MDU Resources (MDU) 10-K 19 Feb 21 2020 FY Annual report Financial data

Cover page

Cover page - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 11, 2021	Jun. 30, 2020
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
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 Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4,447,584,104
Entity Common Stock, Shares Outstanding		200,522,277
Entity Central Index Key	0000067716
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating revenues:
Operating revenues	$ 5,532,750	$ 5,336,776	$ 4,531,552
Operating expenses:
Operation and maintenance:	4,028,262	3,895,294	3,256,121
Purchased natural gas sold	390,269	421,545	404,153
Depreciation, depletion and amortization	285,100	256,017	220,205
Taxes, other than income	217,253	196,143	168,638
Electric fuel and purchased power	66,941	86,557	80,712
Total operating expenses	4,987,825	4,855,556	4,129,829
Operating income	544,925	481,220	401,723
Other income (expense)	26,711	15,812	(238)
Interest expense	96,519	98,587	84,614
Income before income taxes	475,117	398,445	316,871
Income taxes	84,590	63,279	47,485
Income from continuing operations	390,527	335,166	269,386
Income (loss) from discontinued operations, net of tax	(322)	287	2,932
Net income	$ 390,205	$ 335,453	$ 272,318
Earnings per share - basic:
Income from continuing operations	$ 1.95	$ 1.69	$ 1.38
Discontinued operations, net of tax	0	0	0.01
Earnings per share - basic	1.95	1.69	1.39
Earnings per share - diluted:
Income from continuing operations	1.95	1.69	1.38
Discontinued operations, net of tax	0	0	0.01
Earnings per share - diluted	$ 1.95	$ 1.69	$ 1.39
Weighted average common shares outstanding - basic	200,502	198,612	195,720
Weighted Average Number of Shares Outstanding, Diluted	200,571	198,626	196,150
Regulated Operation
Operating revenues:
Operating revenues	$ 1,249,146	$ 1,279,304	$ 1,213,227
Operating expenses:
Operation and maintenance:	353,184	356,132	340,331
Income from continuing operations	135,103	122,535	111,637
Unregulated Operation
Operating revenues:
Operating revenues	4,283,604	4,057,472	3,318,325
Operating expenses:
Operation and maintenance:	3,675,078	3,539,162	2,915,790
Income from continuing operations	$ 255,424	$ 212,631	$ 157,749

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 390,205	$ 335,453	$ 272,318
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, tax	145	(140)	429
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	446	731	162
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, tax	(2,606)	(2,012)	1,471
Postretirment liability gains (losses) arising during the period, net of tax	(8,395)	(6,151)	4,441
Amortization of postretirement liability losses in net periodic benefit cost, tax	630	476	721
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	1,922	1,486	2,173
Postretirement liability adjustment	(6,473)	(4,665)	6,614
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, tax	0	0	(14)
Foreign currency translation adjustment recognized during the period, net of tax	0	0	(61)
Reclassification adjustment for foreign currency translation adjustment included in net income, tax	0	0	75
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax	0	0	249
Foreign currency translation adjustment	0	0	188
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	0	35	(38)
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(1)	134	(144)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	14	10	35
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	52	40	131
Net unrealized gain (loss) on available-for-sale investments	51	174	(13)
Other comprehensive income (loss)	(5,976)	(3,760)	6,951
Comprehensive income attributable to common stockholders	$ 384,229	$ 331,693	$ 279,269

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 59,547	$ 66,459
Receivables, net	873,986	836,605
Inventories	291,167	278,407
Current regulatory assets	68,527	63,613
Prepayments and other current assets	44,120	52,617
Total current assets	1,337,347	1,297,701
Noncurrent assets:
Property, plant and equipment	8,300,770	7,908,628
Less accumulated depreciation, depletion and amortization	3,133,831	2,991,486
Net property, plant and equipment	5,166,939	4,917,142
Goodwill	714,963	681,358
Other intangible assets, net	25,496	15,246
Regulatory assets	379,381	353,784
Investments	165,022	148,656
Operating lease right-of-use assets	120,113	115,323
Other	144,111	153,849
Total noncurrent assets	6,716,025	6,385,358
Total assets	8,053,372	7,683,059
Current liabilities:
Short-term borrowings	50,000	0
Long-term debt due within one year	1,555	16,540
Accounts payable	426,264	403,391
Taxes payable	88,844	48,970
Dividends payable	42,611	41,580
Accrued compensation	90,629	99,269
Regulatory liabilities due within one year	31,450	42,935
Operating lease liabilities due within one year	33,655	31,664
Other accrued liabilities	198,514	182,078
Total current liabilities	963,522	866,427
Noncurrent liabilities:
Long-term debt	2,211,575	2,226,567
Deferred income taxes	516,098	506,583
Regulatory liabilities	428,075	447,370
Asset retirement obligations	440,356	413,298
Operating lease liabilities	86,868	83,742
Other	327,773	291,826
Total noncurrent liabilities	4,010,745	3,969,386
Commitments and contingencies
Stockholders' equity:
Common stock	201,061	200,923
Other paid-in capital	1,371,385	1,355,404
Retained earnings	1,558,363	1,336,647
Accumulated other comprehensive loss	(48,078)	(42,102)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	3,079,105	2,847,246
Total liabilities and stockholders' equity	$ 8,053,372	$ 7,683,059
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Par Value Per Share	$ 1	$ 1
Common Stock, Shares, Issued	201,061,198	200,922,790
Treasury Stock, Common, Shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 8,781	$ 12,326
Receivables, net	4,865	4,727
Prepayments and other current assets	1,612	501
Total current assets	65,797	67,497
Noncurrent assets:
Investments	52,000	46,294
Operating lease right-of-use assets	56	153
Other	28,866	27,098
Total noncurrent assets	3,160,569	2,922,882
Total assets	3,226,366	2,990,379
Current liabilities:
Accounts payable	2,135	2,981
Taxes payable	4,056	1,253
Dividends payable	42,611	41,580
Accrued compensation	7,825	8,812
Operating lease liabilities due within one year	40	96
Other accrued liabilities	6,881	7,690
Total current liabilities	68,960	67,164
Noncurrent liabilities:
Operating lease liabilities	16	56
Other	78,285	75,913
Total noncurrent liabilities	78,301	75,969
Commitments and contingencies
Stockholders' equity:
Common stock	201,061	200,923
Other paid-in capital	1,371,385	1,355,404
Retained earnings	1,558,363	1,336,647
Accumulated other comprehensive loss	(48,078)	(42,102)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	3,079,105	2,847,246
Total liabilities and stockholders' equity	$ 3,226,366	$ 2,990,379
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Par Value Per Share	$ 1	$ 1
Common Stock, Shares, Issued	201,061,198	200,922,790
Treasury Stock, Common, Shares	538,921	538,921

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Previously Reported	Revision of Prior Period, Accounting Standards Update, Adjustment	Common stock	Common stockPreviously Reported	Other paid-in capital	Other paid-in capitalPreviously Reported	Retained earnings	Retained earningsPreviously Reported	Retained earningsRevision of Prior Period, Accounting Standards Update, Adjustment	Total accumulated other comprehensive loss	Total accumulated other comprehensive lossPreviously Reported	Total accumulated other comprehensive lossRevision of Prior Period, Accounting Standards Update, Adjustment	Treasury stock	Treasury stockPreviously Reported
Balance (in shares) at Dec. 31, 2017				195,843,297
Treasury stock (in shares) at Dec. 31, 2017														(538,921)
Balance at Dec. 31, 2017	$ 2,428,073	$ 2,429,043		$ 195,843	$ 195,843	$ 1,233,412	$ 1,233,412	$ 1,039,778	$ 1,040,748		$ (37,334)	$ (37,334)		$ (3,626)	$ (3,626)
Balance (Accounting Standards Update 2014-09) at Dec. 31, 2017			$ (970)							$ (970)
Net income	272,318							272,318
Other comprehensive income (loss)	6,951										6,951
Dividends declared on common stock	(156,453)							(156,453)
Stock-based compensation	5,060					5,060
Repurchase of common stock (in shares)														(182,424)
Repurchase of common stock	(5,020)													$ (5,020)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings						(7,350)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (in shares)														182,424
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings														$ 5,020
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(2,330)
Issuance of common stock (in shares)				721,610
Issuance of common stock	18,176			$ 722		17,454
Balance at Dec. 31, 2018	2,566,775		$ 0	$ 196,565		1,248,576		1,163,602		$ 7,959	(38,342)		$ (7,959)	$ (3,626)
Balance (in shares) at Dec. 31, 2018				196,564,907
Treasury stock (in shares) at Dec. 31, 2018														(538,921)
Net income	335,453							335,453
Other comprehensive income (loss)	(3,760)										(3,760)
Dividends declared on common stock	(162,408)							(162,408)
Stock-based compensation	7,353					7,353
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings						(3,261)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(3,015)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (in shares)				246,214
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings				$ 246
Issuance of common stock (in shares)				4,111,669
Issuance of common stock	106,848			$ 4,112		102,736
Balance at Dec. 31, 2019	$ 2,847,246			$ 200,923		1,355,404		1,336,647			(42,102)			$ (3,626)
Balance (in shares) at Dec. 31, 2019	200,922,790			200,922,790
Treasury stock (in shares) at Dec. 31, 2019														(538,921)
Net income	$ 390,205							390,205
Other comprehensive income (loss)	(5,976)										(5,976)
Dividends declared on common stock	(168,489)							(168,489)
Stock-based compensation	13,096					13,096
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 upon vesting of stock-based compensation, net of shares used for tax withholdings (in shares)				26,406
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings				$ 26
Issuance of common stock (in shares)				112,002
Issuance of common stock	3,385			$ 112		3,273
Balance at Dec. 31, 2020	$ 3,079,105			$ 201,061		$ 1,371,385		$ 1,558,363			$ (48,078)			$ (3,626)
Balance (in shares) at Dec. 31, 2020	201,061,198			201,061,198
Treasury stock (in shares) at Dec. 31, 2020														(538,921)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net income	$ 390,205	$ 335,453	$ 272,318
Income (loss) from discontinued operations, net of tax	(322)	287	2,932
Income from continuing operations	390,527	335,166	269,386
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	285,100	256,017	220,205
Deferred income taxes	(1,801)	63,415	59,735
Changes in current assets and liabilities, net of acquisitions:
Receivables	7,796	(104,374)	28,234
Inventories	(7,221)	9,331	(46,796)
Other current assets	31,601	(38,283)	(31,814)
Accounts payable	15,955	30,079	21,109
Other current liabilities	35,591	51,278	22,285
Other noncurrent changes	12,201	(60,813)	(38,521)
Net cash provided by continuing operations	769,749	541,816	503,823
Net cash provided by (used in) discontinued operations	(1,375)	464	(3,942)
Net cash provided by operating activities	768,374	542,280	499,881
Investing activities:
Capital expenditures	(558,007)	(576,065)	(568,230)
Acquisitions, net of cash acquired	(105,979)	(55,597)	(167,692)
Net proceeds from sale or disposition of property and other	35,557	29,812	26,100
Investments	(1,814)	(2,011)	(2,321)
Net cash used in continuing operations	(630,243)	(603,861)	(712,143)
Net cash provided by discontinued operations	0	0	1,236
Net cash used in investing activities	(630,243)	(603,861)	(710,907)
Financing activities:
Issuance of short-term borrowings	75,000	169,977	0
Repayment of short-term borrowings	(25,000)	(170,000)	0
Issuance of long-term debt	116,973	599,455	566,829
Repayment of long-term debt	(148,634)	(468,917)	(174,520)
Proceeds from issuance of common stock	3,385	106,848	0
Payments of stock issuance costs	0	0	(10)
Dividends paid	(166,405)	(160,256)	(154,573)
Repurchase of common stock	0	0	(5,020)
Tax withholding on stock-based compensation	(362)	(3,015)	(2,330)
Net cash provided by (used in) financing activities	(145,043)	74,092	230,376
Effect of exchange rate changes on cash and cash equivalents	0	0	(1)
Increase (decrease) in cash and cash equivalents	(6,912)	12,511	19,349
Cash and cash equivalents - beginning of year	66,459	53,948	34,599
Cash and cash equivalents - end of year	$ 59,547	$ 66,459	$ 53,948

Basis of Presentation

Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Organization, Consolidation and Presentation of Financial Statements Disclosure	Basis of Presentation The abbreviations and acronyms used throughout are defined following the Notes to Consolidated Financial Statements. The consolidated financial statements of the Company include the accounts of the following businesses: electric, natural gas distribution, pipeline, construction materials and contracting, construction services and other. The electric and natural gas distribution businesses, as well as a portion of the pipeline business, are regulated. Construction materials and contracting, construction services and the other businesses, as well as a portion of the pipeline business, are non-regulated. For further descriptions of the Company's businesses, see Note 17. On January 2, 2019, the Company announced the completion of the Holding Company Reorganization, which resulted in Montana-Dakota becoming a subsidiary of the Company. The purpose of the reorganization was to make the public utility division into a subsidiary of the holding company, just as the other operating companies are wholly owned subsidiaries. In March 2020, the World Health Organization declared the outbreak of COVID-19 a pandemic, and the President of the United States declared the COVID-19 outbreak as a national emergency. 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 and accordingly operations have been generally allowed to continue. The Company has experienced some inefficiency impacts, including operation suspensions and interruptions at some locations to carry out preventive measures or in response to instances of positive tests or quarantines. The Company has assessed the impacts of the COVID-19 pandemic on its results of operations for the twelve months ended December 31, 2020, and determined there were no material adverse impacts. In the first quarter of 2020, the Company recorded an out-of-period adjustment to correct the recognition of revenue on a construction contract, which was the result of an overstatement of operating revenue and receivables of $7.7 million and an understatement of operating expense and accounts payable of $1.2 million in the year ended December 31, 2019. This adjustment resulted in an after-tax reduction to net income of $6.7 million in the first quarter of 2020. The Company evaluated the impact of the out-of-period adjustment and concluded it was not material to any previously issued interim and annual consolidated financial statements and the adjustment was not material to the three months ended March 31, 2020, or the twelve months ended December 31, 2020. In the fourth quarter of 2020, the Company recorded an out-of-period adjustment to correct the recognition of net periodic benefit cost and other comprehensive income associated with the Company's benefit plans, which was the result of the previous overstatement of benefit plan expenses of $6.5 million, understatement of accumulated other comprehensive loss of $2.7 million and overstatement of regulatory liabilities of $3.8 million from 2006 through 2020. This adjustment resulted in an after-tax increase to net income of $4.4 million in the fourth quarter of 2020. The Company evaluated the impact of the out-of-period adjustment, individually and in the aggregate with the previously mentioned out-of-period adjustment, and concluded it was not material to any previously issued interim and annual consolidated financial statements and the adjustment was not material to the three and twelve months ended December 31, 2020. Effective January 1, 2020, the Company adopted the requirements of the ASU on the measurement of credit losses on certain financial instruments following a modified retrospective approach, as further discussed in Note 2. As such, results for reporting periods beginning on January 1, 2020, are presented under the new guidance, while prior period amounts are not adjusted and continue to be reported in accordance with historic accounting. The Company's adoption of this guidance did not have a material impact on its financial reporting. 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 and are not material to the financial statements for any period presented. 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. Management has also evaluated the impact of events occurring after December 31, 2020, up to the date of issuance of these consolidated financial statements. Principles of consolidation The consolidated financial statements were prepared in accordance with GAAP and include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation, except for certain transactions related to the Company's regulated operations in accordance with GAAP. For more information on intercompany revenues, see Note 17. The statements also include the Company's ownership interests in the assets, liabilities and expenses of jointly owned electric transmission and generating facilities. See Note 19 for additional information. Use of estimates The preparation of financial statements in conformity with GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, at the date of the financial statements, as well as the reported amounts of revenues and expenses during the reporting period. Estimates are used for items such as long-lived assets and goodwill; fair values of acquired assets and liabilities under the acquisition method of accounting; aggregate reserves; property depreciable lives; tax provisions; revenue recognized using the cost-to-cost measure of progress for contracts; expected credit losses; environmental and other loss contingencies; regulatory assets expected to be recovered in rates charged to customers; costs on construction contracts; unbilled revenues; actuarially determined benefit costs; asset retirement obligations; lease classification; present value of right-of-use assets and lease liabilities; and the valuation of stock-based compensation. As additional information becomes available, or actual amounts are determinable, the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates.

Accounting Policies

Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Significant Accounting Policies [Text Block]	Significant Accounting Policies New accounting standards Recently adopted accounting standards ASU 2016-13 - Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued guidance on the measurement of credit losses on certain financial instruments. The guidance introduced a new impairment model known as the current expected credit loss model that replaced the incurred loss impairment methodology previously included under GAAP. This guidance required entities to present certain investments in debt securities, trade accounts receivable and other financial assets at their net carrying value of the amount expected to be collected on the financial statements. The Company adopted the guidance on January 1, 2020, using a modified retrospective approach. The Company formed an implementation team to review and assess existing financial assets to identify and evaluate the financial assets subject to the new current expected credit loss model. The Company assessed the impact of the guidance on its processes and internal controls and has identified and updated existing internal controls and processes to ensure compliance with the new guidance; such modifications were deemed insignificant. During the assessment phase, the Company identified the complete portfolio of assets subject to the current expected credit loss model. The Company determined the guidance did not have a material impact on its results of operations, financial position, cash flows or disclosures and did not record a material cumulative effect adjustment upon adoption. See Receivables and allowance for expected credit losses within this note for additional information on the Company's expected credit losses. ASU 2018-13 - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued guidance on modifying the disclosure requirements on fair value measurements as part of the disclosure framework project. The guidance modified, among other things, the disclosures required for Level 3 fair value measurements, including the range and weighted average of significant unobservable inputs. The guidance removed, among other things, the disclosure requirement to disclose transfers between Levels 1 and 2. The Company adopted the guidance on January 1, 2020, and determined it did not have a material impact on its disclosures. Recently issued accounting standards not yet adopted 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, clarifies 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 effects a one percentage point change in assumed health care cost trend rates will have on certain benefit components. The guidance was effective for the Company on January 1, 2021, and must be applied 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 and disclosures. 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 applies 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 new reference rates, 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. Cash and cash equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. Revenue recognition 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. The electric and natural gas distribution segments generate revenue from the sales of electric and natural gas products and services, which includes retail and transportation services. These segments establish a customer's retail or transportation service account based on the customer's application/contract for service, which indicates approval of a contract for service. The contract identifies an obligation to provide service in exchange for delivering or standing ready to deliver the identified commodity; and the customer is obligated to pay for the service as provided in the applicable tariff. The product sales are based on a fixed rate that includes a base and per-unit rate, which are included in approved tariffs as determined by state or federal regulatory agencies. The quantity of the commodity consumed or transported determines the total per-unit revenue. The service provided, along with the product consumed or transported, are a single performance obligation because both are required in combination to successfully transfer the contracted product or service to the customer. Revenues are recognized over time as customers receive and consume the products and services. The method of measuring progress toward the completion of the single performance obligation is on a per-unit output method basis, with revenue recognized based on the direct measurement of the value to the customer of the goods or services transferred to date. For contracts governed by the Company’s utility tariffs, amounts are billed monthly with the amount due between 15 and 22 days of receipt of the invoice depending on the applicable state’s tariff. For other contracts not governed by tariff, payment terms are net 30 days. At this time, the segment has no material obligations for returns, refunds or other similar obligations. The pipeline segment generates revenue from providing natural gas transportation, gathering and underground storage services, as well as other energy-related services to both third parties and internal customers, largely the natural gas distribution segment. The pipeline segment establishes a contract with a customer based upon the customer’s request for firm or interruptible natural gas transportation, storage or gathering service(s). The contract identifies an obligation for the segment to provide the requested service(s) in exchange for consideration from the customer over a specified term. Depending on the type of service(s) requested and contracted, the service provided may include transporting, gathering or storing an identified quantity of natural gas and/or standing ready to deliver or store an identified quantity of natural gas. Natural gas transportation, gathering and storage revenues are based on fixed rates, which may include reservation fees and/or per-unit commodity rates. The services provided by the segment are generally treated as single performance obligations satisfied over time simultaneous to when the service is provided and revenue is recognized. Rates for the segment’s regulated services are based on its FERC approved tariff or customer negotiated rates, and rates for its non-regulated services are negotiated with its customers and set forth in the contract. For contracts governed by the company’s tariff, amounts are billed on or before the ninth business day of the following month and the amount is due within 12 days of receipt of the invoice. For gathering contracts not governed by the tariff, amounts are due within 20 days of invoice receipt. For other contracts not governed by the tariff, payment terms are net 30 days. At this time, the segment has no material obligations for returns, refunds or other similar obligations. The construction materials and contracting segment generates revenue from contracting services and construction materials sales. This segment focuses on the vertical integration of its contracting services with its construction materials to support the aggregate-based product lines. This segment provides contracting services to a customer when a contract has been signed by both the customer and a representative of the segment obligating a service to be provided in exchange for the consideration identified in the contract. The nature of the services this segment provides generally includes integrating a set of services and related construction materials into a single project to create a distinct bundle of goods and services, which the Company evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include liquidated damages; performance bonuses or incentives and penalties; claims; unapproved/unpriced change orders; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using an input method based on the cost-to-cost measure of progress on a project. This is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction materials to third parties and internal customers. The contract for material sales is the use of a sales order or an invoice, which includes the pricing and payment terms. All material contracts contain a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 30 days of receipt. There are no material obligations for returns, refunds or other similar obligations. The construction services segment generates revenue from specialty contracting services which also includes the sale of construction equipment and other supplies. This segment provides specialty contracting services to a customer when a contract has been signed by both the customer and a representative of the segment obligating a service to be provided in exchange for the consideration identified in the contract. The nature of the services this segment provides generally includes multiple promised goods and services in a single project to create a distinct bundle of goods and services, which the Company evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include claims, unapproved/unpriced change orders, bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using the input method based on the measurement of progress on a project. The input method is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction equipment and other supplies to third parties and internal customers. The contract for these sales is the use of a sales order or invoice, which includes the pricing and payment terms. All such contracts include a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 30 days of receipt. There are no material obligations for returns, refunds or other similar obligations. The Company recognizes all other revenues when services are rendered or goods are delivered. Legal costs The Company expenses external legal fees as they are incurred. Business combinations For all business combinations, the Company preliminarily allocates the purchase price of the acquisitions to the assets acquired and liabilities assumed based on their estimated fair values as of the acquisition dates and are considered provisional until final fair values are determined or the measurement period has passed. The Company expects to record adjustments as it accumulates the information needed to estimate the fair value of assets acquired and liabilities assumed, including working capital balances, estimated fair value of identifiable intangible assets, property, plant and equipment, total consideration and goodwill. The excess of the purchase price over the aggregate fair values is recorded as goodwill. The Company calculated the fair value of the assets acquired in 2020 and 2019 using a market or cost approach (or a combination of both). Fair values for some of the assets were determined based on Level 3 inputs including estimated future cash flows, discount rates, growth rates, sales projections, retention rates and terminal values, all of which require significant management judgment and are susceptible to change. The final fair value of the net assets acquired may result in adjustments to the assets and liabilities, including goodwill, and will be made as soon as practical, but no later than 12 months from the respective acquisition dates. Any subsequent measurement period adjustments are not expected to have a material impact on the Company's results of operations. Receivables and allowance for expected credit losses Receivable 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 3. 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 $43.9 million and $46.7 million at December 31, 2020 and 2019, 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 and general economic conditions of the various local economies, among others. Specific account balances are written off when management determines the amounts to be uncollectible. The Company conducted additional analysis of its receivables and allowance for expected credit losses due to the impacts of COVID-19. As more customer balances enter arrears, further analysis supported increasing the uncollectible factors used in determining the expected credit losses of certain segments during 2020. 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 January 1, 2020 $ 328 $ 1,056 $ — $ 5,357 $ 1,756 $ 8,497 Current expected credit loss provision 1,517 3,187 2 1,447 4,832 10,985 Less write-offs charged against the allowance 1,289 2,511 — 640 866 5,306 Credit loss recoveries collected 343 839 — — — 1,182 At December 31, 2020 $ 899 $ 2,571 $ 2 $ 6,164 $ 5,722 $ 15,358 The Company's allowance for doubtful accounts at December 31, 2019, was $8.5 million. Receivables also consist of accrued unbilled revenue representing revenues recognized in excess of amounts billed. Accrued unbilled revenue at MDU Energy Capital was $94.0 million and $100.8 million at December 31, 2020 and 2019, respectively. Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows: 2020 2019 (In thousands) Short-term retainage* $ 100,054 $ 75,590 Long-term retainage** 2,761 14,228 Total retainage $ 102,815 $ 89,818 * Expected to be paid within 12 months or less and included in receivables, net. ** Included in noncurrent assets - other. 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 at December 31 consisted of: 2020 2019 (In thousands) Aggregates held for resale $ 175,782 $ 147,723 Asphalt oil 28,238 41,912 Materials and supplies 25,142 22,512 Merchandise for resale 21,087 22,232 Natural gas in storage (current) 21,919 22,058 Other 18,999 21,970 Total $ 291,167 $ 278,407 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 and $48.4 million at December 31, 2020 and 2019, respectively. Property, plant and equipment Additions to property, plant and equipment are recorded at cost. When regulated assets are retired, or otherwise disposed of in the ordinary course of business, the original cost of the asset is charged to accumulated depreciation. With respect to the retirement or disposal of all other assets, the resulting gains or losses are recognized as a component of income. The Company is permitted to capitalize AFUDC on regulated construction projects and to include such amounts in rate base when the related facilities are placed in service. In addition, the Company capitalizes interest, when applicable, on certain construction projects associated with its other operations. The amount of AFUDC for the years ended December 31 was as follows: 2020 2019 2018 (In thousands) AFUDC - borrowed $ 2,640 $ 2,807 $ 2,290 AFUDC - equity $ 1,270 $ 698 $ 1,897 Generally, property, plant and equipment are depreciated on a straight-line basis over the average useful lives of the assets, except for depletable aggregate reserves, which are depleted based on the units-of-production method. The Company collects removal costs for plant assets in regulated utility rates. These amounts are recorded as regulatory liabilities on the Consolidated Balance Sheets. Impairment of long-lived assets The Company reviews the carrying values of its long-lived assets, excluding goodwill, whenever events or changes in circumstances indicate that such carrying values may not be recoverable. The determination of whether an impairment has occurred is based on an estimate of undiscounted future cash flows attributable to the assets, compared to the carrying value of the assets. If impairment has occurred, the amount of the impairment recognized is determined by estimating the fair value of the assets and recording a loss if the carrying value is greater than the fair value. The impairments are recorded in operation and maintenance expense on the Consolidated Statements of Income. No significant impairment losses were recorded in 2020, 2019 or 2018. Unforeseen events and changes in circumstances could require the recognition of impairment losses at some future date. Regulatory assets and liabilities The Company's regulated businesses are subject to various state and federal agency regulations. The accounting policies followed by these businesses are generally subject to the Uniform System of Accounts of the FERC as well as the provisions of ASC 980 - Regulated Operations . These accounting policies differ in some respects from those used by the Company's non-regulated businesses. The Company's regulated businesses account for certain income and expense items under the provisions of regulatory accounting, which requires these businesses to defer as regulatory assets or liabilities certain items that would have otherwise been reflected as expense or income, respectively. The Company records regulatory assets or liabilities at the time the Company determines the amounts to be recoverable in current or future rates. Regulatory assets and liabilities are being amortized consistently with the regulatory treatment established by the FERC and the applicable state public service commission. See Note 6 for more information regarding the nature and amounts of these regulatory deferrals. Natural gas costs recoverable or refundable through rate adjustments Under the terms of certain orders of the applicable state public service commissions, the Company is deferring natural gas commodity, transportation and storage costs that are greater or less than amounts presently being recovered through its existing rate schedules. Such orders generally provide that these amounts are recoverable or refundable through rate adjustments. Natural gas costs refundable through rate adjustments were $18.6 million and $23.8 million at December 31, 2020 and 2019, respectively, which was included in regulatory liabilities due within one year on the Consolidated Balance Sheets. Natural gas costs recoverable through rate adjustments were $64.0 million and $89.2 million at December 31, 2020 and 2019, respectively, which was included in current regulatory assets and noncurrent assets - regulatory assets on the Consolidated Balance Sheets. Goodwill Goodwill represents the excess of the purchase price over the fair value of identifiable net tangible and intangible assets acquired in a business combination. Goodwill is required to be tested for impairment annually, which the Company completes in the fourth quarter, or more frequently if events or changes in circumstances indicate that goodwill may be impaired. The Company has determined that the reporting units for its goodwill impairment test are its operating segments, or components of an operating segment, that constitute a business for which discrete financial information is available and for which segment management regularly reviews the operating results. For more information on the Company's operating segments, see Note 17. Goodwill impairment, if any, is measured by comparing the fair value of each reporting unit to its carrying value. If the fair value of a reporting unit exceeds its carrying value, the goodwill of the reporting unit is not impaired. If the carrying value of a reporting unit exceeds its fair value, the Company must record an impairment loss for the amount that the carrying value of the reporting unit, including goodwill, exceeds the fair value of the reporting unit. For the years ended December 31, 2020, 2019 and 2018, there were no impairment losses recorded. The Company performed its annual goodwill impairment test in the fourth quarter of 2020 and determined the fair value substantially exceeded the carrying value at all reporting units at October 31, 2020. Investments The Company's investments include the cash surrender value of life insurance policies, an insurance contract, mortgage-backed securities and U.S. Treasury securities. The Company measures its investment in the insurance contract at fair value with any unrealized gains and losses recorded on the Consolidated Statements of Income. The Company has not elected the fair value option for its mortgage-backed securities and U.S. Treasury securities and, as a result, the unrealized gains and losses on these investments are recorded in accumulated other comprehensive income (loss). For more information, see Notes 8 and 18. Derivative instruments The Company enters into commodity price derivative contracts in order to minimize the price volatility associated with natural gas costs at its natural gas distribution segment. These derivatives are not designated as hedging instruments and are recorded in the Consolidated Balance Sheets at fair value, as discussed in Note 8. Changes in the fair value of these derivatives along with any contract settlements are recorded each period in regulatory assets or liabilities in accordance with regulatory accounting. The Company does not enter into any derivatives for trading or other speculative purposes. In 2017, the WUTC issued a requirement for gas providers to implement robust, risk-responsive hedging programs in order to minimize volatility in natural gas prices for natural gas utility customers and in 2019, the Company implemented policies and procedures that met these requirements. During 2020 and 2019, the Company entered into commodity price derivative contracts securing the purchase of 1.4 million MMBtu and 535,000 MMBtu of natural gas, respectively. Leases Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The Company recognizes leases with an original lease term of 12 months or less in income on a straight-line basis over the term of the lease and does not recognize a corresponding right-of-use asset or lease liability. The Company determines the lease term based on the non-cancelable and cancelable periods in each contract. The non-cancelable period consists of the term of the contract that is legally enforceable and cannot be canceled by either party without incurring a significant penalty. The cancelable period is determined by various factors that are based on who has the right to cancel a contract. If only the lessor has the right to cancel the contract, the Company will assume the contract will continue. If the lessee is the only party that has the right to cancel the contract, the Company looks to asset, entity and market-based factors. If both the lessor and the lessee have the right to cancel the contract, the Company assumes the contract will not continue. The discount rate used to calculate the present value of the lease liabilities is based upon the implied rate within each contract. If the rate is unknown or cannot be determined, the Company uses an incremental borrowing rate, which is determined by the length of the contract, asset class and the Company's borrowing rates, as of the commencement date of the contract. Asset retirement obligations The Company records the fair value of a liability for an asset retirement obligation in the period in which it is incurred. When the liability is initially recorded, the Company capitalizes a cost by increasing the carrying amount of the related long-lived asset. Over time, the liability is accreted to its present value each period and the capitalized cost is depreciated over the useful life of the related asset. Upon settlement of the liability, the Company either settles the obligation for the recorded amount or incurs a gain or loss at its non-regulated operations or incurs a regulatory asset or liability at its regulated operations. Stock-based compensation The Company determines compensation expense for stock-based awards based on the estimated fair values at the grant date and recognizes the related compensation expense over the vesting period. The Company uses the straight-line amortization method to recognize compensation expense related to restricted stock, which only has a service condition. This method recognizes stock compensation expense on a straight-line basis over the requisite service period for the entire award. The Company recognizes compensation expense related to performance awards that vest based on performance metrics and service conditions on a straight-line basis over the service period. Inception-to-date expense is adjusted based upon the determination of the potential achievement of the performance target at each reporting date. The Company recognizes compensation expense related to performance awards with market-based performance metrics on a straight-line basis over the requisite service period. The Company records the compensation expense for performance share awards using an estimated forfeiture rate. The estimated forfeiture rate is calculated based on an average of actual historical forfeitures. The Company also performs an analysis of any know

Revenue from Contract with Cust

Revenue from Contract with Customer	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue from contracts from 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. As part of the adoption of ASC 606 - Revenue from Contracts with Customers , the Company elected the practical expedient to recognize the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that the Company otherwise would have recognized is 12 months or less. Disaggregation In the following table, 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. Year ended December 31, 2020 Electric Natural gas distribution Pipeline Construction materials and contracting Construction services Other Total (In thousands) Residential utility sales $ 122,663 $ 476,388 $ — $ — $ — $ — $ 599,051 Commercial utility sales 131,477 277,873 — — — — 409,350 Industrial utility sales 36,744 26,243 — — — — 62,987 Other utility sales 6,634 — — — — — 6,634 Natural gas transportation — 45,546 111,686 — — — 157,232 Natural gas gathering — — 4,865 — — — 4,865 Natural gas storage — — 14,918 — — — 14,918 Contracting services — — — 1,069,665 — — 1,069,665 Construction materials — — — 1,659,152 — — 1,659,152 Intrasegment eliminations — — — (550,815) — — (550,815) Inside specialty contracting — — — — 1,397,124 — 1,397,124 Outside specialty contracting — — — — 649,486 — 649,486 Other 32,452 10,753 12,216 — 1,541 11,903 68,865 Intersegment eliminations (491) (534) (58,531) (417) (5,038) (11,958) (76,969) Revenues from contracts with customers 329,479 836,269 85,154 2,177,585 2,043,113 (55) 5,471,545 Revenues out of scope 2,059 11,382 192 — 47,572 — 61,205 Total external operating revenues $ 331,538 $ 847,651 $ 85,346 $ 2,177,585 $ 2,090,685 $ (55) $ 5,532,750 Year ended December 31, 2019 Electric Natural gas distribution Pipeline Construction materials and contracting Construction services Other Total (In thousands) Residential utility sales $ 125,369 $ 483,452 $ — $ — $ — $ — $ 608,821 Commercial utility sales 141,596 296,835 — — — — 438,431 Industrial utility sales 37,765 26,895 — — — — 64,660 Other utility sales 7,408 — — — — — 7,408 Natural gas transportation — 45,449 101,665 — — — 147,114 Natural gas gathering — — 9,164 — — — 9,164 Natural gas storage — — 11,708 — — — 11,708 Contracting services — — — 1,088,633 — — 1,088,633 Construction materials — — — 1,627,833 — — 1,627,833 Intrasegment eliminations — — — (525,749) — — (525,749) Inside specialty contracting — — — — 1,266,196 — 1,266,196 Outside specialty contracting — — — — 531,882 — 531,882 Other 35,574 12,726 17,687 — 131 16,551 82,669 Intersegment eliminations — — (56,252) (1,066) (3,370) (16,461) (77,149) Revenues from contracts with customers 347,712 865,357 83,972 2,189,651 1,794,839 90 5,281,621 Revenues out of scope 4,013 (135) 220 — 51,057 — 55,155 Total external operating revenues $ 351,725 $ 865,222 $ 84,192 $ 2,189,651 $ 1,845,896 $ 90 $ 5,336,776 Year ended December 31, 2018 Electric Natural gas distribution Pipeline Construction materials and contracting Construction services Other Total (In thousands) Residential utility sales $ 121,477 $ 457,959 $ — $ — $ — $ — $ 579,436 Commercial utility sales 136,236 276,716 — — — — 412,952 Industrial utility sales 34,353 24,603 — — — — 58,956 Other utility sales 7,556 — — — — — 7,556 Natural gas transportation — 43,238 89,159 — — — 132,397 Natural gas gathering — — 9,159 — — — 9,159 Natural gas storage — — 11,543 — — — 11,543 Contracting services — — — 968,755 — — 968,755 Construction materials — — — 1,423,068 — — 1,423,068 Intrasegment eliminations — — — (465,969) — — (465,969) Inside specialty contracting — — — — 926,875 — 926,875 Outside specialty contracting — — — — 392,544 — 392,544 Other 31,568 14,579 18,865 — 525 11,259 76,796 Intersegment eliminations — — (50,905) (669) (1,681) (11,052) (64,307) Revenues from contracts with customers 331,190 817,095 77,821 1,925,185 1,318,263 207 4,469,761 Revenues out of scope 3,933 6,152 197 — 51,509 — 61,791 Total external operating revenues $ 335,123 $ 823,247 $ 78,018 $ 1,925,185 $ 1,369,772 $ 207 $ 4,531,552 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: December 31, 2020 December 31, 2019 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 104,345 $ 109,078 $ (4,733) Receivables, net Contract liabilities - current (158,603) (142,768) (15,835) Accounts payable Contract liabilities - noncurrent (52) (19) (33) Noncurrent liabilities - other Net contract liabilities $ (54,310) $ (33,709) $ (20,601) December 31, 2019 December 31, 2018 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 109,078 $ 104,239 $ 4,839 Receivables, net Contract liabilities - current (142,768) (93,901) (48,867) Accounts payable Contract liabilities - noncurrent (19) (135) 116 Noncurrent liabilities - other Net contract assets (liabilities) $ (33,709) $ 10,203 $ (43,912) The Company recognized $138.2 million and $89.0 million in revenue for the years ended December 31, 2020 and 2019, respectively, which was previously included in contract liabilities at December 31, 2019 and 2018, respectively. The Company recognized a net increase in revenues of $58.8 million and $44.1 million for the years ended December 31, 2020 and 2019, 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 that 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 five and one years, respectively.

Business Combinations

Business Combinations	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
Business combination disclosure	Business Combinations The following acquisitions were accounted for as business combinations in accordance with ASC 805 - Business Combinations. The results of the business combinations have been included in the Company's 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. The following acquisitions were made during 2020 and 2019 at the construction materials and contracting segment: • In December 2020, the Company acquired the assets of McMurry Ready-Mix Co., an aggregates and concrete supplier in Wyoming. • In February 2020, the Company acquired the assets of Oldcastle Infrastructure Spokane, a prestressed-concrete business in Washington. • In December 2019, the Company acquired the assets Roadrunner Ready Mix, Inc., a provider of ready-mixed concrete in Idaho. • In March 2019, the Company acquired Viesko Redi-Mix, Inc., a provider of ready-mixed concrete in Oregon. The following acquisitions were made during 2020 and 2019 at the construction services segment: • In February 2020, the Company acquired PerLectric, Inc., an electrical construction company in Virginia. • In September 2019, the Company acquired the assets of Pride Electric, Inc., an electrical construction company in Washington. The total purchase price for acquisitions that occurred in 2020 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. At December 31, 2020, the purchase price adjustments for Oldcastle Infrastructure Spokane had been settled and no material adjustments were made to the provisional accounting. Purchase price allocations for PerLectric, Inc. and McMurry Ready-Mix Co. are preliminary and will be finalized within 12 months of the respective acquisition dates. The Company issued debt to finance these acquisitions. In 2019, the gross aggregate consideration for acquisitions was $56.8 million, subject to certain adjustments, and includes $1.2 million of debt assumed. The amounts allocated to the aggregated assets acquired and liabilities assumed during 2019 were as follows: $15.8 million to current assets; $16.7 million to property, plant and equipment; $23.1 million to goodwill; $6.7 million to other intangible assets; $500,000 to noncurrent assets - other; $5.9 million to current liabilities and $100,000 to noncurrent liabilities - other. At December 31, 2020, the purchase price adjustments for all 2019 acquisitions had been settled and no material adjustments were made to the provisional accounting. The Company issued debt and equity securities to finance these acquisitions. During 2020, measurement period adjustments were made to previously reported provisional amounts, which increased goodwill by $391,000. Costs incurred for acquisitions are included in operation and maintenance expense on the Consolidated Statements of Income and were not material for the years ended December 31, 2020 and 2019.

Property, Plant, and Equipment

Property, Plant, and Equipment	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment Disclosure	Property, Plant and Equipment Property, plant and equipment at December 31 was as follows: 2020 2019 Weighted (Dollars in thousands, where applicable) Regulated: Electric: Generation $ 1,133,390 $ 1,139,059 48 Distribution 464,442 443,780 46 Transmission 524,155 445,485 64 Construction in progress 61,766 66,664 — Other 139,650 132,157 14 Natural gas distribution: Distribution 2,302,121 2,133,249 47 Transmission 104,695 104,401 51 Storage 33,014 31,484 24 General 198,211 191,446 14 Construction in progress 16,836 39,506 — Other 213,976 188,037 16 Pipeline: Transmission 665,567 636,796 46 Gathering — 35,661 — Storage 52,632 50,001 53 Construction in progress 46,690 22,597 — Other 49,640 48,340 17 Non-regulated: Pipeline: Gathering — 31,148 — Construction in progress 4 154 — Other 7,164 9,518 11 Construction materials and contracting: Land 132,948 127,729 — Buildings and improvements 130,417 122,064 21 Machinery, vehicles and equipment 1,284,604 1,180,343 12 Construction in progress 23,803 25,018 — Aggregate reserves 456,704 455,408 ** Construction services: Land 7,218 7,146 — Buildings and improvements 41,674 31,735 25 Machinery, vehicles and equipment 163,080 156,537 7 Other 8,824 17,952 4 Other: Land 2,648 2,648 — Other 34,897 32,565 12 Less accumulated depreciation, depletion and amortization 3,133,831 2,991,486 Net property, plant and equipment $ 5,166,939 $ 4,917,142 * Depleted on the units-of-production method based on recoverable aggregate reserves.

Regulatory assets and liabiliti

Regulatory assets and liabilities	12 Months Ended
	Dec. 31, 2020
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 as of December 31: Estimated Recovery or Refund Period * 2020 2019 (In thousands) Regulatory assets: Current: Natural gas costs recoverable through rate adjustments Up to 1 year $ 42,481 $ 42,823 Cost recovery mechanisms Up to 1 year 10,645 6,288 Conservation programs Up to 1 year 7,117 6,963 Other Up to 1 year 8,284 7,539 68,527 63,613 Noncurrent: Pension and postretirement benefits ** 155,942 157,069 Plant costs/asset retirement obligations Over plant lives 71,740 66,000 Plant to be retired - 65,919 32,931 Manufactured gas plant sites remediation - 26,429 15,126 Natural gas costs recoverable through rate adjustments Up to 2 years 21,539 46,381 Cost recovery mechanisms Up to 10 years 16,245 13,108 Taxes recoverable from customers Over plant lives 10,785 11,486 Long-term debt refinancing costs Up to 40 years 4,426 4,286 Other Up to 18 years 6,356 7,397 379,381 353,784 Total regulatory assets $ 447,908 $ 417,397 Regulatory liabilities: Current: Natural gas costs refundable through rate adjustments Up to 1 year $ 18,565 $ 23,825 Electric fuel and purchased power deferral Up to 1 year 3,667 5,824 Taxes refundable to customers Up to 1 year 3,557 3,472 Other Up to 1 year 5,661 9,814 31,450 42,935 Noncurrent: Taxes refundable to customers Over plant lives 227,850 246,034 Plant removal and decommissioning costs Over plant lives 167,171 173,722 Pension and postretirement benefits ** 16,989 18,065 Other Up to 21 years 16,065 9,549 428,075 447,370 Total regulatory liabilities $ 459,525 $ 490,305 Net regulatory position $ (11,617) $ (72,908) * Estimated recovery or refund period for amounts currently being recovered or refunded in rates to customers. ** Recovered as expense is incurred or cash contributions are made. As of December 31, 2020 and 2019, approximately $332.5 million and $276.5 million, respectively, of regulatory assets were not earning a rate of return but 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 to be retired and the estimated future cost of manufactured gas plant site remediation. 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 that it intends to retire one aging coal-fired electric generating unit in March 2021 and two units in early 2022. The Company has accelerated the depreciation related to these facilities in property, plant and equipment and has recorded the difference between the accelerated depreciation, in accordance with GAAP, and the depreciation approved for rate-making purposes as regulatory assets. The Company expects to recover the regulatory assets related to the plants to be retired in future rates.

Goodwill and other intangible a

Goodwill and other intangible assets	12 Months Ended
	Dec. 31, 2020
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, 2020 Goodwill Measurement Period 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 Balance at January 1, 2019 Goodwill Acquired Measurement Period Balance at December 31, 2019 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 209,421 14,482 (6,669) 217,234 Construction services 109,765 8,623 — 118,388 Total $ 664,922 $ 23,105 $ (6,669) $ 681,358 Other amortizable intangible assets at December 31 were as follows: 2020 2019 (In thousands) Customer relationships $ 28,836 $ 17,958 Less accumulated amortization 6,887 6,268 21,949 11,690 Noncompete agreements 3,941 3,439 Less accumulated amortization 2,309 1,957 1,632 1,482 Other 12,927 8,094 Less accumulated amortization 11,012 6,020 1,915 2,074 Total $ 25,496 $ 15,246 The previous tables include goodwill and intangible assets associated with the business combinations completed during 2020 and 2019. For more information related to these business combinations, see Note 4. Amortization expense for amortizable intangible assets for the years ended December 31, 2020, 2019 and 2018, was $9.0 million, $2.4 million and $1.2 million, respectively. The amounts of estimated amortization expense for identifiable intangible assets as of December 31, 2020, were: 2021 2022 2023 2024 2025 Thereafter (In thousands) Amortization expense $ 4,911 $ 4,394 $ 4,121 $ 3,799 $ 1,862 $ 6,409

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 2020
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 an insurance contract, to satisfy its obligations under its unfunded, nonqualified defined benefit 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 $100.1 million and $87.0 million at December 31, 2020 and 2019, respectively, are classified as investments on the Consolidated Balance Sheets. The net unrealized gains on these investments for the years ended December 31, 2020 and 2019, were $13.1 million and $13.2 million, respectively. The net unrealized loss on these investments for the year ended December 31, 2018, was $3.6 million. 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: 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 December 31, 2019 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 9,804 $ 87 $ 10 $ 9,881 U.S. Treasury securities 1,228 1 — 1,229 Total $ 11,032 $ 88 $ 10 $ 11,110 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 assets measured at fair value on a recurring basis were as follows: Fair Value Measurements Quoted Prices 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 Measurements Quoted Prices Significant Significant Balance at December 31, 2019 (In thousands) Assets: Money market funds $ — $ 8,440 $ — $ 8,440 Insurance contract* — 87,009 — 87,009 Available-for-sale securities: Mortgage-backed securities — 9,881 — 9,881 U.S. Treasury securities — 1,229 — 1,229 Total assets measured at fair value $ — $ 106,559 $ — $ 106,559 * The insurance contract invests approximately 51 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 12 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 3 percent in target date investments and 1 percent in cash equivalents. The Company's money market funds are valued at the net asset value of shares held at the end of the period, 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 contract is 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 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. In the second quarter of 2019, the Company reviewed a non-utility investment at its electric and natural gas distribution segments for impairment. This was a cost-method investment and was written down to zero using the income approach to determine its fair value, requiring the Company to record a write-down of $2.0 million, before tax. The fair value of this investment was categorized as Level 3 in the fair value hierarchy. The reduction is reflected in investments on the Consolidated Balance Sheet, as well as within other income on the Consolidated Statement of Income. The estimated fair value of the Company's Level 2 commodity derivative instruments is based on futures prices, volatility and time to maturity, among other things. Counterparty statements are utilized to determine the value of the commodity derivative instruments and are reviewed and corroborated using various methodologies and significant observable inputs. The Company's and the counterparties' nonperformance risk is also evaluated. The Company performed a fair value assessment of the assets acquired and liabilities assumed in the business combinations that occurred during 2020 and 2019. For more information on these Level 2 and Level 3 fair value measurements, see Notes 2 and 4. 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 at December 31 was as follows: 2020 2019 (In thousands) Carrying Amount $ 2,213,130 $ 2,243,107 Fair Value $ 2,537,289 $ 2,418,631 The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values.

Debt

Debt	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt	Debt Certain debt instruments of the Company's subsidiaries, including those discussed later, 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 December 31, 2020. In the event the subsidiaries do not comply with the applicable covenants and other conditions, alternative sources of funding may need to be pursued. The following table summarizes the outstanding revolving credit facilities of the Company's subsidiaries: Company Facility Facility Amount Outstanding at December 31, 2020 Amount Outstanding at December 31, Letters of Expiration (In millions) Montana-Dakota Utilities Co. Commercial paper/Revolving credit agreement (a) $ 175.0 $ 87.7 $ 118.6 $ — 12/19/24 Cascade Natural Gas Corporation Revolving credit agreement $ 100.0 (b) $ 54.0 $ 64.6 $ 2.2 (c) 6/7/24 Intermountain Gas Company Revolving credit agreement $ 85.0 (d) $ 41.9 $ 24.5 $ — 6/7/24 Centennial Energy Holdings, Inc. Commercial paper/Revolving credit agreement (e) $ 600.0 $ 37.9 $ 104.3 $ — 12/19/24 (a) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Montana-Dakota on stated conditions, up to a maximum of $225.0 million). There were no amounts outstanding under the revolving credit agreement. (b) Certain provisions allow for increased borrowings, up to a maximum of $125.0 million. (c) Outstanding letter(s) of credit reduce the amount available under the credit agreement. (d) Certain provisions allow for increased borrowings, up to a maximum of $110.0 million. (e) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Centennial on stated conditions, up to a maximum of $700.0 million). There were no amounts outstanding under the revolving credit agreement. The 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 their 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. Short-term debt Montana-Dakota On April 8, 2020, Montana-Dakota entered into a $75.0 million term loan agreement with a LIBOR-based variable interest rate and a maturity date of April 7, 2021. At December 31, 2020, Montana-Dakota had $50.0 million outstanding under the agreement. The agreement contains customary covenants and provisions, including a covenant of Montana-Dakota not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. The covenants also include certain restrictions on the sale of certain assets, loans and investments. Long-term debt Long-term Debt Outstanding Long-term debt outstanding was as follows: Weighted Average Interest Rate at December 31, 2020 2020 2019 (In thousands) Senior Notes due on dates ranging from October 22, 2022 to October 30, 2060 4.40 % $ 1,950,000 $ 1,850,000 Commercial paper supported by revolving credit agreements .28 % 125,600 222,900 Credit agreements due on June 7, 2024 1.88 % 95,900 89,050 Medium-Term Notes due on dates ranging from September 15, 2027 to March 16, 2029 7.32 % 35,000 50,000 Term Loan Agreement due on September 3, 2032 2.00 % 8,400 9,100 Other notes due on dates ranging from July 15, 2021 to November 30, 2038 .75 % 4,034 29,117 Less unamortized debt issuance costs 5,803 7,010 Less discount 1 50 Total long-term debt 2,213,130 2,243,107 Less current maturities 1,555 16,540 Net long-term debt $ 2,211,575 $ 2,226,567 Montana-Dakota On January 1, 2019, the Company's revolving credit agreement and commercial paper program became Montana-Dakota's revolving credit agreement and commercial paper program as a result of the Holding Company Reorganization. The outstanding balance of the revolving credit agreement was also transferred to Montana-Dakota. All of the related terms and covenants of the credit agreements remained the same. Montana-Dakota's revolving credit agreement supports its commercial paper program. Commercial paper borrowings under this agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued commercial paper borrowings. The credit agreement contains customary covenants and provisions, including covenants of Montana-Dakota not to permit, as of the end of any fiscal quarter, the ratio of funded debt to total capitalization (determined on a consolidated basis) to be greater than 65 percent. Other covenants include limitations on the sale of certain assets and on the making of certain loans and investments. Montana-Dakota's ratio of total debt to total capitalization at December 31, 2020, was 49 percent. Cascade Any borrowings under the revolving credit agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued borrowings. The credit agreement contains customary covenants and provisions, including a covenant of Cascade not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. Other covenants include restrictions on the sale of certain assets, limitations on indebtedness and the making of certain investments. On June 15, 2020, Cascade issued $50.0 million of senior notes under a note purchase agreement with maturity dates ranging from June 15, 2050 to June 15, 2060, at a weighted average interest rate of 3.66 percent. The agreement contains customary covenants and provisions, including a covenant of Cascade not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. On October 30, 2020, Cascade issued $25.0 million of senior notes under a note purchase agreement with a maturity date of October 30, 2060, at an interest rate of 3.34 percent. The agreement contains customary covenants and provisions, including a covenant of Cascade not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. Cascade's ratio of total debt to total capitalization at December 31, 2020, was 53 percent. Intermountain Any borrowings under the revolving credit agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued borrowings. The credit agreement contains customary covenants and provisions, including a covenant of Intermountain not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. Other covenants include restrictions on the sale of certain assets, limitations on indebtedness and the making of certain investments. Intermountain's ratio of total debt to total capitalization at December 31, 2020, was 51 percent. Centennial Centennial's revolving credit agreement supports its commercial paper program. Commercial paper borrowings under this agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued commercial paper borrowings. Centennial's revolving credit agreement contains customary covenants and provisions, including a covenant of Centennial, not to permit, as of the end of any fiscal quarter, the ratio of total consolidated debt to total consolidated capitalization to be greater than 65 percent. Other covenants include restricted payments, restrictions on the sale of certain assets, limitations on subsidiary indebtedness, minimum consolidated net worth, limitations on priority debt and the making of certain loans and investments. Centennial's ratio of total debt to total capitalization at December 31, 2020, was 31 percent. Certain of Centennial's financing agreements contain cross-default provisions. These provisions state that if Centennial or any subsidiary of Centennial fails to make any payment with respect to any indebtedness or contingent obligation, in excess of a specified amount, under any agreement that causes such indebtedness to be due prior to its stated maturity or the contingent obligation to become payable, the applicable agreements will be in default. WBI Energy Transmission On July 26, 2019, WBI Energy Transmission amended its uncommitted note purchase and private shelf agreement to increase capacity to $300.0 million and extend the issuance period to May 16, 2022. On December 16, 2020, WBI Energy Transmission issued $25.0 million of senior notes under the private shelf agreement with a maturity date of December 16, 2035, at an interest rate of 3.26 percent. WBI Energy Transmission had $195.0 million of notes outstanding at December 31, 2020, which reduced the remaining capacity under this uncommitted private shelf agreement to $105.0 million. This agreement contains customary covenants and provisions, including a covenant of WBI Energy Transmission not to permit, as of the end of any fiscal quarter, the ratio of total debt to total capitalization to be greater than 55 percent. Other covenants include a limitation on priority debt and restrictions on the sale of certain assets and the making of certain investments. WBI Energy Transmission's ratio of total debt to total capitalization at December 31, 2020, was 41 percent. Schedule of Debt Maturities Long-term debt maturities, which excludes unamortized debt issuance costs and discount, for the five years and thereafter following December 31, 2020, were as follows: 2021 2022 2023 2024 2025 Thereafter (In thousands) Long-term debt maturities $ 1,555 $ 148,021 $ 77,921 $ 282,922 $ 177,802 $ 1,530,713

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases of Lessee Disclosure	Leases Most of the leases the Company enters into are for equipment, buildings, easements and vehicles as part of their ongoing operations. The Company also leases certain equipment to third parties through its utility and construction services segments. The Company determines if an arrangement contains a lease at inception of a contract and accounts for all leases in accordance with ASC 842 - Leases. The recognition of leases requires the Company to make estimates and assumptions that affect the lease classification and the assets and liabilities recorded. The accuracy of lease assets and liabilities reported on the Consolidated Financial Statements depends on, among other things, management's estimates of interest rates used to discount the lease assets and liabilities to their present value, as well as the lease terms based on the unique facts and circumstances of each lease. Lessee accounting The leases the Company has entered into as part of its ongoing operations are considered operating leases and are recognized on the Consolidated Balance Sheets as operating lease right-of-use assets, operating lease liabilities due within one year and, if applicable, noncurrent liabilities - operating lease liabilities. The corresponding lease costs are included in operation and maintenance expense on the Consolidated Statements of Income. Generally, the leases for vehicles and equipment have a term of five years or less and buildings and easements have a longer term of up to 35 years or more. To date, the Company does not have any residual value guarantee amounts probable of being owed to a lessor, financing leases or material agreements with related parties. The following tables provide information on the Company's operating leases at and for the years ended December 31: 2020 2019 (In thousands) Lease costs: Operating lease cost $ 45,319 $ 43,759 Variable lease cost 1,319 1,555 Short-term lease cost 135,376 120,030 Total lease costs $ 182,014 $ 165,344 2020 2019 (Dollars in thousands) Weighted average remaining lease term 2.73 years 3.13 years Weighted average discount rate 4.03 % 4.41 % Cash paid for amounts included in the measurement of lease liabilities $ 45,043 $ 43,477 The reconciliation of the future undiscounted cash flows to the operating lease liabilities presented on the Consolidated Balance Sheet at December 31, 2020, was as follows: (In thousands) 2021 $ 36,632 2022 25,133 2023 16,639 2024 12,140 2025 7,852 Thereafter 48,845 Total 147,241 Less discount 26,718 Total operating lease liabilities $ 120,523
Leases of Lessor Disclosure	Lessor accounting 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 $48.0 million and $51.5 million for the years ended December 31, 2020 and 2019, respectively. At December 31, 2020, the Company had $11.2 million of lease receivables with a majority due within 12 months or less.

Asset retirement obligations

Asset retirement obligations	12 Months Ended
	Dec. 31, 2020
Asset Retirement Obligation [Abstract]
Asset retirement obligations	Asset Retirement Obligations The Company records obligations related to retirement costs of natural gas distribution mains and lines, natural gas transmission lines, natural gas storage wells, decommissioning of certain electric generating facilities, reclamation of certain aggregate properties, special handling and disposal of hazardous materials at certain electric generating facilities, natural gas distribution facilities and buildings, and certain other obligations as asset retirement obligations. A reconciliation of the Company's liability, which the current portion is included in other accrued liabilities on the Consolidated Balance Sheets, for the years ended December 31 was as follows: 2020 2019 (In thousands) Balance at beginning of year $ 417,575 $ 375,553 Liabilities incurred 11,560 25,869 Liabilities acquired 1,378 486 Liabilities settled (5,369) (7,097) Accretion expense* 21,668 19,789 Revisions in estimates 107 2,975 Balance at end of year $ 446,919 $ 417,575 * Includes $20.1 million and $18.3 million in 2020 and 2019, respectively, related to regulatory assets. The Company believes that largely all expenses related to asset retirement obligations at the Company's regulated operations will be recovered in rates over time and, accordingly, defers such expenses as regulatory assets. For more information on the Company's regulatory assets and liabilities, see Note 6.

Equity

Equity	12 Months Ended
	Dec. 31, 2020
Class of Stock Disclosures [Abstract]
Stockholders' Equity Note Disclosure	Equity The Company depends on earnings and dividends from its subsidiaries to pay dividends on common stock. The Company has paid quarterly dividends for 83 consecutive years with an increase in the dividend amount for the last 30 consecutive years. For the years ended December 31, 2020, 2019 and 2018, dividends declared on common stock were $.8350, $.8150 and $.7950 per common share, respectively. Dividends on common stock are paid quarterly to the stockholders of record less than 30 days prior to the distribution date. For the years ended December 31, 2020, 2019 and 2018, the dividends declared to common stockholders were $167.4 million, $162.1 million and $155.7 million, respectively. The declaration and payment of dividends of the Company is at the sole discretion of the board of directors. In addition, the Company's subsidiaries are generally restricted to paying dividends out of capital accounts or net assets. The following discusses the most restrictive limitations. Pursuant to a covenant under a credit agreement, Centennial may only declare or pay distributions if, as of the last day of any fiscal quarter, the ratio of Centennial's average consolidated indebtedness as of the last day of such fiscal quarter and each of the preceding three fiscal quarters to Centennial's Consolidated EBITDA does not exceed 3.5 to 1. In addition, certain credit agreements and regulatory limitations of the Company's subsidiaries also contain restrictions on dividend payments. The most restrictive limitation requires the Company's subsidiaries not to permit the ratio of funded debt to capitalization to be greater than 65 percent. Based on this limitation, approximately $1.4 billion of the net assets of the Company's subsidiaries, which represents common stockholders' equity including retained earnings, would be restricted from use for dividend payments at December 31, 2020. 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. Any public offer and sale of such securities will be made only by means of a prospectus meeting the requirements of the Securities Act and the rules and regulations thereunder. 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. The Distribution Agreement 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 the agreement. The Company did not issue shares of common stock for the year ended December 31, 2020, pursuant to the “at-the-market” offering. The Company issued 3.6 million shares of common stock for the year ended December 31, 2019, pursuant to the "at-the-market" offering. For the year ended December 31, 2019, the Company received net proceeds of $94.0 million and paid commissions to the sales agents of approximately $950,000 in connection with the sales of common stock under the "at-the-market" offering. The net proceeds were used for capital expenditures and acquisitions. As of December 31, 2020, the Company had capacity to issue up to 6.4 million additional shares of common stock under the "at-the-market" offering program.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement, Noncash Expense [Abstract]
Stock-based compensation	Stock-Based Compensation The Company has stock-based compensation plans under which it is currently authorized to grant restricted stock and other stock awards. As of December 31, 2020, there were 4.2 million remaining shares available to grant under these plans. The Company either purchases shares on the open market or issues new shares of common stock to satisfy the vesting of stock-based awards. Total stock-based compensation expense (after tax) was $10.8 million, $6.5 million and $4.6 million in 2020, 2019 and 2018, respectively. As of December 31, 2020, total remaining unrecognized compensation expense related to stock-based compensation was approximately $10.9 million (before income taxes) which will be amortized over a weighted average period of 1.6 years. Stock awards Non-employee directors receive shares of common stock in addition to and in lieu of cash payment for directors' fees. There were 45,273 shares with a fair value of $1.1 million, 41,644 shares with a fair value of $1.2 million and 38,605 shares with a fair value of $1.0 million issued to non-employee directors during the years ended December 31, 2020, 2019 and 2018, respectively. Restricted stock awards In February 2018, the Company granted restricted stock awards under the long-term performance-based incentive plan to certain key employees. The Company granted 22,838 shares at a weighted average grant-date fair value of $27.48 per share. The restricted stock awards vested on December 31, 2020. The fair value of the vested awards was $600,000. Performance share awards Since 2003, key employees of the Company have been granted performance share awards each year under the long-term performance-based incentive plan. Entitlement to performance shares is established by either the market condition or the performance metrics and service condition relative to the designated award. Target grants of performance shares outstanding at December 31, 2020, were as follows: Grant Date Performance Target Grant February 2019 2019-2021 327,194 February 2020 2020-2022 285,980 Under the market condition for these performance share awards, participants may earn from zero to 200 percent of the apportioned target grant of shares based on the Company's total shareholder return relative to that of the selected peer group. Compensation expense is based on the grant-date fair value as determined by Monte Carlo simulation. The blended volatility term structure ranges are comprised of 50 percent historical volatility and 50 percent implied volatility. Risk-free interest rates were based on U.S. Treasury security rates in effect as of the grant date. Assumptions used for grants applicable to the market condition for certain performance shares issued in 2020, 2019 and 2018 were: 2020 2019 2018 Weighted average grant-date fair value $40.75 $35.07 $34.55 Blended volatility range 15.30 % – 15.97 % 19.50 % – 19.69 % 17.87 % – 22.14 % Risk-free interest rate range 1.45 % – 1.62 % 2.46 % – 2.55 % 1.86 % – 2.46 % Weighted average discounted dividends per share $2.91 $2.85 $2.46 Under the performance conditions for these performance share awards, participants may earn from zero to 200 percent of the apportioned target grant of shares. The performance conditions are based on the Company's compound annual growth rate in earnings from continuing operations before interest, taxes, depreciation, depletion and amortization and the Company's compound annual growth rate in earnings from continuing operations. The weighted average grant-date fair value per share for the performance shares applicable to these performance conditions issued in 2020, 2019 and 2018 was $31.63, $26.25 and $27.48, respectively. The fair value of the performance shares that vested during both years ended December 31, 2020 and 2019, was $9.7 million. There were no performance shares that vested in 2018. A summary of the status of the performance share awards for the year ended December 31, 2020, was as follows: Number of Weighted Nonvested at beginning of period 573,503 $ 30.81 Granted 285,980 36.19 Additional performance shares earned 123,147 27.48 Less: Vested 369,456 29.84 Nonvested at end of period 613,174 $ 33.24

Accumulated other comprehensive

Accumulated other comprehensive income (loss)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Comprehensive income (loss)	Accumulated Other Comprehensive Income (Loss) The Company's accumulated other comprehensive income (loss) is comprised of losses on derivative instruments qualifying as hedges, postretirement liability adjustments and gain (loss) on available-for-sale investments. The after-tax changes in the components of accumulated other comprehensive loss at December 31, 2020, 2019 and 2018, were as follows: Net Post- Net Total (In thousands) At December 31, 2018 $ (2,161) $ (36,069) $ (112) $ (38,342) Other comprehensive income (loss) before reclassifications — (6,151) 134 (6,017) Amounts reclassified from accumulated other comprehensive loss 731 1,486 40 2,257 Net current-period other comprehensive income (loss) 731 (4,665) 174 (3,760) At December 31, 2019 (1,430) (40,734) 62 (42,102) Other comprehensive loss before reclassifications — (8,395) (1) (8,396) Amounts reclassified from accumulated other comprehensive loss 446 1,922 52 2,420 Net current-period other comprehensive income (loss) 446 (6,473) 51 (5,976) At December 31, 2020 $ (984) $ (47,207) $ 113 $ (48,078) 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 for the years ended December 31 were as follows: 2020 2019 Location on Consolidated (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (591) $ (591) Interest expense 145 (140) Income taxes (446) (731) Amortization of postretirement liability losses included in net periodic benefit cost (2,552) (1,962) Other income 630 476 Income taxes (1,922) (1,486) Reclassification adjustment for loss on available-for-sale investments included in net income (66) (50) Other income 14 10 Income taxes (52) (40) Total reclassifications $ (2,420) $ (2,257)

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income taxes	Income Taxes The components of income before income taxes from continuing operations for each of the years ended December 31 were as follows: 2020 2019 2018 (In thousands) United States $ 474,856 $ 398,532 $ 317,655 Foreign 261 (87) (784) Income before income taxes from continuing operations $ 475,117 $ 398,445 $ 316,871 Income tax expense (benefit) from continuing operations for the years ended December 31 was as follows: 2020 2019 2018 (In thousands) Current: Federal $ 65,006 $ (3,502) $ (15,901) State 21,234 3,366 3,651 Foreign 151 — — 86,391 (136) (12,250) Deferred: Income taxes: Federal (3,735) 50,218 50,755 State (625) 12,098 7,206 Investment tax credit - net 2,559 1,099 1,774 (1,801) 63,415 59,735 Total income tax expense $ 84,590 $ 63,279 $ 47,485 The TCJA was enacted on December 22, 2017. The SEC issued rules that allowed for a measurement period of up to 12 months after the enactment date of the TCJA to finalize the recording of the related tax impacts. The Company reviewed the impacts of the TCJA and completed its assessment of the transitional impacts during the period ending December 31, 2018, of which there were no such material adjustments. Components of deferred tax assets and deferred tax liabilities at December 31 were as follows: 2020 2019 (In thousands) Deferred tax assets: Postretirement $ 51,495 $ 51,075 Compensation-related 40,477 37,330 Operating lease liabilities 25,963 24,459 Payroll tax deferral 14,010 — Legal and environmental contingencies 9,467 6,601 Asset retirement obligations 8,060 7,450 Customer advances 7,463 7,325 Federal renewable energy credit — 5,343 Other 37,944 32,533 Total deferred tax assets 194,879 172,116 Deferred tax liabilities: Depreciation and basis differences on property, plant and equipment 536,966 511,867 Postretirement 49,233 48,927 Operating lease right-of-use-assets 25,858 24,436 Intangible asset amortization 19,514 18,930 Other 67,922 61,385 Total deferred tax liabilities 699,493 665,545 Valuation allowance 11,484 13,154 Net deferred income tax liability $ 516,098 $ 506,583 As of December 31, 2020 and 2019, the Company had various state income tax net operating loss carryforwards of $151.5 million and $149.8 million, respectively, and federal and state income tax credit carryforwards, excluding alternative minimum tax credit carryforwards, of $37.1 million and $43.7 million, respectively. Included in the state credits are various regulatory investment tax credits of approximately $36.3 million and $37.4 million at December 31, 2020 and 2019, respectively. The state income tax credit carryforwards are due to expire between 2021 and 2034. Changes in tax regulations or assumptions regarding current and future taxable income could require additional valuation allowances in the future. The following table reconciles the change in the net deferred income tax liability from December 31, 2019, to December 31, 2020, to deferred income tax benefit: 2020 (In thousands) Change in net deferred income tax liability from the preceding table $ 9,515 Deferred taxes associated with other comprehensive loss 1,817 Excess deferred income tax amortization (12,517) Other (616) Deferred income tax benefit for the period $ (1,801) Total income tax expense differs from the amount computed by applying the statutory federal income tax rate to income before taxes. The reasons for this difference were as follows: Years ended December 31, 2020 2019 2018 Amount % Amount % Amount % (Dollars in thousands) Computed tax at federal statutory rate $ 99,775 21.0 $ 83,674 21.0 $ 66,543 21.0 Increases (reductions) resulting from: State income taxes, net of federal income tax 17,845 3.8 14,029 3.5 12,190 3.8 Federal renewable energy credit (16,009) (3.4) (15,843) (4.0) (11,759) (3.7) Tax compliance and uncertain tax positions (3,543) (.7) (2,739) (.7) (2,725) (.9) Excess deferred income tax amortization (12,517) (2.6) (11,904) (3.0) (9,319) (2.9) TCJA revaluation — — — — (5,947) (1.9) TCJA revaluation related to accumulated other comprehensive loss balance — — — — (42) — Other (961) (.3) (3,938) (.9) (1,456) (.4) Total income tax expense $ 84,590 17.8 $ 63,279 15.9 $ 47,485 15.0 The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction, and various state, local and foreign jurisdictions. The Company is no longer subject to U.S. federal or non-U.S. income tax examinations by tax authorities for years ending prior to 2016. With few exceptions, as of December 31, 2020, the Company is no longer subject to state and local income tax examinations by tax authorities for years ending prior to 2016. For the years ended December 31, 2020, 2019 and 2018, total reserves for uncertain tax positions were not material. The Company recognizes interest and penalties accrued relative to unrecognized tax benefits in income tax expense.

Cash flow information

Cash flow information	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Information [Abstract]
Cash flow information	Cash Flow Information Cash expenditures for interest and income taxes for the years ended December 31 were as follows: 2020 2019 2018 (In thousands) Interest, net* $ 88,681 $ 93,414 $ 83,009 Income taxes paid (refunded), net** $ 65,536 $ (8,475) $ 16,041 * AFUDC - borrowed was $2.6 million, $2.8 million and $2.3 million for the years ended December 31, 2020, 2019 and 2018, respectively. ** Income taxes paid (refunded), including discontinued operations, were $59.4 million, $(9.4) million and $5.5 million for the years ended December 31, 2020, 2019 and 2018, respectively. Noncash investing and financing transactions at December 31 were as follows: 2020 2019 2018 (In thousands) Property, plant and equipment additions in accounts payable $ 26,082 $ 46,119 $ 42,355 Right-of-use assets obtained in exchange for new operating lease liabilities $ 54,356 $ 54,880 $ — Issuance of common stock in connection with acquisition $ — $ — $ 18,186 Debt assumed in connection with a business combination $ — $ 1,163 $ — Accrual for holdback payment related to a business combination $ 2,500 $ — $ —

Business segment data

Business segment data	12 Months Ended
	Dec. 31, 2020
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 has 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, including Alaska and Hawaii. The construction services segment provides inside and outside specialty contracting services in 44 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 Dakota Prairie Refining and 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. Information on the Company's segments as of December 31 and for the years then ended was as follows: 2020 2019 2018 (In thousands) External operating revenues: Regulated operations: Electric $ 331,538 $ 351,725 $ 335,123 Natural gas distribution 847,651 865,222 823,247 Pipeline 69,957 62,357 54,857 1,249,146 1,279,304 1,213,227 Non-regulated operations: Pipeline 15,389 21,835 23,161 Construction materials and contracting 2,177,585 2,189,651 1,925,185 Construction services 2,090,685 1,845,896 1,369,772 Other (55) 90 207 4,283,604 4,057,472 3,318,325 Total external operating revenues $ 5,532,750 $ 5,336,776 $ 4,531,552 2020 2019 2018 (In thousands) Intersegment operating revenues: Regulated operations: Electric $ 491 $ — $ — Natural gas distribution 534 — — Pipeline 57,977 56,037 50,580 59,002 56,037 50,580 Non-regulated operations: Pipeline 554 215 325 Construction materials and contracting 417 1,066 669 Construction services 5,038 3,370 1,681 Other 11,958 16,461 11,052 17,967 21,112 13,727 Intersegment eliminations (76,969) (77,149) (64,307) Total intersegment operating revenues $ — $ — $ — Depreciation, depletion and amortization: Electric $ 62,998 $ 58,721 $ 50,982 Natural gas distribution 84,580 79,564 72,486 Pipeline 21,669 21,220 17,896 Construction materials and contracting 89,626 77,450 61,158 Construction services 23,523 17,038 15,728 Other 2,704 2,024 1,955 Total depreciation, depletion and amortization $ 285,100 $ 256,017 $ 220,205 Operating income (loss): Electric $ 63,434 $ 64,039 $ 65,148 Natural gas distribution 73,082 69,188 72,336 Pipeline 49,436 42,796 36,128 Construction materials and contracting 214,498 179,955 141,426 Construction services 147,644 126,426 86,764 Other (3,169) (1,184) (79) Total operating income $ 544,925 $ 481,220 $ 401,723 Interest expense: Electric $ 26,699 $ 25,334 $ 25,860 Natural gas distribution 36,798 35,488 30,768 Pipeline 7,622 7,198 5,964 Construction materials and contracting 20,577 23,792 17,290 Construction services 4,095 5,331 3,551 Other 883 1,859 2,762 Intersegment eliminations (155) (415) (1,581) Total interest expense $ 96,519 $ 98,587 $ 84,614 Income tax expense (benefit): Electric $ (11,636) $ (12,650) $ (6,482) Natural gas distribution 5,746 1,405 4,075 Pipeline 7,650 7,219 2,677 Construction materials and contracting 47,431 37,389 28,357 Construction services 35,797 29,973 20,000 Other (398) (57) (1,142) Total income expense $ 84,590 $ 63,279 $ 47,485 2020 2019 2018 (In thousands) Net income (loss): Regulated operations: Electric $ 55,601 $ 54,763 $ 47,000 Natural gas distribution 44,049 39,517 37,732 Pipeline 35,453 28,255 26,905 135,103 122,535 111,637 Non-regulated operations: Pipeline 1,559 1,348 1,554 Construction materials and contracting 147,325 120,371 92,647 Construction services 109,721 92,998 64,309 Other (3,181) (2,086) (761) 255,424 212,631 157,749 Income from continuing operations 390,527 335,166 269,386 Income (loss) from discontinued operations, net of tax (322) 287 2,932 Net income $ 390,205 $ 335,453 $ 272,318 Capital expenditures: Electric $ 114,676 $ 99,449 $ 186,105 Natural gas distribution 193,048 206,799 205,896 Pipeline 62,224 71,477 70,057 Construction materials and contracting 191,635 190,092 280,396 Construction services 83,651 60,500 25,081 Other 3,045 8,181 1,768 Total capital expenditures (a) $ 648,279 $ 636,498 $ 769,303 Assets: Electric (b) $ 2,123,693 $ 1,680,194 $ 1,613,822 Natural gas distribution (b) 2,302,770 2,574,965 2,375,871 Pipeline 703,377 677,482 616,959 Construction materials and contracting 1,798,493 1,684,161 1,508,032 Construction services 818,662 761,127 604,798 Other (c) 305,157 303,279 266,111 Assets held for sale 1,220 1,851 2,517 Total assets $ 8,053,372 $ 7,683,059 $ 6,988,110 Property, plant and equipment: Electric (b) $ 2,323,403 $ 2,227,145 $ 2,148,569 Natural gas distribution (b) 2,868,853 2,688,123 2,499,093 Pipeline 821,697 834,215 764,959 Construction materials and contracting 2,028,476 1,910,562 1,768,006 Construction services 220,796 213,370 188,586 Other 37,545 35,213 28,108 Less accumulated depreciation, depletion and amortization 3,133,831 2,991,486 2,818,644 Net property, plant and equipment $ 5,166,939 $ 4,917,142 $ 4,578,677 (a) Capital expenditures for 2020, 2019 and 2018 include noncash transactions such as the issuance of the Company's equity securities in connection with acquisitions, capital expenditure-related accounts payable, AFUDC and accrual of holdback payments in connection with acquisitions totaling $(15.7) million, $4.8 million and $33.4 million, respectively. (b) Includes allocations of common utility property. (c) Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets).

Employee benefit plans

Employee benefit plans	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefit Plans	Employee Benefit Plans Pension and other postretirement benefit plans The Company has noncontributory qualified defined benefit pension plans and other postretirement benefit plans for certain eligible employees. The Company uses a measurement date of December 31 for all of its pension and postretirement benefit plans. Prior to 2013, defined benefit pension plan benefits and accruals for all nonunion and certain union plans were frozen and on June 30, 2015, the remaining union plan was frozen. These employees were eligible to receive additional defined contribution plan benefits. In October 2018, the Company transferred the liability of certain participants in the defined benefit pension plan, who are currently receiving benefits, to an annuity company. The transfer of the benefit payments for these participants reduced the Company's liability and future premiums. Effective January 1, 2010, eligibility to receive retiree medical benefits was modified at certain of the Company's businesses. Employees who had attained age 55 with 10 years of continuous service by December 31, 2010, were provided the option to choose between a pre-65 comprehensive medical plan coupled with a Medicare supplement or a specified company funded Retiree Reimbursement Account, regardless of when they retire. All other eligible employees must meet the new eligibility criteria of age 60 and 10 years of continuous service at the time they retire to be eligible for a specified company funded Retiree Reimbursement Account. Employees hired after December 31, 2009, will not be eligible for retiree medical benefits at certain of the Company's businesses. In 2012, the Company modified health care coverage for certain retirees. Effective January 1, 2013, post-65 coverage was replaced by a fixed-dollar subsidy for retirees and spouses to be used to purchase individual insurance through an exchange. Changes in benefit obligation and plan assets and amounts recognized in the Consolidated Balance Sheets at December 31 were as follows: Pension Benefits Other 2020 2019 2020 2019 Change in benefit obligation: (In thousands) Benefit obligation at beginning of year $ 421,166 $ 391,602 $ 83,614 $ 81,201 Service cost — — 1,532 1,142 Interest cost 12,093 15,225 2,437 2,986 Plan participants' contributions — — 752 1,040 Actuarial loss 27,737 40,219 2,203 2,632 Benefits paid (23,636) (25,880) (4,383) (5,387) Benefit obligation at end of year 437,360 421,166 86,155 83,614 Change in net plan assets: Fair value of plan assets at beginning of year 365,264 307,809 94,587 82,516 Actual return on plan assets 42,206 58,409 10,249 15,731 Employer contribution — 24,926 434 687 Plan participants' contributions — — 752 1,040 Benefits paid (23,636) (25,880) (4,383) (5,387) Fair value of net plan assets at end of year 383,834 365,264 101,639 94,587 Funded status - over (under) $ (53,526) $ (55,902) $ 15,484 $ 10,973 Amounts recognized in the Consolidated Balance Sheets at December 31: Noncurrent assets - other $ — $ — $ 36,769 30,475 Other accrued liabilities — — 622 647 Noncurrent liabilities - other 53,526 55,902 20,663 18,855 Benefit obligation assets (liabilities) - net amount recognized $ (53,526) $ (55,902) $ 15,484 $ 10,973 Amounts recognized in accumulated other comprehensive loss: Actuarial loss $ 27,527 $ 27,748 $ 5,557 $ 6,118 Prior service credit — — (634) (731) Total $ 27,527 $ 27,748 $ 4,923 $ 5,387 Amounts recognized in regulatory assets or liabilities: Actuarial (gain) loss $ 154,013 $ 155,484 $ (8,228) $ (4,450) Prior service credit — — (6,808) (8,109) Total $ 154,013 $ 155,484 $ (15,036) $ (12,559) Employer contributions and benefits paid in the preceding table include only those amounts contributed directly to, or paid directly from, plan assets. Amounts related to regulated operations are recorded as regulatory assets or liabilities and are expected to be reflected in rates charged to customers over time. For more information on regulatory assets and liabilities, see Note 6. Unrecognized pension actuarial losses in excess of 10 percent of the greater of the projected benefit obligation or the market-related value of assets are amortized over the average life expectancy of plan participants for frozen plans. The market-related value of assets is determined using a five-year average of assets. The pension plans all have accumulated benefit obligations in excess of plan assets. The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for these plans at December 31 were as follows: 2020 2019 (In thousands) Projected benefit obligation $ 437,360 $ 421,166 Accumulated benefit obligation $ 437,360 $ 421,166 Fair value of plan assets $ 383,834 $ 365,264 The components of net periodic benefit cost (credit), other than the service cost component, are included in other income on the Consolidated Statements of Income. These components related to the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other 2020 2019 2018 2020 2019 2018 Components of net periodic benefit cost (credit): (In thousands) Service cost $ — $ — $ — $ 1,532 $ 1,142 $ 1,494 Interest cost 12,093 15,225 14,591 2,437 2,986 2,899 Expected return on assets (19,949) (18,236) (20,753) (5,019) (4,804) (4,866) Amortization of prior service credit — — — (1,398) (1,398) (1,394) Recognized net actuarial loss 7,172 5,548 7,005 287 353 640 Net periodic benefit cost (credit), including amount capitalized (684) 2,537 843 (2,161) (1,721) (1,227) Less amount capitalized — — — 156 113 153 Net periodic benefit cost (credit) (684) 2,537 843 (2,317) (1,834) (1,380) Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss: Net (gain) loss 934 (144) 991 (259) (127) (1,735) Amortization of actuarial loss (1,155) (904) (1,084) (306) (110) (354) Amortization of prior service (cost) credit — — — 101 100 (220) Total recognized in accumulated other comprehensive loss (221) (1,048) (93) (464) (137) (2,309) Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities: Net (gain) loss 4,546 189 8,263 (3,793) (8,168) (732) Amortization of actuarial gain (loss) (6,017) (4,644) (5,921) 19 (242) (286) Amortization of prior service credit — — — 1,297 1,297 1,614 Total recognized in regulatory assets or liabilities (1,471) (4,455) 2,342 (2,477) (7,113) 596 Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities $ (2,376) $ (2,966) $ 3,092 $ (5,258) $ (9,084) $ (3,093) The estimated net loss for the defined benefit pension plans that will be amortized from accumulated other comprehensive loss and regulatory assets or liabilities into net periodic benefit cost in 2021 is $8.0 million. The estimated net loss and prior service credit for the other postretirement benefit plans that will be amortized from accumulated other comprehensive loss and regulatory assets or liabilities into net periodic benefit credit in 2021 are $24,000 and $1.4 million, respectively. Prior service credit is amortized on a straight-line basis over the average remaining service period of active participants. Weighted average assumptions used to determine benefit obligations at December 31 were as follows: Pension Benefits Other 2020 2019 2020 2019 Discount rate 2.30 % 2.96 % 2.30 % 3.00 % Expected return on plan assets 6.00 % 6.25 % 5.50 % 5.75 % Rate of compensation increase N/A N/A 3.00 % 3.00 % Weighted average assumptions used to determine net periodic benefit cost (credit) for the years ended December 31 were as follows: Pension Benefits Other 2020 2019 2020 2019 Discount rate 2.96 % 4.03 % 3.00 % 4.05 % Expected return on plan assets 6.25 % 6.25 % 5.75 % 5.75 % Rate of compensation increase N/A N/A 3.00 % 3.00 % The expected rate of return on pension plan assets is based on a targeted asset allocation range determined by the funded ratio of the plan. As of December 31, 2020, the expected rate of return on pension plan assets is based on the targeted asset allocation range of 35 percent to 45 percent equity securities and 55 percent to 65 percent fixed-income securities and the expected rate of return from these asset categories. The expected rate of return on other postretirement plan assets is based on the targeted asset allocation range of 10 percent equity securities and 90 percent fixed-income securities and the expected rate of return from these asset categories. The expected return on plan assets for other postretirement benefits reflects insurance-related investment costs. Health care rate assumptions for the Company's other postretirement benefit plans as of December 31 were as follows: 2020 2019 Health care trend rate assumed for next year 7.0 % 7.1 % – 7.4 % Health care cost trend rate - ultimate 4.5 % 4.5 % Year in which ultimate trend rate achieved 2031 2024 The Company's other postretirement benefit plans include health care and life insurance benefits for certain retirees. The plans underlying these benefits may require contributions by the retiree depending on such retiree's age and years of service at retirement or the date of retirement. The Company contributes a flat dollar amount to the monthly premiums which is updated annually on January 1. Assumed health care cost trend rates may have a significant effect on the amounts reported for the health care plans. A one percentage point change in the assumed health care cost trend rates would have had the following effects at December 31, 2020: 1 Percentage 1 Percentage (In thousands) Effect on total of service and interest cost components $ 182 $ (154) Effect on postretirement benefit obligation $ 3,846 $ (3,304) The Company does not expect to contribute to its defined benefit pension plans in 2021 due to an additional $20.0 million contributed to the plans in 2019. The Company expects to contribute approximately $500,000 to its postretirement benefit plans in 2021. The following benefit payments, which reflect future service, as appropriate, and expected Medicare Part D subsidies at December 31, 2020, are as follows: Years Pension Other Expected (In thousands) 2021 $ 24,455 $ 5,404 $ 95 2022 24,507 5,437 89 2023 24,733 5,424 83 2024 24,835 5,377 75 2025 24,713 5,277 70 2026-2030 119,191 25,949 252 Outside investment managers manage the Company's pension and postretirement assets. The Company's investment policy with respect to pension and other postretirement assets is to make investments solely in the interest of the participants and beneficiaries of the plans and for the exclusive purpose of providing benefits accrued and defraying the reasonable expenses of administration. The Company strives to maintain investment diversification to assist in minimizing the risk of large losses. The Company's policy guidelines allow for investment of funds in cash equivalents, fixed-income securities and equity securities. The guidelines prohibit investment in commodities and futures contracts, equity private placement, employer securities, leveraged or derivative securities, options, direct real estate investments, precious metals, venture capital and limited partnerships. The guidelines also prohibit short selling and margin transactions. The Company's practice is to periodically review and rebalance asset categories based on its targeted asset allocation percentage policy. 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 pension plans' assets are determined using the market approach. The carrying value of the pension plans' Level 2 cash equivalents approximates fair value and is determined using observable inputs in active markets or the net asset value of shares held at year end, which is determined using other observable inputs including pricing from outside sources. The estimated fair value of the pension plans' Level 1 and Level 2 equity securities are based on the closing price reported on the active market on which the individual securities are traded or other known sources including pricing from outside sources. The estimated fair value of the pension plans' Level 1 and Level 2 collective and mutual funds are based on the net asset value of shares held at year end, based on either published market quotations on active markets or other known sources including pricing from outside sources. The estimated fair value of the pension plans' Level 2 corporate and municipal bonds is determined using other observable inputs, including benchmark yields, reported trades, broker/dealer quotes, bids, offers, future cash flows and other reference data. The estimated fair value of the pension plans' Level 1 U.S. Government securities are valued based on quoted prices on an active market. The estimated fair value of the pension plans' Level 2 U.S. Government securities are valued mainly using other observable inputs, including benchmark yields, reported trades, broker/dealer quotes, bids, offers, to be announced prices, future cash flows and other reference data. Some of these securities are valued using pricing from outside sources. All investments measured at net asset value in the tables that follow are invested in comingled funds, separate accounts or common collective trusts which do not have publicly quoted prices. The fair value of the comingled funds, separate accounts and common collective trusts are determined based on the net asset value of the underlying investments. The fair value of the underlying investments held by the comingled funds, separate accounts and common collective trusts is generally based on quoted prices in active markets. 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 fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2020 (In thousands) Assets: Cash equivalents $ — $ 7,841 $ — $ 7,841 Equity securities: U.S. companies 12,844 — — 12,844 International companies — 1,727 — 1,727 Collective and mutual funds* 177,397 55,788 — 233,185 Corporate bonds — 92,809 — 92,809 Municipal bonds — 10,126 — 10,126 U.S. Government securities 11,177 2,695 — 13,872 Investments measured at net asset value — — — 11,430 Total assets measured at fair value $ 201,418 $ 170,986 $ — $ 383,834 * Collective and mutual funds invest approximately 36 percent in corporate bonds, 24 percent in common stock of international companies, 18 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 5 percent in U.S. Government securities and 9 percent in other investments. Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2019 (In thousands) Assets: Cash equivalents $ — $ 26,166 $ — $ 26,166 Equity securities: U.S. companies 14,457 — — 14,457 International companies — 938 — 938 Collective and mutual funds* 160,906 58,894 — 219,800 Corporate bonds — 80,768 — 80,768 Municipal bonds — 11,828 — 11,828 U.S. Government securities 7,296 2,082 — 9,378 Total assets measured at fair value $ 182,659 $ 180,676 $ — $ 363,335 * Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in common stock of large-cap U.S. companies, 18 percent in U.S. Government securities, 9 percent in corporate bonds, 6 percent in cash equivalents and 17 percent in other investments. The estimated fair values of the Company's other postretirement benefit plans' assets are determined using the market approach. The estimated fair value of the other postretirement benefit plans' Level 2 cash equivalents is valued at the net asset value of shares held at year end, based on published market quotations on active markets, or using other known sources including pricing from outside sources. The estimated fair value of the other postretirement benefit plans' Level 1 and Level 2 equity securities is based on the closing price reported on the active market on which the individual securities are traded or other known sources including pricing from outside sources . The estimated fair value of the other postretirement benefit plans' Level 2 insurance contract is 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 fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2020 (In thousands) Assets: Cash equivalents $ — $ 3,517 $ — $ 3,517 Equity securities: U.S. companies 1,850 — — 1,850 International companies — 2 — 2 Collective and mutual funds (a) 10 147 — 157 Insurance contract (b) — 96,103 — 96,103 Investments measured at net asset value — — — 10 Total assets measured at fair value $ 1,860 $ 99,769 $ — $ 101,639 (a) Collective and mutual funds invest approximately 36 percent in corporate bonds, 24 percent in common stock of international companies, 18 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 5 percent in U.S. Government securities and 9 percent in other investments. (b) The insurance contract invests approximately 67 percent in corporate bonds, 12 percent in U.S. Government securities, 10 percent in common stock of large-cap U.S. companies, 4 percent in common stock of small-cap U.S. companies, 1 percent in cash equivalents and 6 percent in other investments. Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2019 (In thousands) Assets: Cash equivalents $ — $ 4,017 $ — $ 4,017 Equity securities: U.S. companies 2,073 — — 2,073 International companies — 1 — 1 Collective and mutual funds (a) 10 221 — 231 Insurance contract (b) — 88,265 — 88,265 Total assets measured at fair value $ 2,083 $ 92,504 $ — $ 94,587 (a) Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in common stock of large-cap U.S. companies, 18 percent in U.S. Government securities, 9 percent in corporate bonds, 6 percent in cash equivalents and 17 percent in other investments. (b) The insurance contract invests approximately 50 percent in corporate bonds, 25 percent in common stock of large-cap U.S. companies, 7 percent in U.S. Government securities, 7 percent in common stock of small-cap U.S. companies and 11 percent in other investments. Nonqualified 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 that generally provide for defined benefit payments at age 65 following the employee's retirement or, upon death, to their beneficiaries for a 15-year period. In February 2016, the Company froze the unfunded, nonqualified defined benefit plans to new participants and eliminated benefit increases. Vesting for participants not fully vested was retained. The projected benefit obligation and accumulated benefit obligation for these plans at December 31 were as follows: 2020 2019 (In thousands) Projected benefit obligation $ 101,242 $ 99,245 Accumulated benefit obligation $ 101,242 $ 99,245 The components of net periodic benefit cost are included in other income on the Consolidated Statements of Income. These components related to the Company's nonqualified defined benefit plans for the years ended December 31 were as follows: 2020 2019 2018 (In thousands) Components of net periodic benefit cost: Service cost $ 58 $ 109 $ 185 Interest cost 2,606 3,473 3,157 Recognized net actuarial loss 1,192 764 1,047 Net periodic benefit cost $ 3,856 $ 4,346 $ 4,389 Weighted average assumptions used at December 31 were as follows: 2020 2019 Benefit obligation discount rate 1.97 % 2.73 % Benefit obligation rate of compensation increase N/A N/A Net periodic benefit cost discount rate 2.73 % 3.86 % Net periodic benefit cost rate of compensation increase N/A N/A The amount of future benefit payments for the unfunded, nonqualified defined benefit plans at December 31, 2020, are expected to aggregate as follows: 2021 2022 2023 2024 2025 2026-2030 (In thousands) Nonqualified benefits $ 7,693 $ 6,957 $ 6,933 $ 7,299 $ 7,253 $ 33,938 In 2012, the Company established a nonqualified defined contribution plan for certain key management employees. In 2020, the plan was frozen to new participants and no new Company contributions will be made to the plan after December 31, 2020. A new plan was adopted in 2020 to replace the plan originally established in 2012 with similar provisions. Vesting for participants not fully vested was retained. Expenses incurred under this plan for 2020, 2019 and 2018 were $1.8 million, $1.6 million and $597,000, respectively. The amount of investments that the Company anticipates using to satisfy obligations under these plans at December 31 was as follows: 2020 2019 (In thousands) Investments Insurance contract* $ 100,104 $ 87,009 Life insurance** 39,779 38,659 Other 8,917 8,450 Total investments $ 148,800 $ 134,118 * For more information on the insurance contract, see Note 8. ** Investments of life insurance are carried on plan participants (payable upon the employee's death). Defined contribution plans The Company sponsors various defined contribution plans for eligible employees and the costs incurred under these plans were $50.1 million in 2020, $51.8 million in 2019 and $42.4 million in 2018. Multiemployer plans The Company contributes to a number of MEPPs under the terms of collective-bargaining agreements that cover its union-represented employees. The risks of participating in these multiemployer plans are different from single-employer plans in the following aspects: • Assets contributed to the MEPP by one employer may be used to provide benefits to employees of other participating employers • If a participating employer stops contributing to the plan, the unfunded obligations of the plan may be borne by the remaining participating employers • If the Company chooses to stop participating in some of its MEPPs, the Company may be required to pay those plans an amount based on the underfunded status of the plan, referred to as a withdrawal liability The Company's participation in these plans is outlined in the following table. Unless otherwise noted, the most recent Pension Protection Act zone status available in 2020 and 2019 is for the plan's year-end at December 31, 2019, and December 31, 2018, respectively. The zone status is based on information that the Company received from the plan and is certified by the plan's actuary. Among other factors, plans in the red zone are generally less than 65 percent funded, plans in the yellow zone are between 65 percent and 80 percent funded, and plans in the green zone are at least 80 percent funded. EIN/Pension Plan Number Pension Protection Act Zone Status FIP/RP Status Pending/Implemented Contributions Surcharge Imposed Expiration Date Pension Fund 2020 2019 2020 2019 2018 (In thousands) Alaska Laborers-Employers Retirement Fund 91-6028298-001 Yellow as of 6/30/2020 Yellow as of 6/30/2019 Implemented $ 828 $ 815 $ 732 No 12/31/2020 * Construction Industry and Laborers Joint Pension Trust for So Nevada, Plan A 88-0135695-001 Red Red Implemented 515 544 346 No 6/30/2023 Edison Pension Plan 93-6061681-001 Green Green No 16,121 12,252 12,111 No 12/31/2021 IBEW Local 212 Pension Trust 31-6127280-001 Green as of 4/30/2020 Green as of 4/30/2019 No 1,521 1,110 1,341 No 6/1/2025 IBEW Local 357 Pension Plan A 88-6023284-001 Green Green No 9,913 10,162 3,460 No 5/31/2021 IBEW Local 38 Pension Plan 34-6574238-001 Red as of 4/30/2020 Yellow as of 4/30/2019 Implemented 140 158 116 No 4/23/2023 IBEW Local 648 Pension Plan 31-6134845-001 Yellow as of 2/28/2020 Yellow as of 2/28/2019 Implemented 526 728 2,175 No 8/29/2021 IBEW Local 82 Pension Plan 31-6127268-001 Green as of 6/30/2020 Green as of 6/30/2019 No 1,373 1,662 1,569 No 12/3/2023 Idaho Plumbers and Pipefitters Pension Plan 82-6010346-001 Green as of 5/31/2020 Green as of 5/31/2019 No 1,370 1,307 1,247 No 3/31/2023 Minnesota Teamsters Construction Division Pension Fund 41-6187751-001 Green as of 11/30/2019 Green as of 11/30/2018 No 663 673 740 No 4/30/2021 National Automatic Sprinkler Industry Pension Fund 52-6054620-001 Red Red Implemented 954 1,074 738 No 3/31/2021- 7/31/2024 National Electrical Benefit Fund 53-0181657-001 Green Green No 14,484 12,679 8,468 No 12/31/2021- 8/30/2025 Pension Trust Fund for Operating Engineers 94-6090764-001 Yellow Yellow Implemented 2,680 2,598 2,403 No 6/15/2022- 6/30/2023 Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV 95-6052257-001 Yellow Yellow Implemented 3,255 2,119 1,774 No 6/30/2024 Southwest Marine Pension Trust 95-6123404-001 Red Red Implemented 170 132 81 No 1/31/2024 Other funds 30,931 24,512 21,421 Total contributions $ 85,444 $ 72,525 $ 58,722 * Plan includes contributions required by collective bargaining agreements which have expired, but contain provisions automatically renewing their terms in the absence of a subsequent negotiated agreement. The Company was listed in the plans' Forms 5500 as providing more than 5 percent of the total contributions for the following plans and plan years: Pension Fund Year Contributions to Plan Exceeded More Than 5 Percent Edison Pension Plan 2019 and 2018 IBEW Local 82 Pension Plan 2019 and 2018 IBEW Local 124 Pension Trust Fund 2019 and 2018 IBEW Local 212 Pension Trust Fund 2019 and 2018 IBEW Local 357 Pension Plan A 2019 and 2018 IBEW Local 648 Pension Plan 2019 and 2018 IBEW Local 683 Pension Fund Pension Plan 2019 IBEW Local Union No 226 Open End Pension Fund 2019 and 2018 Idaho Plumbers and Pipefitters Pension Plan 2019 and 2018 International Union of Operating Engineers Local 701 Pension Trust Fund 2019 and 2018 Minnesota Teamsters Construction Division Pension Fund 2019 and 2018 Pension and Retirement Plan of Plumbers and Pipefitters Local 525 2019 and 2018 The Company also contributes to a number of multiemployer other postretirement plans under the terms of collective-bargaining agreements that cover its union-represented employees. These plans provide benefits such as health insurance, disability insurance and life insurance to retired union employees. Many of the multiemployer other postretirement plans are combined with active multiemployer health and welfare plans. The Company's total contributions to its multiemployer other postretirement plans, which also includes contributions to active multiemployer health and welfare plans, were $63.8 million, $59.5 million and $51.9 million for the years ended December 31, 2020, 2019 and 2018, respectively. Amounts contributed in 2020, 2019 and 2018 to defined contribution multiemployer plans were $54.2 million, $49.2 million and $31.1 million, respectively.

Jointly owned facilities

Jointly owned facilities	12 Months Ended
	Dec. 31, 2020
Regulated Operations [Abstract]
Jointly owned facilities	Jointly Owned Facilities The consolidated financial statements include the Company's ownership interests in three coal-fired electric generating facilities (Big Stone Station, Coyote Station and Wygen III) and one major transmission line (BSSE). Each owner of the jointly owned facilities is responsible for financing its investment. The Company's share of the jointly owned facilities operating expenses was reflected in the appropriate categories of operating expenses (electric fuel and purchased power; operation and maintenance; and taxes, other than income) in the Consolidated Statements of Income. At December 31, the Company's share of the cost of utility plant in service, construction work in progress and related accumulated depreciation for the jointly owned facilities was as follows: Ownership Percentage 2020 2019 (In thousands) Big Stone Station: 22.7 % Utility plant in service $ 155,967 $ 152,836 Construction work in progress 104 518 Less accumulated depreciation 45,435 46,266 $ 110,636 $ 107,088 BSSE: 50.0 % Utility plant in service $ 107,442 $ 105,767 Construction work in progress — — Less accumulated depreciation 2,682 1,232 $ 104,760 $ 104,535 Coyote Station: 25.0 % Utility plant in service $ 159,784 $ 160,235 Construction work in progress 323 21 Less accumulated depreciation 108,852 107,638 $ 51,255 $ 52,618 Wygen III: 25.0 % Utility plant in service $ 66,101 $ 67,869 Construction work in progress 232 112 Less accumulated depreciation 10,038 10,482 $ 56,295 $ 57,499

Regulatory matters

Regulatory matters	12 Months Ended
	Dec. 31, 2020
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 regulatory proceedings and cases by jurisdiction, including the status of each open request. 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. MNPUC On September 27, 2019, Great Plains filed an application with the MNPUC for a natural gas rate increase of approximately $2.9 million annually or approximately 12.0 percent above current rates. The requested increase was primarily to recover investments in facilities to enhance safety and reliability and the depreciation and taxes associated with the increase in investment. On November 22, 2019, Great Plains received approval to implement an interim rate increase of approximately $2.6 million or approximately 11.0 percent, subject to refund, effective January 1, 2020. On October 26, 2020, the MNPUC issued an order authorizing an annual increase in revenues of approximately $2.6 million or approximately 11.5 percent. This matter is pending before the MNPUC. MTPSC On May 8, 2020, Montana-Dakota filed a request with the MTPSC to use deferred accounting for costs related to the COVID-19 pandemic. The filing was withdrawn by Montana-Dakota on January 25, 2021. On June 22, 2020, Montana-Dakota filed an application with the MTPSC for a natural gas rate increase of approximately $8.6 million annually or approximately 13.4 percent above current rates. The requested increase was primarily to recover investments in facilities that were made to enhance system safety and reliability, as well as the depreciation, taxes and operation and maintenance costs associated with this increase in investment. On January 14, 2021, Montana-Dakota received approval to implement an interim rate increase of approximately $4.9 million or approximately 7.7 percent, subject to refund, effective February 1, 2021. On February 1, 2021, Montana-Dakota filed a stipulation and settlement agreement with the MTPSC reflecting an updated increase of approximately $7.3 million annually or approximately 11.4 percent above current rates. On February 16, 2021, the MTPSC approved the settlement with rates effective on or after March 15, 2021. NDPSC On April 24, 2020, Montana-Dakota filed a request with the NDPSC to use deferred accounting for costs related to the COVID-19 pandemic. On February 3, 2021, the NDPSC approved this request with an accounting order to track expenses and revenues related to the COVID-19 pandemic. This order had an effective date of April 24, 2020. On August 26, 2020, Montana-Dakota filed an application with the NDPSC for a natural gas rate increase of approximately $9.0 million annually or approximately 7.8 percent above current rates. The requested increase was primarily to recover investments in facilities to enhance system safety and reliability and the depreciation and taxes associated with the increase in investment. On December 16, 2020, Montana-Dakota received approval to implement an interim rate increase of approximately $6.9 million or approximately 6.0 percent, subject to refund, effective January 1, 2021. A hearing is scheduled for March 17, 2021. This matter is pending before the NDPSC. Montana-Dakota has a renewable resource cost adjustment rate tariff that allows for annual adjustments for recent projected capital costs and related expenses for projects determined to be recoverable under the tariff. On November 2, 2020, Montana-Dakota filed an annual update to its renewable resource cost adjustment requesting to recover a revised revenue requirement of approximately $14.4 million annually, not including the prior period true-up adjustment. The update reflects a decrease of approximately $300,000 from the revenues currently included in rates. On January 6, 2021, the NDPSC approved the increase with rates effective February 1, 2021. OPUC On March 31, 2020, Cascade filed a natural gas general rate case with the OPUC requesting an increase in annual revenue of approximately $4.9 million or approximately 7.2 percent, which included a request for an additional recovery of environmental remediation deferred costs of approximately $364,000. On September 30, 2020, Cascade filed a settlement agreement with the OPUC reflecting an annual increase in revenues of approximately $3.2 million or approximately 4.8 percent. On January 6, 2021, the filing was approved with rates effective February 1, 2021. On January 21, 2021, Cascade submitted a compliance filing using final costs for plant additions which resulted in a final increase in revenues of approximately $2.9 million or approximately 4.3 percent. WUTC On May 27, 2020, Cascade filed a request with the WUTC to use deferred accounting for costs related to the COVID-19 pandemic. On December 10, 2020, the WUTC approved this request. On June 19, 2020, Cascade filed an application with the WUTC for a natural gas rate increase of approximately $13.8 million annually or approximately 5.3 percent above current rates. The requested increase was primarily to recover investments made in infrastructure upgrades, as well as increased operation and maintenance costs. Cascade updated its filing on July 24, 2020, to approximately $14.3 million annually or approximately 5.5 percent. Cascade filed a rebuttal case on January 8, 2021, supporting an increase of approximately $7.4 million annually or approximately 2.8 percent. The revised revenue within the rebuttal case reflects several adjustments including depreciation, reduction to return on equity, delays on certain projects, adjustments to income taxes and updates to wages. The WUTC has 11 months to render a final decision on the rate case. A hearing is scheduled for February 24, 2021. This matter is pending before the WUTC. FERC On September 1, 2020, Montana-Dakota filed an update to its transmission formula rate under the MISO tariff for its multi-value project for $12.9 million, which is effective January 1, 2021.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and 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 December 31, 2020 and 2019, the Company accrued liabilities which have not been discounted, including liabilities held for sale, of $41.5 million and $29.1 million, respectively. At December 31, 2020 and 2019, the Company also recorded corresponding insurance receivables of $17.5 million and $16.2 million, respectively, and regulatory assets of $21.3 million and $10.5 million, respectively, related to the accrued liabilities. The accruals are for contingencies, including 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 Portland Harbor Site In December 2000, Knife River - Northwest was named by the EPA as a PRP in connection with the cleanup of the riverbed site adjacent to a commercial property site acquired by Knife River - Northwest from Georgia-Pacific West, Inc. along the Willamette River. The riverbed site is part of the Portland, Oregon, Harbor Superfund Site where the EPA wants responsible parties to share in the costs of cleanup. To date, costs of the overall remedial investigation and feasibility study of the harbor site are being recorded, and initially paid, through an administrative consent order by the LWG. Investigative costs are indicated to be in excess of $100 million. The EPA issued an ROD in January 2017 adopting a selected remedy which is expected to take 13 years to complete with a then estimated present value of approximately $1 billion. Corrective action will not be taken until remedial design/remedial action plans are approved by the EPA. Knife River - Northwest was also notified that the Portland Harbor Natural Resource Trustee Council intends to perform an injury assessment to natural resources resulting from the release of hazardous substances at the Harbor Superfund Site. It is not possible to estimate the costs of natural resource damages until an assessment is completed and allocations are undertaken. At this time, Knife River - Northwest does not believe it is a responsible party and has notified Georgia-Pacific West, Inc., that it intends to seek indemnity for liabilities incurred in relation to the above matters pursuant to the terms of their sale agreement. Knife River - Northwest has entered into an agreement tolling the statute of limitations in connection with the LWG's potential claim for contribution to the costs of the remedial investigation and feasibility study. LWG has stated its intent to file suit against Knife River - Northwest and others to recover LWG's investigation costs to the extent Knife River - Northwest cannot demonstrate its non-liability for the contamination or is unwilling to participate in an alternative dispute resolution process that has been established to address the matter. At this time, Knife River - Northwest has agreed to participate in the alternative dispute resolution process. The Company believes it is not probable that it will incur any material environmental remediation costs or damages in relation to the above referenced matter. Manufactured Gas Plant Sites Claims have been made against Cascade for cleanup of environmental contamination at manufactured gas plant sites operated by Cascade's predecessors and a similar claim has been made against Montana-Dakota for a site operated by Montana-Dakota and its predecessors. Any accruals related to these claims are reflected in regulatory assets. For more information, see Note 6. Demand has been made of Montana-Dakota to participate in investigation and remediation of environmental contamination at a site in Missoula, Montana. The site operated as a former manufactured gas plant from approximately 1907 to 1938 when it was converted to a butane-air plant that operated until 1956. Montana-Dakota or its predecessors owned or controlled the site for a period of the time it operated as a manufactured gas plant and Montana-Dakota operated the butane-air plant from 1940 to 1951, at which time it sold the plant. There are no documented wastes or by-products resulting from the mixing or distribution of butane-air gas. Preliminary assessment of a portion of the site provided a recommended remedial alternative for that portion of approximately $560,000. However, the recommended remediation would not address any potential contamination to adjacent parcels that may be impacted by contamination from the manufactured gas plant. An environmental assessment was started in 2020, which is estimated to cost approximately $800,000. Montana-Dakota and another party agreed to voluntarily investigate and remediate the site and that Montana-Dakota will pay two-thirds of the costs for further investigation and remediation of the site. Montana-Dakota has accrued costs of $800,000 for the remediation and investigation costs, and has incurred costs of $130,000 as of December 31, 2020. Montana-Dakota received notice from a prior insurance carrier that it will participate in payment of defense costs incurred in relation to the claim. A claim was made against Cascade for contamination at the Bremerton Gasworks Superfund Site in Bremerton, Washington, which was received in 1997. A preliminary investigation has found soil and groundwater at the site contain contaminants requiring further investigation and cleanup. The EPA conducted a Targeted Brownfields Assessment of the site and released a report summarizing the results of that assessment in August 2009. The assessment confirmed that contaminants have affected soil and groundwater at the site, as well as sediments in the adjacent Port Washington Narrows. In April 2010, the Washington DOE issued notice it considered Cascade a PRP for hazardous substances at the site. In May 2012, the EPA added the site to the National Priorities List of Superfund sites. Cascade entered into an administrative settlement agreement and consent order with the EPA regarding the scope and schedule for a remedial investigation and feasibility study for the site. Current estimates for the cost to complete the remedial investigation and feasibility study are approximately $7.6 million of which $5.0 million has been incurred as of December 31, 2020. Based on the site investigation, preliminary remediation alternative costs were provided by consultants in August 2020; therefore, the accrual for these costs was increased in the third quarter of 2020 by $11.1 million. The preliminary information received through the completion of the data report allowed for the projection of possible costs for a variety of site configurations, remedial measures and potential natural resource damage claims of between $13.6 million and $71.0 million. At December 31, 2020, Cascade has accrued $2.6 million for the remedial investigation and feasibility study, as well as $17.5 million for remediation of this site. The accrual for remediation cost will be reviewed and adjusted, if necessary, after the completion of the feasibility study. In April 2010, Cascade filed a petition with the WUTC for authority to defer the costs incurred in relation to the environmental remediation of this site. The WUTC approved the petition in September 2010, subject to conditions set forth in the order. A claim was made against Cascade for contamination at a site in Bellingham, Washington. Cascade received notice from a party in May 2008 that Cascade may be a PRP, along with other parties, for contamination from a manufactured gas plant owned by Cascade and its predecessor from about 1946 to 1962. Other PRPs reached an agreed order and work plan with the Washington DOE for completion of a remedial investigation and feasibility study for the site. A feasibility study prepared for one of the PRPs in March 2018 identifies five cleanup action alternatives for the site with estimated costs ranging from $8.0 million to $20.4 million with a selected preferred alternative having an estimated total cost of $9.3 million. The other PRPs will develop a cleanup action plan and, after public review of the cleanup action plan, develop design documents. Cascade believes its proportional share of any liability will be relatively small in comparison to other PRPs. The plant manufactured gas from coal between approximately 1890 and 1946. In 1946, shortly after Cascade's predecessor acquired the plant, the plant converted to a propane-air gas facility. There are no documented wastes or by-products resulting from the mixing or distribution of propane-air gas. Cascade has recorded an accrual for this site for an amount that is not material. The Company has received notices from and entered into agreement with certain of its insurance carriers that they will participate in defense for certain contamination claims subject to full and complete reservations of rights and defenses to insurance coverage. To the extent these claims are not covered by insurance, the Company intends to seek recovery of remediation costs through its natural gas rates charged to customers. Purchase commitments The Company has entered into various commitments largely consisting of contracts for natural gas and coal supply; purchased power; natural gas transportation and storage; royalties; information technology; and construction materials. Certain of these contracts are subject to variability in volume and price. The commitment terms vary in length, up to 39 years. The commitments under these contracts as of December 31, 2020, were: 2021 2022 2023 2024 2025 Thereafter (In thousands) Purchase commitments $ 458,397 $ 235,858 $ 173,306 $ 124,724 $ 98,894 $ 691,613 These commitments were not reflected in the Company's consolidated financial statements. Amounts purchased under various commitments for the years ended December 31, 2020, 2019 and 2018, were $666.0 million, $686.5 million and $548.0 million, respectively. 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 December 31, 2020, the fixed maximum amounts guaranteed under these agreements aggregated $212.4 million. Certain of the guarantees also have no fixed maximum amounts specified. The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate to $178.6 million in 2021; $16.1 million in 2022; $7.0 million in 2023; $500,000 in 2024; $500,000 in 2025; $700,000 thereafter; and $9.0 million, which has no scheduled maturity date. There were no amounts outstanding under the previously mentioned guarantees at December 31, 2020. 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 December 31, 2020, the fixed maximum amounts guaranteed under these letters of credit aggregated $22.7 million. The amounts of scheduled expiration of the maximum amounts guaranteed under these letters of credit aggregate to $22.2 million in 2021 and $500,000 in 2022. There were no amounts outstanding under the previously mentioned letters of credit at December 31, 2020. 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 December 31, 2020. 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 December 31, 2020, approximately $953.9 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 December 31, 2020, 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 $33.7 million.

Quarterly Data

Quarterly Data	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Supplementary financial Information-quarterly data (Unaudited)	Supplementary Financial Information Quarterly Data (Unaudited) The following unaudited information shows selected items by quarter for the years 2020 and 2019: First Quarter Second Third Fourth (In thousands, except per share amounts) 2020 Operating revenues $ 1,197,373 $ 1,362,928 $ 1,587,289 $ 1,385,160 Operating expenses 1,140,303 1,224,674 1,383,578 1,239,270 Operating income 57,070 138,254 203,711 145,890 Income from continuing operations 25,539 99,842 153,015 112,131 Income (loss) from discontinued operations, net of tax (409) (139) 63 163 Net income 25,130 99,703 153,078 112,294 Earnings per share - basic: Income from continuing operations .13 .50 .76 .56 Discontinued operations, net of tax — — — — Earnings per share - basic .13 .50 .76 .56 Earnings per share - diluted: Income from continuing operations .13 .50 .76 .56 Discontinued operations, net of tax — — — — Earnings per share - diluted .13 .50 .76 .56 Weighted average common shares outstanding: Basic 200,440 200,522 200,522 200,522 Diluted 200,456 200,539 200,619 200,923 2019 Operating revenues $ 1,091,191 $ 1,303,573 $ 1,563,799 $ 1,378,213 Operating expenses 1,026,973 1,206,262 1,374,329 1,247,992 Operating income 64,218 97,311 189,470 130,221 Income from continuing operations 41,089 63,145 136,128 94,804 Income (loss) from discontinued operations, net of tax (163) (1,320) 1,509 261 Net income 40,926 61,825 137,637 95,065 Earnings per share - basic: Income from continuing operations .21 .32 .68 .47 Discontinued operations, net of tax — (.01) .01 — Earnings per share - basic .21 .31 .69 .47 Earnings per share - diluted: Income from continuing operations .21 .32 .68 .47 Discontinued operations, net of tax — (.01) .01 — Earnings per share - diluted .21 .31 .69 .47 Weighted average common shares outstanding: Basic 196,401 198,270 199,343 200,383 Diluted 196,414 198,287 199,383 200,478 Certain operations of the Company are highly seasonal and revenues from and certain expenses for such operations may fluctuate significantly among quarterly periods. Accordingly, quarterly financial information may not be indicative of results for a full year.

Schedule I-Condensed Financial

Schedule I-Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Schedule I-Condensed Financial Information of Registrant	Condensed Financial Information of Registrant (Unconsolidated) Condensed Statements of Income and Comprehensive Income Years ended December 31, 2020 2019 2018 (In thousands) Operating revenues $ — $ — $ 628,331 Operating expenses — — 540,125 Operating income — — 88,206 Other income — — 1,504 Interest expense — — 32,761 Income before income taxes — — 56,949 Income taxes — — (4,259) Equity in earnings of subsidiaries from continuing operations 390,527 335,166 208,177 Income from continuing operations 390,527 335,166 269,385 Equity in earnings (loss) of subsidiaries from discontinued operations (322) 287 2,933 Net income $ 390,205 $ 335,453 $ 272,318 Comprehensive income $ 384,229 $ 331,693 $ 279,269 The accompanying notes are an integral part of these condensed financial statements. Condensed Balance Sheets December 31, 2020 2019 (In thousands, except shares and per share amounts) Assets Current assets: Cash and cash equivalents $ 8,781 $ 12,326 Receivables, net 4,865 4,727 Accounts receivable from subsidiaries 50,539 49,943 Prepayments and other current assets 1,612 501 Total current assets 65,797 67,497 Noncurrent assets Investments 52,000 46,294 Investment in subsidiaries 3,069,956 2,842,068 Deferred income taxes 9,691 7,269 Operating lease right-of-use assets 56 153 Other 28,866 27,098 Total noncurrent assets 3,160,569 2,922,882 Total assets $ 3,226,366 $ 2,990,379 Liabilities and Stockholders' Equity Current liabilities: Accounts payable $ 2,135 $ 2,981 Accounts payable to subsidiaries 5,412 4,752 Taxes payable 4,056 1,253 Dividends payable 42,611 41,580 Accrued compensation 7,825 8,812 Operating lease liabilities due within one year 40 96 Other accrued liabilities 6,881 7,690 Total current liabilities 68,960 67,164 Noncurrent liabilities: Operating lease liabilities 16 56 Other 78,285 75,913 Total noncurrent liabilities 78,301 75,969 Commitments and contingencies Stockholders' equity: Common stock Authorized - 500,000,000 shares, $1.00 par value Shares issued - 201,061,198 at December 31, 2020 and 200,922,790 at December 31, 2019 201,061 200,923 Other paid-in capital 1,371,385 1,355,404 Retained earnings 1,558,363 1,336,647 Accumulated other comprehensive loss (48,078) (42,102) Treasury stock at cost - 538,921 shares (3,626) (3,626) Total stockholders' equity 3,079,105 2,847,246 Total liabilities and stockholders' equity $ 3,226,366 $ 2,990,379 The accompanying notes are an integral part of these condensed financial statements. Condensed Statements of Cash Flows Years ended December 31, 2020 2019 2018 (In thousands) Net cash provided by operating activities $ 226,642 $ 168,520 $ 294,379 Investing activities: Capital expenditures — — (242,692) Net proceeds from sale or disposition of property and other — — 5,032 Investments in and advances to subsidiaries (67,000) (120,000) (40,000) Advances from subsidiaries — 17,000 70,000 Investments (4) (236) (528) Net cash used in investing activities (67,004) (103,236) (208,188) Financing activities: Issuance of long-term debt — — 199,422 Repayment of long-term debt — — (125,961) Payments of stock issuance costs — — (10) Proceeds from issuance of common stock 3,385 106,848 — Dividends paid (166,405) (160,256) (154,573) Repurchase of common stock — — (1,920) Tax withholding on stock-based compensation (163) (1,821) (1,721) Net cash used in financing activities (163,183) (55,229) (84,763) Increase (decrease) in cash and cash equivalents (3,545) 10,055 1,428 Cash and cash equivalents - beginning of year 12,326 2,271 843 Cash and cash equivalents - end of year $ 8,781 $ 12,326 $ 2,271 The accompanying notes are an integral part of these condensed financial statements. Note 1 - Summary of Significant Accounting Policies Basis of presentation The condensed financial information reported in Schedule I is being presented to comply with Rule 12-04 of Regulation S-X. The information is unconsolidated and is presented for the parent company only, MDU Resources Group, Inc. (the Company) as of and for the years ended December 31, 2020 and 2019. Prior to the Holding Company Reorganization, the Company included Montana-Dakota and Great Plains, public utility divisions of the Company as of December 31, 2018. On January 2, 2019, the Company announced the completion of the Holding Company Reorganization, which resulted in Montana-Dakota and Great Plains becoming a subsidiary of the Company. Immediately after consummation, the Company had, on a consolidated basis, the same assets, businesses and operations as it had immediately prior to the reorganization. For more information on the reorganization, see Item 8 - Note 1. The prior periods have not been restated and reflect the condensed financial information of Montana-Dakota and Great Plains as of and for the year ended December 31, 2018. Due to the completion of the Holding Company Reorganization, the presentation of the year ended December 31, 2018, will vary from that of and for the years ended December 31, 2020 and 2019. In Schedule I, investments in subsidiaries are presented under the equity method of accounting where the assets and liabilities of the subsidiaries are not consolidated. The investments in net assets of the subsidiaries are recorded on the Condensed Balance Sheets. The income from subsidiaries is reported as equity in earnings of subsidiaries on the Condensed Statements of Income. The material cash inflows on the Condensed Statements of Cash Flows are primarily from the dividends and other payments received from its subsidiaries and the proceeds raised from the issuance of equity securities. The consolidated financial statements of MDU Resources Group, Inc. reflect certain businesses as discontinued operations. These statements should be read in conjunction with the consolidated financial statements and notes thereto of MDU Resources Group, Inc. Earnings per common share Please refer to the Consolidated Statements of Income of the registrant for earnings per common share. In addition, see Item 8 - Note 2 for information on the computation of earnings per common share. Note 2 - Debt At December 31, 2020, the Company had no long-term debt maturities. For more information on debt, see Item 8 - Note 9. Note 3 - Dividends The Company depends on earnings and dividends from its subsidiaries to pay dividends on common stock. Cash dividends paid to the Company by subsidiaries were $228.4 million, $177.1 million and $115.9 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Basis of Presentation (Policies

Basis of Presentation (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of presentation	The abbreviations and acronyms used throughout are defined following the Notes to Consolidated Financial Statements. The consolidated financial statements of the Company include the accounts of the following businesses: electric, natural gas distribution, pipeline, construction materials and contracting, construction services and other. The electric and natural gas distribution businesses, as well as a portion of the pipeline business, are regulated. Construction materials and contracting, construction services and the other businesses, as well as a portion of the pipeline business, are non-regulated. For further descriptions of the Company's businesses, see Note 17. On January 2, 2019, the Company announced the completion of the Holding Company Reorganization, which resulted in Montana-Dakota becoming a subsidiary of the Company. The purpose of the reorganization was to make the public utility division into a subsidiary of the holding company, just as the other operating companies are wholly owned subsidiaries. In March 2020, the World Health Organization declared the outbreak of COVID-19 a pandemic, and the President of the United States declared the COVID-19 outbreak as a national emergency. 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 and accordingly operations have been generally allowed to continue. The Company has experienced some inefficiency impacts, including operation suspensions and interruptions at some locations to carry out preventive measures or in response to instances of positive tests or quarantines. The Company has assessed the impacts of the COVID-19 pandemic on its results of operations for the twelve months ended December 31, 2020, and determined there were no material adverse impacts. In the first quarter of 2020, the Company recorded an out-of-period adjustment to correct the recognition of revenue on a construction contract, which was the result of an overstatement of operating revenue and receivables of $7.7 million and an understatement of operating expense and accounts payable of $1.2 million in the year ended December 31, 2019. This adjustment resulted in an after-tax reduction to net income of $6.7 million in the first quarter of 2020. The Company evaluated the impact of the out-of-period adjustment and concluded it was not material to any previously issued interim and annual consolidated financial statements and the adjustment was not material to the three months ended March 31, 2020, or the twelve months ended December 31, 2020. In the fourth quarter of 2020, the Company recorded an out-of-period adjustment to correct the recognition of net periodic benefit cost and other comprehensive income associated with the Company's benefit plans, which was the result of the previous overstatement of benefit plan expenses of $6.5 million, understatement of accumulated other comprehensive loss of $2.7 million and overstatement of regulatory liabilities of $3.8 million from 2006 through 2020. This adjustment resulted in an after-tax increase to net income of $4.4 million in the fourth quarter of 2020. The Company evaluated the impact of the out-of-period adjustment, individually and in the aggregate with the previously mentioned out-of-period adjustment, and concluded it was not material to any previously issued interim and annual consolidated financial statements and the adjustment was not material to the three and twelve months ended December 31, 2020. Effective January 1, 2020, the Company adopted the requirements of the ASU on the measurement of credit losses on certain financial instruments following a modified retrospective approach, as further discussed in Note 2. As such, results for reporting periods beginning on January 1, 2020, are presented under the new guidance, while prior period amounts are not adjusted and continue to be reported in accordance with historic accounting. The Company's adoption of this guidance did not have a material impact on its financial reporting. 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 and are not material to the financial statements for any period presented. 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. Management has also evaluated the impact of events occurring after December 31, 2020, up to the date of issuance of these consolidated financial statements.
Consolidation, Policy	Principles of consolidation The consolidated financial statements were prepared in accordance with GAAP and include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation, except for certain transactions related to the Company's regulated operations in accordance with GAAP. For more information on intercompany revenues, see Note 17. The statements also include the Company's ownership interests in the assets, liabilities and expenses of jointly owned electric transmission and generating facilities. See Note 19 for additional information.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, at the date of the financial statements, as well as the reported amounts of revenues and expenses during the reporting period. Estimates are used for items such as long-lived assets and goodwill; fair values of acquired assets and liabilities under the acquisition method of accounting; aggregate reserves; property depreciable lives; tax provisions; revenue recognized using the cost-to-cost measure of progress for contracts; expected credit losses; environmental and other loss contingencies; regulatory assets expected to be recovered in rates charged to customers; costs on construction contracts; unbilled revenues; actuarially determined benefit costs; asset retirement obligations; lease classification; present value of right-of-use assets and lease liabilities; and the valuation of stock-based compensation. As additional information becomes available, or actual amounts are determinable, the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates.

Accounting Policies (Policies)

Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
New accounting standards	New accounting standards Recently adopted accounting standards ASU 2016-13 - Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued guidance on the measurement of credit losses on certain financial instruments. The guidance introduced a new impairment model known as the current expected credit loss model that replaced the incurred loss impairment methodology previously included under GAAP. This guidance required entities to present certain investments in debt securities, trade accounts receivable and other financial assets at their net carrying value of the amount expected to be collected on the financial statements. The Company adopted the guidance on January 1, 2020, using a modified retrospective approach. The Company formed an implementation team to review and assess existing financial assets to identify and evaluate the financial assets subject to the new current expected credit loss model. The Company assessed the impact of the guidance on its processes and internal controls and has identified and updated existing internal controls and processes to ensure compliance with the new guidance; such modifications were deemed insignificant. During the assessment phase, the Company identified the complete portfolio of assets subject to the current expected credit loss model. The Company determined the guidance did not have a material impact on its results of operations, financial position, cash flows or disclosures and did not record a material cumulative effect adjustment upon adoption. See Receivables and allowance for expected credit losses within this note for additional information on the Company's expected credit losses. ASU 2018-13 - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued guidance on modifying the disclosure requirements on fair value measurements as part of the disclosure framework project. The guidance modified, among other things, the disclosures required for Level 3 fair value measurements, including the range and weighted average of significant unobservable inputs. The guidance removed, among other things, the disclosure requirement to disclose transfers between Levels 1 and 2. The Company adopted the guidance on January 1, 2020, and determined it did not have a material impact on its disclosures. Recently issued accounting standards not yet adopted 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, clarifies 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 effects a one percentage point change in assumed health care cost trend rates will have on certain benefit components. The guidance was effective for the Company on January 1, 2021, and must be applied 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 and disclosures. 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 applies 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 new reference rates, 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.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents.
Revenue recognition	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. The electric and natural gas distribution segments generate revenue from the sales of electric and natural gas products and services, which includes retail and transportation services. These segments establish a customer's retail or transportation service account based on the customer's application/contract for service, which indicates approval of a contract for service. The contract identifies an obligation to provide service in exchange for delivering or standing ready to deliver the identified commodity; and the customer is obligated to pay for the service as provided in the applicable tariff. The product sales are based on a fixed rate that includes a base and per-unit rate, which are included in approved tariffs as determined by state or federal regulatory agencies. The quantity of the commodity consumed or transported determines the total per-unit revenue. The service provided, along with the product consumed or transported, are a single performance obligation because both are required in combination to successfully transfer the contracted product or service to the customer. Revenues are recognized over time as customers receive and consume the products and services. The method of measuring progress toward the completion of the single performance obligation is on a per-unit output method basis, with revenue recognized based on the direct measurement of the value to the customer of the goods or services transferred to date. For contracts governed by the Company’s utility tariffs, amounts are billed monthly with the amount due between 15 and 22 days of receipt of the invoice depending on the applicable state’s tariff. For other contracts not governed by tariff, payment terms are net 30 days. At this time, the segment has no material obligations for returns, refunds or other similar obligations. The pipeline segment generates revenue from providing natural gas transportation, gathering and underground storage services, as well as other energy-related services to both third parties and internal customers, largely the natural gas distribution segment. The pipeline segment establishes a contract with a customer based upon the customer’s request for firm or interruptible natural gas transportation, storage or gathering service(s). The contract identifies an obligation for the segment to provide the requested service(s) in exchange for consideration from the customer over a specified term. Depending on the type of service(s) requested and contracted, the service provided may include transporting, gathering or storing an identified quantity of natural gas and/or standing ready to deliver or store an identified quantity of natural gas. Natural gas transportation, gathering and storage revenues are based on fixed rates, which may include reservation fees and/or per-unit commodity rates. The services provided by the segment are generally treated as single performance obligations satisfied over time simultaneous to when the service is provided and revenue is recognized. Rates for the segment’s regulated services are based on its FERC approved tariff or customer negotiated rates, and rates for its non-regulated services are negotiated with its customers and set forth in the contract. For contracts governed by the company’s tariff, amounts are billed on or before the ninth business day of the following month and the amount is due within 12 days of receipt of the invoice. For gathering contracts not governed by the tariff, amounts are due within 20 days of invoice receipt. For other contracts not governed by the tariff, payment terms are net 30 days. At this time, the segment has no material obligations for returns, refunds or other similar obligations. The construction materials and contracting segment generates revenue from contracting services and construction materials sales. This segment focuses on the vertical integration of its contracting services with its construction materials to support the aggregate-based product lines. This segment provides contracting services to a customer when a contract has been signed by both the customer and a representative of the segment obligating a service to be provided in exchange for the consideration identified in the contract. The nature of the services this segment provides generally includes integrating a set of services and related construction materials into a single project to create a distinct bundle of goods and services, which the Company evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include liquidated damages; performance bonuses or incentives and penalties; claims; unapproved/unpriced change orders; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using an input method based on the cost-to-cost measure of progress on a project. This is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction materials to third parties and internal customers. The contract for material sales is the use of a sales order or an invoice, which includes the pricing and payment terms. All material contracts contain a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 30 days of receipt. There are no material obligations for returns, refunds or other similar obligations. The construction services segment generates revenue from specialty contracting services which also includes the sale of construction equipment and other supplies. This segment provides specialty contracting services to a customer when a contract has been signed by both the customer and a representative of the segment obligating a service to be provided in exchange for the consideration identified in the contract. The nature of the services this segment provides generally includes multiple promised goods and services in a single project to create a distinct bundle of goods and services, which the Company evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include claims, unapproved/unpriced change orders, bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using the input method based on the measurement of progress on a project. The input method is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction equipment and other supplies to third parties and internal customers. The contract for these sales is the use of a sales order or invoice, which includes the pricing and payment terms. All such contracts include a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 30 days of receipt. There are no material obligations for returns, refunds or other similar obligations. The Company recognizes all other revenues when services are rendered or goods are delivered. 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. As part of the adoption of ASC 606 - Revenue from Contracts with Customers , the Company elected the practical expedient to recognize the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that the Company otherwise would have recognized is 12 months or less.
Legal costs	The Company expenses external legal fees as they are incurred.
Business combinations policy	For all business combinations, the Company preliminarily allocates the purchase price of the acquisitions to the assets acquired and liabilities assumed based on their estimated fair values as of the acquisition dates and are considered provisional until final fair values are determined or the measurement period has passed. The Company expects to record adjustments as it accumulates the information needed to estimate the fair value of assets acquired and liabilities assumed, including working capital balances, estimated fair value of identifiable intangible assets, property, plant and equipment, total consideration and goodwill. The excess of the purchase price over the aggregate fair values is recorded as goodwill. The Company calculated the fair value of the assets acquired in 2020 and 2019 using a market or cost approach (or a combination of both). Fair values for some of the assets were determined based on Level 3 inputs including estimated future cash flows, discount rates, growth rates, sales projections, retention rates and terminal values, all of which require significant management judgment and are susceptible to change. The final fair value of the net assets acquired may result in adjustments to the assets and liabilities, including goodwill, and will be made as soon as practical, but no later than 12 months from the respective acquisition dates. Any subsequent measurement period adjustments are not expected to have a material impact on the Company's results of operations.
Accounts receivable and allowance for doubtful accounts	Receivable 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 3. The Company's trade receivables are all due in 12 months or less.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 and general economic conditions of the various local economies, among others. Specific account balances are written off when management determines the amounts to be uncollectible.
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
Property, Plant and Equipment, Policy	Additions to property, plant and equipment are recorded at cost. When regulated assets are retired, or otherwise disposed of in the ordinary course of business, the original cost of the asset is charged to accumulated depreciation. With respect to the retirement or disposal of all other assets, the resulting gains or losses are recognized as a component of income. The Company is permitted to capitalize AFUDC on regulated construction projects and to include such amounts in rate base when the related facilities are placed in service. In addition, the Company capitalizes interest, when applicable, on certain construction projects associated with its other operations.
Impairment or Disposal of Long-Lived Assets, Including Intangible Assets, Policy	The Company reviews the carrying values of its long-lived assets, excluding goodwill, whenever events or changes in circumstances indicate that such carrying values may not be recoverable. The determination of whether an impairment has occurred is based on an estimate of undiscounted future cash flows attributable to the assets, compared to the carrying value of the assets. If impairment has occurred, the amount of the impairment recognized is determined by estimating the fair value of the assets and recording a loss if the carrying value is greater than the fair value.
Regulatory assets and liabilities	The Company's regulated businesses are subject to various state and federal agency regulations. The accounting policies followed by these businesses are generally subject to the Uniform System of Accounts of the FERC as well as the provisions of ASC 980 - Regulated Operations . These accounting policies differ in some respects from those used by the Company's non-regulated businesses. The Company's regulated businesses account for certain income and expense items under the provisions of regulatory accounting, which requires these businesses to defer as regulatory assets or liabilities certain items that would have otherwise been reflected as expense or income, respectively. The Company records regulatory assets or liabilities at the time the Company determines the amounts to be recoverable in current or future rates. Regulatory assets and liabilities are being amortized consistently with the regulatory treatment established by the FERC and the applicable state public service commission. See Note 6 for more information regarding the nature and amounts of these regulatory deferrals.
Natural gas costs recoverable or refundable through rate adjustments	Under the terms of certain orders of the applicable state public service commissions, the Company is deferring natural gas commodity, transportation and storage costs that are greater or less than amounts presently being recovered through its existing rate schedules. Such orders generally provide that these amounts are recoverable or refundable through rate adjustments
Goodwill	Goodwill represents the excess of the purchase price over the fair value of identifiable net tangible and intangible assets acquired in a business combination. Goodwill is required to be tested for impairment annually, which the Company completes in the fourth quarter, or more frequently if events or changes in circumstances indicate that goodwill may be impaired.
Investment, Policy	The Company's investments include the cash surrender value of life insurance policies, an insurance contract, mortgage-backed securities and U.S. Treasury securities. The Company measures its investment in the insurance contract at fair value with any unrealized gains and losses recorded on the Consolidated Statements of Income. The Company has not elected the fair value option for its mortgage-backed securities and U.S. Treasury securities and, as a result, the unrealized gains and losses on these investments are recorded in accumulated other comprehensive income (loss). For more information, see Notes 8 and 18
Derivatives, Methods of Accounting, Derivatives Not Designated or Qualifying as Hedges	The Company enters into commodity price derivative contracts in order to minimize the price volatility associated with natural gas costs at its natural gas distribution segment. These derivatives are not designated as hedging instruments and are recorded in the Consolidated Balance Sheets at fair value, as discussed in Note 8. Changes in the fair value of these derivatives along with any contract settlements are recorded each period in regulatory assets or liabilities in accordance with regulatory accounting. The Company does not enter into any derivatives for trading or other speculative purposes.
Leases	Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The Company recognizes leases with an original lease term of 12 months or less in income on a straight-line basis over the term of the lease and does not recognize a corresponding right-of-use asset or lease liability. The Company determines the lease term based on the non-cancelable and cancelable periods in each contract. The non-cancelable period consists of the term of the contract that is legally enforceable and cannot be canceled by either party without incurring a significant penalty. The cancelable period is determined by various factors that are based on who has the right to cancel a contract. If only the lessor has the right to cancel the contract, the Company will assume the contract will continue. If the lessee is the only party that has the right to cancel the contract, the Company looks to asset, entity and market-based factors. If both the lessor and the lessee have the right to cancel the contract, the Company assumes the contract will not continue. The discount rate used to calculate the present value of the lease liabilities is based upon the implied rate within each contract. If the rate is unknown or cannot be determined, the Company uses an incremental borrowing rate, which is determined by the length of the contract, asset class and the Company's borrowing rates, as of the commencement date of the contract.
Asset retirement obligation	The Company records the fair value of a liability for an asset retirement obligation in the period in which it is incurred. When the liability is initially recorded, the Company capitalizes a cost by increasing the carrying amount of the related long-lived asset. Over time, the liability is accreted to its present value each period and the capitalized cost is depreciated over the useful life of the related asset. Upon settlement of the liability, the Company either settles the obligation for the recorded amount or incurs a gain or loss at its non-regulated operations or incurs a regulatory asset or liability at its regulated operations.
Stock-based compensation	The Company determines compensation expense for stock-based awards based on the estimated fair values at the grant date and recognizes the related compensation expense over the vesting period. The Company uses the straight-line amortization method to recognize compensation expense related to restricted stock, which only has a service condition. This method recognizes stock compensation expense on a straight-line basis over the requisite service period for the entire award. The Company recognizes compensation expense related to performance awards that vest based on performance metrics and service conditions on a straight-line basis over the service period. Inception-to-date expense is adjusted based upon the determination of the potential achievement of the performance target at each reporting date. The Company recognizes compensation expense related to performance awards with market-based performance metrics on a straight-line basis over the requisite service period.The Company records the compensation expense for performance share awards using an estimated forfeiture rate. The estimated forfeiture rate is calculated based on an average of actual historical forfeitures. The Company also performs an analysis of any known factors at the time of the calculation to identify any necessary adjustments to the average historical forfeiture rate. At the time actual forfeitures become more than estimated forfeitures, the Company records compensation expense using actual forfeitures
Earnings (loss) per common share	Basic earnings per share were computed by dividing net income by the weighted average number of shares of common stock outstanding during the year. Diluted earnings per share were computed by dividing net income by the total of the weighted average number of shares of common stock outstanding during the year, plus the effect of nonvested performance share awards and restricted stock units.
Income Tax, Policy	The Company provides deferred federal and state income taxes on all temporary differences between the book and tax basis of the Company's assets and liabilities by using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. Excess deferred income tax balances associated with the Company's rate-regulated activities have been recorded as a regulatory liability and are included in other liabilities. These regulatory liabilities are expected to be reflected as a reduction in future rates charged to customers in accordance with applicable regulatory procedures. The Company uses the deferral method of accounting for investment tax credits and amortizes the credits on regulated electric and natural gas distribution plant over various periods that conform to the ratemaking treatment prescribed by the applicable state public service commissions.
Income tax uncertainties	The Company records uncertain tax positions in accordance with accounting guidance on accounting for income taxes on the basis of a two-step process in which (1) the Company determines whether it is more-likely-than-not that the tax position will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, the Company recognizes the largest amount of the tax benefit that is more than 50 percent percent likely to be realized upon ultimate settlement with the related tax authority. Tax positions that do not meet the more-likely-than-not criteria are reflected as a tax liability. The Company recognizes interest and penalties accrued related to unrecognized tax benefits in income taxes.
Variable interest entities	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. GAAP provides a framework for identifying VIEs and determining when a company should include the assets, liabilities, noncontrolling interest and results of activities of a VIE in its consolidated financial statements. A VIE should be consolidated if a party with an ownership, contractual or other financial interest in the VIE (a variable interest holder) has the power to direct the VIE's most significant activities and the obligation to absorb losses or right to receive benefits of the VIE that could be significant to the VIE. A variable interest holder that consolidates the VIE is called the primary beneficiary. Upon consolidation, the primary beneficiary generally must initially record all of the VIE's assets, liabilities and noncontrolling interests at fair value and subsequently account for the VIE as if it were consolidated. The Company's evaluation of whether it qualifies as the primary beneficiary of a VIE involves significant judgments, estimates and assumptions and includes a qualitative analysis of the activities that most significantly impact the VIE's economic performance and whether the Company has the power to direct those activities, the design of the entity, the rights of the parties and the purpose of the arrangement.

Revenue from Contract with Cu_2

Revenue from Contract with Customer (Policies)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue recognition	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. The electric and natural gas distribution segments generate revenue from the sales of electric and natural gas products and services, which includes retail and transportation services. These segments establish a customer's retail or transportation service account based on the customer's application/contract for service, which indicates approval of a contract for service. The contract identifies an obligation to provide service in exchange for delivering or standing ready to deliver the identified commodity; and the customer is obligated to pay for the service as provided in the applicable tariff. The product sales are based on a fixed rate that includes a base and per-unit rate, which are included in approved tariffs as determined by state or federal regulatory agencies. The quantity of the commodity consumed or transported determines the total per-unit revenue. The service provided, along with the product consumed or transported, are a single performance obligation because both are required in combination to successfully transfer the contracted product or service to the customer. Revenues are recognized over time as customers receive and consume the products and services. The method of measuring progress toward the completion of the single performance obligation is on a per-unit output method basis, with revenue recognized based on the direct measurement of the value to the customer of the goods or services transferred to date. For contracts governed by the Company’s utility tariffs, amounts are billed monthly with the amount due between 15 and 22 days of receipt of the invoice depending on the applicable state’s tariff. For other contracts not governed by tariff, payment terms are net 30 days. At this time, the segment has no material obligations for returns, refunds or other similar obligations. The pipeline segment generates revenue from providing natural gas transportation, gathering and underground storage services, as well as other energy-related services to both third parties and internal customers, largely the natural gas distribution segment. The pipeline segment establishes a contract with a customer based upon the customer’s request for firm or interruptible natural gas transportation, storage or gathering service(s). The contract identifies an obligation for the segment to provide the requested service(s) in exchange for consideration from the customer over a specified term. Depending on the type of service(s) requested and contracted, the service provided may include transporting, gathering or storing an identified quantity of natural gas and/or standing ready to deliver or store an identified quantity of natural gas. Natural gas transportation, gathering and storage revenues are based on fixed rates, which may include reservation fees and/or per-unit commodity rates. The services provided by the segment are generally treated as single performance obligations satisfied over time simultaneous to when the service is provided and revenue is recognized. Rates for the segment’s regulated services are based on its FERC approved tariff or customer negotiated rates, and rates for its non-regulated services are negotiated with its customers and set forth in the contract. For contracts governed by the company’s tariff, amounts are billed on or before the ninth business day of the following month and the amount is due within 12 days of receipt of the invoice. For gathering contracts not governed by the tariff, amounts are due within 20 days of invoice receipt. For other contracts not governed by the tariff, payment terms are net 30 days. At this time, the segment has no material obligations for returns, refunds or other similar obligations. The construction materials and contracting segment generates revenue from contracting services and construction materials sales. This segment focuses on the vertical integration of its contracting services with its construction materials to support the aggregate-based product lines. This segment provides contracting services to a customer when a contract has been signed by both the customer and a representative of the segment obligating a service to be provided in exchange for the consideration identified in the contract. The nature of the services this segment provides generally includes integrating a set of services and related construction materials into a single project to create a distinct bundle of goods and services, which the Company evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include liquidated damages; performance bonuses or incentives and penalties; claims; unapproved/unpriced change orders; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using an input method based on the cost-to-cost measure of progress on a project. This is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction materials to third parties and internal customers. The contract for material sales is the use of a sales order or an invoice, which includes the pricing and payment terms. All material contracts contain a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 30 days of receipt. There are no material obligations for returns, refunds or other similar obligations. The construction services segment generates revenue from specialty contracting services which also includes the sale of construction equipment and other supplies. This segment provides specialty contracting services to a customer when a contract has been signed by both the customer and a representative of the segment obligating a service to be provided in exchange for the consideration identified in the contract. The nature of the services this segment provides generally includes multiple promised goods and services in a single project to create a distinct bundle of goods and services, which the Company evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include claims, unapproved/unpriced change orders, bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using the input method based on the measurement of progress on a project. The input method is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction equipment and other supplies to third parties and internal customers. The contract for these sales is the use of a sales order or invoice, which includes the pricing and payment terms. All such contracts include a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 30 days of receipt. There are no material obligations for returns, refunds or other similar obligations. The Company recognizes all other revenues when services are rendered or goods are delivered. 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. As part of the adoption of ASC 606 - Revenue from Contracts with Customers , the Company elected the practical expedient to recognize the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that the Company otherwise would have recognized is 12 months or less.

Accumulated other comprehensi_2

Accumulated other comprehensive income (loss) (Policies)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Accumulated other comprehensive income (loss) (Policy)	The Company's accumulated other comprehensive income (loss) is comprised of losses on derivative instruments qualifying as hedges, postretirement liability adjustments and gain (loss) on available-for-sale investments.

Business segment data (Policies

Business segment data (Policies)	12 Months Ended
	Dec. 31, 2020
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.

Commitment and Contingencies (P

Commitment and Contingencies (Policies)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies, Policy	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.

Accounting Policies (Tables)

Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Financing Receivable, Allowance for Credit Loss	Details of the Company's expected credit losses were as follows: 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 1,517 3,187 2 1,447 4,832 10,985 Less write-offs charged against the allowance 1,289 2,511 — 640 866 5,306 Credit loss recoveries collected 343 839 — — — 1,182 At December 31, 2020 $ 899 $ 2,571 $ 2 $ 6,164 $ 5,722 $ 15,358
Contract receivable retainage	Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows: 2020 2019 (In thousands) Short-term retainage* $ 100,054 $ 75,590 Long-term retainage** 2,761 14,228 Total retainage $ 102,815 $ 89,818 * Expected to be paid within 12 months or less and included in receivables, net. ** Included in noncurrent assets - other.
Inventories	Inventories at December 31 consisted of: 2020 2019 (In thousands) Aggregates held for resale $ 175,782 $ 147,723 Asphalt oil 28,238 41,912 Materials and supplies 25,142 22,512 Merchandise for resale 21,087 22,232 Natural gas in storage (current) 21,919 22,058 Other 18,999 21,970 Total $ 291,167 $ 278,407
AFUDC and interest capitailized	The amount of AFUDC for the years ended December 31 was as follows: 2020 2019 2018 (In thousands) AFUDC - borrowed $ 2,640 $ 2,807 $ 2,290 AFUDC - equity $ 1,270 $ 698 $ 1,897
Schedule of earnings per share reconciliation	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculation was as follows: 2020 2019 2018 (In thousands) Weighted average common shares outstanding - basic 200,502 198,612 195,720 Effect of dilutive performance share awards 69 14 430 Weighted average common shares outstanding - diluted 200,571 198,626 196,150 Shares excluded from the calculation of diluted earnings per share 164 164 10

Revenue from contracts with cus

Revenue from contracts with customers (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Disaggregation of revenue	Disaggregation In the following table, 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. Year ended December 31, 2020 Electric Natural gas distribution Pipeline Construction materials and contracting Construction services Other Total (In thousands) Residential utility sales $ 122,663 $ 476,388 $ — $ — $ — $ — $ 599,051 Commercial utility sales 131,477 277,873 — — — — 409,350 Industrial utility sales 36,744 26,243 — — — — 62,987 Other utility sales 6,634 — — — — — 6,634 Natural gas transportation — 45,546 111,686 — — — 157,232 Natural gas gathering — — 4,865 — — — 4,865 Natural gas storage — — 14,918 — — — 14,918 Contracting services — — — 1,069,665 — — 1,069,665 Construction materials — — — 1,659,152 — — 1,659,152 Intrasegment eliminations — — — (550,815) — — (550,815) Inside specialty contracting — — — — 1,397,124 — 1,397,124 Outside specialty contracting — — — — 649,486 — 649,486 Other 32,452 10,753 12,216 — 1,541 11,903 68,865 Intersegment eliminations (491) (534) (58,531) (417) (5,038) (11,958) (76,969) Revenues from contracts with customers 329,479 836,269 85,154 2,177,585 2,043,113 (55) 5,471,545 Revenues out of scope 2,059 11,382 192 — 47,572 — 61,205 Total external operating revenues $ 331,538 $ 847,651 $ 85,346 $ 2,177,585 $ 2,090,685 $ (55) $ 5,532,750 Year ended December 31, 2019 Electric Natural gas distribution Pipeline Construction materials and contracting Construction services Other Total (In thousands) Residential utility sales $ 125,369 $ 483,452 $ — $ — $ — $ — $ 608,821 Commercial utility sales 141,596 296,835 — — — — 438,431 Industrial utility sales 37,765 26,895 — — — — 64,660 Other utility sales 7,408 — — — — — 7,408 Natural gas transportation — 45,449 101,665 — — — 147,114 Natural gas gathering — — 9,164 — — — 9,164 Natural gas storage — — 11,708 — — — 11,708 Contracting services — — — 1,088,633 — — 1,088,633 Construction materials — — — 1,627,833 — — 1,627,833 Intrasegment eliminations — — — (525,749) — — (525,749) Inside specialty contracting — — — — 1,266,196 — 1,266,196 Outside specialty contracting — — — — 531,882 — 531,882 Other 35,574 12,726 17,687 — 131 16,551 82,669 Intersegment eliminations — — (56,252) (1,066) (3,370) (16,461) (77,149) Revenues from contracts with customers 347,712 865,357 83,972 2,189,651 1,794,839 90 5,281,621 Revenues out of scope 4,013 (135) 220 — 51,057 — 55,155 Total external operating revenues $ 351,725 $ 865,222 $ 84,192 $ 2,189,651 $ 1,845,896 $ 90 $ 5,336,776 Year ended December 31, 2018 Electric Natural gas distribution Pipeline Construction materials and contracting Construction services Other Total (In thousands) Residential utility sales $ 121,477 $ 457,959 $ — $ — $ — $ — $ 579,436 Commercial utility sales 136,236 276,716 — — — — 412,952 Industrial utility sales 34,353 24,603 — — — — 58,956 Other utility sales 7,556 — — — — — 7,556 Natural gas transportation — 43,238 89,159 — — — 132,397 Natural gas gathering — — 9,159 — — — 9,159 Natural gas storage — — 11,543 — — — 11,543 Contracting services — — — 968,755 — — 968,755 Construction materials — — — 1,423,068 — — 1,423,068 Intrasegment eliminations — — — (465,969) — — (465,969) Inside specialty contracting — — — — 926,875 — 926,875 Outside specialty contracting — — — — 392,544 — 392,544 Other 31,568 14,579 18,865 — 525 11,259 76,796 Intersegment eliminations — — (50,905) (669) (1,681) (11,052) (64,307) Revenues from contracts with customers 331,190 817,095 77,821 1,925,185 1,318,263 207 4,469,761 Revenues out of scope 3,933 6,152 197 — 51,509 — 61,791 Total external operating revenues $ 335,123 $ 823,247 $ 78,018 $ 1,925,185 $ 1,369,772 $ 207 $ 4,531,552
Contract balances	The changes in contract assets and liabilities were as follows: December 31, 2020 December 31, 2019 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 104,345 $ 109,078 $ (4,733) Receivables, net Contract liabilities - current (158,603) (142,768) (15,835) Accounts payable Contract liabilities - noncurrent (52) (19) (33) Noncurrent liabilities - other Net contract liabilities $ (54,310) $ (33,709) $ (20,601) December 31, 2019 December 31, 2018 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 109,078 $ 104,239 $ 4,839 Receivables, net Contract liabilities - current (142,768) (93,901) (48,867) Accounts payable Contract liabilities - noncurrent (19) (135) 116 Noncurrent liabilities - other Net contract assets (liabilities) $ (33,709) $ 10,203 $ (43,912)

Property, Plant, and Equipment

Property, Plant, and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment at December 31 was as follows: 2020 2019 Weighted (Dollars in thousands, where applicable) Regulated: Electric: Generation $ 1,133,390 $ 1,139,059 48 Distribution 464,442 443,780 46 Transmission 524,155 445,485 64 Construction in progress 61,766 66,664 — Other 139,650 132,157 14 Natural gas distribution: Distribution 2,302,121 2,133,249 47 Transmission 104,695 104,401 51 Storage 33,014 31,484 24 General 198,211 191,446 14 Construction in progress 16,836 39,506 — Other 213,976 188,037 16 Pipeline: Transmission 665,567 636,796 46 Gathering — 35,661 — Storage 52,632 50,001 53 Construction in progress 46,690 22,597 — Other 49,640 48,340 17 Non-regulated: Pipeline: Gathering — 31,148 — Construction in progress 4 154 — Other 7,164 9,518 11 Construction materials and contracting: Land 132,948 127,729 — Buildings and improvements 130,417 122,064 21 Machinery, vehicles and equipment 1,284,604 1,180,343 12 Construction in progress 23,803 25,018 — Aggregate reserves 456,704 455,408 ** Construction services: Land 7,218 7,146 — Buildings and improvements 41,674 31,735 25 Machinery, vehicles and equipment 163,080 156,537 7 Other 8,824 17,952 4 Other: Land 2,648 2,648 — Other 34,897 32,565 12 Less accumulated depreciation, depletion and amortization 3,133,831 2,991,486 Net property, plant and equipment $ 5,166,939 $ 4,917,142 * Depleted on the units-of-production method based on recoverable aggregate reserves.

Regulatory assets and liabili_2

Regulatory assets and liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31: Estimated Recovery or Refund Period * 2020 2019 (In thousands) Regulatory assets: Current: Natural gas costs recoverable through rate adjustments Up to 1 year $ 42,481 $ 42,823 Cost recovery mechanisms Up to 1 year 10,645 6,288 Conservation programs Up to 1 year 7,117 6,963 Other Up to 1 year 8,284 7,539 68,527 63,613 Noncurrent: Pension and postretirement benefits ** 155,942 157,069 Plant costs/asset retirement obligations Over plant lives 71,740 66,000 Plant to be retired - 65,919 32,931 Manufactured gas plant sites remediation - 26,429 15,126 Natural gas costs recoverable through rate adjustments Up to 2 years 21,539 46,381 Cost recovery mechanisms Up to 10 years 16,245 13,108 Taxes recoverable from customers Over plant lives 10,785 11,486 Long-term debt refinancing costs Up to 40 years 4,426 4,286 Other Up to 18 years 6,356 7,397 379,381 353,784 Total regulatory assets $ 447,908 $ 417,397 Regulatory liabilities: Current: Natural gas costs refundable through rate adjustments Up to 1 year $ 18,565 $ 23,825 Electric fuel and purchased power deferral Up to 1 year 3,667 5,824 Taxes refundable to customers Up to 1 year 3,557 3,472 Other Up to 1 year 5,661 9,814 31,450 42,935 Noncurrent: Taxes refundable to customers Over plant lives 227,850 246,034 Plant removal and decommissioning costs Over plant lives 167,171 173,722 Pension and postretirement benefits ** 16,989 18,065 Other Up to 21 years 16,065 9,549 428,075 447,370 Total regulatory liabilities $ 459,525 $ 490,305 Net regulatory position $ (11,617) $ (72,908) * Estimated recovery or refund period for amounts currently being recovered or refunded in rates to customers. ** Recovered as expense is incurred or cash contributions are made.
Regulatory liabilities	The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31: Estimated Recovery or Refund Period * 2020 2019 (In thousands) Regulatory assets: Current: Natural gas costs recoverable through rate adjustments Up to 1 year $ 42,481 $ 42,823 Cost recovery mechanisms Up to 1 year 10,645 6,288 Conservation programs Up to 1 year 7,117 6,963 Other Up to 1 year 8,284 7,539 68,527 63,613 Noncurrent: Pension and postretirement benefits ** 155,942 157,069 Plant costs/asset retirement obligations Over plant lives 71,740 66,000 Plant to be retired - 65,919 32,931 Manufactured gas plant sites remediation - 26,429 15,126 Natural gas costs recoverable through rate adjustments Up to 2 years 21,539 46,381 Cost recovery mechanisms Up to 10 years 16,245 13,108 Taxes recoverable from customers Over plant lives 10,785 11,486 Long-term debt refinancing costs Up to 40 years 4,426 4,286 Other Up to 18 years 6,356 7,397 379,381 353,784 Total regulatory assets $ 447,908 $ 417,397 Regulatory liabilities: Current: Natural gas costs refundable through rate adjustments Up to 1 year $ 18,565 $ 23,825 Electric fuel and purchased power deferral Up to 1 year 3,667 5,824 Taxes refundable to customers Up to 1 year 3,557 3,472 Other Up to 1 year 5,661 9,814 31,450 42,935 Noncurrent: Taxes refundable to customers Over plant lives 227,850 246,034 Plant removal and decommissioning costs Over plant lives 167,171 173,722 Pension and postretirement benefits ** 16,989 18,065 Other Up to 21 years 16,065 9,549 428,075 447,370 Total regulatory liabilities $ 459,525 $ 490,305 Net regulatory position $ (11,617) $ (72,908) * Estimated recovery or refund period for amounts currently being recovered or refunded in rates to customers. ** Recovered as expense is incurred or cash contributions are made.

Goodwill and other intangible_2

Goodwill and other intangible assets (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 Goodwill Measurement Period 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 Balance at January 1, 2019 Goodwill Acquired Measurement Period Balance at December 31, 2019 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 209,421 14,482 (6,669) 217,234 Construction services 109,765 8,623 — 118,388 Total $ 664,922 $ 23,105 $ (6,669) $ 681,358
Other amortizable intangible assets	Other amortizable intangible assets at December 31 were as follows: 2020 2019 (In thousands) Customer relationships $ 28,836 $ 17,958 Less accumulated amortization 6,887 6,268 21,949 11,690 Noncompete agreements 3,941 3,439 Less accumulated amortization 2,309 1,957 1,632 1,482 Other 12,927 8,094 Less accumulated amortization 11,012 6,020 1,915 2,074 Total $ 25,496 $ 15,246
Estimated amortization expense	The amounts of estimated amortization expense for identifiable intangible assets as of December 31, 2020, were: 2021 2022 2023 2024 2025 Thereafter (In thousands) Amortization expense $ 4,911 $ 4,394 $ 4,121 $ 3,799 $ 1,862 $ 6,409

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: 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 December 31, 2019 Cost Gross Gross Fair Value (In thousands) Mortgage-backed securities $ 9,804 $ 87 $ 10 $ 9,881 U.S. Treasury securities 1,228 1 — 1,229 Total $ 11,032 $ 88 $ 10 $ 11,110
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 Quoted Prices 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 Measurements Quoted Prices Significant Significant Balance at December 31, 2019 (In thousands) Assets: Money market funds $ — $ 8,440 $ — $ 8,440 Insurance contract* — 87,009 — 87,009 Available-for-sale securities: Mortgage-backed securities — 9,881 — 9,881 U.S. Treasury securities — 1,229 — 1,229 Total assets measured at fair value $ — $ 106,559 $ — $ 106,559 * The insurance contract invests approximately 51 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 12 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 3 percent in target date investments and 1 percent in cash equivalents.
Fair value of long-term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt at December 31 was as follows: 2020 2019 (In thousands) Carrying Amount $ 2,213,130 $ 2,243,107 Fair Value $ 2,537,289 $ 2,418,631

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Outstanding credit facilities	The following table summarizes the outstanding revolving credit facilities of the Company's subsidiaries: Company Facility Facility Amount Outstanding at December 31, 2020 Amount Outstanding at December 31, Letters of Expiration (In millions) Montana-Dakota Utilities Co. Commercial paper/Revolving credit agreement (a) $ 175.0 $ 87.7 $ 118.6 $ — 12/19/24 Cascade Natural Gas Corporation Revolving credit agreement $ 100.0 (b) $ 54.0 $ 64.6 $ 2.2 (c) 6/7/24 Intermountain Gas Company Revolving credit agreement $ 85.0 (d) $ 41.9 $ 24.5 $ — 6/7/24 Centennial Energy Holdings, Inc. Commercial paper/Revolving credit agreement (e) $ 600.0 $ 37.9 $ 104.3 $ — 12/19/24 (a) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Montana-Dakota on stated conditions, up to a maximum of $225.0 million). There were no amounts outstanding under the revolving credit agreement. (b) Certain provisions allow for increased borrowings, up to a maximum of $125.0 million. (c) Outstanding letter(s) of credit reduce the amount available under the credit agreement. (d) Certain provisions allow for increased borrowings, up to a maximum of $110.0 million. (e) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Centennial on stated conditions, up to a maximum of $700.0 million). There were no amounts outstanding under the revolving credit agreement.
Long term debt outstanding	Long-term debt outstanding was as follows: Weighted Average Interest Rate at December 31, 2020 2020 2019 (In thousands) Senior Notes due on dates ranging from October 22, 2022 to October 30, 2060 4.40 % $ 1,950,000 $ 1,850,000 Commercial paper supported by revolving credit agreements .28 % 125,600 222,900 Credit agreements due on June 7, 2024 1.88 % 95,900 89,050 Medium-Term Notes due on dates ranging from September 15, 2027 to March 16, 2029 7.32 % 35,000 50,000 Term Loan Agreement due on September 3, 2032 2.00 % 8,400 9,100 Other notes due on dates ranging from July 15, 2021 to November 30, 2038 .75 % 4,034 29,117 Less unamortized debt issuance costs 5,803 7,010 Less discount 1 50 Total long-term debt 2,213,130 2,243,107 Less current maturities 1,555 16,540 Net long-term debt $ 2,211,575 $ 2,226,567
Schedule of debt maturities	Long-term debt maturities, which excludes unamortized debt issuance costs and discount, for the five years and thereafter following December 31, 2020, were as follows: 2021 2022 2023 2024 2025 Thereafter (In thousands) Long-term debt maturities $ 1,555 $ 148,021 $ 77,921 $ 282,922 $ 177,802 $ 1,530,713

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Lease, Cost	The following tables provide information on the Company's operating leases at and for the years ended December 31: 2020 2019 (In thousands) Lease costs: Operating lease cost $ 45,319 $ 43,759 Variable lease cost 1,319 1,555 Short-term lease cost 135,376 120,030 Total lease costs $ 182,014 $ 165,344 2020 2019 (Dollars in thousands) Weighted average remaining lease term 2.73 years 3.13 years Weighted average discount rate 4.03 % 4.41 % Cash paid for amounts included in the measurement of lease liabilities $ 45,043 $ 43,477
Lessee, Operating Lease, Liability, Maturity	The reconciliation of the future undiscounted cash flows to the operating lease liabilities presented on the Consolidated Balance Sheet at December 31, 2020, was as follows: (In thousands) 2021 $ 36,632 2022 25,133 2023 16,639 2024 12,140 2025 7,852 Thereafter 48,845 Total 147,241 Less discount 26,718 Total operating lease liabilities $ 120,523

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2020
Asset Retirement Obligation [Abstract]
Reconciliation of the company's asset retirement obligation	A reconciliation of the Company's liability, which the current portion is included in other accrued liabilities on the Consolidated Balance Sheets, for the years ended December 31 was as follows: 2020 2019 (In thousands) Balance at beginning of year $ 417,575 $ 375,553 Liabilities incurred 11,560 25,869 Liabilities acquired 1,378 486 Liabilities settled (5,369) (7,097) Accretion expense* 21,668 19,789 Revisions in estimates 107 2,975 Balance at end of year $ 446,919 $ 417,575 * Includes $20.1 million and $18.3 million in 2020 and 2019, respectively, related to regulatory assets.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement, Noncash Expense [Abstract]
Target grants performance share	Target grants of performance shares outstanding at December 31, 2020, were as follows: Grant Date Performance Target Grant February 2019 2019-2021 327,194 February 2020 2020-2022 285,980
Schedule of share-based payment award, performance shares, valuation assumptions	Assumptions used for grants applicable to the market condition for certain performance shares issued in 2020, 2019 and 2018 were: 2020 2019 2018 Weighted average grant-date fair value $40.75 $35.07 $34.55 Blended volatility range 15.30 % – 15.97 % 19.50 % – 19.69 % 17.87 % – 22.14 % Risk-free interest rate range 1.45 % – 1.62 % 2.46 % – 2.55 % 1.86 % – 2.46 % Weighted average discounted dividends per share $2.91 $2.85 $2.46
Summary of the status of the performance share awards	A summary of the status of the performance share awards for the year ended December 31, 2020, was as follows: Number of Weighted Nonvested at beginning of period 573,503 $ 30.81 Granted 285,980 36.19 Additional performance shares earned 123,147 27.48 Less: Vested 369,456 29.84 Nonvested at end of period 613,174 $ 33.24

Accumulated other comprehensi_3

Accumulated other comprehensive income (loss) (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Accumulated comprehensive loss	The after-tax changes in the components of accumulated other comprehensive loss at December 31, 2020, 2019 and 2018, were as follows: Net Post- Net Total (In thousands) At December 31, 2018 $ (2,161) $ (36,069) $ (112) $ (38,342) Other comprehensive income (loss) before reclassifications — (6,151) 134 (6,017) Amounts reclassified from accumulated other comprehensive loss 731 1,486 40 2,257 Net current-period other comprehensive income (loss) 731 (4,665) 174 (3,760) At December 31, 2019 (1,430) (40,734) 62 (42,102) Other comprehensive loss before reclassifications — (8,395) (1) (8,396) Amounts reclassified from accumulated other comprehensive loss 446 1,922 52 2,420 Net current-period other comprehensive income (loss) 446 (6,473) 51 (5,976) At December 31, 2020 $ (984) $ (47,207) $ 113 $ (48,078)
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 for the years ended December 31 were as follows: 2020 2019 Location on Consolidated (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (591) $ (591) Interest expense 145 (140) Income taxes (446) (731) Amortization of postretirement liability losses included in net periodic benefit cost (2,552) (1,962) Other income 630 476 Income taxes (1,922) (1,486) Reclassification adjustment for loss on available-for-sale investments included in net income (66) (50) Other income 14 10 Income taxes (52) (40) Total reclassifications $ (2,420) $ (2,257)

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Components of income before income taxes	The components of income before income taxes from continuing operations for each of the years ended December 31 were as follows: 2020 2019 2018 (In thousands) United States $ 474,856 $ 398,532 $ 317,655 Foreign 261 (87) (784) Income before income taxes from continuing operations $ 475,117 $ 398,445 $ 316,871
Income tax expense	Income tax expense (benefit) from continuing operations for the years ended December 31 was as follows: 2020 2019 2018 (In thousands) Current: Federal $ 65,006 $ (3,502) $ (15,901) State 21,234 3,366 3,651 Foreign 151 — — 86,391 (136) (12,250) Deferred: Income taxes: Federal (3,735) 50,218 50,755 State (625) 12,098 7,206 Investment tax credit - net 2,559 1,099 1,774 (1,801) 63,415 59,735 Total income tax expense $ 84,590 $ 63,279 $ 47,485
Deferred tax assets and deferred tax liabilities	Components of deferred tax assets and deferred tax liabilities at December 31 were as follows: 2020 2019 (In thousands) Deferred tax assets: Postretirement $ 51,495 $ 51,075 Compensation-related 40,477 37,330 Operating lease liabilities 25,963 24,459 Payroll tax deferral 14,010 — Legal and environmental contingencies 9,467 6,601 Asset retirement obligations 8,060 7,450 Customer advances 7,463 7,325 Federal renewable energy credit — 5,343 Other 37,944 32,533 Total deferred tax assets 194,879 172,116 Deferred tax liabilities: Depreciation and basis differences on property, plant and equipment 536,966 511,867 Postretirement 49,233 48,927 Operating lease right-of-use-assets 25,858 24,436 Intangible asset amortization 19,514 18,930 Other 67,922 61,385 Total deferred tax liabilities 699,493 665,545 Valuation allowance 11,484 13,154 Net deferred income tax liability $ 516,098 $ 506,583
Schedule of change in net deferred income tax liability reconciliation	The following table reconciles the change in the net deferred income tax liability from December 31, 2019, to December 31, 2020, to deferred income tax benefit: 2020 (In thousands) Change in net deferred income tax liability from the preceding table $ 9,515 Deferred taxes associated with other comprehensive loss 1,817 Excess deferred income tax amortization (12,517) Other (616) Deferred income tax benefit for the period $ (1,801)
Reconciliation of income tax expense (benefit) at statutory federal rate versus actual rate	Total income tax expense differs from the amount computed by applying the statutory federal income tax rate to income before taxes. The reasons for this difference were as follows: Years ended December 31, 2020 2019 2018 Amount % Amount % Amount % (Dollars in thousands) Computed tax at federal statutory rate $ 99,775 21.0 $ 83,674 21.0 $ 66,543 21.0 Increases (reductions) resulting from: State income taxes, net of federal income tax 17,845 3.8 14,029 3.5 12,190 3.8 Federal renewable energy credit (16,009) (3.4) (15,843) (4.0) (11,759) (3.7) Tax compliance and uncertain tax positions (3,543) (.7) (2,739) (.7) (2,725) (.9) Excess deferred income tax amortization (12,517) (2.6) (11,904) (3.0) (9,319) (2.9) TCJA revaluation — — — — (5,947) (1.9) TCJA revaluation related to accumulated other comprehensive loss balance — — — — (42) — Other (961) (.3) (3,938) (.9) (1,456) (.4) Total income tax expense $ 84,590 17.8 $ 63,279 15.9 $ 47,485 15.0

Cash flow information (Tables)

Cash flow information (Tables)	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Information [Abstract]
Cash expenditures for interest and income taxes and noncash investing transactions	Cash expenditures for interest and income taxes for the years ended December 31 were as follows: 2020 2019 2018 (In thousands) Interest, net* $ 88,681 $ 93,414 $ 83,009 Income taxes paid (refunded), net** $ 65,536 $ (8,475) $ 16,041 * AFUDC - borrowed was $2.6 million, $2.8 million and $2.3 million for the years ended December 31, 2020, 2019 and 2018, respectively. ** Income taxes paid (refunded), including discontinued operations, were $59.4 million, $(9.4) million and $5.5 million for the years ended December 31, 2020, 2019 and 2018, respectively. Noncash investing and financing transactions at December 31 were as follows: 2020 2019 2018 (In thousands) Property, plant and equipment additions in accounts payable $ 26,082 $ 46,119 $ 42,355 Right-of-use assets obtained in exchange for new operating lease liabilities $ 54,356 $ 54,880 $ — Issuance of common stock in connection with acquisition $ — $ — $ 18,186 Debt assumed in connection with a business combination $ — $ 1,163 $ — Accrual for holdback payment related to a business combination $ 2,500 $ — $ —

Business segment data (Tables)

Business segment data (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Information on the Company's businesses	Information on the Company's segments as of December 31 and for the years then ended was as follows: 2020 2019 2018 (In thousands) External operating revenues: Regulated operations: Electric $ 331,538 $ 351,725 $ 335,123 Natural gas distribution 847,651 865,222 823,247 Pipeline 69,957 62,357 54,857 1,249,146 1,279,304 1,213,227 Non-regulated operations: Pipeline 15,389 21,835 23,161 Construction materials and contracting 2,177,585 2,189,651 1,925,185 Construction services 2,090,685 1,845,896 1,369,772 Other (55) 90 207 4,283,604 4,057,472 3,318,325 Total external operating revenues $ 5,532,750 $ 5,336,776 $ 4,531,552 2020 2019 2018 (In thousands) Intersegment operating revenues: Regulated operations: Electric $ 491 $ — $ — Natural gas distribution 534 — — Pipeline 57,977 56,037 50,580 59,002 56,037 50,580 Non-regulated operations: Pipeline 554 215 325 Construction materials and contracting 417 1,066 669 Construction services 5,038 3,370 1,681 Other 11,958 16,461 11,052 17,967 21,112 13,727 Intersegment eliminations (76,969) (77,149) (64,307) Total intersegment operating revenues $ — $ — $ — Depreciation, depletion and amortization: Electric $ 62,998 $ 58,721 $ 50,982 Natural gas distribution 84,580 79,564 72,486 Pipeline 21,669 21,220 17,896 Construction materials and contracting 89,626 77,450 61,158 Construction services 23,523 17,038 15,728 Other 2,704 2,024 1,955 Total depreciation, depletion and amortization $ 285,100 $ 256,017 $ 220,205 Operating income (loss): Electric $ 63,434 $ 64,039 $ 65,148 Natural gas distribution 73,082 69,188 72,336 Pipeline 49,436 42,796 36,128 Construction materials and contracting 214,498 179,955 141,426 Construction services 147,644 126,426 86,764 Other (3,169) (1,184) (79) Total operating income $ 544,925 $ 481,220 $ 401,723 Interest expense: Electric $ 26,699 $ 25,334 $ 25,860 Natural gas distribution 36,798 35,488 30,768 Pipeline 7,622 7,198 5,964 Construction materials and contracting 20,577 23,792 17,290 Construction services 4,095 5,331 3,551 Other 883 1,859 2,762 Intersegment eliminations (155) (415) (1,581) Total interest expense $ 96,519 $ 98,587 $ 84,614 Income tax expense (benefit): Electric $ (11,636) $ (12,650) $ (6,482) Natural gas distribution 5,746 1,405 4,075 Pipeline 7,650 7,219 2,677 Construction materials and contracting 47,431 37,389 28,357 Construction services 35,797 29,973 20,000 Other (398) (57) (1,142) Total income expense $ 84,590 $ 63,279 $ 47,485 2020 2019 2018 (In thousands) Net income (loss): Regulated operations: Electric $ 55,601 $ 54,763 $ 47,000 Natural gas distribution 44,049 39,517 37,732 Pipeline 35,453 28,255 26,905 135,103 122,535 111,637 Non-regulated operations: Pipeline 1,559 1,348 1,554 Construction materials and contracting 147,325 120,371 92,647 Construction services 109,721 92,998 64,309 Other (3,181) (2,086) (761) 255,424 212,631 157,749 Income from continuing operations 390,527 335,166 269,386 Income (loss) from discontinued operations, net of tax (322) 287 2,932 Net income $ 390,205 $ 335,453 $ 272,318 Capital expenditures: Electric $ 114,676 $ 99,449 $ 186,105 Natural gas distribution 193,048 206,799 205,896 Pipeline 62,224 71,477 70,057 Construction materials and contracting 191,635 190,092 280,396 Construction services 83,651 60,500 25,081 Other 3,045 8,181 1,768 Total capital expenditures (a) $ 648,279 $ 636,498 $ 769,303 Assets: Electric (b) $ 2,123,693 $ 1,680,194 $ 1,613,822 Natural gas distribution (b) 2,302,770 2,574,965 2,375,871 Pipeline 703,377 677,482 616,959 Construction materials and contracting 1,798,493 1,684,161 1,508,032 Construction services 818,662 761,127 604,798 Other (c) 305,157 303,279 266,111 Assets held for sale 1,220 1,851 2,517 Total assets $ 8,053,372 $ 7,683,059 $ 6,988,110 Property, plant and equipment: Electric (b) $ 2,323,403 $ 2,227,145 $ 2,148,569 Natural gas distribution (b) 2,868,853 2,688,123 2,499,093 Pipeline 821,697 834,215 764,959 Construction materials and contracting 2,028,476 1,910,562 1,768,006 Construction services 220,796 213,370 188,586 Other 37,545 35,213 28,108 Less accumulated depreciation, depletion and amortization 3,133,831 2,991,486 2,818,644 Net property, plant and equipment $ 5,166,939 $ 4,917,142 $ 4,578,677 (a) Capital expenditures for 2020, 2019 and 2018 include noncash transactions such as the issuance of the Company's equity securities in connection with acquisitions, capital expenditure-related accounts payable, AFUDC and accrual of holdback payments in connection with acquisitions totaling $(15.7) million, $4.8 million and $33.4 million, respectively. (b) Includes allocations of common utility property. (c) Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets).

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Schedule of amounts recognized in balance sheet	Changes in benefit obligation and plan assets and amounts recognized in the Consolidated Balance Sheets at December 31 were as follows: Pension Benefits Other 2020 2019 2020 2019 Change in benefit obligation: (In thousands) Benefit obligation at beginning of year $ 421,166 $ 391,602 $ 83,614 $ 81,201 Service cost — — 1,532 1,142 Interest cost 12,093 15,225 2,437 2,986 Plan participants' contributions — — 752 1,040 Actuarial loss 27,737 40,219 2,203 2,632 Benefits paid (23,636) (25,880) (4,383) (5,387) Benefit obligation at end of year 437,360 421,166 86,155 83,614 Change in net plan assets: Fair value of plan assets at beginning of year 365,264 307,809 94,587 82,516 Actual return on plan assets 42,206 58,409 10,249 15,731 Employer contribution — 24,926 434 687 Plan participants' contributions — — 752 1,040 Benefits paid (23,636) (25,880) (4,383) (5,387) Fair value of net plan assets at end of year 383,834 365,264 101,639 94,587 Funded status - over (under) $ (53,526) $ (55,902) $ 15,484 $ 10,973 Amounts recognized in the Consolidated Balance Sheets at December 31: Noncurrent assets - other $ — $ — $ 36,769 30,475 Other accrued liabilities — — 622 647 Noncurrent liabilities - other 53,526 55,902 20,663 18,855 Benefit obligation assets (liabilities) - net amount recognized $ (53,526) $ (55,902) $ 15,484 $ 10,973 Amounts recognized in accumulated other comprehensive loss: Actuarial loss $ 27,527 $ 27,748 $ 5,557 $ 6,118 Prior service credit — — (634) (731) Total $ 27,527 $ 27,748 $ 4,923 $ 5,387 Amounts recognized in regulatory assets or liabilities: Actuarial (gain) loss $ 154,013 $ 155,484 $ (8,228) $ (4,450) Prior service credit — — (6,808) (8,109) Total $ 154,013 $ 155,484 $ (15,036) $ (12,559)
The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets	The pension plans all have accumulated benefit obligations in excess of plan assets. The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for these plans at December 31 were as follows: 2020 2019 (In thousands) Projected benefit obligation $ 437,360 $ 421,166 Accumulated benefit obligation $ 437,360 $ 421,166 Fair value of plan assets $ 383,834 $ 365,264 The projected benefit obligation and accumulated benefit obligation for these plans at December 31 were as follows: 2020 2019 (In thousands) Projected benefit obligation $ 101,242 $ 99,245 Accumulated benefit obligation $ 101,242 $ 99,245
Components of net periodic benefit cost	The components of net periodic benefit cost (credit), other than the service cost component, are included in other income on the Consolidated Statements of Income. These components related to the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other 2020 2019 2018 2020 2019 2018 Components of net periodic benefit cost (credit): (In thousands) Service cost $ — $ — $ — $ 1,532 $ 1,142 $ 1,494 Interest cost 12,093 15,225 14,591 2,437 2,986 2,899 Expected return on assets (19,949) (18,236) (20,753) (5,019) (4,804) (4,866) Amortization of prior service credit — — — (1,398) (1,398) (1,394) Recognized net actuarial loss 7,172 5,548 7,005 287 353 640 Net periodic benefit cost (credit), including amount capitalized (684) 2,537 843 (2,161) (1,721) (1,227) Less amount capitalized — — — 156 113 153 Net periodic benefit cost (credit) (684) 2,537 843 (2,317) (1,834) (1,380) Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss: Net (gain) loss 934 (144) 991 (259) (127) (1,735) Amortization of actuarial loss (1,155) (904) (1,084) (306) (110) (354) Amortization of prior service (cost) credit — — — 101 100 (220) Total recognized in accumulated other comprehensive loss (221) (1,048) (93) (464) (137) (2,309) Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities: Net (gain) loss 4,546 189 8,263 (3,793) (8,168) (732) Amortization of actuarial gain (loss) (6,017) (4,644) (5,921) 19 (242) (286) Amortization of prior service credit — — — 1,297 1,297 1,614 Total recognized in regulatory assets or liabilities (1,471) (4,455) 2,342 (2,477) (7,113) 596 Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities $ (2,376) $ (2,966) $ 3,092 $ (5,258) $ (9,084) $ (3,093) The components of net periodic benefit cost are included in other income on the Consolidated Statements of Income. These components related to the Company's nonqualified defined benefit plans for the years ended December 31 were as follows: 2020 2019 2018 (In thousands) Components of net periodic benefit cost: Service cost $ 58 $ 109 $ 185 Interest cost 2,606 3,473 3,157 Recognized net actuarial loss 1,192 764 1,047 Net periodic benefit cost $ 3,856 $ 4,346 $ 4,389
Weighted average assumptions used to determine benefit obligations and net periodic benefit costs	Weighted average assumptions used to determine benefit obligations at December 31 were as follows: Pension Benefits Other 2020 2019 2020 2019 Discount rate 2.30 % 2.96 % 2.30 % 3.00 % Expected return on plan assets 6.00 % 6.25 % 5.50 % 5.75 % Rate of compensation increase N/A N/A 3.00 % 3.00 % Weighted average assumptions used to determine net periodic benefit cost (credit) for the years ended December 31 were as follows: Pension Benefits Other 2020 2019 2020 2019 Discount rate 2.96 % 4.03 % 3.00 % 4.05 % Expected return on plan assets 6.25 % 6.25 % 5.75 % 5.75 % Rate of compensation increase N/A N/A 3.00 % 3.00 % Weighted average assumptions used at December 31 were as follows: 2020 2019 Benefit obligation discount rate 1.97 % 2.73 % Benefit obligation rate of compensation increase N/A N/A Net periodic benefit cost discount rate 2.73 % 3.86 % Net periodic benefit cost rate of compensation increase N/A N/A
Health care rate assumptions for the Company's other postretirement benefit plans	Health care rate assumptions for the Company's other postretirement benefit plans as of December 31 were as follows: 2020 2019 Health care trend rate assumed for next year 7.0 % 7.1 % – 7.4 % Health care cost trend rate - ultimate 4.5 % 4.5 % Year in which ultimate trend rate achieved 2031 2024
Assumed health care cost trend rates	Assumed health care cost trend rates may have a significant effect on the amounts reported for the health care plans. A one percentage point change in the assumed health care cost trend rates would have had the following effects at December 31, 2020: 1 Percentage 1 Percentage (In thousands) Effect on total of service and interest cost components $ 182 $ (154) Effect on postretirement benefit obligation $ 3,846 $ (3,304)
Benefit payments expected to be paid	The following benefit payments, which reflect future service, as appropriate, and expected Medicare Part D subsidies at December 31, 2020, are as follows: Years Pension Other Expected (In thousands) 2021 $ 24,455 $ 5,404 $ 95 2022 24,507 5,437 89 2023 24,733 5,424 83 2024 24,835 5,377 75 2025 24,713 5,277 70 2026-2030 119,191 25,949 252 The amount of future benefit payments for the unfunded, nonqualified defined benefit plans at December 31, 2020, are expected to aggregate as follows: 2021 2022 2023 2024 2025 2026-2030 (In thousands) Nonqualified benefits $ 7,693 $ 6,957 $ 6,933 $ 7,299 $ 7,253 $ 33,938
The fair value of the pension and postretirement net plan assets by class	The fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2020 (In thousands) Assets: Cash equivalents $ — $ 7,841 $ — $ 7,841 Equity securities: U.S. companies 12,844 — — 12,844 International companies — 1,727 — 1,727 Collective and mutual funds* 177,397 55,788 — 233,185 Corporate bonds — 92,809 — 92,809 Municipal bonds — 10,126 — 10,126 U.S. Government securities 11,177 2,695 — 13,872 Investments measured at net asset value — — — 11,430 Total assets measured at fair value $ 201,418 $ 170,986 $ — $ 383,834 * Collective and mutual funds invest approximately 36 percent in corporate bonds, 24 percent in common stock of international companies, 18 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 5 percent in U.S. Government securities and 9 percent in other investments. Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2019 (In thousands) Assets: Cash equivalents $ — $ 26,166 $ — $ 26,166 Equity securities: U.S. companies 14,457 — — 14,457 International companies — 938 — 938 Collective and mutual funds* 160,906 58,894 — 219,800 Corporate bonds — 80,768 — 80,768 Municipal bonds — 11,828 — 11,828 U.S. Government securities 7,296 2,082 — 9,378 Total assets measured at fair value $ 182,659 $ 180,676 $ — $ 363,335 * Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in common stock of large-cap U.S. companies, 18 percent in U.S. Government securities, 9 percent in corporate bonds, 6 percent in cash equivalents and 17 percent in other investments. The fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2020 (In thousands) Assets: Cash equivalents $ — $ 3,517 $ — $ 3,517 Equity securities: U.S. companies 1,850 — — 1,850 International companies — 2 — 2 Collective and mutual funds (a) 10 147 — 157 Insurance contract (b) — 96,103 — 96,103 Investments measured at net asset value — — — 10 Total assets measured at fair value $ 1,860 $ 99,769 $ — $ 101,639 (a) Collective and mutual funds invest approximately 36 percent in corporate bonds, 24 percent in common stock of international companies, 18 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 5 percent in U.S. Government securities and 9 percent in other investments. (b) The insurance contract invests approximately 67 percent in corporate bonds, 12 percent in U.S. Government securities, 10 percent in common stock of large-cap U.S. companies, 4 percent in common stock of small-cap U.S. companies, 1 percent in cash equivalents and 6 percent in other investments. Fair Value Measurements Quoted Prices Significant Significant Balance at December 31, 2019 (In thousands) Assets: Cash equivalents $ — $ 4,017 $ — $ 4,017 Equity securities: U.S. companies 2,073 — — 2,073 International companies — 1 — 1 Collective and mutual funds (a) 10 221 — 231 Insurance contract (b) — 88,265 — 88,265 Total assets measured at fair value $ 2,083 $ 92,504 $ — $ 94,587 (a) Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in common stock of large-cap U.S. companies, 18 percent in U.S. Government securities, 9 percent in corporate bonds, 6 percent in cash equivalents and 17 percent in other investments. (b) The insurance contract invests approximately 50 percent in corporate bonds, 25 percent in common stock of large-cap U.S. companies, 7 percent in U.S. Government securities, 7 percent in common stock of small-cap U.S. companies and 11 percent in other investments. The amount of investments that the Company anticipates using to satisfy obligations under these plans at December 31 was as follows: 2020 2019 (In thousands) Investments Insurance contract* $ 100,104 $ 87,009 Life insurance** 39,779 38,659 Other 8,917 8,450 Total investments $ 148,800 $ 134,118 * For more information on the insurance contract, see Note 8. ** Investments of life insurance are carried on plan participants (payable upon the employee's death).
Schedule of Multiemployer Plans [Table Text Block]	EIN/Pension Plan Number Pension Protection Act Zone Status FIP/RP Status Pending/Implemented Contributions Surcharge Imposed Expiration Date Pension Fund 2020 2019 2020 2019 2018 (In thousands) Alaska Laborers-Employers Retirement Fund 91-6028298-001 Yellow as of 6/30/2020 Yellow as of 6/30/2019 Implemented $ 828 $ 815 $ 732 No 12/31/2020 * Construction Industry and Laborers Joint Pension Trust for So Nevada, Plan A 88-0135695-001 Red Red Implemented 515 544 346 No 6/30/2023 Edison Pension Plan 93-6061681-001 Green Green No 16,121 12,252 12,111 No 12/31/2021 IBEW Local 212 Pension Trust 31-6127280-001 Green as of 4/30/2020 Green as of 4/30/2019 No 1,521 1,110 1,341 No 6/1/2025 IBEW Local 357 Pension Plan A 88-6023284-001 Green Green No 9,913 10,162 3,460 No 5/31/2021 IBEW Local 38 Pension Plan 34-6574238-001 Red as of 4/30/2020 Yellow as of 4/30/2019 Implemented 140 158 116 No 4/23/2023 IBEW Local 648 Pension Plan 31-6134845-001 Yellow as of 2/28/2020 Yellow as of 2/28/2019 Implemented 526 728 2,175 No 8/29/2021 IBEW Local 82 Pension Plan 31-6127268-001 Green as of 6/30/2020 Green as of 6/30/2019 No 1,373 1,662 1,569 No 12/3/2023 Idaho Plumbers and Pipefitters Pension Plan 82-6010346-001 Green as of 5/31/2020 Green as of 5/31/2019 No 1,370 1,307 1,247 No 3/31/2023 Minnesota Teamsters Construction Division Pension Fund 41-6187751-001 Green as of 11/30/2019 Green as of 11/30/2018 No 663 673 740 No 4/30/2021 National Automatic Sprinkler Industry Pension Fund 52-6054620-001 Red Red Implemented 954 1,074 738 No 3/31/2021- 7/31/2024 National Electrical Benefit Fund 53-0181657-001 Green Green No 14,484 12,679 8,468 No 12/31/2021- 8/30/2025 Pension Trust Fund for Operating Engineers 94-6090764-001 Yellow Yellow Implemented 2,680 2,598 2,403 No 6/15/2022- 6/30/2023 Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV 95-6052257-001 Yellow Yellow Implemented 3,255 2,119 1,774 No 6/30/2024 Southwest Marine Pension Trust 95-6123404-001 Red Red Implemented 170 132 81 No 1/31/2024 Other funds 30,931 24,512 21,421 Total contributions $ 85,444 $ 72,525 $ 58,722 * Plan includes contributions required by collective bargaining agreements which have expired, but contain provisions automatically renewing their terms in the absence of a subsequent negotiated agreement. The Company was listed in the plans' Forms 5500 as providing more than 5 percent of the total contributions for the following plans and plan years: Pension Fund Year Contributions to Plan Exceeded More Than 5 Percent Edison Pension Plan 2019 and 2018 IBEW Local 82 Pension Plan 2019 and 2018 IBEW Local 124 Pension Trust Fund 2019 and 2018 IBEW Local 212 Pension Trust Fund 2019 and 2018 IBEW Local 357 Pension Plan A 2019 and 2018 IBEW Local 648 Pension Plan 2019 and 2018 IBEW Local 683 Pension Fund Pension Plan 2019 IBEW Local Union No 226 Open End Pension Fund 2019 and 2018 Idaho Plumbers and Pipefitters Pension Plan 2019 and 2018 International Union of Operating Engineers Local 701 Pension Trust Fund 2019 and 2018 Minnesota Teamsters Construction Division Pension Fund 2019 and 2018 Pension and Retirement Plan of Plumbers and Pipefitters Local 525 2019 and 2018

Jointly owned facilities (Table

Jointly owned facilities (Tables)	12 Months Ended
	Dec. 31, 2020
Regulated Operations [Abstract]
Company's share of the cost of utility plant in service and related accumulated depreciation for the stations	At December 31, the Company's share of the cost of utility plant in service, construction work in progress and related accumulated depreciation for the jointly owned facilities was as follows: Ownership Percentage 2020 2019 (In thousands) Big Stone Station: 22.7 % Utility plant in service $ 155,967 $ 152,836 Construction work in progress 104 518 Less accumulated depreciation 45,435 46,266 $ 110,636 $ 107,088 BSSE: 50.0 % Utility plant in service $ 107,442 $ 105,767 Construction work in progress — — Less accumulated depreciation 2,682 1,232 $ 104,760 $ 104,535 Coyote Station: 25.0 % Utility plant in service $ 159,784 $ 160,235 Construction work in progress 323 21 Less accumulated depreciation 108,852 107,638 $ 51,255 $ 52,618 Wygen III: 25.0 % Utility plant in service $ 66,101 $ 67,869 Construction work in progress 232 112 Less accumulated depreciation 10,038 10,482 $ 56,295 $ 57,499

Commitment and Contingencies Di

Commitment and Contingencies Disclosure (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Purchase commitments	The commitment terms vary in length, up to 39 years. The commitments under these contracts as of December 31, 2020, were: 2021 2022 2023 2024 2025 Thereafter (In thousands) Purchase commitments $ 458,397 $ 235,858 $ 173,306 $ 124,724 $ 98,894 $ 691,613

Quarterly Data (Tables)

Quarterly Data (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly financial data	The following unaudited information shows selected items by quarter for the years 2020 and 2019: First Quarter Second Third Fourth (In thousands, except per share amounts) 2020 Operating revenues $ 1,197,373 $ 1,362,928 $ 1,587,289 $ 1,385,160 Operating expenses 1,140,303 1,224,674 1,383,578 1,239,270 Operating income 57,070 138,254 203,711 145,890 Income from continuing operations 25,539 99,842 153,015 112,131 Income (loss) from discontinued operations, net of tax (409) (139) 63 163 Net income 25,130 99,703 153,078 112,294 Earnings per share - basic: Income from continuing operations .13 .50 .76 .56 Discontinued operations, net of tax — — — — Earnings per share - basic .13 .50 .76 .56 Earnings per share - diluted: Income from continuing operations .13 .50 .76 .56 Discontinued operations, net of tax — — — — Earnings per share - diluted .13 .50 .76 .56 Weighted average common shares outstanding: Basic 200,440 200,522 200,522 200,522 Diluted 200,456 200,539 200,619 200,923 2019 Operating revenues $ 1,091,191 $ 1,303,573 $ 1,563,799 $ 1,378,213 Operating expenses 1,026,973 1,206,262 1,374,329 1,247,992 Operating income 64,218 97,311 189,470 130,221 Income from continuing operations 41,089 63,145 136,128 94,804 Income (loss) from discontinued operations, net of tax (163) (1,320) 1,509 261 Net income 40,926 61,825 137,637 95,065 Earnings per share - basic: Income from continuing operations .21 .32 .68 .47 Discontinued operations, net of tax — (.01) .01 — Earnings per share - basic .21 .31 .69 .47 Earnings per share - diluted: Income from continuing operations .21 .32 .68 .47 Discontinued operations, net of tax — (.01) .01 — Earnings per share - diluted .21 .31 .69 .47 Weighted average common shares outstanding: Basic 196,401 198,270 199,343 200,383 Diluted 196,414 198,287 199,383 200,478

Basis of Presentation (Details)

Basis of Presentation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Receivables, net	873,986				836,605				873,986	836,605
Operating expenses	1,239,270	1,383,578	1,224,674	1,140,303	1,247,992	1,374,329	1,206,262	1,026,973	4,987,825	4,855,556	4,129,829
Net income	112,294	$ 153,078	$ 99,703	25,130	95,065	$ 137,637	$ 61,825	$ 40,926	390,205	335,453	$ 272,318
Accumulated other comprehensive loss	(48,078)				(42,102)				(48,078)	(42,102)
Regulatory liabilities	428,075				$ 447,370				428,075	$ 447,370
Revision of Prior Period, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenues				(7,700)
Receivables, net				(7,700)
Operating expenses				1,200
Accounts Payable				1,200
Net income	4,400			$ (6,700)
Other Income	6,500
Accumulated other comprehensive loss	(2,700)								(2,700)
Regulatory liabilities	$ (3,800)								$ (3,800)

Accounting Policies - Receivabl

Accounting Policies - Receivables Past Due 90 Days (Details 1) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Accounts Receivable, Noncurrent, 90 Days or More Past Due, Still Accruing	$ 43.9	$ 46.7

Accounting Policies - CECL Tabl

Accounting Policies - CECL Table (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for doubtful accounts	$ 15,358	$ 8,497
Accounts Receivable, Credit Loss Expense (Reversal)	10,985
Accounts Receivable, Allowance for Credit Loss, Writeoff	5,306
Accounts Receivable, Allowance for Credit Loss, Recovery	1,182
Electric
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for doubtful accounts	899	328
Accounts Receivable, Credit Loss Expense (Reversal)	1,517
Accounts Receivable, Allowance for Credit Loss, Writeoff	1,289
Accounts Receivable, Allowance for Credit Loss, Recovery	343
Natural gas distribution
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for doubtful accounts	2,571	1,056
Accounts Receivable, Credit Loss Expense (Reversal)	3,187
Accounts Receivable, Allowance for Credit Loss, Writeoff	2,511
Accounts Receivable, Allowance for Credit Loss, Recovery	839
Pipeline
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for doubtful accounts	2	0
Accounts Receivable, Credit Loss Expense (Reversal)	2
Accounts Receivable, Allowance for Credit Loss, Writeoff	0
Accounts Receivable, Allowance for Credit Loss, Recovery	0
Construction materials and contracting
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for doubtful accounts	6,164	5,357
Accounts Receivable, Credit Loss Expense (Reversal)	1,447
Accounts Receivable, Allowance for Credit Loss, Writeoff	640
Accounts Receivable, Allowance for Credit Loss, Recovery	0
Construction services
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for doubtful accounts	5,722	$ 1,756
Accounts Receivable, Credit Loss Expense (Reversal)	4,832
Accounts Receivable, Allowance for Credit Loss, Writeoff	866
Accounts Receivable, Allowance for Credit Loss, Recovery	$ 0

Accounting Policies - Accrued U

Accounting Policies - Accrued Unbilled Revenue (Details 3) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Accrued unbilled revenue at MDU Energy Capital	$ 94	$ 100.8

Accounting Policies - Retainage

Accounting Policies - Retainage (Details 4) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Contract Receivable Retainage, Next Twelve Months	$ 100,054	$ 75,590
Construction Contractor, Receivable, Retainage, after Year One	2,761	14,228
Total Retainage	$ 102,815	$ 89,818

Accounting Policies - Inventory

Accounting Policies - Inventory (Details 5) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Inventory and natural gas in storage [Line Items]
Aggregates held for resale	$ 175,782	$ 147,723
Asphalt oil	28,238	41,912
Materials and supplies	25,142	22,512
Merchandise for resale	21,087	22,232
Natural gas in storage (current)	21,919	22,058
Other	18,999	21,970
Total	291,167	278,407
Natural gas in storage (noncurrent)	$ 47,500	$ 48,400

Accounting Policies - PPE (Deta

Accounting Policies - PPE (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, plant and equipment [Line Items]
Allowance for Funds Used During Construction, Capitalized Interest	$ 2,640	$ 2,807	$ 2,290
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	$ 1,270	$ 698	$ 1,897

Accounting Policies - Impairmen

Accounting Policies - Impairment of Long-Lived Assets (Details 7) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Impairment of Long-Lived Assets to be Disposed of	$ 0	$ 0	$ 0

Natural Gas Costs Recoverable o

Natural Gas Costs Recoverable or Refundable through Rate Adjustments (Details 8) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Natural gas costs refundable	$ 18.6	$ 23.8
Natural gas costs recoverable	$ 64	$ 89.2

Accounting Policies - Goodwill

Accounting Policies - Goodwill (Details 9) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Goodwill, Impairment Loss	$ 0	$ 0	$ 0

Accounting Policies - Derivativ

Accounting Policies - Derivatives (Details 10) - MMBTU	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Commodity Contract [Member] | Not Designated as Hedging Instrument, Economic Hedge [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	1,400,000	535,000

Accounting Policies - EPS (Deta

Accounting Policies - EPS (Details 11) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Weighted average common shares outstanding - basic	200,522	200,522	200,522	200,440	200,383	199,343	198,270	196,401	200,502	198,612	195,720
Effect of dilutive performance share awards									69	14	430
Weighted average common shares outstanding - diluted	200,923	200,619	200,539	200,456	200,478	199,383	198,287	196,414	200,571	198,626	196,150
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									164	164	10

Accounting Policies - Income Ta

Accounting Policies - Income Taxes (Details 12)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Threshold of likelihood of tax positions being realized upon ultimate settlement with a taxing authority	50.00%

Disaggregation of revenue (Deta

Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(76,969)	(77,149)	(64,307)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(550,815)	(525,749)	(465,969)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									157,232	147,114	132,397
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									4,865	9,164	9,159
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									14,918	11,708	11,543
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									1,069,665	1,088,633	968,755
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,659,152	1,627,833	1,423,068
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,397,124	1,266,196	926,875
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									649,486	531,882	392,544
Other
Disaggregation of Revenue [Line Items]
Operating revenues									68,865	82,669	76,796
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									5,471,545	5,281,621	4,469,761
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									61,205	55,155	61,791
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									599,051	608,821	579,436
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									409,350	438,431	412,952
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									62,987	64,660	58,956
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									6,634	7,408	7,556
Electric
Disaggregation of Revenue [Line Items]
Operating revenues									331,538	351,725	335,123
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(491)	0	0
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues									32,452	35,574	31,568
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									329,479	347,712	331,190
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									2,059	4,013	3,933
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									122,663	125,369	121,477
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									131,477	141,596	136,236
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									36,744	37,765	34,353
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									6,634	7,408	7,556
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues									847,651	865,222	823,247
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(534)	0	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									45,546	45,449	43,238
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues									10,753	12,726	14,579
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									836,269	865,357	817,095
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									11,382	(135)	6,152
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									476,388	483,452	457,959
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									277,873	296,835	276,716
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									26,243	26,895	24,603
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Operating revenues									85,346	84,192	78,018
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(58,531)	(56,252)	(50,905)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									111,686	101,665	89,159
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									4,865	9,164	9,159
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									14,918	11,708	11,543
Pipeline | Other
Disaggregation of Revenue [Line Items]
Operating revenues									12,216	17,687	18,865
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									85,154	83,972	77,821
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									192	220	197
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues									2,177,585	2,189,651	1,925,185
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(417)	(1,066)	(669)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(550,815)	(525,749)	(465,969)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									1,069,665	1,088,633	968,755
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,659,152	1,627,833	1,423,068
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									2,177,585	2,189,651	1,925,185
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues									2,090,685	1,845,896	1,369,772
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(5,038)	(3,370)	(1,681)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,397,124	1,266,196	926,875
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									649,486	531,882	392,544
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues									1,541	131	525
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									2,043,113	1,794,839	1,318,263
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									47,572	51,057	51,509
Other
Disaggregation of Revenue [Line Items]
Operating revenues									(55)	90	207
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(11,958)	(16,461)	(11,052)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues									11,903	16,551	11,259
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									(55)	90	207
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									$ 0	$ 0	$ 0

Contract balances (Details 2)

Contract balances (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Contract assets	$ 104,345	$ 109,078	$ 104,239
Change in contract assets	(4,733)	4,839
Contract liabilities - current	(158,603)	(142,768)	(93,901)
Change in contract liabilities - current	(15,835)	(48,867)
Contract liabilities - noncurrent	(52)	(19)	(135)
Change in contract liabilities - noncurrent	(33)	116
Net contract assets (liabilities)	(54,310)	(33,709)	$ 10,203
Change in net contract assets (liabilities)	(20,601)	(43,912)
Amounts included in contract liability at the beginning of the period	138,200	89,000
Amounts from performance obligations satisfied in prior periods	$ 58,800	$ 44,100

Revenue from contracts with c_2

Revenue from contracts with customers Remaining performance obligations (Details 3) $ in Millions	Dec. 31, 2020USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, amount	$ 2,100
Remaining performance obligation, expected timing of satisfaction, start date: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, amount	$ 1,600
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	12 months
Remaining performance obligation, expected timing of satisfaction, start date: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, amount	$ 298.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	13 months
Remaining performance obligation, expected timing of satisfaction, start date: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, amount	$ 234.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	25 months

Business Combinations (Details)

Business Combinations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Goodwill	$ 714,963,000	$ 681,358,000	$ 664,922,000
Asset Retirement Obligation	446,919,000	$ 417,575,000	$ 375,553,000
2020 Acquisition [Member]
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Gross Aggregate Consideration	110,200,000
Cash Assumed	1,700,000
Business Combination, Consideration Transferred	106,000,000
Business Combination, Indemnity Holdback Liability	2,500,000
Business Combination, Current Assets	54,800,000
Business Combination, Property, Plant, and Equipment	27,100,000
Goodwill	33,600,000
Business Combination, Other Intangible Assets	19,000,000
Business Combination, Current Liabilities	22,600,000
Business Combination, Noncurrent Liabilities	300,000
Asset Retirement Obligation	$ 1,400,000

Business Combinations - Assets

Business Combinations - Assets Acquired & Liabilities Assumed (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 681,358,000	$ 714,963,000	$ 664,922,000
2019 Acquisition
Business Acquisition [Line Items]
Gross Aggregate Consideration	56,800,000
Business Combination, Long-Term Debt Assumed	1,200,000
Business Combination, Current Assets	15,800,000
Business Combination, Property, Plant, and Equipment	16,700,000
Goodwill	23,100,000
Business Combination, Other Intangible Assets	6,700,000
Deferred charges and other assets - other	500,000
Business Combination, Current Liabilities	5,900,000
Business Combination, Noncurrent Liabilities	$ 100,000

Business Combinations Measureme

Business Combinations Measurement Period Adjustments (Details 3) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Business Combinations [Abstract]
Goodwill, Purchase Accounting Adjustments	$ 391,000	$ (6,669,000)

Property, Plant, and Equipmen_2

Property, Plant, and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 8,300,770	$ 7,908,628
Less accumulated depreciation, depletion and amortization	3,133,831	2,991,486	$ 2,818,644
Net property, plant and equipment	5,166,939	4,917,142	4,578,677
Electric
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	2,323,403	2,227,145	2,148,569
Natural gas distribution
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	2,868,853	2,688,123	2,499,093
Pipeline
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	821,697	834,215	764,959
Construction materials and contracting
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	2,028,476	1,910,562	1,768,006
Construction services
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	220,796	213,370	188,586
Other
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	37,545	35,213	$ 28,108
Regulated Operation | Electric | Electric Generation Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,133,390	1,139,059
Weighted average depreciable life in years	48 years
Regulated Operation | Electric | Electric Distribution
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 464,442	443,780
Weighted average depreciable life in years	46 years
Regulated Operation | Electric | Electric Transmission
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 524,155	445,485
Weighted average depreciable life in years	64 years
Regulated Operation | Electric | Construction in Progress
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 61,766	66,664
Regulated Operation | Electric | Other Capitalized Property Plant and Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 139,650	132,157
Weighted average depreciable life in years	14 years
Regulated Operation | Natural gas distribution | Construction in Progress
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 16,836	39,506
Regulated Operation | Natural gas distribution | Other Capitalized Property Plant and Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 213,976	188,037
Weighted average depreciable life in years	16 years
Regulated Operation | Natural gas distribution | Gas Distribution [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 2,302,121	2,133,249
Weighted average depreciable life in years	47 years
Regulated Operation | Natural gas distribution | Gas Transmission [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 104,695	104,401
Weighted average depreciable life in years	51 years
Regulated Operation | Natural gas distribution | Natural gas storage
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 33,014	31,484
Weighted average depreciable life in years	24 years
Regulated Operation | Natural gas distribution | General
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 198,211	191,446
Weighted average depreciable life in years	14 years
Regulated Operation | Pipeline | Construction in Progress
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 46,690	22,597
Regulated Operation | Pipeline | Other Capitalized Property Plant and Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 49,640	48,340
Weighted average depreciable life in years	17 years
Regulated Operation | Pipeline | Gas Transmission [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 665,567	636,796
Weighted average depreciable life in years	46 years
Regulated Operation | Pipeline | Gathering
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 0	35,661
Regulated Operation | Pipeline | Gas Storage [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 52,632	50,001
Weighted average depreciable life in years	53 years
Unregulated Operation | Pipeline | Construction in Progress
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 4	154
Unregulated Operation | Pipeline | Other Capitalized Property Plant and Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 7,164	9,518
Weighted average depreciable life in years	11 years
Unregulated Operation | Pipeline | Gathering and processing
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 0	31,148
Unregulated Operation | Construction materials and contracting | Construction in Progress
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	23,803	25,018
Unregulated Operation | Construction materials and contracting | Land [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	132,948	127,729
Unregulated Operation | Construction materials and contracting | Building and Building Improvements [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 130,417	122,064
Weighted average depreciable life in years	21 years
Unregulated Operation | Construction materials and contracting | Machinery and Equipment [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,284,604	1,180,343
Weighted average depreciable life in years	12 years
Unregulated Operation | Construction materials and contracting | Aggregate reserves [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 456,704	455,408
Unregulated Operation | Construction services | Other Capitalized Property Plant and Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 8,824	17,952
Weighted average depreciable life in years	4 years
Unregulated Operation | Construction services | Land [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 7,218	7,146
Unregulated Operation | Construction services | Building and Building Improvements [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 41,674	31,735
Weighted average depreciable life in years	25 years
Unregulated Operation | Construction services | Machinery and Equipment [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 163,080	156,537
Weighted average depreciable life in years	7 years
Unregulated Operation | Other | Other Capitalized Property Plant and Equipment
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 34,897	32,565
Weighted average depreciable life in years	12 years
Unregulated Operation | Other | Land [Member]
Property, plant and equipment [Line Items]
Property, Plant and Equipment, Gross	$ 2,648	$ 2,648

Regulatory assets and liabili_3

Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory assets and liabilities
Current regulatory assets	$ 68,527	$ 63,613
Regulatory assets	379,381	353,784
Regulatory assets:	447,908	417,397
Regulatory liabilities due within one year	31,450	42,935
Regulatory liabilities	428,075	447,370
Regulatory liabilities:	459,525	490,305
Net regulatory position	(11,617)	(72,908)
Regulatory assets not earning a rate of return	$ 332,500	276,500
Natural gas costs refundable through rate adjustments
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Regulatory liabilities due within one year	$ 18,565	23,825
Liabilities Electric fuel and purchased power deferral
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Regulatory liabilities due within one year	$ 3,667	5,824
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Regulatory liabilities due within one year	3,557	3,472
Regulatory liabilities	$ 227,850	246,034
Taxes recoverable from/refundable to customers | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Taxes recoverable from/refundable to customers | Maximum
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Plants to be retired
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory liabilities	$ 167,171	173,722
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	**
Regulatory liabilities	$ 16,989	18,065
Other regulatory assets/liabilities
Regulatory assets and liabilities
Regulatory liabilities due within one year	5,661	9,814
Regulatory liabilities	$ 16,065	9,549
Other regulatory assets/liabilities | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Other regulatory assets/liabilities | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 21 years
Natural gas costs recoverable through rate adjustments
Regulatory assets and liabilities
Current regulatory assets	$ 42,481	42,823
Regulatory assets	$ 21,539	46,381
Natural gas costs recoverable through rate adjustments | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Natural gas costs recoverable through rate adjustments | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 2 years
Cost recovery mechanisms
Regulatory assets and liabilities
Current regulatory assets	$ 10,645	6,288
Regulatory assets	$ 16,245	13,108
Cost recovery mechanisms | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Cost recovery mechanisms | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 10 years
Conservation programs
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Current regulatory assets	$ 7,117	6,963
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	**
Regulatory assets	$ 155,942	157,069
Asset retirement obligation costs
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets	$ 71,740	66,000
Plants to be retired
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets	$ 65,919	32,931
Manufactured gas plant sites remediation
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets	$ 26,429	15,126
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets	$ 10,785	11,486
Long-term debt refinancing costs
Regulatory assets and liabilities
Estimated recovery period	Up to 40 years
Regulatory assets	$ 4,426	4,286
Other regulatory assets/liabilities
Regulatory assets and liabilities
Current regulatory assets	8,284	7,539
Regulatory assets	$ 6,356	$ 7,397
Other regulatory assets/liabilities | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Other regulatory assets/liabilities | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 18 years

Goodwill rollforward (Details)

Goodwill rollforward (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Balance at beginning of period	$ 681,358,000	$ 664,922,000
Goodwill acquired during the year	33,214,000	23,105,000
Goodwill, Purchase Accounting Adjustments	391,000	(6,669,000)
Balance at end of period	714,963,000	681,358,000
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736,000	345,736,000
Goodwill acquired during the year	0	0
Goodwill, Purchase Accounting Adjustments	0	0
Balance at end of period	345,736,000	345,736,000
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	217,234,000	209,421,000
Goodwill acquired during the year	8,778,000	14,482,000
Goodwill, Purchase Accounting Adjustments	(9,000)	(6,669,000)
Balance at end of period	226,003,000	217,234,000
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	118,388,000	109,765,000
Goodwill acquired during the year	24,436,000	8,623,000
Goodwill, Purchase Accounting Adjustments	400,000	0
Balance at end of period	$ 143,224,000	$ 118,388,000

Other intangible assets (Detail

Other intangible assets (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 25,496	$ 15,246
Amortization of intangible assets	9,000	2,400	$ 1,200
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	28,836	17,958
Intangible assets, less accumulated amortization	6,887	6,268
Intangible assets, net (excluding goodwill)	21,949	11,690
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	3,941	3,439
Intangible assets, less accumulated amortization	2,309	1,957
Intangible assets, net (excluding goodwill)	1,632	1,482
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	12,927	8,094
Intangible assets, less accumulated amortization	11,012	6,020
Intangible assets, net (excluding goodwill)	$ 1,915	$ 2,074

Goodwill and other intangible_3

Goodwill and other intangible assets Future amortization expense (Details 3) $ in Thousands	Dec. 31, 2020USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2021	$ 4,911
2022	4,394
2023	4,121
2024	3,799
2025	1,862
Thereafter	$ 6,409

Fair value measurements insuran

Fair value measurements insurance contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 100.1	$ 87
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans	$ 13.1	$ 13.2	$ (3.6)

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale securities [Line Items]
Cost	$ 11,185	$ 11,032
Gross Unrealized Gains	156	88
Gross Unrealized Losses	14	10
Fair Value	11,327	11,110
Mortgage-backed securities
Available-for-sale securities [Line Items]
Cost	9,799	9,804
Gross Unrealized Gains	156	87
Gross Unrealized Losses	9	10
Fair Value	9,946	9,881
U.S. Treasury securities
Available-for-sale securities [Line Items]
Cost	1,386	1,228
Gross Unrealized Gains	0	1
Gross Unrealized Losses	5	0
Fair Value	$ 1,381	$ 1,229

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Concentration risks, percentage [Abstract]
Percentage in fixed-income investments	57.00%	51.00%
Percentage investment in common stock of large-cap companies	18.00%	23.00%
Percentage investment in common stock of mid-cap companies	9.00%	12.00%
Percentage investment in common stock of small-cap companies	9.00%	10.00%
Percentage investment in target date investments	5.00%	3.00%
Percentage investment in cash and cash equivalents	2.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 120,348	$ 106,559
Fair value, measurements, recurring | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	8,917	8,440
Fair value, measurements, recurring | Insurance contract*
Fair value measurements [Line Items]
Assets, fair value disclosure	100,104	87,009
Fair value, measurements, recurring | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	9,946	9,881
Fair value, measurements, recurring | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	1,381	1,229
Fair value, measurements, recurring | Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	120,348	106,559
Fair value, measurements, recurring | Fair value, inputs, level 2 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	8,917	8,440
Fair value, measurements, recurring | Fair value, inputs, level 2 | Insurance contract*
Fair value measurements [Line Items]
Assets, fair value disclosure	100,104	87,009
Fair value, measurements, recurring | Fair value, inputs, level 2 | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	9,946	9,881
Fair value, measurements, recurring | Fair value, inputs, level 2 | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 1,381	$ 1,229

Fair value measurements Fair va

Fair value measurements Fair value measurements (Details 4)	3 Months Ended
	Jun. 30, 2019USD ($)
Fair Value Disclosures [Abstract]
Cost Method Investment Fair Value	$ 0
Asset Impairment Charges	$ 2,000,000

Fair value measurements (Deta_2

Fair value measurements (Details 5) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 2,213,130	$ 2,243,107
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	2,213,130	2,243,107
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 2,537,289	$ 2,418,631

Credit facilities (Details)

Credit facilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Line of credit facility and other debt [Abstract]
Letters of credit at end of period	$ 0
Long-term debt | Commercial paper revolving credit agreement | Montana-Dakota Utilities Co. [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	175,000,000
Amount outstanding, end of period	87,700,000	$ 118,600,000
Letters of credit at end of period	$ 0
Expiration date	Dec. 19, 2024
Long-term debt | Commercial paper revolving credit agreement | Centennial Energy Holdings, Inc.
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	$ 600,000,000
Amount outstanding, end of period	37,900,000	104,300,000
Letters of credit at end of period	$ 0
Expiration date	Dec. 19, 2024
Long-term debt | Revolving credit facility | Montana-Dakota Utilities Co. [Member]
Line of credit facility and other debt [Abstract]
Amount outstanding, end of period	$ 0
Option to increase borrowings, maximum amount	225,000,000
Long-term debt | Revolving credit facility | Cascade Natural Gas Corporation
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	100,000,000
Amount outstanding, end of period	54,000,000	64,600,000
Letters of credit at end of period	$ 2,200,000
Expiration date	Jun. 7, 2024
Option to increase borrowings, maximum amount	$ 125,000,000
Long-term debt | Revolving credit facility | Intermountain Gas Company
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	85,000,000
Amount outstanding, end of period	41,900,000	$ 24,500,000
Letters of credit at end of period	$ 0
Expiration date	Jun. 7, 2024
Option to increase borrowings, maximum amount	$ 110,000,000
Long-term debt | Revolving credit facility | Centennial Energy Holdings, Inc.
Line of credit facility and other debt [Abstract]
Amount outstanding, end of period	0
Option to increase borrowings, maximum amount	$ 700,000,000

Short-term Debt (Details 2)

Short-term Debt (Details 2) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term borrowings	$ 50,000	$ 0
Montana-Dakota Utilities Co. [Member]
Short-term Debt [Line Items]
Short-term borrowings	50,000
Term Loan Agreement [Member] | Montana-Dakota Utilities Co. [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 75,000
Ratio of total debt to total capitalization as specified in debt covenants	65.00%

Long-term debt outstanding (Det

Long-term debt outstanding (Details 3) - USD ($) $ in Thousands	Dec. 31, 2020	Oct. 30, 2020	Jun. 15, 2020	Dec. 31, 2019
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,213,130			$ 2,243,107
Long-term debt due within one year	1,555			16,540
Long-term debt	$ 2,211,575			2,226,567
Senior Notes
Long-term debt outstanding [Line Items]
Weighted average interest rate	4.40%
Long-term debt	$ 1,950,000			1,850,000
Senior Notes | Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Weighted average interest rate		3.34%	3.66%
Long-term debt		$ 25,000	$ 50,000
Commercial paper
Long-term debt outstanding [Line Items]
Weighted average interest rate	0.28%
Long-term debt	$ 125,600			222,900
Term Loan Agreements
Long-term debt outstanding [Line Items]
Weighted average interest rate	2.00%
Long-term debt	$ 8,400			9,100
Credit Agreements
Long-term debt outstanding [Line Items]
Weighted average interest rate	1.88%
Long-term debt	$ 95,900			89,050
Medium-term notes
Long-term debt outstanding [Line Items]
Weighted average interest rate	7.32%
Long-term debt	$ 35,000			50,000
Other notes
Long-term debt outstanding [Line Items]
Weighted average interest rate	0.75%
Long-term debt	$ 4,034			29,117
Long-term debt
Long-term debt outstanding [Line Items]
Unamortized debt issuance costs	5,803			7,010
Discount	$ 1			$ 50

Long-term borrowings (Details 4

Long-term borrowings (Details 4) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 16, 2020	Oct. 30, 2020	Jun. 15, 2020	Dec. 31, 2019	Jul. 26, 2019
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,213,130				$ 2,243,107
Montana-Dakota Utilities Co. [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	49.00%
Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	53.00%
Intermountain Gas Company
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	51.00%
Centennial Energy Holdings, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	31.00%
WBI Energy Transmission, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	41.00%
Senior Notes
Long-term debt outstanding [Line Items]
Long-term debt	$ 1,950,000				$ 1,850,000
Weighted average interest rate	4.40%
Senior Notes | Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Long-term debt			$ 25,000	$ 50,000
Weighted average interest rate			3.34%	3.66%
Ratio of total debt to total capitalization as specified in debt covenants			65.00%	65.00%
Senior Notes | WBI Energy Transmission, Inc.
Long-term debt outstanding [Line Items]
Long-term debt	$ 195,000	$ 25,000
Weighted average interest rate		3.26%
Ratio of total debt to total capitalization as specified in debt covenants	55.00%
Long-term private shelf agreement issuance capacity						$ 300,000
Remaining capacity under uncommitted private shelf agreement	$ 105,000
Revolving credit facility | Montana-Dakota Utilities Co. [Member]
Long-term debt outstanding [Line Items]
Ratio of funded debt to total capitalization as specified in debt covenants	65.00%
Revolving credit facility | Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Revolving credit facility | Intermountain Gas Company
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Revolving credit facility | Centennial Energy Holdings, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%

Schedule of debt maturities (De

Schedule of debt maturities (Details 5) $ in Thousands	Dec. 31, 2020USD ($)
Long-term debt maturities [Line Items]
2021	$ 1,555
2022	148,021
2023	77,921
2024	282,922
2025	177,802
Thereafter	$ 1,530,713

Lease costs (Details)

Lease costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 45,319	$ 43,759
Variable lease cost	1,319	1,555
Short-term lease cost	135,376	120,030
Total lease costs	$ 182,014	$ 165,344
Weighted average remaining lease term	2 years 8 months 23 days	3 years 1 month 17 days
Weighted average discount rate	4.03%	4.41%
Cash paid for amounts included in the measurement of lease liabilities	$ 45,043	$ 43,477

Operating lease liabilities und

Operating lease liabilities undiscounted cash flows maturity schedule (Details 2) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
2021	$ 36,632
2022	25,133
2023	16,639
2024	12,140
2025	7,852
Thereafter	48,845
Total	147,241
Less discount	26,718
Total operating lease liabilities	$ 120,523

Lessor accounting (Details 3)

Lessor accounting (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating Lease, Lease Income	$ 48	$ 51.5
Lessor, Operating Lease, Payments to be Received, Next Twelve Months	$ 11.2

Asset Retirement Obligations (D

Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset retirement obligation [Roll Forward]
Balance at beginning of year	$ 417,575	$ 375,553
Liabilities incurred	11,560	25,869
Liabilities acquired	1,378	486
Liabilities settled	(5,369)	(7,097)
Accretion expense*	21,668	19,789
Revisions in estimates	107	2,975
Balance at end of year	446,919	417,575
Asset retirement obligation costs
Asset retirement obligation [Roll Forward]
Accretion expense, including asset retirement obligations	$ 20,100	$ 18,300

Common stock (Details)

Common stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Dividends declared per common share	$ 0.8350	$ 0.8150	$ 0.7950
Dividends declared to common stockholders	$ 168,489	$ 162,408	$ 156,453
Credit agreement limitation on company's subsidiaries ratio of funded debt to capitalization	65.00%
Company's subsidiaries net assets restricted from use for dividend payments	$ 1,400,000
Common stock
Dividends declared to common stockholders	$ 167,400	$ 162,100	$ 155,700
Centennial [Member]
Maximum distributions to the company as a ratio of average consolidated indebtedness to consolidated EBITDA	350.00%

Common stock Common stock issua

Common stock Common stock issuance (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Shares, Issued	201,061,198	200,922,790
Proceeds from Issuance of Common Stock	$ 3,385,000	$ 106,848,000	$ 0
At-the-market offering
Class of Stock [Line Items]
Common Stock, Shares Authorized	6,400,000
Common Stock, Shares, Issued	0	3,600,000
Proceeds from Issuance of Common Stock		$ 94,000,000
Sales Commissions Paid from Issuance of Common Stock through the ATM Program		$ 950,000
Common Stock, Authorized Shares Remaining	6,400,000
K-Plan
Class of Stock [Line Items]
Common Stock, Authorized Shares Remaining	7,200,000

Preferred Stock (Details)

Preferred Stock (Details) - Preferred Stock [Member] - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Preferred stock, shares authorized	2,000,000
Preferred stock, par value per share	$ 100
Preferred stock, shares oustanding	0	0

Stock based compensation plans

Stock based compensation plans (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement, Noncash Expense [Abstract]
Share-based compensation arrangement, number of shares available for grant	4.2
Share-based compensation expense, net of tax	$ 10.8	$ 6.5	$ 4.6
Share-based compensation nonvested awards total compensation cost not yet recognized	$ 10.9
Share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition	1 year 7 months 6 days

Stock awards (Details 2)

Stock awards (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement, Noncash Expense [Abstract]
Nonemployee director stock compensation shares issued	45,273	41,644	38,605
Fair value of nonemployee director stock compensation shares issued	$ 1.1	$ 1.2	$ 1

Restricted stock awards (Detail

Restricted stock awards (Details 3) - Restricted Stock Units (RSUs) [Member] $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards, shares issued | shares	22,838
Weighted average grant-date fair value | $ / shares	$ 27.48
Fair Value of Restricted Stock Awards, Vested | $	$ 600

Performance share awards (Detai

Performance share awards (Details 4) - Performance shares [Member] - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based compensation arrangement by share-based payment award [Line Items]
Weighted average grant-date fair value	$ 36.19
Fair value of vested shares	$ 9,700,000	$ 9,700,000	$ 0
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Nonvested at beginning of period	573,503
Nonvested at beginning of period	$ 30.81
Granted	285,980
Weighted average grant-date fair value	$ 36.19
Additional performance shares earned	123,147
Additional performance shares earned	$ 27.48
Vested	369,456
Vested	$ 29.84
Nonvested at end of period	613,174	573,503
Nonvested at end of period	$ 33.24	$ 30.81
Grant date February 2019 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	327,194
Grant Date February 2020 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	285,980
Market Condition [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Historical volatility rate	50.00%
Implied volatility rate	50.00%
Weighted average grant-date fair value	$ 40.75	$ 35.07	$ 34.55
Expected volatility rate, minimum	15.30%	19.50%	17.87%
Expected volatility rate, maximum	15.97%	19.69%	22.14%
Risk-free interest rate, minimum	1.45%	2.46%	1.86%
Risk-free interest rate, maximum	1.62%	2.55%	2.46%
Weighted average discounted dividends per share	$ 2.91	$ 2.85	$ 2.46
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Weighted average grant-date fair value	$ 40.75	$ 35.07	$ 34.55
Market Condition [Member] | Minimum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	0.00%
Market Condition [Member] | Maximum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	200.00%
Performance Condition [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Weighted average grant-date fair value	$ 31.63	26.25	27.48
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Weighted average grant-date fair value	$ 31.63	$ 26.25	$ 27.48
Performance Condition [Member] | Minimum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	0.00%
Performance Condition [Member] | Maximum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	200.00%

Accumulated other comprehensi_4

Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,847,246
Other comprehensive income (loss)	(5,976)	$ (3,760)	$ 6,951
Balance	3,079,105	2,847,246
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,430)	(2,161)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	446	731
Other comprehensive income (loss)	446	731
Balance	(984)	(1,430)	(2,161)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(40,734)	(36,069)
Other comprehensive income (loss) before reclassifications	(8,395)	(6,151)
Amounts reclassified from accumulated other comprehensive loss	1,922	1,486
Other comprehensive income (loss)	(6,473)	(4,665)
Balance	(47,207)	(40,734)	(36,069)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	62	(112)
Other comprehensive income (loss) before reclassifications	(1)	134
Amounts reclassified from accumulated other comprehensive loss	52	40
Other comprehensive income (loss)	51	174
Balance	113	62	(112)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(42,102)	(38,342)
Other comprehensive income (loss) before reclassifications	(8,396)	(6,017)
Amounts reclassified from accumulated other comprehensive loss	2,420	2,257
Other comprehensive income (loss)	(5,976)	(3,760)	6,951
Balance	$ (48,078)	$ (42,102)	$ (38,342)

Reclassification out of accumul

Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									$ (96,519)	$ (98,587)	$ (84,614)
Income taxes									(84,590)	(63,279)	(47,485)
Other income (expense)									26,711	15,812	(238)
Net income	$ 112,294	$ 153,078	$ 99,703	$ 25,130	$ 95,065	$ 137,637	$ 61,825	$ 40,926	390,205	335,453	$ 272,318
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									(2,420)	(2,257)
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive income | Interest rate contract
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									(591)	(591)
Income taxes									145	(140)
Net income									(446)	(731)
Amortization of postretirement liability losses included in net periodic benefit cost | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									630	476
Other income (expense)									(2,552)	(1,962)
Net income									(1,922)	(1,486)
Reclassification adjustment for loss on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									14	10
Other income (expense)									(66)	(50)
Net income									$ (52)	$ (40)

Components of income before inc

Components of income before income taxes from continuing operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ 474,856	$ 398,532	$ 317,655
Foreign	261	(87)	(784)
Income before income taxes	$ 475,117	$ 398,445	$ 316,871

Income tax expense (benefit) (D

Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 65,006	$ (3,502)	$ (15,901)
State	21,234	3,366	3,651
Foreign	151	0	0
Current income taxes	86,391	(136)	(12,250)
Deferred:
Federal	(3,735)	50,218	50,755
State	(625)	12,098	7,206
Investment tax credit - net	2,559	1,099	1,774
Deferred income taxes	(1,801)	63,415	59,735
Total income tax expense	$ 84,590	$ 63,279	$ 47,485

Components of deferred tax asse

Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Postretirement	$ 51,495	$ 51,075
Compensation-related	40,477	37,330
Operating lease liabilities	25,963	24,459
Payroll tax deferral	14,010	0
Legal and environmental contingencies	9,467	6,601
Asset retirement obligations	8,060	7,450
Customer advances	7,463	7,325
Federal renewable energy credit	0	5,343
Other	37,944	32,533
Total deferred tax assets	194,879	172,116
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	536,966	511,867
Postretirement	49,233	48,927
Operating lease right-of-use-assets	25,858	24,436
Intangible asset amortization	19,514	18,930
Other	67,922	61,385
Total deferred tax liabilities	699,493	665,545
Valuation allowance	11,484	13,154
Net deferred income tax liability	$ 516,098	$ 506,583

Carryforwards (Details 4)

Carryforwards (Details 4) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
State and local jurisdiction
Carryforwards [Line Items]
Operating loss carryforwards	$ 151.5	$ 149.8
Investment tax credit carryforward, amount	36.3	37.4
Federal and State Taxing Authority
Carryforwards [Line Items]
Tax credit carryforward, amount	$ 37.1	$ 43.7

Deferred tax reconciliation (De

Deferred tax reconciliation (Details 5) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Change in net deferred income tax liability from the preceding table	$ 9,515
Deferred taxes associated with other comprehensive loss	1,817
Excess deferred income tax amortization	(12,517)
Other	(616)
Deferred income taxes	$ (1,801)	$ 63,415	$ 59,735

Income tax expense (benefit) st

Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ 99,775	$ 83,674	$ 66,543
Federal statutory rate	21.00%	21.00%	21.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax	$ 17,845	$ 14,029	$ 12,190
State income tax, rate	3.80%	3.50%	3.80%
Federal renewable energy credit	$ (16,009)	$ (15,843)	$ (11,759)
Federal renewable energy credit, rate	(3.40%)	(4.00%)	(3.70%)
Tax compliance and uncertain tax positions	$ (3,543)	$ (2,739)	$ (2,725)
Tax compliance and uncertain tax positions, rate	(0.70%)	(0.70%)	(0.90%)
Excess deferred income tax amortization	$ (12,517)	$ (11,904)	$ (9,319)
Excess deferred income tax amortization, rate	(2.60%)	(3.00%)	(2.90%)
TCJA revaluation	$ 0	$ 0	$ (5,947)
TCJA revaluation, rate	0.00%	0.00%	(1.90%)
TCJA revaluation related to accumulated other comprehensive loss balance	$ 0	$ 0	$ (42)
TCJA revaluation related to accumulated other comprehensive loss balance, rate	0.00%	0.00%	0.00%
Other	$ (961)	$ (3,938)	$ (1,456)
Other, rate	(0.30%)	(0.90%)	(0.40%)
Income taxes	$ 84,590	$ 63,279	$ 47,485
Total income tax expense, rate	17.80%	15.90%	15.00%

Cash flow information (Details)

Cash flow information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest, net*	$ 88,681,000	$ 93,414,000	$ 83,009,000
Income taxes paid (refunded), net**	65,536,000	(8,475,000)	16,041,000
AFUDC borrowed	2,600,000	2,800,000	2,300,000
Property, plant and equipment additions in accounts payable	26,082,000	46,119,000	42,355,000
Right-of-use assets obtained in exchange for new operating lease liabilities	54,356,000	54,880,000	0
Issuance of common stock in connection with acquisition	0	0	18,186,000
Debt assumed in connection with a business combination	0	1,163,000	0
Accrual for holdback payment related to a business combination	2,500,000	0	0
Continuing and discontinued operations
Income taxes paid (refunded), net**	$ 59,400,000	$ (9,400,000)	$ 5,500,000

Business segment data (Details)

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

Change in benefit obligations a

Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Underfunded Plan | Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 421,166	$ 391,602
Service cost	0	0	$ 0
Interest cost	12,093	15,225	14,591
Plan participants' contributions	0	0
Actuarial loss	27,737	40,219
Benefits paid	(23,636)	(25,880)
Benefit obligation at end of year	437,360	421,166	391,602
Change in net plan assets:
Fair value of plan assets at beginning of year	365,264	307,809
Actual return on plan assets	42,206	58,409
Employer contribution	0	24,926
Plan participants' contributions	0	0
Benefits paid	(23,636)	(25,880)
Fair value of net plan assets at end of year	383,834	365,264	307,809
Funded status - over (under)	(53,526)	(55,902)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	0	0
Other accrued liabilities	0	0
Noncurrent liabilities - other	53,526	55,902
Benefit obligation assets (liabilities) - net amount recognized	(53,526)	(55,902)
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	27,527	27,748
Prior service credit	0	0
Total	27,527	27,748
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	154,013	155,484
Prior service credit	0	0
Total	154,013	155,484
Overfunded Plan | Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	83,614	81,201
Service cost	1,532	1,142	1,494
Interest cost	2,437	2,986	2,899
Plan participants' contributions	752	1,040
Actuarial loss	2,203	2,632
Benefits paid	(4,383)	(5,387)
Benefit obligation at end of year	86,155	83,614	81,201
Change in net plan assets:
Fair value of plan assets at beginning of year	94,587	82,516
Actual return on plan assets	10,249	15,731
Employer contribution	434	687
Plan participants' contributions	752	1,040
Benefits paid	(4,383)	(5,387)
Fair value of net plan assets at end of year	101,639	94,587	$ 82,516
Funded status - over (under)	15,484	10,973
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	36,769	30,475
Other accrued liabilities	622	647
Noncurrent liabilities - other	20,663	18,855
Benefit obligation assets (liabilities) - net amount recognized	15,484	10,973
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	5,557	6,118
Prior service credit	(634)	(731)
Total	4,923	5,387
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	(8,228)	(4,450)
Prior service credit	(6,808)	(8,109)
Total	$ (15,036)	$ (12,559)

Benefit obligations in excess o

Benefit obligations in excess of plan assets (Details 2) - Qualified Plan - Pension Benefits - Underfunded Plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 437,360	$ 421,166
Accumulated benefit obligation	437,360	421,166
Fair value of plan assets	$ 383,834	$ 365,264

Components of net periodic bene

Components of net periodic benefit cost (Details 3) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Underfunded Plan | Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 0	$ 0
Interest cost	12,093	15,225	14,591
Expected return on assets	(19,949)	(18,236)	(20,753)
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	7,172	5,548	7,005
Net periodic benefit cost (credit), including amount capitalized	(684)	2,537	843
Less amount capitalized	0	0	0
Net periodic benefit cost (credit)	(684)	2,537	843
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	934	(144)	991
Amortization of actuarial loss	(1,155)	(904)	(1,084)
Amortization of prior service (cost) credit	0	0	0
Total recognized in accumulated other comprehensive loss	(221)	(1,048)	(93)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	4,546	189	8,263
Amortization of actuarial gain (loss)	(6,017)	(4,644)	(5,921)
Amortization of prior service credit	0	0	0
Total recognized in regulatory assets or liabilities	(1,471)	(4,455)	2,342
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	(2,376)	(2,966)	3,092
Overfunded Plan | Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,532	1,142	1,494
Interest cost	2,437	2,986	2,899
Expected return on assets	(5,019)	(4,804)	(4,866)
Amortization of prior service credit	(1,398)	(1,398)	(1,394)
Recognized net actuarial loss	287	353	640
Net periodic benefit cost (credit), including amount capitalized	(2,161)	(1,721)	(1,227)
Less amount capitalized	156	113	153
Net periodic benefit cost (credit)	(2,317)	(1,834)	(1,380)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	(259)	(127)	(1,735)
Amortization of actuarial loss	(306)	(110)	(354)
Amortization of prior service (cost) credit	101	100	(220)
Total recognized in accumulated other comprehensive loss	(464)	(137)	(2,309)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	(3,793)	(8,168)	(732)
Amortization of actuarial gain (loss)	19	(242)	(286)
Amortization of prior service credit	1,297	1,297	1,614
Total recognized in regulatory assets or liabilities	(2,477)	(7,113)	596
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	$ (5,258)	$ (9,084)	$ (3,093)

Estimated net loss and prior se

Estimated net loss and prior service credit (Details 4) - Qualified Plan	Dec. 31, 2020USD ($)
Underfunded Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	$ (8,000,000)
Overfunded Plan | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	(24,000)
Defined benefit plan, amortization of net prior service cost (credit)	$ (1,400,000)

Weighted average assumptions (D

Weighted average assumptions (Details 5) - Qualified Plan	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Underfunded Plan | Pension Benefits
Used to determine benefit obligation
Discount rate	2.30%	2.96%
Expected return on plan assets	6.00%	6.25%
Used to determine benefit cost
Discount rate	2.96%	4.03%
Expected return on plan assets	6.25%	6.25%
Overfunded Plan | Other Postretirement Benefits
Used to determine benefit obligation
Discount rate	2.30%	3.00%
Expected return on plan assets	5.50%	5.75%
Rate of compensation increase	3.00%	3.00%
Used to determine benefit cost
Discount rate	3.00%	4.05%
Expected return on plan assets	5.75%	5.75%
Rate of compensation increase	3.00%	3.00%

Investment Allocations (Details

Investment Allocations (Details 6) - Qualified Plan	Dec. 31, 2020
Underfunded Plan | Minimum | Equity securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	35.00%
Underfunded Plan | Minimum | Fixed Income Securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	55.00%
Underfunded Plan | Maximum | Equity securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	45.00%
Underfunded Plan | Maximum | Fixed Income Securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	65.00%
Overfunded Plan | Equity securities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	10.00%
Overfunded Plan | Fixed Income Securities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	90.00%

Health care rate assumptions an

Health care rate assumptions and cost trend rate (Details 7) - Overfunded Plan - Qualified Plan - Other Postretirement Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year	7.00%
Health care cost trend rate - ultimate	4.50%	4.50%
Year in which ultimate trend rate achieved	2031	2024
Minimum
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year		7.10%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year		7.40%
Effect on total of service and interest cost components, one percentage point increase	$ 182
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(154)
Effect on postretirement benefit obligations, one percentage point increase	3,846
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	$ (3,304)

Estimated future benefit paymen

Estimated future benefit payments and subsidies (Details 8) - Qualified Plan - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020
Underfunded Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 0
Defined Benefit Plan, Plan Assets, Additional Contributions by Employer	$ 20,000,000
2021		24,455,000
2022		24,507,000
2023		24,733,000
2024		24,835,000
2025		24,713,000
2026-2030		119,191,000
Overfunded Plan | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		500,000
2021		5,404,000
2022		5,437,000
2023		5,424,000
2024		5,377,000
2025		5,277,000
2026-2030		25,949,000
2021		95,000
2022		89,000
2023		83,000
2024		75,000
2025		70,000
2026-2030		$ 252,000

Fair value - pension (Details 9

Fair value - pension (Details 9) - Qualified Plan - Underfunded Plan - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 383,834	$ 365,264	$ 307,809
Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	8.00%	6.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 7,841	$ 26,166
Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 12,844	$ 14,457
Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	24.00%	29.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 1,727	$ 938
Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 233,185	$ 219,800
Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	36.00%	9.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 92,809	$ 80,768
Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 10,126	$ 11,828
U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	5.00%	18.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 13,872	$ 9,378
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 383,834	$ 363,335
Defined Benefit Plan, Equity Securities, US, Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	18.00%	21.00%
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	9.00%	17.00%
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 12,844	$ 14,457
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	177,397	160,906
Quoted Prices in Active Markets for Identical Assets  (Level 1) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	11,177	7,296
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	201,418	182,659
Significant Other Observable Inputs  (Level 2) | Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	7,841	26,166
Significant Other Observable Inputs  (Level 2) | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	1,727	938
Significant Other Observable Inputs  (Level 2) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	55,788	58,894
Significant Other Observable Inputs  (Level 2) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	92,809	80,768
Significant Other Observable Inputs  (Level 2) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	10,126	11,828
Significant Other Observable Inputs  (Level 2) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	2,695	2,082
Significant Other Observable Inputs  (Level 2) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	170,986	$ 180,676
Fair Value Measured at Net Asset Value Per Share | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 11,430

Fair value - other postretireme

Fair value - other postretirement (Details 10) - Other Postretirement Benefits - Qualified Plan - Overfunded Plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 101,639	$ 94,587	$ 82,516
Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	8.00%	6.00%
Postretirement Investments in Insurance Contracts	1.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 3,517	$ 4,017
Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 1,850	$ 2,073
Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	24.00%	29.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 2	$ 1
Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	157	231
Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	96,103	88,265
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 101,639	$ 94,587
Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	36.00%	9.00%
Postretirement Investments in Insurance Contracts	67.00%	50.00%
Defined Benefit Plan, Equity Securities, US, Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	18.00%	21.00%
Postretirement Investments in Insurance Contracts	10.00%	25.00%
Defined Benefit Plan, Equity Securities, US, Small Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Insurance Contracts	4.00%	7.00%
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	9.00%	17.00%
Postretirement Investments in Insurance Contracts	6.00%	11.00%
U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	5.00%	18.00%
Postretirement Investments in Insurance Contracts	12.00%	7.00%
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 1,850	$ 2,073
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	10	10
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1,860	2,083
Significant Other Observable Inputs  (Level 2) | Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	3,517	4,017
Significant Other Observable Inputs  (Level 2) | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	2	1
Significant Other Observable Inputs  (Level 2) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	147	221
Significant Other Observable Inputs  (Level 2) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	96,103	88,265
Significant Other Observable Inputs  (Level 2) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	99,769	$ 92,504
Fair Value Measured at Net Asset Value Per Share | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 10

Nonqualified Benefit Plans Bene

Nonqualified Benefit Plans Benefit Obligations (Details 11) - Unfunded Plan - Nonqualified Plan - Supplemental Employee Retirement Plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 101,242	$ 99,245
Accumulated benefit obligation	$ 101,242	$ 99,245

Nonqualified Benefit Plans Comp

Nonqualified Benefit Plans Components of NPBC (Details 12) - Unfunded Plan - Nonqualified Plan - Supplemental Employee Retirement Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 58	$ 109	$ 185
Interest cost	2,606	3,473	3,157
Recognized net actuarial loss	1,192	764	1,047
Net periodic benefit cost	$ 3,856	$ 4,346	$ 4,389

Nonqualified Benefit Plans Weig

Nonqualified Benefit Plans Weighted Average Assumptions (Details 13) - Unfunded Plan - Nonqualified Plan - Supplemental Employee Retirement Plan	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.97%	2.73%
Discount rate	2.73%	3.86%

Nonqualified Benefit Plans Futu

Nonqualified Benefit Plans Future Benefit Payments (Details 14) - Unfunded Plan - Nonqualified Plan - Supplemental Employee Retirement Plan $ in Thousands	Dec. 31, 2020USD ($)
Defined Benefit Plan Disclosure [Line Items]
2021	$ 7,693
2022	6,957
2023	6,933
2024	7,299
2025	7,253
2026-2030	$ 33,938

Nonqualified Benefit Plans Cont

Nonqualified Benefit Plans Contributions (Details 15) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Unfunded Plan | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Nonqualified Defined Contribution Plan, Cost	$ 1,800,000	$ 1,600,000	$ 597,000

Nonqualified Benefit Plans Inve

Nonqualified Benefit Plans Investments (Details 16) - Unfunded Plan - Supplemental Employee Retirement Plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	$ 148,800	$ 134,118
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	100,104	87,009
Life insurance carried on plan participants
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	39,779	38,659
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	$ 8,917	$ 8,450

Defined Contribution Benefit Pl

Defined Contribution Benefit Plans Contribution (Details 17) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan
Defined Benefit Plan Disclosure [Line Items]
Nonqualified Defined Contribution Plan, Cost	$ 50.1	$ 51.8	$ 42.4

Multiemployer Plans Zones (Deta

Multiemployer Plans Zones (Details 18)	12 Months Ended
	Dec. 31, 2020
Maximum
Multiemployer Plans [Line Items]
Multiemployer plan red zone status funded percentage	65.00%
Mulitiemployer plan yellow status funded percentage	80.00%
Minimum
Multiemployer Plans [Line Items]
Multiemployer plan yellow zone status funded percentage	65.00%
Multiemployer plan green zone status funded percentage	80.00%

Multiemployer Plans Participati

Multiemployer Plans Participation by Plan (Details 19) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Multiemployer Plans [Line Items]
Entity tax ID number	30-1133956
Pension Benefits
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Insignificant, Employer Contribution, Cost	$ 30,931	$ 24,512	$ 21,421
Multiemployer Plan, Employer Contribution, Cost	$ 85,444	$ 72,525	58,722
Alaska Laborers-Employers Retirement Fund | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	91-6028298
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	Jun. 30, 2020	Jun. 30, 2019
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 828	$ 815	732
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Dec. 31, 2020
Construction Industry and Laborers Joint Pension Trust for So Nevada, Plan A | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	88-0135695
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Red	Red
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 515	$ 544	346
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jun. 30, 2023
Edison Pension Plan | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	93-6061681
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 16,121	$ 12,252	12,111
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Dec. 31, 2021
IBEW Local 212 Pension Trust | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	31-6127280
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	Apr. 30, 2020	Apr. 30, 2019
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 1,521	$ 1,110	1,341
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jun. 1, 2025
IBEW Local 357 Pension Plan A | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	88-6023284
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 9,913	$ 10,162	3,460
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	May 31, 2021
IBEW Local 38 Pension Plan | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	34-6574238
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Red	Yellow
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	Apr. 30, 2020	Apr. 30, 2019
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 140	$ 158	116
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Apr. 23, 2023
IBEW Local 648 Pension Plan | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	31-6134845
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	Feb. 28, 2020	Feb. 28, 2019
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 526	$ 728	2,175
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Aug. 29, 2021
IBEW Local 82 Pension Plan | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	31-6127268
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	Jun. 30, 2020	Jun. 30, 2019
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 1,373	$ 1,662	1,569
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Dec. 3, 2023
Idaho Plumbers and Pipefitters Pension Plan | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	82-6010346
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	May 31, 2020	May 31, 2019
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 1,370	$ 1,307	1,247
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Mar. 31, 2023
Minnesota Teamsters Construction Division Pension Fund | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	41-6187751
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date	Nov. 30, 2019	Nov. 30, 2018
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 663	$ 673	740
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Apr. 30, 2021
National Automatic Sprinkler Industry Pension Fund | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	52-6054620
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Red	Red
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 954	$ 1,074	738
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
National Automatic Sprinkler Industry Pension Fund | Pension Benefits | Minimum
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Mar. 31, 2021
National Automatic Sprinkler Industry Pension Fund | Pension Benefits | Maximum
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jul. 31, 2024
National Electrical Benefit Fund | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	53-0181657
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	No
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 14,484	$ 12,679	8,468
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
National Electrical Benefit Fund | Pension Benefits | Minimum
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Dec. 31, 2021
National Electrical Benefit Fund | Pension Benefits | Maximum
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Aug. 30, 2025
Pension Trust Fund for Operating Engineers | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	94-6090764
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 2,680	$ 2,598	2,403
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Pension Trust Fund for Operating Engineers | Pension Benefits | Minimum
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jun. 15, 2022
Pension Trust Fund for Operating Engineers | Pension Benefits | Maximum
Multiemployer Plans [Line Items]
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jun. 30, 2023
Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	95-6052257
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 3,255	$ 2,119	1,774
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jun. 30, 2024
Southwest Marine Pension Trust | Pension Benefits
Multiemployer Plans [Line Items]
Entity tax ID number	95-6123404
Multiemployer Plan, Pension, Significant, Plan Number	001
Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List]	Red	Red
Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List]	Implemented
Multiemployer Plan, Pension, Significant, Employer Contribution, Cost	$ 170	$ 132	$ 81
Multiemployer Plan, Pension, Significant, Surcharge [Fixed List]	No
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement, Expiration Date	Jan. 31, 2024

Multiemployer Plan Contribution

Multiemployer Plan Contributions (Details 20) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Multiemployer Plans, Defined Contribution [Member]
Multiemployer Plans [Line Items]
Multiemployer Plan, Employer Contribution, Cost	$ 54.2	$ 49.2	$ 31.1
Other Postretirement Benefits
Multiemployer Plans [Line Items]
Multiemployer Plan, Employer Contribution, Cost	$ 63.8	$ 59.5	$ 51.9

Jointly owned facilities (Detai

Jointly owned facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Jointly owned electricity generation plant | Big Stone Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	22.70%
Utility plant in service	$ 155,967	$ 152,836
Construction work in progress	104	518
Less accumulated depreciation	45,435	46,266
Utility plant in services net	$ 110,636	107,088
Jointly owned electricity generation plant | Coyote Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 159,784	160,235
Construction work in progress	323	21
Less accumulated depreciation	108,852	107,638
Utility plant in services net	$ 51,255	52,618
Jointly owned electricity generation plant | Wygen III
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 66,101	67,869
Construction work in progress	232	112
Less accumulated depreciation	10,038	10,482
Utility plant in services net	$ 56,295	57,499
Jointly Owned Electricity Transmission and Distribution System | BSSE
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	50.00%
Utility plant in service	$ 107,442	105,767
Construction work in progress	0	0
Less accumulated depreciation	2,682	1,232
Utility plant in services net	$ 104,760	$ 104,535

MNPUC (Details)

MNPUC (Details) - Pending Rate Case [Member] - MNPUC [Member] - Gas Distribution [Member] - Great Plains Natural Gas Co. [Member] - USD ($) $ in Millions	Oct. 26, 2020	Jan. 01, 2020	Sep. 27, 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 2.9
Public Utilities, Requested Rate Increase (Decrease), Percentage			12.00%
Public Utilities, Interim Rate Increase (Decrease), Amount		$ 2.6
Public Utilities, Interim Rate Increase (Decrease), Percentage		11.00%
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 2.6
Public Utilities, Approved Rate Increase (Decrease), Percentage	11.50%

MTPSC (Details 2)

MTPSC (Details 2) - Gas Distribution [Member] - MTPSC [Member] - Montana-Dakota Utilities Co. [Member] - USD ($) $ in Millions	Mar. 15, 2021	Feb. 01, 2021	Jun. 22, 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 8.6
Public Utilities, Requested Rate Increase (Decrease), Percentage			13.40%
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Interim Rate Increase (Decrease), Amount		$ 4.9
Public Utilities, Interim Rate Increase (Decrease), Percentage		7.70%
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 7.3
Public Utilities, Approved Rate Increase (Decrease), Percentage	11.40%

NDPSC (Details 3)

NDPSC (Details 3) - NDPSC [Member] - Montana-Dakota Utilities Co. [Member] - USD ($)	Feb. 01, 2021	Jan. 01, 2021	Aug. 26, 2020
Electricity [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Renewable Resource Cost Adjustment Rate Tariff	$ 14,400,000
Public Utilities, Approved Rate Increase (Decrease), Amount	$ (300,000)
Pending Rate Case [Member] | Gas Distribution [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 9,000,000
Public Utilities, Requested Rate Increase (Decrease), Percentage			7.80%
Pending Rate Case [Member] | Gas Distribution [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Interim Rate Increase (Decrease), Amount		$ 6,900,000
Public Utilities, Interim Rate Increase (Decrease), Percentage		6.00%

OPUC (Details 4)

OPUC (Details 4) - OPUC [Member] - Gas Distribution [Member] - Cascade Natural Gas [Member] - USD ($)	Feb. 01, 2021	Sep. 30, 2020	Mar. 31, 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 4,900,000
Public Utilities, Requested Rate Increase (Decrease), Percentage			7.20%
Environmental Remediation Deferred Costs Increase (Decrease) Amount			$ 364,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount		$ 3,200,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage		4.80%
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount	$ 2,900,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage	4.30%

WUTC (Details 5)

WUTC (Details 5) - Pending Rate Case [Member] - Gas Distribution [Member] - WUTC [Member] - Cascade Natural Gas [Member] - USD ($) $ in Millions	Jan. 08, 2021	Jul. 24, 2020	Jun. 19, 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 13.8
Public Utilities, Requested Rate Increase (Decrease), Percentage			5.30%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount		$ 14.3
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage		5.50%
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount	$ 7.4
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage	2.80%

FERC (Details 6)

FERC (Details 6) $ in Millions	Jan. 01, 2021USD ($)
MISO [Member] | Montana-Dakota Utilities Co. [Member] | Electric | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Transmission Formula Revenue Requirement	$ 12.9

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 41.5	$ 29.1
Loss Contingency, Receivable	17.5	16.2
Loss Contingency, Regulated	$ 21.3	$ 10.5

Enviromental matters (Details 2

Enviromental matters (Details 2) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020
Portland Harbor Site
Site Contingency [Line Items]
Environmental matters investigative costs		$ 100,000,000
Environmental matters, estimated costs		1,000,000,000
Missoula, MT manufactured gas plant site
Site Contingency [Line Items]
Total estimated costs for site remediation		560,000
Environmental Matters Estimated Investigative Costs		800,000
Environmental matters accrual for site remediation		800,000
Incurred Costs For Site Remedial Investigation		130,000
Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Environmental matters accrual for site remediation		17,500,000
Total estimated costs for site remedial investigation and feasibility study		7,600,000
Incurred costs for site remedial investigation and feasibility study		5,000,000
Environmental Matters Accrual For Site Remediation, Period Increase	$ 11,100,000
Environmental matters accrual of investigative costs		2,600,000
Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, preferred alternative estimate		9,300,000
Minimum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		13,600,000
Minimum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		8,000,000
Maximum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		71,000,000
Maximum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		$ 20,400,000

Purchase commitments (Details 3

Purchase commitments (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Purchase commitments due
2021	$ 458,397
2022	235,858
2023	173,306
2024	124,724
2025	98,894
Thereafter	691,613
Amounts purchased under various commitments	$ 666,000	$ 686,500	$ 548,000
Maximum
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Term	39 years

Guarantees (Details 4)

Guarantees (Details 4)	Dec. 31, 2020USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 212,400,000
Fixed maximum amounts guaranteed by year 2021	178,600,000
Fixed maximum amounts guaranteed by year 2022	16,100,000
Fixed maximum amounts guaranteed by year 2023	7,000,000
Fixed maximum amounts guaranteed by year 2024	500,000
Fixed maximum amounts guaranteed by year 2025	500,000
Fixed maximum amounts guaranteed, thereafter	700,000
No scheduled maturity date	9,000,000
Amount outstanding under guarantees that is reflected on balance sheet	0
Letters of credit	22,700,000
Letters of credit set to expire - 2021	22,200,000
Letters of credit set to expire - 2022	500,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	$ 953,900,000

Variable interest entities (Det

Variable interest entities (Details 5) $ in Millions	Dec. 31, 2020USD ($)
Fuel contract
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 33.7

Quarterly Data (Details)

Quarterly Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Operating expenses	1,239,270	1,383,578	1,224,674	1,140,303	1,247,992	1,374,329	1,206,262	1,026,973	4,987,825	4,855,556	4,129,829
Operating income	145,890	203,711	138,254	57,070	130,221	189,470	97,311	64,218	544,925	481,220	401,723
Income from continuing operations	112,131	153,015	99,842	25,539	94,804	136,128	63,145	41,089	390,527	335,166	269,386
Income (loss) from discontinued operations, net of tax	163	63	(139)	(409)	261	1,509	(1,320)	(163)	(322)	287	2,932
Net income	$ 112,294	$ 153,078	$ 99,703	$ 25,130	$ 95,065	$ 137,637	$ 61,825	$ 40,926	$ 390,205	$ 335,453	$ 272,318
Income from continuing operations	$ 0.56	$ 0.76	$ 0.50	$ 0.13	$ 0.47	$ 0.68	$ 0.32	$ 0.21	$ 1.95	$ 1.69	$ 1.38
Discontinued operations, net of tax	0	0	0	0	0	0.01	(0.01)	0	0	0	0.01
Earnings per share - basic	0.56	0.76	0.50	0.13	0.47	0.69	0.31	0.21	1.95	1.69	1.39
Income from continuing operations	0.56	0.76	0.50	0.13	0.47	0.68	0.32	0.21	1.95	1.69	1.38
Discontinued operations, net of tax	0	0	0	0	0	0.01	(0.01)	0	0	0	0.01
Earnings per share - diluted	$ 0.56	$ 0.76	$ 0.50	$ 0.13	$ 0.47	$ 0.69	$ 0.31	$ 0.21	$ 1.95	$ 1.69	$ 1.39
Weighted average common shares outstanding - basic	200,522	200,522	200,522	200,440	200,383	199,343	198,270	196,401	200,502	198,612	195,720
Weighted average common shares outstanding - diluted	200,923	200,619	200,539	200,456	200,478	199,383	198,287	196,414	200,571	198,626	196,150

Condensed Statements of Income

Condensed Statements of Income and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Operating expenses	1,239,270	1,383,578	1,224,674	1,140,303	1,247,992	1,374,329	1,206,262	1,026,973	4,987,825	4,855,556	4,129,829
Operating income	145,890	203,711	138,254	57,070	130,221	189,470	97,311	64,218	544,925	481,220	401,723
Other income									26,711	15,812	(238)
Interest expense									96,519	98,587	84,614
Income before income taxes									475,117	398,445	316,871
Income taxes									84,590	63,279	47,485
Net income from continuing operations	$ 112,131	$ 153,015	$ 99,842	$ 25,539	$ 94,804	$ 136,128	$ 63,145	$ 41,089	390,527	335,166	269,386
Comprehensive income									384,229	331,693	279,269
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	628,331
Operating expenses									0	0	540,125
Operating income									0	0	88,206
Other income									0	0	1,504
Interest expense									0	0	32,761
Income before income taxes									0	0	56,949
Income taxes									0	0	(4,259)
Equity in earnings of subsidiaries from continuing operations									390,527	335,166	208,177
Net income from continuing operations									390,527	335,166	269,385
Equity in earnings (loss) of subsidiaries from discontinued operations									(322)	287	2,933
Net income									390,205	335,453	272,318
Comprehensive income									$ 384,229	$ 331,693	$ 279,269

Condensed Balance Sheets (Detai

Condensed Balance Sheets (Details 2) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 59,547	$ 66,459	$ 53,948	$ 34,599
Receivables, net	873,986	836,605
Prepayments and other current assets	44,120	52,617
Total current assets	1,337,347	1,297,701
Noncurrent Assets
Investments	165,022	148,656
Operating lease right-of-use assets	120,113	115,323
Other	144,111	153,849
Total noncurrent assets	6,716,025	6,385,358
Total assets	8,053,372	7,683,059	6,988,110
Current liabilities:
Accounts payable	426,264	403,391
Taxes payable	88,844	48,970
Dividends payable	42,611	41,580
Accrued compensation	90,629	99,269
Operating lease liabilities due within one year	33,655	31,664
Other accrued liabilities	198,514	182,078
Total current liabilities	963,522	866,427
Noncurrent liabilities:
Operating lease liabilities	86,868	83,742
Other	327,773	291,826
Total noncurrent liabilities	4,010,745	3,969,386
Commitments and contingencies
Commitments and Contingencies
Stockholders' equity:
Common stock	201,061	200,923
Other paid-in capital	1,371,385	1,355,404
Retained earnings	1,558,363	1,336,647
Accumulated other comprehensive loss	(48,078)	(42,102)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Stockholders' Equity Attributable to Parent	3,079,105	2,847,246
Total liabilities and stockholders' equity	$ 8,053,372	$ 7,683,059
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Par or Stated Value Per Share	$ 1	$ 1
Common Stock, Shares, Issued	201,061,198	200,922,790
Treasury Stock, Common, Shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 8,781	$ 12,326	$ 2,271	$ 843
Receivables, net	4,865	4,727
Accounts receivable from subsidiaries	50,539	49,943
Prepayments and other current assets	1,612	501
Total current assets	65,797	67,497
Noncurrent Assets
Investments	52,000	46,294
Investment in subsidiaries	3,069,956	2,842,068
Deferred income taxes	9,691	7,269
Operating lease right-of-use assets	56	153
Other	28,866	27,098
Total noncurrent assets	3,160,569	2,922,882
Total assets	3,226,366	2,990,379
Current liabilities:
Accounts payable	2,135	2,981
Accounts payable to subsidiaries	5,412	4,752
Taxes payable	4,056	1,253
Dividends payable	42,611	41,580
Accrued compensation	7,825	8,812
Operating lease liabilities due within one year	40	96
Other accrued liabilities	6,881	7,690
Total current liabilities	68,960	67,164
Noncurrent liabilities:
Operating lease liabilities	16	56
Other	78,285	75,913
Total noncurrent liabilities	78,301	75,969
Commitments and contingencies
Commitments and Contingencies
Stockholders' equity:
Common stock	201,061	200,923
Other paid-in capital	1,371,385	1,355,404
Retained earnings	1,558,363	1,336,647
Accumulated other comprehensive loss	(48,078)	(42,102)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Stockholders' Equity Attributable to Parent	3,079,105	2,847,246
Total liabilities and stockholders' equity	$ 3,226,366	$ 2,990,379
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Par or Stated Value Per Share	$ 1	$ 1
Common Stock, Shares, Issued	201,061,198	200,922,790
Treasury Stock, Common, Shares	538,921	538,921

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Operating activities:	$ 768,374	$ 542,280	$ 499,881
Capital expenditures	(558,007)	(576,065)	(568,230)
Net proceeds from sale or disposition of property and other	35,557	29,812	26,100
Investments	(1,814)	(2,011)	(2,321)
Net cash used in investing activities	(630,243)	(603,861)	(710,907)
Issuance of long-term debt	116,973	599,455	566,829
Repayment of long-term debt	(148,634)	(468,917)	(174,520)
Payments of stock issuance costs	0	0	(10)
Proceeds from issuance of common stock	3,385	106,848	0
Dividends paid	(166,405)	(160,256)	(154,573)
Repurchase of common stock	0	0	(5,020)
Tax withholding on stock-based compensation	(362)	(3,015)	(2,330)
Net cash used in financing activities	(145,043)	74,092	230,376
Increase (decrease) in cash and cash equivalents	(6,912)	12,511	19,349
Cash and cash equivalents - beginning of year	66,459	53,948	34,599
Cash and cash equivalents - end of year	59,547	66,459	53,948
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating activities:	226,642	168,520	294,379
Capital expenditures	0	0	(242,692)
Net proceeds from sale or disposition of property and other	0	0	5,032
Investments in and advances to subsidiaries	(67,000)	(120,000)	(40,000)
Advances from subsidiaries	0	17,000	70,000
Investments	(4)	(236)	(528)
Net cash used in investing activities	(67,004)	(103,236)	(208,188)
Issuance of long-term debt	0	0	199,422
Repayment of long-term debt	0	0	(125,961)
Payments of stock issuance costs	0	0	(10)
Proceeds from issuance of common stock	3,385	106,848	0
Dividends paid	(166,405)	(160,256)	(154,573)
Repurchase of common stock	0	0	(1,920)
Tax withholding on stock-based compensation	(163)	(1,821)	(1,721)
Net cash used in financing activities	(163,183)	(55,229)	(84,763)
Increase (decrease) in cash and cash equivalents	(3,545)	10,055	1,428
Cash and cash equivalents - beginning of year	12,326	2,271	843
Cash and cash equivalents - end of year	$ 8,781	$ 12,326	$ 2,271

Notes to Condensed Financial St

Notes to Condensed Financial Statements (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Cash dividends paid to parent company by consolidated subsidiaries	$ 228.4	$ 177.1	$ 115.9