)	(440)	(2,385)	(1,727)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 769	$ 1,000	$ 2,300	$ 2,900

IPUC (Details)

IPUC (Details) - IPUC - Intermountain Gas - Gas Distribution [Member] - USD ($) $ in Millions	Jan. 12, 2021	Jan. 01, 2021
Public Utilities, General Disclosures [Line Items]
Depreciation and Amortization Rate Case	$ (2.9)
Depreciation and Amortization, Rate, Percent	3.00%
Depreciation and Amortization, Requested Rate, Percent	2.60%
Depreciation and Amortization , Approved Rate Increase (Decrease), Amount		$ (3.8)
Depreciation and Amortization, Approved Rate, Percent		2.40%

MNPUC (Details 2)

MNPUC (Details 2) $ in Millions	Sep. 01, 2021USD ($)
MNPUC [Member] | Gas Distribution [Member] | Great Plains Natural Gas Co. [Member] | Pending Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 8.8

NDPSC (Details 3)

NDPSC (Details 3) - Electric - NDPSC [Member] - Montana-Dakota Utilities Co. [Member] - Subsequent Event [Member] $ in Millions	Nov. 01, 2021USD ($)
Public Utilities, General Disclosures [Line Items]
Total Transmission Cost Adjustment Rider	$ 14.5
Public Utilities, Requested Rate Increase (Decrease), Amount	(1.1)
Transmission Capital Projects Increase (Decrease)	$ 5.1

SDPUC (Details 4)

SDPUC (Details 4)	Mar. 11, 2021USD ($)
SDPUC | Pending Rate Case [Member] | Electricity [Member] | Montana-Dakota Utilities Co. [Member]
Public Utilities, General Disclosures [Line Items]
Total Infrastructure Rider	$ 0

WUTC (Details 5)

WUTC (Details 5) - Pending Rate Case [Member] - WUTC [Member] - Cascade Natural Gas [Member] - USD ($) $ in Millions	Oct. 15, 2021	Sep. 30, 2021	Jun. 01, 2021
Pipeline Cost Recovery Mechanism [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 2.1
Public Utilities, Requested Rate Increase (Decrease), Percentage			0.80%
Pipeline Cost Recovery Mechanism [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount	$ 1.7
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage	0.60%
Gas Distribution [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 13.7
Public Utilities, Requested Rate Increase (Decrease), Percentage		5.10%

FERC (Details 6)

FERC (Details 6) $ in Millions	Jan. 01, 2022USD ($)
MISO [Member] | Montana-Dakota Utilities Co. [Member] | Electric | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Transmission Formula Revenue Requirement	$ 13.4

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 34.7	$ 41.5	$ 72.7
Insurance Receivable	11.7	17.5	49.1
Regulatory Assets	$ 21.1	$ 21.3	$ 20.9

Guarantees (Details 2)

Guarantees (Details 2)	Sep. 30, 2021USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 147,600,000
Fixed maximum amounts guaranteed by year 2021	900,000
Fixed maximum amounts guaranteed by year 2022	39,300,000
Fixed maximum amounts guaranteed by year 2023	50,300,000
Fixed maximum amounts guaranteed by year 2024	46,600,000
Fixed maximum amounts guaranteed by year 2025	600,000
Fixed maximum amounts guaranteed, thereafter	900,000
No scheduled maturity date	9,000,000
Amount outstanding under guarantees that is reflected on balance sheet	0
Letters of credit	25,500,000
Letters of credit set to expire - 2021	21,200,000
Letters of credit set to expire - 2022	4,300,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	$ 897,200,000

Variable interest entities (Det

Variable interest entities (Details 3) $ in Millions	Sep. 30, 2021USD ($)
Fuel contract
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 32.1