MDU Resources (MDU) 10-K 21 Feb 20 2019 FY Annual report Financial data

Cover page

Cover page - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 13, 2020	Jun. 28, 2019
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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
Entity Shell Company	false
Entity Public Float			$ 5,134,204,876
Entity Common Stock, Shares Outstanding		200,389,708
Entity Central Index Key	0000067716
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues:
Operating revenues	$ 5,336,776	$ 4,531,552	$ 4,443,351
Operating expenses:
Operation and maintenance:	3,895,294	3,256,121	3,135,466
Purchased natural gas sold	421,545	404,153	430,954
Depreciation, depletion and amortization	256,017	220,205	207,486
Taxes, other than income	196,143	168,638	166,673
Electric fuel and purchased power	86,557	80,712	78,724
Total operating expenses	4,855,556	4,129,829	4,019,303
Operating income	481,220	401,723	424,048
Other income (expense)	15,812	(238)	8,767
Interest expense	98,587	84,614	82,788
Income before income taxes	398,445	316,871	350,027
Income taxes	63,279	47,485	65,041
Income from continuing operations	335,166	269,386	284,986
Income (loss) from discontinued operations, net of tax	287	2,932	(3,783)
Net income	335,453	272,318	281,203
Loss on redemption of preferred stock	0	0	600
Dividends declared on preferred stock	0	0	171
Earnings on common stock	$ 335,453	$ 272,318	$ 280,432
Earnings per common share - basic:
Earnings before discontinued operations	$ 1.69	$ 1.38	$ 1.46
Discontinued operations, net of tax	0	0.01	(0.02)
Earnings per common share - basic	1.69	1.39	1.44
Earnings per common share - diluted:
Earnings before discontinued operations	1.69	1.38	1.45
Discontinued operations, net of tax	0	0.01	(0.02)
Earnings per common share - diluted	$ 1.69	$ 1.39	$ 1.43
Weighted average common shares outstanding - basic	198,612	195,720	195,304
Weighted average common shares outstanding - diluted	198,626	196,150	195,687
Regulated:
Operating revenues:
Operating revenues	$ 1,279,304	$ 1,213,227	$ 1,244,759
Operating expenses:
Operation and maintenance:	356,132	340,331	326,687
Nonregulated:
Operating revenues:
Operating revenues	4,057,472	3,318,325	3,198,592
Operating expenses:
Operation and maintenance:	$ 3,539,162	$ 2,915,790	$ 2,808,779

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 335,453	$ 272,318	$ 281,203
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $(140), $429 and $224 in 2019, 2018 and 2017, respectively	731	162	366
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $(2,012), $1,471 and $(1,162) in 2019, 2018 and 2017, respectively	(6,151)	4,441	(1,812)
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $476, $721 and $645 in 2019, 2018 and 2017, respectively	1,486	2,173	1,013
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0, $0 and $(876) in 2019, 2018 and 2017, respectively	0	0	(1,143)
Postretirement liability adjustment	(4,665)	6,614	(1,942)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0, $(14) and $(3) in 2019, 2018 and 2017, respectively	0	(61)	(6)
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0, $75 and $0 in 2019, 2018 and 2017, respectively	0	249	0
Foreign currency translation adjustment	0	188	(6)
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $35, $(38) and $(75) in 2019, 2018 and 2017, respectively	134	(144)	(139)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $10, $35 and $65 in 2019, 2018 and 2017, respectively	40	131	120
Net unrealized gain (loss) on available-for-sale investments	174	(13)	(19)
Other comprehensive income (loss)	(3,760)	6,951	(1,601)
Comprehensive income attributable to common stockholders	$ 331,693	$ 279,269	$ 279,602

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification adjustment for loss on derivative instruments included in net income, tax	$ (140)	$ 429	$ 224
Postretirement liability gains (losses) arising during the period, tax	(2,012)	1,471	(1,162)
Amortization of postretirement liability losses included in net periodic benefit cost, tax	476	721	645
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	(876)
Foreign currency translation adjustment recognized during the period, tax	0	(14)	(3)
Reclassification adjustment for foreign currency translation adjustment included in net income, tax	0	75	0
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	35	(38)	(75)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	$ 10	$ 35	$ 65

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 66,459	$ 53,948	$ 34,599	$ 46,107
Receivables, net	836,605	722,945
Inventories	278,407	287,309
Prepayments and other current assets	115,805	119,500
Current assets held for sale	425	430
Total current assets	1,297,701	1,184,132
Investments	148,656	138,620
Property, plant and equipment	7,908,628	7,397,321
Less accumulated depreciation, depletion and amortization	2,991,486	2,818,644	2,691,641
Net property, plant and equipment	4,917,142	4,578,677	4,079,188
Deferred charges and other assets:
Goodwill	681,358	664,922	631,791
Other intangible assets, net	15,246	10,815
Operating lease right-of-use assets	115,323
Other	506,207	408,857
Noncurrent assets held for sale	1,426	2,087
Total deferred charges and other assets	1,319,560	1,086,681
Total assets	7,683,059	6,988,110	$ 6,334,666
Current liabilities:
Long-term debt due within one year	16,540	251,854
Accounts payable	403,391	358,505
Taxes payable	48,970	41,929
Dividends payable	41,580	39,695
Accrued compensation	99,269	69,007
Current operating lease liabilities	31,664
Other accrued liabilities	221,502	221,059
Current liabilities held for sale	3,511	4,001
Total current liabilities	866,427	986,050
Long-term debt	2,226,567	1,856,841
Deferred credits and other liabilities:
Deferred income taxes	506,583	430,085
Noncurrent operating lease liabilities	83,742
Other	1,152,494	1,148,359
Total deferred credits and other liabilities	1,742,819	1,578,444
Commitments and contingencies (Note 20)
Stockholders' equity:
Authorized - 500,000,000 shares, $1.00 par value Shares issued - 200,922,790 at December 31, 2019 and 196,564,907 at December 31, 2018	200,923	196,565
Other paid-in capital	1,355,404	1,248,576
Retained earnings	1,336,647	1,163,602
Accumulated other comprehensive loss	(42,102)	(38,342)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	2,847,246	2,566,775
Total liabilities and stockholders' equity	$ 7,683,059	$ 6,988,110

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Stockholders' equity:
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1
Common stock, shares issued	200,922,790	196,564,907
Treasury stock, shares	538,921	538,921

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Other paid-in capital	Retained earnings	Total accumulated other comprehensive loss	Treasury stock
Balance (in shares) at Dec. 31, 2017		0
Balance (in shares) at Dec. 31, 2016			195,843,297
Treasury stock (in shares) at Dec. 31, 2016							(538,921)
Balance at Dec. 31, 2016	$ 2,316,244	$ 15,000	$ 195,843	$ 1,232,478	$ 912,282	$ (35,733)	$ (3,626)
Net income	281,203				281,203
Other comprehensive income (loss)	(1,601)					(1,601)
Dividends declared on preferred stocks	(171)				(171)
Dividends declared on common stock	(151,966)				(151,966)
Stock-based compensation	3,375			3,375
Repurchase of common stock (in shares)							(64,384)
Repurchase of common stock	(1,684)						$ (1,684)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings				(2,441)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (in shares)							64,384
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings							$ 1,684
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(757)
Redemption of preferred stock (in shares)		(150,000)
Redemption of preferred stock - amount		$ (15,000)
Redemption of preferred stock - premium	(600)				(600)
Redemption of preferred stock	(15,600)
Balance (in shares) at Dec. 31, 2016		150,000
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	2,429,043	$ 0	$ 195,843	1,233,412	1,040,748	(37,334)	$ (3,626)
Cumulative effect of adoption of ASU 2014-09 | Accounting Standards Update 2014-09	(970)				(970)
Adjusted balance at January 1, 2018	2,428,073		195,843	1,233,412	1,039,778	(37,334)	$ (3,626)
Balance (in shares) at Dec. 31, 2018		0
Net income	272,318				272,318
Other comprehensive income (loss)	6,951					6,951
Reclassification of certain prior period tax effects from accumulated other comprehensive loss | Adjustments for New Accounting Principle, Early Adoption	0				7,959	(7,959)
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	18,176		$ 722	17,454
Issuance of common stock (in shares)			721,610
Redemption of preferred stock - premium	0
Redemption of preferred stock	$ 0
Balance (in shares) at Dec. 31, 2017		0
Balance (in shares) at Dec. 31, 2018	196,564,907		196,564,907
Treasury stock (in shares) at Dec. 31, 2018							(538,921)
Balance at Dec. 31, 2018	$ 2,566,775	$ 0	$ 196,565	1,248,576	1,163,602	(38,342)	$ (3,626)
Balance (in shares) at Dec. 31, 2019		0
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 (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 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	106,848		$ 4,112	102,736
Issuance of common stock (in shares)			4,111,669
Redemption of preferred stock - premium	0
Redemption of preferred stock	$ 0
Balance (in shares) at Dec. 31, 2018		0
Balance (in shares) at Dec. 31, 2019	200,922,790		200,922,790
Treasury stock (in shares) at Dec. 31, 2019							(538,921)
Balance at Dec. 31, 2019	$ 2,847,246	$ 0	$ 200,923	$ 1,355,404	$ 1,336,647	$ (42,102)	$ (3,626)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net income	$ 335,453	$ 272,318	$ 281,203
Income (loss) from discontinued operations, net of tax	287	2,932	(3,783)
Income from continuing operations	335,166	269,386	284,986
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	256,017	220,205	207,486
Deferred income taxes	63,415	59,735	(25,423)
Changes in current assets and liabilities, net of acquisitions:
Receivables	(104,374)	28,234	(108,255)
Inventories	9,331	(46,796)	9,135
Other current assets	(38,283)	(31,814)	(30,588)
Accounts payable	30,079	21,109	26,013
Other current liabilities	51,278	22,285	4,648
Other noncurrent changes	(60,813)	(38,521)	(18,790)
Net cash provided by continuing operations	541,816	503,823	349,212
Net cash provided by (used in) discontinued operations	464	(3,942)	98,799
Net cash provided by operating activities	542,280	499,881	448,011
Investing activities:
Capital expenditures	(576,065)	(568,230)	(341,382)
Acquisitions, net of cash acquired	(55,597)	(167,692)	0
Net proceeds from sale or disposition of property and other	29,812	26,100	126,588
Investments	(2,011)	(2,321)	(1,608)
Net cash used in continuing operations	(603,861)	(712,143)	(216,402)
Net cash provided by discontinued operations	0	1,236	2,234
Net cash used in investing activities	(603,861)	(710,907)	(214,168)
Financing activities:
Issuance of short-term borrowings	169,977	0	0
Repayment of short-term borrowings	(170,000)	0	0
Issuance of long-term debt	599,455	566,829	140,812
Repayment of long-term debt	(468,917)	(174,520)	(217,394)
Proceeds from issuance of common stock	106,848	0	0
Payments of stock issuance costs	0	(10)	0
Dividends paid	(160,256)	(154,573)	(150,727)
Redemption of preferred stock	0	0	(15,600)
Repurchase of common stock	0	(5,020)	(1,684)
Tax withholding on stock-based compensation	(3,015)	(2,330)	(757)
Net cash provided by (used in) financing activities	74,092	230,376	(245,350)
Effect of exchange rate changes on cash and cash equivalents	0	(1)	(1)
Increase (decrease) in cash and cash equivalents	12,511	19,349	(11,508)
Cash and cash equivalents - beginning of year	53,948	34,599	46,107
Cash and cash equivalents - end of year	$ 66,459	$ 53,948	$ 34,599

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of Significant Accounting Policies 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 and midstream, construction materials and contracting, construction services and other. The electric and natural gas distribution businesses, as well as a portion of the pipeline and midstream business, are regulated. Construction materials and contracting, construction services and the other businesses, as well as a portion of the pipeline and midstream business, are nonregulated. For further descriptions of the Company's businesses, see Note 16 . Intercompany balances and transactions have been eliminated in consolidation, except for certain transactions related to the Company's regulated operations in accordance with GAAP. The statements also include the ownership interests in the assets, liabilities and expenses of jointly owned electric generating facilities. 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. These accounting policies differ in some respects from those used by the Company's nonregulated 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, based on the expected regulatory treatment in future rates. The expected recovery or flowback of these deferred items generally is based on specific ratemaking decisions or precedent for each item. Regulatory assets and liabilities are being amortized consistently with the regulatory treatment established by the FERC and the applicable state public service commissions. See Note 7 for more information regarding the nature and amounts of these regulatory deferrals. 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. On December 22, 2017, President Trump signed into law the TCJA which includes lower corporate tax rates, repealing the domestic production deduction, disallowance of immediate expensing for regulated utility property and modifying or repealing many other business deductions and credits. The reduction in the corporate tax rate was effective on January 1, 2018. The effects of the change in tax laws or rates must be accounted for in the period of enactment, which resulted in the Company making reasonable estimates of the impact of the reduction in corporate tax rate on the Company's net deferred tax liabilities during the fourth quarter of 2017. The SEC issued rules that allowed for a measurement period of up to one year after the enactment date of the TCJA to finalize the recording of the related tax impacts. At December 31, 2018, the Company finalized the estimates from the fourth quarter of 2017 and no material adjustments were recorded to income from continuing operations during the twelve months ended December 31, 2018. Effective January 1, 2019, the Company adopted the requirements of the ASU on leases, as further discussed in this note, as well as in Note 5 . As such, results for reporting periods beginning January 1, 2019, are presented under the new guidance, while prior period amounts are not adjusted and continue to be reported in accordance with the historic accounting for leases. The assets and liabilities for the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. At the time the assets were classified as held for sale, depreciation, depletion and amortization expense was no longer recorded. Unless otherwise indicated, the amounts presented in the accompanying notes to the consolidated financial statements relate to the Company's continuing operations. For more information on the Company's discontinued operations, see Note 4 . Management has also evaluated the impact of events occurring after December 31, 2019 , up to the date of issuance of these consolidated financial statements. For more information on the Company's subsequent events, see N ote 21 . 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. Accounts receivable and allowance for doubtful accounts Accounts receivable consists primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts. For more information, see Note 2 . The total balance of receivables past due 90 days or more was $46.7 million and $30.0 million at December 31, 2019 and 2018 , respectively. The allowance for doubtful accounts is determined through a review of past due balances and other specific account data. Account balances are written off when management determines the amounts to be uncollectible. The Company's allowance for doubtful accounts at December 31, 2019 and 2018 , was $8.5 million and $8.9 million , respectively. Accounts receivable also consists of accrued unbilled revenue representing revenues recognized in excess of amounts billed. Accrued unbilled revenue at MDU Energy Capital was $100.8 million and $96.2 million at December 31, 2019 and 2018 , respectively. Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows: 2019 2018 (In thousands) Short-term retainage* $ 75,590 $ 56,228 Long-term retainage** 14,228 4,152 Total retainage $ 89,818 $ 60,380 * Expected to be paid within one year or less and included in receivables, net. ** Included in deferred charges and other 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 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: 2019 2018 (In thousands) Aggregates held for resale $ 147,723 $ 139,681 Asphalt oil 41,912 54,741 Materials and supplies 22,512 23,611 Merchandise for resale 22,232 22,552 Natural gas in storage (current) 22,058 22,117 Other 21,970 24,607 Total $ 278,407 $ 287,309 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 deferred charges and other assets - other and was $48.4 million and $48.5 million at December 31, 2019 and 2018 , respectively. 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 17 . 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 were as follows: 2019 2018 2017 (In thousands) AFUDC - borrowed $ 2,807 $ 2,290 $ 966 AFUDC - equity $ 698 $ 1,897 $ 909 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, which are included in deferred credits and other liabilities - other. Property, plant and equipment at December 31 was as follows: 2019 2018 Weighted Average Depreciable Life in Years (Dollars in thousands, where applicable) Regulated: Electric: Generation $ 1,139,059 $ 1,131,484 48 Distribution 443,780 430,750 46 Transmission 445,485 302,315 65 Construction in progress 66,664 161,893 — Other 132,157 122,127 15 Natural gas distribution: Distribution 2,133,249 1,981,356 47 Construction in progress 39,506 21,028 — Other 515,368 496,708 17 Pipeline and midstream: Transmission 636,796 585,594 46 Gathering 35,661 37,829 20 Storage 50,001 49,101 53 Construction in progress 22,597 5,915 — Other 48,340 45,763 16 Nonregulated: Pipeline and midstream: Gathering and processing 31,148 31,094 19 Construction in progress 154 86 — Other 9,518 9,577 10 Construction materials and contracting: Land 127,729 109,541 — Buildings and improvements 122,064 114,905 20 Machinery, vehicles and equipment 1,180,343 1,090,790 12 Construction in progress 25,018 22,507 — Aggregate reserves 455,408 430,263 * Construction services: Land 7,146 5,216 — Buildings and improvements 31,735 29,795 24 Machinery, vehicles and equipment 156,537 145,859 6 Other 17,952 7,716 2 Other: Land 2,648 2,648 — Other 32,565 25,461 14 Less accumulated depreciation, depletion and amortization 2,991,486 2,818,644 Net property, plant and equipment $ 4,917,142 $ 4,578,677 * Depleted on the units-of-production method based on recoverable aggregate reserves. Impairment of long-lived assets The Company reviews the carrying values of its long-lived assets, excluding goodwill and assets held for sale, 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 2019 , 2018 or 2017 . Unforeseen events and changes in circumstances could require the recognition of impairment losses at some future date. 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. Regulatory assets and liabilities 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. The Company records regulatory assets or liabilities at the time the Company determines the amounts to be recoverable in current or future rates. 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 16 . 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, 2019 , 2018 and 2017 , there were no impairment losses recorded . At December 31, 2019 , the fair value substantially exceeded the carrying value at all reporting units. Determining the fair value of a reporting unit requires judgment and the use of significant estimates which include assumptions about the Company's future revenue, profitability and cash flows, amount and timing of estimated capital expenditures, inflation rates, risk adjusted capital cost, operational plans, and current and future economic conditions, among others. The fair value of each reporting unit is determined using a weighted combination of income and market approaches. The Company uses a discounted cash flow methodology for its income approach. Under the income approach, the discounted cash flow model determines fair value based on the present value of projected cash flows over a specified period and a residual value related to future cash flows beyond the projection period. Both values are discounted using a rate which reflects the best estimate of the risk adjusted capital cost at each reporting unit. Risk adjusted capital cost, which varies by reporting unit and was in the range of 4 percent to 9 percent was utilized in the goodwill impairment test performed in the fourth quarter of 2019 . The goodwill impairment test also utilizes a long-term growth rate projection, which varies by reporting unit and was in the range of approximately 2 percent to 3 percent in the goodwill impairment test performed in the fourth quarter of 2019. Under the market approach, the Company estimates fair value using various multiples derived from enterprise value to EBITDA for comparative peer companies for each respective reporting unit. These multiples are applied to operating data for each reporting unit to arrive at an indication of fair value. In addition, the Company adds a reasonable control premium when calculating the fair value utilizing the peer multiples, which is estimated as the premium that would be received in a sale in an orderly transaction between market participants. The Company believes that the estimates and assumptions used in its impairment assessments are reasonable and based on available market information. 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 and midstream 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 and midstream 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. 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 nonregulated operations or incurs a regulatory asset or liability at its regulated operations. Legal costs The Company expenses external legal fees as they are incurred. 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 within a period ranging from 12 to 36 months from the time such costs are paid. Natural gas costs refundable through rate adjustments were $23.8 million and $30.0 million at December 31, 2019 and 2018 , respectively, which is included in other accrued liabilities on the Consolidated Balance Sheets. Natural gas costs recoverable through rate adjustments were $89.2 million and $42.7 million at December 31, 2019 and 2018 , respectively, which is included in prepayments and other current assets and deferred charges and other assets - other on the Consolidated Balance Sheets. 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. Income taxes 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. The Company records uncertain tax positions in accordance with accounting guidance on accounting for income taxes on the basis

Revenue from contracts with cus

Revenue from contracts with customers	12 Months Ended
	Dec. 31, 2019
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 16 . Year ended December 31, 2019 Electric Natural gas distribution Pipeline and midstream 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 * Intrasegment revenues are presented 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. Year ended December 31, 2018 Electric Natural gas distribution Pipeline and midstream 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 * Intrasegment revenues are presented 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, 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 Deferred credits and other liabilities - other Net contract assets (liabilities) $ (33,709 ) $ 10,203 $ (43,912 ) December 31, 2018 December 31, 2017 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 104,239 $ 109,540 $ (5,301 ) Receivables, net Contract liabilities - current (93,901 ) (84,123 ) (9,778 ) Accounts payable Contract liabilities - noncurrent (135 ) — (135 ) Deferred credits and other liabilities - other Net contract assets $ 10,203 $ 25,417 $ (15,214 ) The Company recognized $89.0 million and $78.6 million in revenue for the years ended December 31, 2019 and 2018, respectively, which was previously included in contract liabilities at December 31, 2018 and 2017, respectively. The Company recognized a net increase in revenues of $44.1 million and $36.7 million for the years ended December 31, 2019 and 2018, respectively, from performance obligations satisfied in prior periods. Remaining performance obligations The remaining performance obligations at the construction materials and contracting and construction services segments include unrecognized revenues, also referred to as backlog, 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 remaining performance obligations at the pipeline and midstream segment include firm transportation and storage contracts with fixed pricing and fixed volumes. At December 31, 2019 , the Company's remaining performance obligations were $2.0 billion . The Company expects to recognize the following revenue amounts in future periods related to these remaining performance obligations: $1.5 billion within the next 12 months or less; $229.4 million within the next 13 to 24 months; and $259.3 million thereafter.

Business Combinations

Business Combinations	12 Months Ended
	Dec. 31, 2019
Business Combinations [Abstract]
Business combination disclosure	Business Combinations The acquisitions below were accounted for as business combinations in accordance with ASC 805 - Business Combinations. The results of the acquired businesses 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 were material to the Company's financial position or results of operations. 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 2019 and 2018 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 one year 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. The discount rate used in calculating the fair value of the common stock issued was determined by a Black-Scholes-Merton model. The model used Level 2 inputs including risk-free interest rate, volatility range and dividend yield. 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 are the acquisitions made during 2019 and 2018 at the construction materials and contracting segment: • In December 2019, the Company acquired 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. • In October 2018, the Company acquired Sweetman Construction Company, a provider of aggregates, asphalt and ready-mixed concrete in South Dakota. • In July 2018, the Company acquired Molalla Redi-Mix and Rock Products, Inc., a producer of ready-mixed concrete in Oregon. • In June 2018, the Company acquired Tri-City Paving, Inc., a general contractor and aggregate, asphalt and ready-mixed concrete supplier in Minnesota. • In April 2018, the Company acquired Teevin & Fischer Quarry, LLC, an aggregate producer that provides crushed rock and gravel to construction and retail customers in Oregon. In addition to the above acquisitions, in September 2019, the Company purchased the assets of Pride Electric, Inc., an electrical construction company in Washington. The results of Pride Electric, Inc. are included in the constructions services segment. 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 deferred charges and other assets - other; $5.9 million to current liabilities and $100,000 to deferred credits and other liabilities - other. At December 31, 2019, the purchase price adjustments for Viesko Redi-Mix, Inc. have been settled and no material adjustments were made to the provisional accounting. Purchase price allocations for Pride Electric, Inc. and Roadrunner Ready Mix, Inc. are preliminary and will be finalized within one year of the respective acquisition dates. The Company issued debt and equity securities to finance these acquisitions. In 2018, the gross aggregate consideration for acquisitions was $168.1 million in cash, subject to certain adjustments, and 721,610 shares of common stock with a market value of $20.3 million as of the respective acquisition date. Due to the holding period restriction on the common stock, the share consideration was discounted to a fair value of approximately $18.2 million , as reflected in the Company's financial statements. In addition to the issuance of the Company's equity securities, the Company issued debt to finance these acquisitions. During the third quarter of 2019, the Company finalized its valuation of the assets acquired and liabilities assumed in conjunction with the acquisition in 2018 of Sweetman Construction Company. As a result, measurement period adjustments were made to the previously disclosed provisional fair values. At December 31, 2019, the purchase price adjustments for all business combinations that occurred in 2018 had been finalized. These adjustments did not have a material impact on the Company's consolidated results of operations. The aggregate total consideration for the 2018 acquisitions and the final amounts allocated to the assets acquired and liabilities assumed were as follows: December 31, 2018 Measurement Period Adjustments December 31, 2019 (In thousands) Assets Current assets: Receivables, net $ 18,984 $ — $ 18,984 Inventories 10,329 (228 ) 10,101 Other current assets 515 (14 ) 501 Total current assets 29,828 (242 ) 29,586 Property, plant and equipment 131,766 6,669 138,435 Deferred charges and other assets: Goodwill 33,131 (6,669 ) 26,462 Other intangible assets, net 8,227 — 8,227 Other 927 — 927 Total deferred charges and other assets 42,285 (6,669 ) 35,616 Total assets acquired $ 203,879 $ (242 ) $ 203,637 Liabilities Current liabilities $ 11,122 $ (242 ) $ 10,880 Deferred credits and other liabilities: Asset retirement obligation 914 — 914 Deferred income taxes 5,565 — 5,565 Total deferred credits and other liabilities 6,479 — 6,479 Total liabilities assumed $ 17,601 $ (242 ) $ 17,359 Total consideration (fair value) $ 186,278 $ — $ 186,278 For the years ended December 31, 2019 and 2018, costs incurred for acquisitions were $655,000 and $1.5 million

Discontinued operations

Discontinued operations	12 Months Ended
	Dec. 31, 2019
Discontinued Operations and Disposal Groups [Abstract]
Discontinued operations	Discontinued Operations The assets and liabilities of the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. At the time the assets were classified as held for sale, depreciation, depletion and amortization expense was no longer recorded. On June 27, 2016, the Company sold Dakota Prairie Refining to Tesoro. During 2015 and 2016, the Company sold substantially all of Fidelity's oil and natural gas assets. In July 2018, the Company completed the sale of a majority of the remaining property, plant and equipment of Fidelity. The sales of Dakota Prairie Refining and Fidelity were part of the Company's strategic plan to grow its capital investments in the remaining business segments, reduce exposure to commodity pricing and to focus on creating a greater long-term value. At December 31, 2019 and 2018 , the Company’s deferred tax assets included in assets held for sale of $1.3 million and $1.9 million , respectively, were largely comprised of state alternative minimum tax credits. The carrying amounts of the major classes of assets and liabilities classified as held for sale on the Consolidated Balance Sheets at December 31 were as follows: 2019 2018 (In thousands) Assets Current assets: Receivables, net $ 425 $ 430 Total current assets held for sale 425 430 Noncurrent assets: Deferred income taxes 1,265 1,926 Other 161 161 Total noncurrent assets held for sale 1,426 2,087 Total assets held for sale $ 1,851 $ 2,517 Liabilities Current liabilities: Accounts payable $ — $ 80 Taxes payable 1,279 1,451 Other accrued liabilities 2,232 2,470 Total current liabilities held for sale 3,511 4,001 Total liabilities held for sale $ 3,511 $ 4,001 The reconciliation of the major classes of income and expense constituting pretax income (loss) from discontinued operations to the after-tax income (loss) from discontinued operations on the Consolidated Statements of Income for the years ended December 31 were as follows: 2019 2018 2017 (In thousands) Operating revenues $ 103 $ (459 ) $ 465 Operating expenses 290 921 (4,607 ) Operating income (loss) (187 ) (1,380 ) 5,072 Other income (expense) — 12 (13 ) Interest expense — 575 250 Income (loss) from discontinued operations before income taxes (187 ) (1,943 ) 4,809 Income taxes* (474 ) (4,875 ) 8,592 Income (loss) from discontinued operations $ 287 $ 2,932 $ (3,783 ) * Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations .

Leases

Leases	12 Months Ended
	Dec. 31, 2019
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. For more information on the adoption of ASC 842, see Note 1 . 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 right-of-use assets, current lease liabilities and, if applicable, noncurrent 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 year ended December 31, 2019: (In thousands) Lease costs: Operating lease cost $ 43,759 Variable lease cost 1,555 Short-term lease cost 120,030 Total lease costs $ 165,344 (Dollars in thousands) Weighted average remaining lease term 3.13 years Weighted average discount rate 4.41 % Cash paid for amounts included in the measurement of lease liabilities $ 43,477 The reconciliation of the future undiscounted cash flows to the operating lease liabilities presented on the Consolidated Balance Sheet at December 31, 2019 , was as follows: (In thousands) 2020 $ 35,156 2021 24,893 2022 16,932 2023 10,227 2024 7,368 Thereafter 47,926 Total 142,502 Less discount 27,096 Total operating lease liabilities $ 115,406 The undiscounted annual minimum lease payments due under the Company's leases following the previous lease accounting standard as of December 31, 2018 , were as follows: 2019 2020 2021 2022 2023 Thereafter (In thousands) Operating leases $ 37,740 $ 26,255 $ 17,868 $ 11,647 $ 7,278 $ 49,098
Leases of Lessor Disclosure	Lessor accounting The Company leases certain equipment to third parties, which are considered operating leases. The Company recognized revenue from operating leases of $51.5 million for the year ended December 31, 2019 . The majority of the Company's operating leases are short-term leases of less than 12 months. At December 31, 2019 , the Company had $11.3 million of lease receivables with a majority due within 12 months or less.

Goodwill and other intangible a

Goodwill and other intangible assets	12 Months Ended
	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and other intangible assets	Goodwill and Other Intangible Assets The changes in the carrying amount of goodwill for the year ended December 31, 2019 , were as follows: Balance at January 1, 2019 Goodwill Acquired During the Year 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 The changes in the carrying amount of goodwill for the year ended December 31, 2018 , were as follows: Balance at January 1, 2018 Goodwill Acquired During the Year Measurement Period Balance at December 31, 2018 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 176,290 33,131 — 209,421 Construction services 109,765 — — 109,765 Total $ 631,791 $ 33,131 $ — $ 664,922 During 2019 and 2018, the Company completed three and four business combinations, respectively, and the results of these acquisitions have been included in the Company's construction materials and contracting and construction services segments. These business combinations increased the construction materials and contracting segment's goodwill balance at December 31, 2019 and 2018 , respectively, and increased the construction services segment's goodwill balance at December 31, 2019 , as noted in the previous tables. At December 31, 2019 and 2018 , the impacts of these business combinations on other intangible assets resulted in an increase of $6.8 million and $8.2 million , respectively. For more information related to these business combinations, see Note 3 . Other amortizable intangible assets at December 31 were as follows: 2019 2018 (In thousands) Customer relationships $ 17,958 $ 22,720 Less accumulated amortization 6,268 13,535 11,690 9,185 Noncompete agreements 3,439 2,605 Less accumulated amortization 1,957 1,956 1,482 649 Other 8,094 6,458 Less accumulated amortization 6,020 5,477 2,074 981 Total $ 15,246 $ 10,815 Amortization expense for amortizable intangible assets for the years ended December 31, 2019 , 2018 and 2017 , was $2.4 million , $1.2 million and $2.0 million , respectively. The amounts of estimated amortization expense for identifiable intangible assets as of December 31, 2019, were: 2020 2021 2022 2023 2024 Thereafter (In thousands) Amortization expense $ 3,365 $ 2,016 $ 1,968 $ 1,924 $ 1,610 $ 4,363

Regulatory assets and liabiliti

Regulatory assets and liabilities	12 Months Ended
	Dec. 31, 2019
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 Period * 2019 2018 (In thousands) Regulatory assets: Pension and postretirement benefits (a) (e) $ 157,069 $ 165,898 Natural gas costs recoverable through rate adjustments (a) (b) Up to 3 years 89,204 42,652 Asset retirement obligations (a) Over plant lives 66,000 60,097 Plants to be retired (a) - 32,931 — Cost recovery mechanisms (a) (b) Up to 3 years 19,396 17,948 Manufactured gas plant sites remediation (a) - 15,347 17,068 Taxes recoverable from customers (a) Over plant lives 11,486 11,946 Conservation programs (a) (b) Up to 3 years 7,405 7,494 Long-term debt refinancing costs (a) Up to 18 years 4,286 4,898 Costs related to identifying generation development (a) Up to 7 years 2,052 2,508 Other (a) (b) Up to 19 years 12,221 9,608 Total regulatory assets $ 417,397 $ 340,117 Regulatory liabilities: Taxes refundable to customers (c) (d) $ 249,506 $ 277,833 Plant removal and decommissioning costs (c) 173,722 173,143 Natural gas costs refundable through rate adjustments (d) 23,825 29,995 Pension and postretirement benefits (c) 18,065 15,264 Other (c) (d) 25,187 25,197 Total regulatory liabilities $ 490,305 $ 521,432 Net regulatory position $ (72,908 ) $ (181,315 ) * Estimated recovery period for regulatory assets currently being recovered in rates charged to customers. (a) Included in deferred charges and other assets - other on the Consolidated Balance Sheets. (b) Included in prepayments and other current assets on the Consolidated Balance Sheets. (c) Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets. (d) Included in other accrued liabilities on the Consolidated Balance Sheets. (e) Recovered as expense is incurred or cash contributions are made. The regulatory assets are expected to be recovered in rates charged to customers. A portion of the Company's regulatory assets are not earning a return; however, these regulatory assets are expected to be recovered from customers in future rates. As of December 31, 2019 and 2018 , approximately $276.5 million and $313.5 million , respectively, of regulatory assets were not earning a rate of return. During the first quarter of 2019 and the fourth quarter of 2018, the Company experienced increased natural gas costs in certain jurisdictions where it supplies natural gas. The Company has recorded these natural gas costs as regulatory assets as they are expected to be recovered from customers, as discussed in Note 19. In February 2019, the Company announced that it intends to retire three aging coal-fired electric generating units in early 2021 and 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. If, for any reason, the Company's regulated businesses cease to meet the criteria for application of regulatory accounting for all or part of their operations, the regulatory assets and liabilities relating to those portions ceasing to meet such criteria would be removed from the balance sheet and included in the statement of income or accumulated other comprehensive income (loss) in the period in which the discontinuance of regulatory accounting occurs.

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 2019
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 $87.0 million and $73.8 million at December 31, 2019 and 2018 , respectively, are classified as investments on the Consolidated Balance Sheets. The net unrealized gains on these investments for the years ended December 31, 2019 and 2017 , were $13.2 million and $9.3 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, 2019 Cost Gross Unrealized Gains Gross Unrealized Losses 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 December 31, 2018 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,473 $ 21 $ 162 $ 10,332 U.S. Treasury securities 179 — — 179 Total $ 10,652 $ 21 $ 162 $ 10,511 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 Level 2 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 Level 2 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 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. For the years ended December 31, 2019 and 2018 , there were no transfers between Levels 1 and 2. The Company's assets measured at fair value on a recurring basis were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) 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 Measurements at December 31, 2018, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at December 31, 2018 (In thousands) Assets: Money market funds $ — $ 10,799 $ — $ 10,799 Insurance contract* — 73,838 — 73,838 Available-for-sale securities: Mortgage-backed securities — 10,332 — 10,332 U.S. Treasury securities — 179 — 179 Total assets measured at fair value $ — $ 95,148 $ — $ 95,148 * The insurance contract invests approximately 53 percent in fixed-income investments, 21 percent in common stock of large-cap companies, 11 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents. The Company applies the provisions of the fair value measurement standard to its nonrecurring, non-financial measurements, including long-lived asset impairments. These assets are not measured at fair value on an ongoing basis but are subject to fair value adjustments only in certain circumstances. The Company reviews the carrying value of its long-lived assets, excluding goodwill, whenever events or changes in circumstances indicate that such carrying amounts may not be recoverable. 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 Company performed a fair value assessment of the assets acquired and liabilities assumed in the business combinations that occurred during 2019 and 2018. For more information on these Level 2 and Level 3 fair value measurements, see Note 3 . 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: 2019 2018 (In thousands) Carrying Amount $ 2,243,107 $ 2,108,695 Fair Value $ 2,418,631 $ 2,183,819 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, 2019
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, 2019. 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 Limit Amount Outstanding at December 31, 2019 Amount Outstanding at December 31, 2018 Letters of Credit at December 31, 2019 Expiration Date (In millions) Montana-Dakota Utilities Co. Commercial paper/Revolving credit agreement (a) $ 175.0 $ 118.6 (b) $ 48.5 $ — 12/19/24 Cascade Natural Gas Corporation Revolving credit agreement $ 100.0 (c) $ 64.6 $ 53.8 $ 2.2 (d) 6/7/24 Intermountain Gas Company Revolving credit agreement $ 85.0 (e) $ 24.5 $ 56.3 $ 1.4 (d) 6/7/24 Centennial Energy Holdings, Inc. Commercial paper/Revolving credit agreement (f) $ 600.0 $ 104.3 (b) $ 289.6 (b) $ — 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 at December 31, 2019, and $48.5 million was outstanding at December 31, 2018. (b) Amount outstanding under commercial paper program. (c) Certain provisions allow for increased borrowings, up to a maximum of $125.0 million . (d) Outstanding letter(s) of credit reduce the amount available under the credit agreement. (e) Certain provisions allow for increased borrowings, up to a maximum of $110.0 million . (f) 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. The following includes information related to the preceding table. Long-term debt Long-term Debt Outstanding Long-term debt outstanding was as follows: Weighted Average Interest Rate at December 31, 2019 2019 2018 (In thousands) Senior Notes due on dates ranging from October 22, 2022 to November 18, 2059 4.45 % $ 1,850,000 $ 1,381,000 Commercial paper supported by revolving credit agreements 2.04 % 222,900 338,100 Term Loan Agreement due on September 3, 2032 2.00 % 9,100 209,800 Credit agreements due on June 7, 2024 4.40 % 89,050 110,100 Medium-Term Notes due on dates ranging from September 1, 2020 to March 16, 2029 6.68 % 50,000 50,000 Other notes due on dates ranging from July 15, 2021 to November 30, 2038 4.48 % 29,117 25,229 Less unamortized debt issuance costs 7,010 5,207 Less discount 50 327 Total long-term debt 2,243,107 2,108,695 Less current maturities 16,540 251,854 Net long-term debt $ 2,226,567 $ 1,856,841 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. For more information on the reorganization, see Note 1 . On December 19, 2019, Montana-Dakota amended and restated its revolving credit agreement extending the maturity date to December 19, 2024. 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. On July 24, 2019, Montana-Dakota entered into a $200.0 million note purchase agreement with maturity dates ranging from October 17, 2039 to November 18, 2059, at a weighted average interest rate of 3.95 percent. 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. Montana-Dakota's ratio of total debt to total capitalization at December 31, 2019 , was 52 percent. Cascade On June 7, 2019, Cascade amended its revolving credit agreement to increase the borrowing capacity to $100.0 million and extend the maturity date to June 7, 2024. 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 13, 2019, Cascade issued $75.0 million of senior notes with maturity dates ranging from June 13, 2029 to June 13, 2049, at a weighted average interest rate of 3.93 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, 2019 , was 53 percent. Intermountain On June 7, 2019, Intermountain amended its revolving credit agreement to extend the maturity date to June 7, 2024. 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. On June 13, 2019, Intermountain issued $50.0 million of senior notes with maturity dates ranging from June 13, 2029 to June 13, 2049, at a weighted average interest rate of 3.92 percent. The 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. Intermountain's ratio of total debt to total capitalization at December 31, 2019 , was 50 percent. Centennial On December 19, 2019, Centennial amended and restated its revolving credit agreement to increase the borrowing capacity to $600.0 million and extend the maturity date to December 19, 2024. 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. On April 4, 2019, Centennial issued $150.0 million of senior notes with maturity dates ranging from April 4, 2029 to April 4, 2034, at a weighted average interest rate of 4.60 percent. The agreement c ontains customary covenants and provisions, including a covenant of Centennial not to permit, at any time, the ratio of total debt to total capitalization to be greater than 60 percent. Centennial's ratio of total debt to total capitalization at December 31, 2019 , was 34 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, 2019, WBI Energy Transmission issued $45.0 million of senior notes under the private shelf agreement with a maturity date of December 16, 2034, at an interest rate of 4.17 percent. WBI Energy Transmission had $170.0 million of notes outstanding at December 31, 2019 , which reduced the remaining capacity under this uncommitted private shelf agreement to $130.0 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, 2019 , was 40 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, 2019 , were as follows: 2020 2021 2022 2023 2024 Thereafter (In thousands) Long-term debt maturities $ 16,540 $ 1,528 $ 148,021 $ 77,921 $ 373,372 $ 1,632,785

Asset retirement obligations

Asset retirement obligations	12 Months Ended
	Dec. 31, 2019
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 is included in other accrued liabilities and deferred credits and other liabilities - other on the Consolidated Balance Sheets, for the years ended December 31 was as follows: 2019 2018 (In thousands) Balance at beginning of year $ 375,553 $ 341,969 Liabilities incurred 25,869 13,424 Liabilities acquired 486 1,002 Liabilities settled (7,097 ) (3,699 ) Accretion expense* 19,789 18,242 Revisions in estimates 2,975 4,615 Balance at end of year $ 417,575 $ 375,553 * Includes $18.3 million and $16.8 million in 2019 and 2018, 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 7 .

Preferred stocks

Preferred stocks	12 Months Ended
	Dec. 31, 2019
Class of Stock Disclosures [Abstract]
Preferred stocks	Preferred Stock The Company currently has 2.0 million shares of preferred stock authorized to be issued with a $100 par value. At December 31, 2019 , there were no shares outstanding. At December 31, 2018 , there were no shares outstanding. On April 1, 2017, the Company redeemed all outstanding 4.50% Series and 4.70% Series preferred stocks at $105 per share and $102 per share, respectively, for a repurchase price of approximately $15.6 million and $300,000 of redeemable preferred stock classified as long-term debt.

Common stock

Common stock	12 Months Ended
	Dec. 31, 2019
Class of Stock Disclosures [Abstract]
Common Stock	Common Stock The Company depends on earnings and dividends from its subsidiaries to pay dividends on common stock. The Company has paid quarterly dividends for more than 80 consecutive years with an increase in the dividend amount for the last 29 consecutive years. For the years ended December 31, 2019 , 2018 and 2017 , dividends declared on common stock were $.8150 , $.7950 and $.7750 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, 2019 , 2018 and 2017 , the dividends declared to common stockholders were $162.1 million , $155.7 million and $151.5 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, 2019 . 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. The Company's board of directors currently has authorized the issuance and sale of up to an aggregate of $1.0 billion worth of such securities. On February 22, 2019, the Company entered into a Distribution Agreement with J.P. Morgan Securities LLC and MUFG Securities Americas Inc., as sales agents, with respect to the issuance and sale of up to 10.0 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 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, 2019 , the Company had remaining capacity to issue up to 6.4 million additional shares of common stock under the "at-the-market" offering program. The K-Plan provides participants the option to invest in the Company's common stock. For the years ended December 31, 2019, 2018 and 2017, the K-Plan purchased shares of common stock on the open market or issued original issue common stock of the Company. At December 31, 2019 , there were 7.3 million

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
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, 2019 , there were 4.6 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 $6.5 million , $4.6 million and $2.7 million in 2019 , 2018 and 2017 , respectively. As of December 31, 2019 , total remaining unrecognized compensation expense related to stock-based compensation was approximately $9.7 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. Shares of common stock were issued under the non-employee director stock compensation plan or the non-employee director long-term incentive compensation plan in 2019 , 2018 and 2017 . There were 41,644 shares with a fair value of $1.2 million , 38,605 shares with a fair value of $1.0 million and 40,572 shares with a fair value of $1.1 million issued to non-employee directors during the years ended December 31, 2019 , 2018 and 2017 , 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 restricted stock awards granted will vest after three years. The grant-date fair value is the market price of the Company's stock on the grant date. At December 31, 2019 , the total nonvested shares were 22,838 with a weighted average grant-date fair value of $27.48 per share. 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, 2019 , were as follows: Grant Date Performance Period Target Grant of Shares February 2018 2018-2020 246,309 February 2019 2019-2021 327,194 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 2019 , 2018 and 2017 were: 2019 2018 2017 Weighted average grant-date fair value $35.07 $34.55 $24.31 Blended volatility range 19.50 % – 19.69 % 17.87 % – 22.14 % 22.70 % – 25.56 % Risk-free interest rate range 2.46 % – 2.55 % 1.86 % – 2.46 % .69 % – 1.61 % Weighted average discounted dividends per share $2.85 $2.46 $1.70 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 2019 and 2018 was $26.25 and $27.48 , respectively. The fair value of the performance shares that vested during the years ended December 31, 2019 and 2017, was $9.7 million and $9.6 million , respectively. 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, 2019 , was as follows: Number of Shares Weighted Average Grant-Date Fair Value Nonvested at beginning of period 668,791 $ 23.03 Granted 327,194 30.66 Additional performance shares earned 103,159 14.60 Less: Vested 398,919 15.52 Forfeited 126,722 24.31 Nonvested at end of period 573,503 $ 30.81

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
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: 2019 2018 2017 (In thousands) United States $ 398,532 $ 317,655 $ 350,064 Foreign (87 ) (784 ) (37 ) Income before income taxes from continuing operations $ 398,445 $ 316,871 $ 350,027 Income tax expense (benefit) from continuing operations for the years ended December 31 was as follows: 2019 2018 2017 (In thousands) Current: Federal $ (3,502 ) $ (15,901 ) $ 74,272 State 3,366 3,651 16,192 (136 ) (12,250 ) 90,464 Deferred: Income taxes: Federal 50,218 50,755 (24,497 ) State 12,098 7,206 (864 ) Investment tax credit - net 1,099 1,774 (62 ) 63,415 59,735 (25,423 ) Total income tax expense $ 63,279 $ 47,485 $ 65,041 In accordance with the accounting guidance on accounting for income taxes, the tax effects of the change in tax laws or rates are to be recorded in the period of enactment. The TCJA was enacted on December 22, 2017, as discussed in Note 1 . Therefore, the reduction in the corporate tax rate from 35 percent to 21 percent required the Company to prepare a one-time revaluation of the Company's deferred tax assets and liabilities in the fourth quarter of 2017, the period of enactment. The deferred taxes were revalued at the new tax rate because deferred taxes should reflect what the Company expects to pay or receive in future periods under the applicable tax rate. As a result of the revaluation, the Company reduced the value of these assets and liabilities and recorded a tax benefit from continuing operations of $39.5 million on the Consolidated Statements of Income for the year ended December 31, 2017. Included in the tax benefit from continuing operations was income tax expense of $7.7 million related to amounts in accumulated other comprehensive loss and $1.0 million related to the Company's assets held for sale. The Company's regulated operations prepared a one-time revaluation of the Company's regulatory deferred tax assets and liabilities in the fourth quarter of 2017 related to the enactment of the TCJA. The revaluation was deferred under regulatory accounting as the Company worked with the various regulators to determine the amount and timing of amounts to be returned to customers. In the third quarter of 2018, the Company reversed a regulatory liability recorded in 2017 based on a FERC final accounting order being issued, which resulted in a $4.2 million tax benefit. The changes included in the TCJA were broad and complex. The SEC issued rules that allowed for a measurement period of up to one year 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: 2019 2018 (In thousands) Deferred tax assets: Postretirement $ 51,075 $ 51,930 Compensation-related 37,330 29,885 Operating lease liabilities 24,459 — Asset retirement obligations 7,450 7,083 Customer advances 7,325 7,734 Legal and environmental contingencies 6,601 6,729 Federal renewable energy credit 5,343 8,015 Alternative minimum tax credit carryforward — 13,404 Other 32,533 37,347 Total deferred tax assets 172,116 162,127 Deferred tax liabilities: Depreciation and basis differences on property, plant and equipment 511,867 476,832 Postretirement 48,927 44,432 Operating lease right-of-use-assets 24,436 — Intangible asset amortization 18,930 17,752 Other 61,385 39,712 Total deferred tax liabilities 665,545 578,728 Valuation allowance 13,154 13,484 Net deferred income tax liability $ 506,583 $ 430,085 As of December 31, 2019 and 2018 , the Company had various state income tax net operating loss carryforwards of $149.8 million and $153.2 million , respectively, and federal and state income tax credit carryforwards, excluding alternative minimum tax credit carryforwards, of $43.7 million and $43.5 million , respectively. Included in the state credits are various regulatory investment tax credits of approximately $37.4 million and $32.2 million at December 31, 2019 and 2018 , respectively. The federal income tax credit carryforwards expire in 2040 if not utilized and state income tax credit carryforwards are due to expire between 2020 and 2033. 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, 2018 , to December 31, 2019 , to deferred income tax expense: 2019 (In thousands) Change in net deferred income tax liability from the preceding table $ 76,498 Deferred taxes associated with other comprehensive loss 1,631 Deferred taxes associated with TCJA enactment for regulated activities (11,904 ) Other (2,810 ) Deferred income tax expense for the period $ 63,415 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, 2019 2018 2017 Amount % Amount % Amount % (Dollars in thousands) Computed tax at federal statutory rate $ 83,674 21.0 $ 66,543 21.0 $ 122,509 35.0 Increases (reductions) resulting from: State income taxes, net of federal income tax 14,029 3.5 12,190 3.8 10,724 3.1 Federal renewable energy credit (15,843 ) (4.0 ) (11,759 ) (3.7 ) (13,958 ) (4.0 ) Tax compliance and uncertain tax positions (2,739 ) (.7 ) (2,725 ) (.9 ) (643 ) (.2 ) Domestic production deduction — — — — (6,849 ) (2.0 ) Excess deferred income tax amortization (11,904 ) (3.0 ) (9,319 ) (2.9 ) (397 ) — TCJA revaluation — — (5,947 ) (1.9 ) (47,242 ) (13.5 ) TCJA revaluation related to accumulated other comprehensive loss balance — — (42 ) — 7,735 2.2 Other (3,938 ) (.9 ) (1,456 ) (.4 ) (6,838 ) (2.0 ) Total income tax expense $ 63,279 15.9 $ 47,485 15.0 $ 65,041 18.6 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 2015. With few exceptions, as of December 31, 2019 , the Company is no longer subject to state and local income tax examinations by tax authorities for years ending prior to 2015. For the years ended December 31, 2019 , 2018 and 2017 , 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, 2019
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: 2019 2018 2017 (In thousands) Interest, net* $ 93,414 $ 83,009 $ 79,638 Income taxes paid (refunded), net** $ (8,475 ) $ 16,041 $ 112,137 * AFUDC - borrowed was $2.8 million , $2.3 million and $966,000 for the years ended December 31, 2019 , 2018 and 2017 , respectively. ** Income taxes paid (refunded), including discontinued operations, were $(9.4) million , $5.5 million and $9.7 million for the years ended December 31, 2019 , 2018 and 2017 , respectively. Noncash investing and financing transactions at December 31 were as follows: 2019 2018 2017 (In thousands) Property, plant and equipment additions in accounts payable $ 46,119 $ 42,355 $ 29,263 Issuance of common stock in connection with acquisition $ — $ 18,186 $ — Debt assumed in connection with a business combination $ 1,163 $ — $ — Right-of-use assets obtained in exchange for new operating lease liabilities $ 54,880 $ — $ —

Business segment data

Business segment data	12 Months Ended
	Dec. 31, 2019
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 vast majority of 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 and midstream segment provides natural gas transportation, underground storage and gathering services through regulated and nonregulated pipeline systems primarily in the Rocky Mountain and northern Great Plains regions of the United States. This segment also provides cathodic protection and other energy-related services. 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. 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 government 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. For more information on discontinued operations, see Note 4 . The information below follows the same accounting policies as described in Note 1. Information on the Company's segments as of December 31 and for the years then ended was as follows: 2019 2018 2017 (In thousands) External operating revenues: Regulated operations: Electric $ 351,725 $ 335,123 $ 342,805 Natural gas distribution 865,222 823,247 848,388 Pipeline and midstream 62,357 54,857 53,566 1,279,304 1,213,227 1,244,759 Nonregulated operations: Pipeline and midstream 21,835 23,161 19,602 Construction materials and contracting 2,189,651 1,925,185 1,811,964 Construction services 1,845,896 1,369,772 1,366,317 Other 90 207 709 4,057,472 3,318,325 3,198,592 Total external operating revenues $ 5,336,776 $ 4,531,552 $ 4,443,351 Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — Natural gas distribution — — — Pipeline and midstream 56,037 50,580 48,867 56,037 50,580 48,867 Nonregulated operations: Pipeline and midstream 215 325 178 Construction materials and contracting 1,066 669 565 Construction services 3,370 1,681 1,285 Other 16,461 11,052 7,165 21,112 13,727 9,193 Intersegment eliminations (77,149 ) (64,307 ) (58,060 ) Total intersegment operating revenues $ — $ — $ — Depreciation, depletion and amortization: Electric $ 58,721 $ 50,982 $ 47,715 Natural gas distribution 79,564 72,486 69,381 Pipeline and midstream 21,220 17,896 16,788 Construction materials and contracting 77,450 61,158 55,862 Construction services 17,038 15,728 15,739 Other 2,024 1,955 2,001 Total depreciation, depletion and amortization $ 256,017 $ 220,205 $ 207,486 Operating income (loss): Electric $ 64,039 $ 65,148 $ 79,902 Natural gas distribution 69,188 72,336 84,239 Pipeline and midstream 42,796 36,128 36,004 Construction materials and contracting 179,955 141,426 143,230 Construction services 126,426 86,764 81,292 Other (1,184 ) (79 ) (619 ) Total operating income $ 481,220 $ 401,723 $ 424,048 2019 2018 2017 (In thousands) Interest expense: Electric $ 25,334 $ 25,860 $ 25,377 Natural gas distribution 35,488 30,768 31,234 Pipeline and midstream 7,198 5,964 4,990 Construction materials and contracting 23,792 17,290 14,778 Construction services 5,331 3,551 3,742 Other 1,859 2,762 3,564 Intersegment eliminations (415 ) (1,581 ) (897 ) Total interest expense $ 98,587 $ 84,614 $ 82,788 Income taxes: Electric $ (12,650 ) $ (6,482 ) $ 7,699 Natural gas distribution 1,405 4,075 22,756 Pipeline and midstream 7,219 2,677 12,281 Construction materials and contracting 37,389 28,357 5,405 Construction services 29,973 20,000 25,558 Other (57 ) (1,142 ) (1,809 ) Intersegment eliminations — — (6,849 ) Total income taxes $ 63,279 $ 47,485 $ 65,041 Earnings on common stock: Regulated operations: Electric $ 54,763 $ 47,000 $ 49,366 Natural gas distribution 39,517 37,732 32,225 Pipeline and midstream 28,255 26,905 20,620 122,535 111,637 102,211 Nonregulated operations: Pipeline and midstream 1,348 1,554 (127 ) Construction materials and contracting 120,371 92,647 123,398 Construction services 92,998 64,309 53,306 Other (2,086 ) (761 ) (1,422 ) 212,631 157,749 175,155 Intersegment eliminations (a) — — 6,849 Earnings on common stock before income (loss) from discontinued operations 335,166 269,386 284,215 Income (loss) from discontinued operations, net of tax (a) 287 2,932 (3,783 ) Earnings on common stock $ 335,453 $ 272,318 $ 280,432 Capital expenditures: Electric $ 99,449 $ 186,105 $ 109,107 Natural gas distribution 206,799 205,896 146,981 Pipeline and midstream 71,477 70,057 31,054 Construction materials and contracting 190,092 280,396 44,302 Construction services 60,500 25,081 18,630 Other 8,181 1,768 1,850 Total capital expenditures (b) $ 636,498 $ 769,303 $ 351,924 2019 2018 2017 (In thousands) Assets: Electric (c) $ 1,680,194 $ 1,613,822 $ 1,470,922 Natural gas distribution (c) 2,574,965 2,375,871 2,201,081 Pipeline and midstream 677,482 616,959 566,295 Construction materials and contracting 1,684,161 1,508,032 1,238,696 Construction services 761,127 604,798 591,382 Other (d) 303,279 266,111 261,419 Assets held for sale 1,851 2,517 4,871 Total assets $ 7,683,059 $ 6,988,110 $ 6,334,666 Property, plant and equipment: Electric (c) $ 2,227,145 $ 2,148,569 $ 1,982,264 Natural gas distribution (c) 2,688,123 2,499,093 2,319,845 Pipeline and midstream 834,215 764,959 700,284 Construction materials and contracting 1,910,562 1,768,006 1,560,048 Construction services 213,370 188,586 177,265 Other 35,213 28,108 31,123 Less accumulated depreciation, depletion and amortization 2,991,486 2,818,644 2,691,641 Net property, plant and equipment $ 4,917,142 $ 4,578,677 $ 4,079,188 (a) Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations. (b) Capital expenditures for 2019, 2018 and 2017 include noncash transactions such as the issuance of the Company's equity securities in connection with acquisitions, capital expenditure-related accounts payable and AFUDC, totaling $4.8 million , $33.4 million and $10.5 million , respectively. (c) Includes allocations of common utility property. (d) 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, 2019
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 for the years ended December 31, 2019 and 2018 , and amounts recognized in the Consolidated Balance Sheets at December 31, 2019 and 2018 , were as follows: Pension Benefits Other Postretirement Benefits 2019 2018 2019 2018 (In thousands) Change in benefit obligation: Benefit obligation at beginning of year $ 391,602 $ 445,923 $ 81,201 $ 91,206 Service cost — — 1,142 1,494 Interest cost 15,225 14,591 2,986 2,899 Plan participants' contributions — — 1,040 1,282 Actuarial (gain) loss 40,219 (32,637 ) 2,632 (10,115 ) Benefits paid (25,880 ) (36,275 ) (5,387 ) (5,565 ) Benefit obligation at end of year 421,166 391,602 83,614 81,201 Change in net plan assets: Fair value of plan assets at beginning of year 307,809 354,384 82,516 88,739 Actual gain (loss) on plan assets 58,409 (21,138 ) 15,731 (2,781 ) Employer contribution 24,926 10,838 687 842 Plan participants' contributions — — 1,040 1,281 Benefits paid (25,880 ) (36,275 ) (5,387 ) (5,565 ) Fair value of net plan assets at end of year 365,264 307,809 94,587 82,516 Funded status - over (under) $ (55,902 ) $ (83,793 ) $ 10,973 $ 1,315 Amounts recognized in the Consolidated Balance Sheets at December 31: Deferred charges and other assets - other $ — $ — $ 30,475 20,843 Other accrued liabilities — — 647 660 Deferred credits and other liabilities - other 55,902 83,793 18,855 18,868 Benefit obligation assets (liabilities) - net amount recognized $ (55,902 ) $ (83,793 ) $ 10,973 $ 1,315 Amounts recognized in accumulated other comprehensive loss: Actuarial loss $ 27,748 $ 28,796 $ 6,118 $ 6,372 Prior service credit — — (731 ) (848 ) Total $ 27,748 $ 28,796 $ 5,387 $ 5,524 Amounts recognized in regulatory assets or liabilities: Actuarial (gain) loss $ 155,484 $ 159,939 $ (4,450 ) $ 3,944 Prior service credit — — (8,109 ) (9,390 ) Total $ 155,484 $ 159,939 $ (12,559 ) $ (5,446 ) 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 7 . 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: 2019 2018 (In thousands) Projected benefit obligation $ 421,166 $ 391,602 Accumulated benefit obligation $ 421,166 $ 391,602 Fair value of plan assets $ 365,264 $ 307,809 Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2019 2018 2017 2019 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ — $ 1,142 $ 1,494 $ 1,508 Interest cost 15,225 14,591 16,207 2,986 2,899 3,265 Expected return on assets (18,236 ) (20,753 ) (20,528 ) (4,804 ) (4,866 ) (4,641 ) Amortization of prior service credit — — — (1,398 ) (1,394 ) (1,371 ) Recognized net actuarial loss 5,548 7,005 6,355 353 640 857 Net periodic benefit cost (credit), including amount capitalized 2,537 843 2,034 (1,721 ) (1,227 ) (382 ) Less amount capitalized — — 310 113 153 (370 ) Net periodic benefit cost (credit) 2,537 843 1,724 (1,834 ) (1,380 ) (12 ) Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss: Net (gain) loss (144 ) 991 (1,091 ) (127 ) (1,735 ) 1,742 Amortization of actuarial loss (904 ) (1,084 ) (1,040 ) (110 ) (354 ) (289 ) Amortization of prior service (cost) credit — — — 100 (220 ) 161 Total recognized in accumulated other comprehensive loss (1,048 ) (93 ) (2,131 ) (137 ) (2,309 ) 1,614 Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities: Net (gain) loss 189 8,263 (4,736 ) (8,168 ) (732 ) (4,932 ) Amortization of actuarial loss (4,644 ) (5,921 ) (5,315 ) (242 ) (286 ) (568 ) Amortization of prior service credit — — — 1,297 1,614 1,210 Total recognized in regulatory assets or liabilities (4,455 ) 2,342 (10,051 ) (7,113 ) 596 (4,290 ) Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities $ (2,966 ) $ 3,092 $ (10,458 ) $ (9,084 ) $ (3,093 ) $ (2,688 ) 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 2020 is $7.2 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 2020 are $250,000 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 2019 2018 2019 2018 Discount rate 2.96 % 4.03 % 3.00 % 4.05 % Expected return on plan assets 6.25 % 6.75 % 5.75 % 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 2019 2018 2019 2018 Discount rate 4.03 % 3.38 % 4.05 % 3.41 % Expected return on plan assets 6.25 % 6.75 % 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, 2019 , the expected rate of return on pension plan assets is based on the targeted asset allocation range of 40 percent to 50 percent equity securities and 50 percent to 60 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 30 percent equity securities and 70 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: 2019 2018 Health care trend rate assumed for next year 7.1 % – 7.4 % 7.5 % – 8.1 % Health care cost trend rate - ultimate 4.5 % 4.5 % Year in which ultimate trend rate achieved 2024 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, 2019 : 1 Percentage Point Increase 1 Percentage Point Decrease (In thousands) Effect on total of service and interest cost components $ 245 $ (203 ) Effect on postretirement benefit obligation $ 3,751 $ (3,155 ) In 2019, the Company contributed an additional $20.0 million to its defined benefit pension plans, which increased the funded status and decreased future expenses for the plans. The Company does not expect to contribute to its defined benefit pension plans and expects to contribute approximately $660,000 to its postretirement benefit plans in 2020 . The following benefit payments, which reflect future service, as appropriate, and expected Medicare Part D subsidies at December 31, 2019, are as follows: Years Pension Benefits Other Postretirement Benefits Expected Medicare Part D Subsidy (In thousands) 2020 $ 24,128 $ 5,024 $ 92 2021 24,432 5,073 86 2022 24,642 5,098 80 2023 24,874 5,091 73 2024 24,924 5,000 65 2025-2029 121,205 24,242 222 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 equity securities is based on the closing price reported on the active market on which the individual securities are traded. 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. 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. For the years ended December 31, 2019 and 2018 , there were no transfers between Levels 1 and 2. The fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) 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. Fair Value Measurements at December 31, 2018, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at December 31, 2018 (In thousands) Assets: Cash equivalents $ — $ 4,930 $ — $ 4,930 Equity securities: U.S. companies 11,038 — — 11,038 International companies — 967 — 967 Collective and mutual funds* 145,960 51,600 — 197,560 Corporate bonds — 73,110 — 73,110 Municipal bonds — 10,624 — 10,624 U.S. Government securities 479 5,896 — 6,375 Total assets measured at fair value $ 157,477 $ 147,127 $ — $ 304,604 * Collective and mutual funds invest approximately 27 percent in common stock of international companies, 31 percent in corporate bonds, 18 percent in common stock of large-cap U.S. companies, 5 percent in cash equivalents and 19 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 equity securities is based on the closing price reported on the active market on which the individual securities are traded. 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. For the years ended December 31, 2019 and 2018 , there were no transfers between Levels 1 and 2. The fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices Significant Significant Unobservable 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 Insurance contract* 10 88,486 — 88,496 Total assets measured at fair value $ 2,083 $ 92,504 $ — $ 94,587 * 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. Fair Value Measurements at December 31, 2018, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2018 (In thousands) Assets: Cash equivalents $ — $ 3,866 $ — $ 3,866 Equity securities: U.S. companies 1,767 — — 1,767 International companies — 2 — 2 Insurance contract* 1 76,880 — 76,881 Total assets measured at fair value $ 1,768 $ 80,748 $ — $ 82,516 * The insurance contract invests approximately 51 percent in corporate bonds, 23 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 12 percent in other investments.

Nonqualified Benefit Plans

Nonqualified Benefit Plans	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Nonqualified Benefit Plans	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: 2019 2018 (In thousands) Projected benefit obligation $ 99,245 $ 93,988 Accumulated benefit obligation $ 99,245 $ 93,988 Components of net periodic benefit cost for these plans for the years ended December 31 were as follows: 2019 2018 2017 (In thousands) Components of net periodic benefit cost: Service cost $ 109 $ 185 $ 289 Interest cost 3,473 3,157 3,494 Recognized net actuarial loss 764 1,047 883 Net periodic benefit cost $ 4,346 $ 4,389 $ 4,666 Weighted average assumptions used at December 31 were as follows: 2019 2018 Benefit obligation discount rate 2.73 % 3.86 % Benefit obligation rate of compensation increase N/A N/A Net periodic benefit cost discount rate 3.86 % 3.20 % 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, 2019 , are expected to aggregate as follows: 2020 2021 2022 2023 2024 2025-2029 (In thousands) Nonqualified benefits $ 7,774 $ 7,795 $ 7,023 $ 7,219 $ 7,597 $ 35,998 In 2012, the Company established a nonqualified defined contribution plan for certain key management employees. Expenses incurred under this plan for 2019 , 2018 and 2017 were $1.6 million , $597,000 and $736,000 , respectively. The amount of investments that the Company anticipates using to satisfy obligations under these plans at December 31 was as follows: 2019 2018 (In thousands) Investments Insurance contract* $ 87,009 $ 73,838 Life insurance** 38,659 37,274 Other 8,450 10,818 Total investments $ 134,118 $ 121,930 * 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 $51.8 million in 2019 , $42.4 million in 2018 and $41.2 million in 2017 .

Multiemployer Plans

Multiemployer Plans	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Schedule of Defined Benefit Plans Disclosures [Table Text Block]	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 2019 and 2018 is for the plan's year-end at December 31, 2018 , and December 31, 2017 , 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 of Collective Bargaining Agreement Pension Fund 2019 2018 2019 2018 2017 (In thousands) Alaska Laborers-Employers Retirement Fund 91-6028298-001 Yellow as of 6/30/2019 Yellow as of 6/30/2018 Implemented $ 815 $ 732 $ 690 No 12/31/2020 Construction Industry and Laborers Joint Pension Trust for So Nevada, Plan A 88-0135695-001 Red Red Implemented 544 346 377 No 6/30/2020 Edison Pension Plan 93-6061681-001 Green Green No 12,252 12,111 12,725 No 12/31/2020 IBEW Local 212 Pension Trust 31-6127280-001 Green as of 4/30/2019 Green as of 4/30/2018 No 1,110 1,341 1,312 No 6/1/2025 IBEW Local 357 Pension Plan A 88-6023284-001 Green Green No 10,162 3,460 3,286 No 5/31/2021 IBEW Local 648 Pension Plan 31-6134845-001 Yellow as of 2/28/2019 Yellow as of 2/28/2018 Implemented 728 2,175 2,254 No 8/29/2021 IBEW Local 82 Pension Plan 31-6127268-001 Green as of 6/30/2019 Green as of 6/30/2018 No 1,662 1,569 1,757 No 12/3/2023 Idaho Plumbers and Pipefitters Pension Plan 82-6010346-001 Green as of 5/31/2019 Green as of 5/31/2018 No 1,307 1,247 1,156 No 3/31/2023 Minnesota Teamsters Construction Division Pension Fund 41-6187751-001 Green as of 11/30/2018 Green as of 11/30/2017 No 673 740 826 No 4/30/2021 National Automatic Sprinkler Industry Pension Fund 52-6054620-001 Red Red Implemented 1,074 738 718 No 3/31/2021- National Electrical Benefit Fund 53-0181657-001 Green Green No 12,679 8,468 8,891 No 8/31/2019- * Pension Trust Fund for Operating Engineers 94-6090764-001 Yellow Yellow Implemented 2,598 2,403 2,391 No 3/31/2020- Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV 95-6052257-001 Yellow Yellow Implemented 2,119 1,774 1,016 No 6/30/2020 Southwest Marine Pension Trust 95-6123404-001 Red Red Implemented 132 81 48 No 1/31/2024 Other funds 24,670 21,537 19,298 Total contributions $ 72,525 $ 58,722 $ 56,745 * 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 of Total Contributions (as of December 31 of the Plan's Year-End) Edison Pension Plan 2018 and 2017 IBEW Local 82 Pension Plan 2018 and 2017 IBEW Local 124 Pension Trust Fund 2018 and 2017 IBEW Local 212 Pension Trust Fund 2018 and 2017 IBEW Local 357 Pension Plan A 2018 and 2017 IBEW Local 648 Pension Plan 2018 and 2017 IBEW Local Union No 226 Open End Pension Fund 2018 Idaho Plumbers and Pipefitters Pension Plan 2018 and 2017 International Union of Operating Engineers Local 701 Pension Trust Fund 2018 and 2017 Minnesota Teamsters Construction Division Pension Fund 2018 and 2017 Pension and Retirement Plan of Plumbers and Pipefitters Local 525 2018 and 2017 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 $59.5 million , $51.9 million and $50.8 million for the years ended December 31, 2019 , 2018 and 2017 , respectively. Amounts contributed in 2019 , 2018 and 2017 to defined contribution multiemployer plans were $49.2 million , $31.1 million and $32.2 million , respectively.

Jointly owned facilities

Jointly owned facilities	12 Months Ended
	Dec. 31, 2019
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 2019 2018 (In thousands) Big Stone Station: 22.7 % Utility plant in service $ 152,836 $ 156,534 Construction work in progress 518 92 Less accumulated depreciation 46,266 49,345 $ 107,088 $ 107,281 BSSE: 50.0 % Utility plant in service $ 105,767 $ — Construction work in progress — 105,846 Less accumulated depreciation 1,232 — $ 104,535 $ 105,846 Coyote Station: 25.0 % Utility plant in service $ 160,235 $ 155,236 Construction work in progress 21 1,920 Less accumulated depreciation 107,638 105,565 $ 52,618 $ 51,591 Wygen III: 25.0 % Utility plant in service $ 67,869 $ 65,382 Construction work in progress 112 220 Less accumulated depreciation 10,482 9,174 $ 57,499 $ 56,428

Regulatory matters

Regulatory matters	12 Months Ended
	Dec. 31, 2019
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. As indicated below, 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 summarizes 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. This matter is pending before the MNPUC. MTPSC On November 1, 2019, Montana-Dakota submitted an application with the MTPSC requesting the use of deferred accounting for the treatment of costs related to the retirement of Lewis & Clark Station in Sidney, Montana, and units 1 and 2 at Heskett Station near Mandan, North Dakota. This matter is pending before the MTPSC. NDPSC Montana-Dakota has a transmission cost adjustment rider that allows annual updates to rates for actual costs for transmission-related projects and services. On July 19, 2019, Montana-Dakota filed a change to its transmission cost adjustment rates to reflect projected charges for July 2019 through June 2020 assessed to Montana-Dakota for transmission-related services provided by MISO and Southwest Power Pool, along with the projected transmission service revenues or credits received for the same time period. Montana-Dakota also requested recovery of six transmission capital projects. Total revenues of approximately $9.2 million , which reflects a true-up of the prior period adjustment, were requested resulting in an increase of approximately $600,000 or approximately 7.2 percent over current rates, which includes approximately $1.5 million related to transmission capital projects. On October 22, 2019, the NDPSC approved the rates as requested. The rates were effective October 28, 2019. 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 1, 2019, Montana-Dakota filed an annual update to its renewable resource cost adjustment requesting to recover a revised revenue requirement of approximately $14.7 million annually, not including the prior period true-up adjustment. The update reflects a decrease of approximately $800,000 from the revenues currently included in rates. On February 19, 2020, the NDPSC approved the increase with rates effective on March 1, 2020. On August 28, 2019, Montana-Dakota filed an application with the NDPSC for an advanced determination of prudence and a certificate of public convenience and necessity to construct, own and operate Heskett Unit 4, an 88-MW simple-cycle natural gas-fired combustion turbine peaking unit at the existing Heskett Station near Mandan, North Dakota. This matter is pending before the NDPSC. On September 16, 2019, Montana-Dakota submitted an application with the NDPSC requesting the use of deferred accounting for the treatment of costs related to the retirement of Lewis & Clark Station in Sidney, Montana, and units 1 and 2 at Heskett Station near Mandan, North Dakota. This matter is pending before the NDPSC. OPUC On December 29, 2017, Cascade filed a request with the OPUC to use deferred accounting for the 2018 net benefits associated with the implementation of the TCJA. On September 12, 2019, the OPUC approved the request, including a settlement to refund to customers approximately $1.4 million related to TJCA impacts for the period from January 2018 through March 2019. These refunds will be reflected in customers' rates over a 12-month period beginning November 1, 2019. On June 14, 2019, Cascade filed a request with the OPUC to implement a new pipeline safety cost recovery mechanism to recover investments to replace Cascade's highest risk infrastructure which would have required Cascade to file a report annually with the OPUC detailing actual projects undertaken and the related costs incurred. This matter was denied by the OPUC on January 15, 2020. SDPUC On November 8, 2019, Montana-Dakota submitted an application with the SDPUC requesting the use of deferred accounting for the treatment of costs related to the retirement of Lewis & Clark Station in Sidney, Montana, and units 1 and 2 at Heskett Station near Mandan, North Dakota. The SDPUC approved the use of deferred accounting treatment as requested on January 7, 2020. WUTC On March 29, 2019, Cascade filed a natural gas general rate case with the WUTC requesting an increase in annual revenue of $12.7 million or approximately 5.5 percent. On September 20, 2019, Cascade filed a joint settlement agreement with the WUTC reflecting a revised annual increase of approximately $6.5 million or approximately 2.8 percent with an effective date of March 1, 2020. A settlement hearing was held on November 5, 2019. On February 3, 2020, the WUTC approved the increase with rates effective on March 1, 2020. Cascade has a pipeline replacement cost recovery mechanism, which is designed to recover the replacement cost of the Company's most at risk pipelines. The mechanism requires an annual filing on May 31, as well as two update filings for actual costs before the November 1 effective date. On May 31, 2019, Cascade filed its seventh annual update to its pipeline cost recovery mechanism requesting an increase in revenue of approximately $1.6 million or approximately 0.7 percent. On October 10, 2019, Cascade filed a final update to the cost recovery mechanism with a revised increase in revenue of approximately $440,000 or approximately 0.2 percent annually. On October 24, 2019, the WUTC approved the increase with rates effective for services provided on or after November 1, 2019. Cascade defers the actual cost of gas spent to serve customers and annually records a true-up to their purchased gas adjustment tariff. On September 13, 2019, Cascade filed its annual update to its purchased gas adjustment with the WUTC requesting an annual increase of approximately $12.8 million or approximately 5.7 percent for a period of three years . The requested increase is primarily due to unrecovered purchased gas costs as a result of the rupture of the Enbridge pipeline in Canada on October 9, 2018, causing increased natural gas costs. On October 24, 2019, the WUTC approved the increase with rates effective for services provided on or after November 1, 2019. WYPSC On May 23, 2019, Montana-Dakota filed an application with the WYPSC for a natural gas rate increase of approximately $1.1 million annually or approximately 7.0 percent above current rates. The requested increase was to recover increased operating expenses and investments in distribution facilities to improve system safety and reliability. On December 17, 2019, Montana-Dakota filed a settlement agreement with the WYPSC reflecting an annual increase in revenues of approximately $830,000 or approximately 5.5 percent with rates effective March 1, 2020. This matter is pending before the WYPSC. FERC On December 9, 2019, MISO accepted Montana-Dakota's annual revenue requirement update to its transmission formula rates under the MISO tariff for its multi-value project for approximately $13.1 million , which was effective January 1, 2020. The update effective January 1, 2020, reflects the reduced return on equity order issued by the FERC on November 21, 2019.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018, the Company accrued liabilities which have not been discounted, including liabilities held for sale, of $29.1 million and $30.4 million , respectively. 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 . Remediation is expected to take up to 13 years with a present value cost estimate 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 7 . 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. 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 received notice from a prior insurance carrier that it will participate in payment of defense costs incurred in relation to the claim. Montana-Dakota has accrued $375,000 for the remediation of this site. 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 confirms that contaminants have affected soil and groundwater at the site, as well as sediments in the adjacent Port Washington Narrows. Alternative remediation options have been identified with preliminary cost estimates ranging from $340,000 to $6.4 million . Data developed through the assessment and previous investigations indicates the contamination likely derived from multiple different sources and multiple current and former owners of properties and businesses in the vicinity of the site may be responsible for the contamination. 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 has 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 $4.4 million has been incurred. Cascade has accrued $3.2 million for the remedial investigation and feasibility study, as well as $6.4 million for remediation of this site; however, the accrual for remediation costs will be reviewed and adjusted, if necessary, after completion of the remedial investigation and 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. Cascade has received notices from and entered into agreement with certain of its insurance carriers that they will participate in defense of Cascade for certain of the 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, Cascade intends to seek recovery of remediation costs through the OPUC and WUTC in 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; employee service; 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 41 years. The commitments under these contracts as of December 31, 2019 , were: 2020 2021 2022 2023 2024 Thereafter (In thousands) Purchase commitments $ 405,535 $ 250,266 $ 184,225 $ 123,166 $ 87,297 $ 678,432 These commitments were not reflected in the Company's consolidated financial statements. Amounts purchased under various commitments for the years ended December 31, 2019 , 2018 and 2017 , were $686.5 million , $548.0 million and $516.1 million , respectively. Guarantees In June 2016, WBI Energy sold all of the outstanding membership interests in Dakota Prairie Refining. In connection with the sale, Centennial agreed to continue to guarantee certain debt obligations of Dakota Prairie Refining which were expected to mature in 2023. Tesoro agreed to indemnify Centennial for any losses and litigation expenses arising from the guarantee. Continuation of the guarantee was required as a condition to the sale of Dakota Prairie Refining. On October 17, 2018, Centennial was released from this guarantee of certain debt obligations of Dakota Prairie Refining. 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, 2019 , the fixed maximum amounts guaranteed under these agreements aggregated $174.8 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 $162.6 million in 2020 ; $700,000 in 2021 ; $400,000 in 2022 ; $500,000 in 2023 ; $500,000 in 2024 ; $1.1 million thereafter; and $9.0 million , which has no scheduled maturity date. There were no amounts outstanding under the above guarantees at December 31, 2019 . 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, 2019 , the fixed maximum amounts guaranteed under these letters of credit aggregated $33.2 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these letters of credit aggregate to $32.7 million in 2020 and $500,000 in 2021. There were no amounts outstanding under the above letters of credit at December 31, 2019 . 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, 2019 . 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, 2019 , approximately $1.1 billion 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, 2019 , 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 $36.0 million .

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events [Text Block]	Subsequent Events On February 3, 2020, the Company acquired PerLectric, Inc., a leading electrical construction company in Fairfax, Virginia, which will be included in the Company's construction services segment. On February 14, 2020, the Company acquired the assets of Oldcastle Infrastructure Spokane, a prestressed-concrete business located in Spokane, Washington, which will be included in the Company's construction materials and contracting segment. To date, the initial accounting for these acquisitions is incomplete. Due to the limited time since the date of these acquisitions, it is impracticable for the Company to make business combination disclosures related to these acquisitions. The Company is still gathering the necessary information to provide such disclosures in future filings.

Quarterly data

Quarterly data	12 Months Ended
	Dec. 31, 2019
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 2019 and 2018 : First Quarter Second Quarter Third Quarter Fourth Quarter (In thousands, except per share amounts) 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 2018 Operating revenues $ 976,293 $ 1,064,597 $ 1,280,787 $ 1,209,875 Operating expenses 906,917 990,605 1,140,783 1,091,524 Operating income 69,376 73,992 140,004 118,351 Income from continuing operations 41,960 44,075 107,369 75,982 Income (loss) from discontinued operations, net of tax 477 (273 ) (118 ) 2,846 Net income 42,437 43,802 107,251 78,828 Earnings per share - basic: Income from continuing operations .22 .22 .55 .39 Discontinued operations, net of tax — — — .01 Earnings per share - basic .22 .22 .55 .40 Earnings per share - diluted: Income from continuing operations .22 .22 .55 .39 Discontinued operations, net of tax — — — .01 Earnings per share - diluted .22 .22 .55 .40 Weighted average common shares outstanding: Basic 195,304 195,524 196,018 196,023 Diluted 195,982 196,169 196,265 196,385 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, 2019
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, 2019 2018 2017 (In thousands) Operating revenues $ — $ 628,331 $ 623,693 Operating expenses — 540,125 520,069 Operating income — 88,206 103,624 Other income — 1,504 4,876 Interest expense — 32,761 31,997 Income before income taxes — 56,949 76,503 Income taxes — (4,259 ) 13,800 Equity in earnings of subsidiaries from continuing operations 335,166 208,177 222,283 Net income from continuing operations 335,166 269,385 284,986 Equity in earnings (loss) of subsidiaries from discontinued operations 287 2,933 (3,783 ) Loss on redemption of preferred stock — — 600 Dividends declared on preferred stock — — 171 Earnings on common stock $ 335,453 $ 272,318 $ 280,432 Comprehensive income $ 331,693 $ 279,269 $ 279,602 The accompanying notes are an integral part of these condensed financial statements. Condensed Balance Sheets December 31, 2019 2018 (In thousands, except shares and per share amounts) Assets Current assets: Cash and cash equivalents $ 12,326 $ 2,271 Receivables, net 4,727 92,724 Accounts receivable from subsidiaries 49,943 36,015 Inventories — 13,293 Prepayments and other current assets 501 14,488 Total current assets 67,497 158,791 Investments 46,294 76,202 Investment in subsidiaries 2,842,068 1,790,886 Property, plant and equipment — 2,846,715 Less accumulated depreciation, depletion and amortization — 836,735 Net property, plant and equipment — 2,009,980 Deferred charges and other assets: Goodwill — 4,812 Operating lease right-of-use assets 153 — Other 34,367 180,473 Total deferred charges and other assets 34,520 185,285 Total assets $ 2,990,379 $ 4,221,144 Liabilities and Stockholders' Equity Current liabilities: Long-term debt due within one year $ — $ 200,711 Accounts payable 2,981 50,051 Accounts payable to subsidiaries 4,752 12,438 Taxes payable 1,253 24,704 Dividends payable 41,580 39,695 Accrued compensation 8,812 14,346 Current operating lease liabilities 96 — Other accrued liabilities 7,690 54,099 Total current liabilities 67,164 396,044 Long-term debt — 586,012 Deferred credits and other liabilities: Deferred income taxes — 165,122 Noncurrent operating lease liabilities 56 — Other 75,913 507,191 Total deferred credits and other liabilities 75,969 672,313 Commitments and contingencies Stockholders' equity: Common stock Authorized - 500,000,000 shares, $1.00 par value Shares issued - 200,922,790 at December 31, 2019 and 196,564,907 at December 31, 2018 200,923 196,565 Other paid-in capital 1,355,404 1,248,576 Retained earnings 1,336,647 1,163,602 Accumulated other comprehensive loss (42,102 ) (38,342 ) Treasury stock at cost - 538,921 shares (3,626 ) (3,626 ) Total stockholders' equity 2,847,246 2,566,775 Total liabilities and stockholders' equity $ 2,990,379 $ 4,221,144 The accompanying notes are an integral part of these condensed financial statements. Condensed Statements of Cash Flows Years ended December 31, 2019 2018 2017 (In thousands) Net cash provided by operating activities $ 168,520 $ 294,379 $ 284,075 Investing activities: Capital expenditures — (242,692 ) (146,370 ) Net proceeds from sale or disposition of property and other — 5,032 (5,665 ) Investments in and advances to subsidiaries (120,000 ) (40,000 ) (40,000 ) Advances from subsidiaries 17,000 70,000 40,000 Investments (236 ) (528 ) (468 ) Net cash used in investing activities (103,236 ) (208,188 ) (152,503 ) Financing activities: Issuance of long-term debt — 199,422 70,080 Repayment of long-term debt — (125,961 ) (37,569 ) Payments of stock issuance costs — (10 ) — Proceeds from issuance of common stock 106,848 — — Dividends paid (160,256 ) (154,573 ) (150,727 ) Redemption of preferred stock — — (15,600 ) Repurchase of common stock — (1,920 ) (564 ) Tax withholding on stock-based compensation (1,821 ) (1,721 ) (508 ) Net cash used in financing activities (55,229 ) (84,763 ) (134,888 ) Increase (decrease) in cash and cash equivalents 10,055 1,428 (3,316 ) Cash and cash equivalents - beginning of year 2,271 843 4,159 Cash and cash equivalents - end of year $ 12,326 $ 2,271 $ 843 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 year ended December 31, 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 years ended December 31, 2018 and 2017. Due to the completion of the Holding Company Reorganization, the presentation of prior periods will vary from that of and for the year ended December 31, 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 1 for information on the computation of earnings per common share. Note 2 - Debt At December 31, 2019 , 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 $177.1 million , $115.9 million and $116.1 million for the years ended December 31, 2019 , 2018 and 2017 , respectively.

Schedule II - Consolidated Valu

Schedule II - Consolidated Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2019
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Schedule II - Valuation and Qualifying Accounts	Consolidated Valuation and Qualifying Accounts For the years ended December 31, 2019 , 2018 and 2017 Additions Description Balance at Beginning of Year Charged to Costs and Expenses Other * Deductions ** Balance at End of Year (In thousands) Allowance for doubtful accounts: 2019 $ 8,850 $ 7,864 $ 980 $ 9,197 $ 8,497 2018 8,069 7,532 1,121 7,872 8,850 2017 10,479 7,024 989 10,423 8,069 * Recoveries. ** Uncollectible accounts written off. All other schedules are omitted because of the absence of the conditions under which they are required, or because the information required is included in the Company's Consolidated Financial Statements and Notes thereto.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2019
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 and midstream, construction materials and contracting, construction services and other. The electric and natural gas distribution businesses, as well as a portion of the pipeline and midstream business, are regulated. Construction materials and contracting, construction services and the other businesses, as well as a portion of the pipeline and midstream business, are nonregulated. For further descriptions of the Company's businesses, see Note 16 . Intercompany balances and transactions have been eliminated in consolidation, except for certain transactions related to the Company's regulated operations in accordance with GAAP. The statements also include the ownership interests in the assets, liabilities and expenses of jointly owned electric generating facilities. 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. These accounting policies differ in some respects from those used by the Company's nonregulated 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, based on the expected regulatory treatment in future rates. The expected recovery or flowback of these deferred items generally is based on specific ratemaking decisions or precedent for each item. Regulatory assets and liabilities are being amortized consistently with the regulatory treatment established by the FERC and the applicable state public service commissions. See Note 7 for more information regarding the nature and amounts of these regulatory deferrals. 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. On December 22, 2017, President Trump signed into law the TCJA which includes lower corporate tax rates, repealing the domestic production deduction, disallowance of immediate expensing for regulated utility property and modifying or repealing many other business deductions and credits. The reduction in the corporate tax rate was effective on January 1, 2018. The effects of the change in tax laws or rates must be accounted for in the period of enactment, which resulted in the Company making reasonable estimates of the impact of the reduction in corporate tax rate on the Company's net deferred tax liabilities during the fourth quarter of 2017. The SEC issued rules that allowed for a measurement period of up to one year after the enactment date of the TCJA to finalize the recording of the related tax impacts. At December 31, 2018, the Company finalized the estimates from the fourth quarter of 2017 and no material adjustments were recorded to income from continuing operations during the twelve months ended December 31, 2018. Effective January 1, 2019, the Company adopted the requirements of the ASU on leases, as further discussed in this note, as well as in Note 5 . As such, results for reporting periods beginning January 1, 2019, are presented under the new guidance, while prior period amounts are not adjusted and continue to be reported in accordance with the historic accounting for leases. The assets and liabilities for the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. At the time the assets were classified as held for sale, depreciation, depletion and amortization expense was no longer recorded. Unless otherwise indicated, the amounts presented in the accompanying notes to the consolidated financial statements relate to the Company's continuing operations. For more information on the Company's discontinued operations, see Note 4 . Management has also evaluated the impact of events occurring after December 31, 2019 , up to the date of issuance of these consolidated financial statements. For more information on the Company's subsequent events, see N ote 21 .
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.
Accounts receivable and allowance for doubtful accounts	Accounts receivable consists primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts.The allowance for doubtful accounts is determined through a review of past due balances and other specific account data. 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 lower of cost or net realizable value using the average cost method.
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).
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.
Impairment of long-lived assets	The Company reviews the carrying values of its long-lived assets, excluding goodwill and assets held for sale, 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.
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.
Regulatory assets and liabilities	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. The Company records regulatory assets or liabilities at the time the Company determines the amounts to be recoverable in current or future rates.
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.
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 and midstream 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 and midstream 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.
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 nonregulated operations or incurs a regulatory asset or liability at its regulated operations.
Legal costs	The Company expenses external legal fees as they are incurred.
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 within a period ranging from 12 to 36 months from the time such costs are paid.
Stock-based compensation	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.
Income taxes	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 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.
Earnings (loss) per common share	Basic earnings per common share were computed by dividing earnings on common stock by the weighted average number of shares of common stock outstanding during the year. Diluted earnings per common share were computed by dividing earnings on common stock 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.
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; uncollectible accounts; 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.
New accounting standards	Recently adopted accounting standards ASU 2016-02 - Leases In February 2016, the FASB issued this ASU guidance relating to ASC 842 - Leases . The guidance required lessees to recognize a lease liability and a right-of-use asset on the balance sheet for operating and financing leases. The guidance remained largely the same for lessors, although some changes were made to better align lessor accounting with the new lessee accounting and to align with the revenue recognition standard. The guidance also required additional disclosures, both quantitative and qualitative, related to operating and financing leases for the lessee and sales-type, direct financing and operating leases for the lessor. The Company adopted the standard on January 1, 2019. In July 2018, the FASB issued ASU 2018-11 - Leases: Targeted Improvements , an accounting standard update to ASU 2016-02. This ASU provided an entity the option to adopt the guidance using one of two modified retrospective approaches. An entity could adopt the guidance using the modified retrospective transition approach beginning in the earliest year presented in the financial statements. This method of adoption would have required the restatement of prior periods reported and the presentation of lease disclosures under the new guidance for all periods reported. The additional transition method of adoption, introduced by ASU 2018-11, allowed entities the option to apply the guidance on the date of adoption by recognizing a cumulative effect adjustment to retained earnings during the period of adoption and did not require prior comparative periods to be restated. The Company adopted the standard on January 1, 2019, utilizing the additional transition method of adoption applied on the date of adoption and the practical expedient that allowed the Company to not reassess whether an expired or existing contract contained a lease, the classification of leases or initial direct costs. The Company did not identify any cumulative effect adjustments. The Company also adopted a short-term leasing policy as the lessee where leases with a term of 12 months or less are not included on the Consolidated Balance Sheet. As a practical expedient, a lessee may choose not to separate nonlease components from lease components and instead account for lease and nonlease components as a single lease component. The election shall be made by asset class. The Company has elected to adopt the lease/nonlease component practical expedient for all asset classes as the lessee. The Company did not elect the practical expedient to use hindsight when assessing the lease term or impairment of right-of-use assets for the existing leases on the date of adoption. In January 2018, the FASB issued a practical expedient for land easements under the new lease guidance. The practical expedient permits an entity to elect the option to not evaluate land easements under the new guidance if they existed or expired before the adoption of the new lease guidance and were not previously accounted for as leases under the previous lease guidance. Once an entity adopts the new guidance, the entity should apply the new guidance on a prospective basis to all new or modified land easements. The Company has adopted this practical expedient. The Company formed a lease implementation team to review and assess existing contracts to identify and evaluate those containing leases. Additionally, the team implemented new and revised existing software to meet the reporting and disclosure requirements of the standard. The Company also assessed the impact the standard had on its processes and internal controls and identified new and updated existing internal controls and processes to ensure compliance with the new lease standard; such modifications were not deemed to be significant. During the assessment phase, the Company used various surveys, reconciliations and analytic methodologies to ensure the completeness of the lease inventory. The Company determined that most of the current operating leases were subject to the guidance and were recognized as operating lease liabilities and right-of-use assets on the Consolidated Balance Sheet upon adoption. On January 1, 2019, the Company recorded approximately $112 million to right-of-use assets and lease liabilities as a result of the initial adoption of the guidance. In addition, the Company evaluated the impact the new guidance had on lease contracts where the Company is the lessor and determined it did not have a significant impact to the Company's financial statements. ASU 2017-04 - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued guidance on simplifying the test for goodwill impairment by eliminating Step 2, which required an entity to measure the amount of impairment loss by comparing the implied fair value of reporting unit goodwill with the carrying amount of such goodwill. This guidance requires entities to perform a quantitative impairment test, previously Step 1, to identify both the existence of impairment and the amount of impairment loss by comparing the fair value of a reporting unit to its carrying amount. Entities will continue to have the option of performing a qualitative assessment to determine if the quantitative impairment test is necessary. The guidance also requires additional disclosures if an entity has one or more reporting units with zero or negative carrying amounts of net assets. The Company early adopted the guidance on a prospective basis beginning with the preparation of its 2019 goodwill impairment test in the fourth quarter of 2019. The adoption of the guidance did not have a material impact on its results of operations, financial position, cash flows or disclosures. ASU 2018-15 - Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract In August 2018, the FASB issued guidance on the accounting for implementation costs of a hosting arrangement that is a service contract. The guidance aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract similar to the costs incurred to develop or obtain internal-use software and such capitalized costs to be expensed over the term of the hosting arrangement. Costs incurred during the preliminary and postimplementation stages should continue to be expensed as activities are performed. The capitalized costs are required to be presented on the balance sheet in the same line the prepayment for the fees associated with the hosting arrangement would be presented. In addition, the expense related to the capitalized implementation costs should be presented in the same line on the income statement as the fees associated with the hosting element of the arrangements. The Company adopted the guidance effective January 1, 2019, on a prospective basis. The adoption of the guidance did not have a material impact on its results of operations, financial position, cash flows or disclosures. Recently issued accounting standards not yet adopted ASU 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 introduces a new impairment model known as the current expected credit loss model that will replace the incurred loss impairment methodology currently included under GAAP. This guidance requires 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. 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 completed checklists to identify 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. 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 modifies, among other things, the disclosures required for Level 3 fair value measurements, including the range and weighted average of significant unobservable inputs. The guidance removes, among other things, the disclosure requirement to disclose transfers between Levels 1 and 2. The guidance will be effective for the Company on January 1, 2020, including interim periods, with early adoption permitted. Level 3 fair value measurement disclosures should be applied prospectively while all other amendments should be applied retrospectively. The Company continues to evaluate the effects the adoption of the new guidance will have on its disclosures in the first quarter of 2020. 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 removes disclosures that are no longer considered cost beneficial, clarifies the specific requirements of disclosures and adds disclosure requirements identified as relevant. The guidance adds, 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 removes, 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 will be effective for the Company on January 1, 2021, and must be applied on a retrospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on its 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 removes exceptions on intraperiod tax allocations and reporting and provides simplification on accounting for franchise taxes, tax basis goodwill and tax law changes. The guidance will be effective for the Company on January 1, 2021, with early adoption permitted. Transition requirements vary among the exceptions and amendments which include retrospective, modified retrospective and prospective application. The Company does not expect the guidance to have a material impact on its results of operations, financial position, cash flows and disclosures.
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.
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, foreign currency translation adjustments and gain (loss) on available-for-sale investments.

Business Combinations Business

Business Combinations Business Combinations (Policies)	12 Months Ended
	Dec. 31, 2019
Business Combinations [Abstract]
Business combinations policy	The acquisitions below were accounted for as business combinations in accordance with ASC 805 - Business Combinations. The results of the acquired businesses 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 were material to the Company's financial position or results of operations.

Discontinued operations (Polici

Discontinued operations (Policies)	12 Months Ended
	Dec. 31, 2019
Discontinued Operations and Disposal Groups [Abstract]
Discontinued operations	The assets and liabilities of the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. At the time the assets were classified as held for sale, depreciation, depletion and amortization expense was no longer recorded.

Business segment data (Policies

Business segment data (Policies)	12 Months Ended
	Dec. 31, 2019
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.

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Contract receivable retainage	Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows: 2019 2018 (In thousands) Short-term retainage* $ 75,590 $ 56,228 Long-term retainage** 14,228 4,152 Total retainage $ 89,818 $ 60,380 * Expected to be paid within one year or less and included in receivables, net. ** Included in deferred charges and other assets - other.
Inventories	Inventories at December 31 consisted of: 2019 2018 (In thousands) Aggregates held for resale $ 147,723 $ 139,681 Asphalt oil 41,912 54,741 Materials and supplies 22,512 23,611 Merchandise for resale 22,232 22,552 Natural gas in storage (current) 22,058 22,117 Other 21,970 24,607 Total $ 278,407 $ 287,309
AFUDC and interest capitailized	The amount of AFUDC for the years ended December 31 were as follows: 2019 2018 2017 (In thousands) AFUDC - borrowed $ 2,807 $ 2,290 $ 966 AFUDC - equity $ 698 $ 1,897 $ 909
Property, plant and equipment	Property, plant and equipment at December 31 was as follows: 2019 2018 Weighted Average Depreciable Life in Years (Dollars in thousands, where applicable) Regulated: Electric: Generation $ 1,139,059 $ 1,131,484 48 Distribution 443,780 430,750 46 Transmission 445,485 302,315 65 Construction in progress 66,664 161,893 — Other 132,157 122,127 15 Natural gas distribution: Distribution 2,133,249 1,981,356 47 Construction in progress 39,506 21,028 — Other 515,368 496,708 17 Pipeline and midstream: Transmission 636,796 585,594 46 Gathering 35,661 37,829 20 Storage 50,001 49,101 53 Construction in progress 22,597 5,915 — Other 48,340 45,763 16 Nonregulated: Pipeline and midstream: Gathering and processing 31,148 31,094 19 Construction in progress 154 86 — Other 9,518 9,577 10 Construction materials and contracting: Land 127,729 109,541 — Buildings and improvements 122,064 114,905 20 Machinery, vehicles and equipment 1,180,343 1,090,790 12 Construction in progress 25,018 22,507 — Aggregate reserves 455,408 430,263 * Construction services: Land 7,146 5,216 — Buildings and improvements 31,735 29,795 24 Machinery, vehicles and equipment 156,537 145,859 6 Other 17,952 7,716 2 Other: Land 2,648 2,648 — Other 32,565 25,461 14 Less accumulated depreciation, depletion and amortization 2,991,486 2,818,644 Net property, plant and equipment $ 4,917,142 $ 4,578,677 * Depleted on the units-of-production method based on recoverable aggregate reserves.
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: 2019 2018 2017 (In thousands) Weighted average common shares outstanding - basic 198,612 195,720 195,304 Effect of dilutive performance share awards 14 430 383 Weighted average common shares outstanding - diluted 198,626 196,150 195,687 Shares excluded from the calculation of diluted earnings per share 164 10 —
After-tax changes in the components of accumulated other comprehensive income (loss)	The after-tax changes in the components of accumulated other comprehensive loss at December 31, 2019 , 2018 and 2017 , were as follows: Net Unrealized Loss on Derivative Instruments Qualifying as Hedges Post- retirement Liability Adjustment Foreign Currency Translation Adjustment Net Unrealized Gain (Loss) on Available- for-sale Investments Total Accumulated Other Comprehensive Loss (In thousands) At December 31, 2017 $ (1,934 ) $ (35,163 ) $ (155 ) $ (82 ) $ (37,334 ) Other comprehensive income (loss) before reclassifications — 4,441 (61 ) (144 ) 4,236 Amounts reclassified from accumulated other comprehensive loss 162 2,173 249 131 2,715 Net current-period other comprehensive income (loss) 162 6,614 188 (13 ) 6,951 Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss (389 ) (7,520 ) (33 ) (17 ) (7,959 ) 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 )
Reclassification out of accumulated other comprehensive income (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: 2019 2018 Location on Consolidated Statements of Income (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (591 ) $ (591 ) Interest expense (140 ) 429 Income taxes (731 ) (162 ) Amortization of postretirement liability losses included in net periodic benefit cost (1,962 ) (2,894 ) Other income 476 721 Income taxes (1,486 ) (2,173 ) Reclassification adjustment for foreign currency translation adjustment included in net income — (324 ) Other income — 75 Income taxes — (249 ) Reclassification adjustment for loss on available-for-sale investments included in net income (50 ) (166 ) Other income 10 35 Income taxes (40 ) (131 ) Total reclassifications $ (2,257 ) $ (2,715 )

Revenue from contracts with c_2

Revenue from contracts with customers (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Disaggregation of revenue	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 16 . Year ended December 31, 2019 Electric Natural gas distribution Pipeline and midstream 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 * Intrasegment revenues are presented 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. Year ended December 31, 2018 Electric Natural gas distribution Pipeline and midstream 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 * Intrasegment revenues are presented 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 changes in contract assets and liabilities were as follows: 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 Deferred credits and other liabilities - other Net contract assets (liabilities) $ (33,709 ) $ 10,203 $ (43,912 ) December 31, 2018 December 31, 2017 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 104,239 $ 109,540 $ (5,301 ) Receivables, net Contract liabilities - current (93,901 ) (84,123 ) (9,778 ) Accounts payable Contract liabilities - noncurrent (135 ) — (135 ) Deferred credits and other liabilities - other Net contract assets $ 10,203 $ 25,417 $ (15,214 )

Business Combinations (Tables)

Business Combinations (Tables)	12 Months Ended
	Dec. 31, 2019
Business Acquisition [Line Items]
Schedule of Business Acquisitions, by Acquisition	The aggregate total consideration for the 2018 acquisitions and the final amounts allocated to the assets acquired and liabilities assumed were as follows: December 31, 2018 Measurement Period Adjustments December 31, 2019 (In thousands) Assets Current assets: Receivables, net $ 18,984 $ — $ 18,984 Inventories 10,329 (228 ) 10,101 Other current assets 515 (14 ) 501 Total current assets 29,828 (242 ) 29,586 Property, plant and equipment 131,766 6,669 138,435 Deferred charges and other assets: Goodwill 33,131 (6,669 ) 26,462 Other intangible assets, net 8,227 — 8,227 Other 927 — 927 Total deferred charges and other assets 42,285 (6,669 ) 35,616 Total assets acquired $ 203,879 $ (242 ) $ 203,637 Liabilities Current liabilities $ 11,122 $ (242 ) $ 10,880 Deferred credits and other liabilities: Asset retirement obligation 914 — 914 Deferred income taxes 5,565 — 5,565 Total deferred credits and other liabilities 6,479 — 6,479 Total liabilities assumed $ 17,601 $ (242 ) $ 17,359 Total consideration (fair value) $ 186,278 $ — $ 186,278

Discontinued operations (Tables

Discontinued operations (Tables)	12 Months Ended
	Dec. 31, 2019
Discontinued Operations and Disposal Groups [Abstract]
Disposal groups, including discontinued operations	The carrying amounts of the major classes of assets and liabilities classified as held for sale on the Consolidated Balance Sheets at December 31 were as follows: 2019 2018 (In thousands) Assets Current assets: Receivables, net $ 425 $ 430 Total current assets held for sale 425 430 Noncurrent assets: Deferred income taxes 1,265 1,926 Other 161 161 Total noncurrent assets held for sale 1,426 2,087 Total assets held for sale $ 1,851 $ 2,517 Liabilities Current liabilities: Accounts payable $ — $ 80 Taxes payable 1,279 1,451 Other accrued liabilities 2,232 2,470 Total current liabilities held for sale 3,511 4,001 Total liabilities held for sale $ 3,511 $ 4,001
Reconciliation of major classes of income and expense	The reconciliation of the major classes of income and expense constituting pretax income (loss) from discontinued operations to the after-tax income (loss) from discontinued operations on the Consolidated Statements of Income for the years ended December 31 were as follows: 2019 2018 2017 (In thousands) Operating revenues $ 103 $ (459 ) $ 465 Operating expenses 290 921 (4,607 ) Operating income (loss) (187 ) (1,380 ) 5,072 Other income (expense) — 12 (13 ) Interest expense — 575 250 Income (loss) from discontinued operations before income taxes (187 ) (1,943 ) 4,809 Income taxes* (474 ) (4,875 ) 8,592 Income (loss) from discontinued operations $ 287 $ 2,932 $ (3,783 ) * Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations .

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Lease, Cost	The following tables provide information on the Company's operating leases at and for the year ended December 31, 2019: (In thousands) Lease costs: Operating lease cost $ 43,759 Variable lease cost 1,555 Short-term lease cost 120,030 Total lease costs $ 165,344 (Dollars in thousands) Weighted average remaining lease term 3.13 years Weighted average discount rate 4.41 % Cash paid for amounts included in the measurement of lease liabilities $ 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, 2019 , was as follows: (In thousands) 2020 $ 35,156 2021 24,893 2022 16,932 2023 10,227 2024 7,368 Thereafter 47,926 Total 142,502 Less discount 27,096 Total operating lease liabilities $ 115,406 The undiscounted annual minimum lease payments due under the Company's leases following the previous lease accounting standard as of December 31, 2018 , were as follows: 2019 2020 2021 2022 2023 Thereafter (In thousands) Operating leases $ 37,740 $ 26,255 $ 17,868 $ 11,647 $ 7,278 $ 49,098

Goodwill and other intangible_2

Goodwill and other intangible assets (Tables)	12 Months Ended
	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying amount of goodwill	The changes in the carrying amount of goodwill for the year ended December 31, 2019 , were as follows: Balance at January 1, 2019 Goodwill Acquired During the Year 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 The changes in the carrying amount of goodwill for the year ended December 31, 2018 , were as follows: Balance at January 1, 2018 Goodwill Acquired During the Year Measurement Period Balance at December 31, 2018 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Construction materials and contracting 176,290 33,131 — 209,421 Construction services 109,765 — — 109,765 Total $ 631,791 $ 33,131 $ — $ 664,922
Other amortizable intangible assets	Other amortizable intangible assets at December 31 were as follows: 2019 2018 (In thousands) Customer relationships $ 17,958 $ 22,720 Less accumulated amortization 6,268 13,535 11,690 9,185 Noncompete agreements 3,439 2,605 Less accumulated amortization 1,957 1,956 1,482 649 Other 8,094 6,458 Less accumulated amortization 6,020 5,477 2,074 981 Total $ 15,246 $ 10,815
Estimated amortization expense	The amounts of estimated amortization expense for identifiable intangible assets as of December 31, 2019, were: 2020 2021 2022 2023 2024 Thereafter (In thousands) Amortization expense $ 3,365 $ 2,016 $ 1,968 $ 1,924 $ 1,610 $ 4,363

Regulatory assets and liabili_2

Regulatory assets and liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
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 Period * 2019 2018 (In thousands) Regulatory assets: Pension and postretirement benefits (a) (e) $ 157,069 $ 165,898 Natural gas costs recoverable through rate adjustments (a) (b) Up to 3 years 89,204 42,652 Asset retirement obligations (a) Over plant lives 66,000 60,097 Plants to be retired (a) - 32,931 — Cost recovery mechanisms (a) (b) Up to 3 years 19,396 17,948 Manufactured gas plant sites remediation (a) - 15,347 17,068 Taxes recoverable from customers (a) Over plant lives 11,486 11,946 Conservation programs (a) (b) Up to 3 years 7,405 7,494 Long-term debt refinancing costs (a) Up to 18 years 4,286 4,898 Costs related to identifying generation development (a) Up to 7 years 2,052 2,508 Other (a) (b) Up to 19 years 12,221 9,608 Total regulatory assets $ 417,397 $ 340,117 Regulatory liabilities: Taxes refundable to customers (c) (d) $ 249,506 $ 277,833 Plant removal and decommissioning costs (c) 173,722 173,143 Natural gas costs refundable through rate adjustments (d) 23,825 29,995 Pension and postretirement benefits (c) 18,065 15,264 Other (c) (d) 25,187 25,197 Total regulatory liabilities $ 490,305 $ 521,432 Net regulatory position $ (72,908 ) $ (181,315 ) * Estimated recovery period for regulatory assets currently being recovered in rates charged to customers. (a) Included in deferred charges and other assets - other on the Consolidated Balance Sheets. (b) Included in prepayments and other current assets on the Consolidated Balance Sheets. (c) Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets. (d) Included in other accrued liabilities on the Consolidated Balance Sheets. (e) 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 Period * 2019 2018 (In thousands) Regulatory assets: Pension and postretirement benefits (a) (e) $ 157,069 $ 165,898 Natural gas costs recoverable through rate adjustments (a) (b) Up to 3 years 89,204 42,652 Asset retirement obligations (a) Over plant lives 66,000 60,097 Plants to be retired (a) - 32,931 — Cost recovery mechanisms (a) (b) Up to 3 years 19,396 17,948 Manufactured gas plant sites remediation (a) - 15,347 17,068 Taxes recoverable from customers (a) Over plant lives 11,486 11,946 Conservation programs (a) (b) Up to 3 years 7,405 7,494 Long-term debt refinancing costs (a) Up to 18 years 4,286 4,898 Costs related to identifying generation development (a) Up to 7 years 2,052 2,508 Other (a) (b) Up to 19 years 12,221 9,608 Total regulatory assets $ 417,397 $ 340,117 Regulatory liabilities: Taxes refundable to customers (c) (d) $ 249,506 $ 277,833 Plant removal and decommissioning costs (c) 173,722 173,143 Natural gas costs refundable through rate adjustments (d) 23,825 29,995 Pension and postretirement benefits (c) 18,065 15,264 Other (c) (d) 25,187 25,197 Total regulatory liabilities $ 490,305 $ 521,432 Net regulatory position $ (72,908 ) $ (181,315 ) * Estimated recovery period for regulatory assets currently being recovered in rates charged to customers. (a) Included in deferred charges and other assets - other on the Consolidated Balance Sheets. (b) Included in prepayments and other current assets on the Consolidated Balance Sheets. (c) Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets. (d) Included in other accrued liabilities on the Consolidated Balance Sheets. (e) Recovered as expense is incurred or cash contributions are made.

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: December 31, 2019 Cost Gross Unrealized Gains Gross Unrealized Losses 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 December 31, 2018 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,473 $ 21 $ 162 $ 10,332 U.S. Treasury securities 179 — — 179 Total $ 10,652 $ 21 $ 162 $ 10,511
Assets and liabilities measured at fair value on a recurring basis	The Company's assets measured at fair value on a recurring basis were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) 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 Measurements at December 31, 2018, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at December 31, 2018 (In thousands) Assets: Money market funds $ — $ 10,799 $ — $ 10,799 Insurance contract* — 73,838 — 73,838 Available-for-sale securities: Mortgage-backed securities — 10,332 — 10,332 U.S. Treasury securities — 179 — 179 Total assets measured at fair value $ — $ 95,148 $ — $ 95,148 * The insurance contract invests approximately 53 percent in fixed-income investments, 21 percent in common stock of large-cap companies, 11 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents.
Fair value of long-term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt at December 31 was as follows: 2019 2018 (In thousands) Carrying Amount $ 2,243,107 $ 2,108,695 Fair Value $ 2,418,631 $ 2,183,819

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Outstanding credit facilities	The following table summarizes the outstanding revolving credit facilities of the Company's subsidiaries: Company Facility Facility Limit Amount Outstanding at December 31, 2019 Amount Outstanding at December 31, 2018 Letters of Credit at December 31, 2019 Expiration Date (In millions) Montana-Dakota Utilities Co. Commercial paper/Revolving credit agreement (a) $ 175.0 $ 118.6 (b) $ 48.5 $ — 12/19/24 Cascade Natural Gas Corporation Revolving credit agreement $ 100.0 (c) $ 64.6 $ 53.8 $ 2.2 (d) 6/7/24 Intermountain Gas Company Revolving credit agreement $ 85.0 (e) $ 24.5 $ 56.3 $ 1.4 (d) 6/7/24 Centennial Energy Holdings, Inc. Commercial paper/Revolving credit agreement (f) $ 600.0 $ 104.3 (b) $ 289.6 (b) $ — 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 at December 31, 2019, and $48.5 million was outstanding at December 31, 2018. (b) Amount outstanding under commercial paper program. (c) Certain provisions allow for increased borrowings, up to a maximum of $125.0 million . (d) Outstanding letter(s) of credit reduce the amount available under the credit agreement. (e) Certain provisions allow for increased borrowings, up to a maximum of $110.0 million . (f) 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, 2019 2019 2018 (In thousands) Senior Notes due on dates ranging from October 22, 2022 to November 18, 2059 4.45 % $ 1,850,000 $ 1,381,000 Commercial paper supported by revolving credit agreements 2.04 % 222,900 338,100 Term Loan Agreement due on September 3, 2032 2.00 % 9,100 209,800 Credit agreements due on June 7, 2024 4.40 % 89,050 110,100 Medium-Term Notes due on dates ranging from September 1, 2020 to March 16, 2029 6.68 % 50,000 50,000 Other notes due on dates ranging from July 15, 2021 to November 30, 2038 4.48 % 29,117 25,229 Less unamortized debt issuance costs 7,010 5,207 Less discount 50 327 Total long-term debt 2,243,107 2,108,695 Less current maturities 16,540 251,854 Net long-term debt $ 2,226,567 $ 1,856,841
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, 2019 , were as follows: 2020 2021 2022 2023 2024 Thereafter (In thousands) Long-term debt maturities $ 16,540 $ 1,528 $ 148,021 $ 77,921 $ 373,372 $ 1,632,785

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2019
Asset Retirement Obligation [Abstract]
Reconciliation of the company's asset retirement obligation	A reconciliation of the Company's liability, which is included in other accrued liabilities and deferred credits and other liabilities - other on the Consolidated Balance Sheets, for the years ended December 31 was as follows: 2019 2018 (In thousands) Balance at beginning of year $ 375,553 $ 341,969 Liabilities incurred 25,869 13,424 Liabilities acquired 486 1,002 Liabilities settled (7,097 ) (3,699 ) Accretion expense* 19,789 18,242 Revisions in estimates 2,975 4,615 Balance at end of year $ 417,575 $ 375,553 * Includes $18.3 million and $16.8 million in 2019 and 2018, respectively, related to regulatory assets.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement, Noncash Expense [Abstract]
Target grants performance share	Target grants of performance shares outstanding at December 31, 2019 , were as follows: Grant Date Performance Period Target Grant of Shares February 2018 2018-2020 246,309 February 2019 2019-2021 327,194
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 2019 , 2018 and 2017 were: 2019 2018 2017 Weighted average grant-date fair value $35.07 $34.55 $24.31 Blended volatility range 19.50 % – 19.69 % 17.87 % – 22.14 % 22.70 % – 25.56 % Risk-free interest rate range 2.46 % – 2.55 % 1.86 % – 2.46 % .69 % – 1.61 % Weighted average discounted dividends per share $2.85 $2.46 $1.70
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, 2019 , was as follows: Number of Shares Weighted Average Grant-Date Fair Value Nonvested at beginning of period 668,791 $ 23.03 Granted 327,194 30.66 Additional performance shares earned 103,159 14.60 Less: Vested 398,919 15.52 Forfeited 126,722 24.31 Nonvested at end of period 573,503 $ 30.81

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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: 2019 2018 2017 (In thousands) United States $ 398,532 $ 317,655 $ 350,064 Foreign (87 ) (784 ) (37 ) Income before income taxes from continuing operations $ 398,445 $ 316,871 $ 350,027
Income tax expense	Income tax expense (benefit) from continuing operations for the years ended December 31 was as follows: 2019 2018 2017 (In thousands) Current: Federal $ (3,502 ) $ (15,901 ) $ 74,272 State 3,366 3,651 16,192 (136 ) (12,250 ) 90,464 Deferred: Income taxes: Federal 50,218 50,755 (24,497 ) State 12,098 7,206 (864 ) Investment tax credit - net 1,099 1,774 (62 ) 63,415 59,735 (25,423 ) Total income tax expense $ 63,279 $ 47,485 $ 65,041
Deferred tax assets and deferred tax liabilities	Components of deferred tax assets and deferred tax liabilities at December 31 were as follows: 2019 2018 (In thousands) Deferred tax assets: Postretirement $ 51,075 $ 51,930 Compensation-related 37,330 29,885 Operating lease liabilities 24,459 — Asset retirement obligations 7,450 7,083 Customer advances 7,325 7,734 Legal and environmental contingencies 6,601 6,729 Federal renewable energy credit 5,343 8,015 Alternative minimum tax credit carryforward — 13,404 Other 32,533 37,347 Total deferred tax assets 172,116 162,127 Deferred tax liabilities: Depreciation and basis differences on property, plant and equipment 511,867 476,832 Postretirement 48,927 44,432 Operating lease right-of-use-assets 24,436 — Intangible asset amortization 18,930 17,752 Other 61,385 39,712 Total deferred tax liabilities 665,545 578,728 Valuation allowance 13,154 13,484 Net deferred income tax liability $ 506,583 $ 430,085
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, 2018 , to December 31, 2019 , to deferred income tax expense: 2019 (In thousands) Change in net deferred income tax liability from the preceding table $ 76,498 Deferred taxes associated with other comprehensive loss 1,631 Deferred taxes associated with TCJA enactment for regulated activities (11,904 ) Other (2,810 ) Deferred income tax expense for the period $ 63,415
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, 2019 2018 2017 Amount % Amount % Amount % (Dollars in thousands) Computed tax at federal statutory rate $ 83,674 21.0 $ 66,543 21.0 $ 122,509 35.0 Increases (reductions) resulting from: State income taxes, net of federal income tax 14,029 3.5 12,190 3.8 10,724 3.1 Federal renewable energy credit (15,843 ) (4.0 ) (11,759 ) (3.7 ) (13,958 ) (4.0 ) Tax compliance and uncertain tax positions (2,739 ) (.7 ) (2,725 ) (.9 ) (643 ) (.2 ) Domestic production deduction — — — — (6,849 ) (2.0 ) Excess deferred income tax amortization (11,904 ) (3.0 ) (9,319 ) (2.9 ) (397 ) — TCJA revaluation — — (5,947 ) (1.9 ) (47,242 ) (13.5 ) TCJA revaluation related to accumulated other comprehensive loss balance — — (42 ) — 7,735 2.2 Other (3,938 ) (.9 ) (1,456 ) (.4 ) (6,838 ) (2.0 ) Total income tax expense $ 63,279 15.9 $ 47,485 15.0 $ 65,041 18.6

Cash flow information (Tables)

Cash flow information (Tables)	12 Months Ended
	Dec. 31, 2019
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: 2019 2018 2017 (In thousands) Interest, net* $ 93,414 $ 83,009 $ 79,638 Income taxes paid (refunded), net** $ (8,475 ) $ 16,041 $ 112,137 * AFUDC - borrowed was $2.8 million , $2.3 million and $966,000 for the years ended December 31, 2019 , 2018 and 2017 , respectively. ** Income taxes paid (refunded), including discontinued operations, were $(9.4) million , $5.5 million and $9.7 million for the years ended December 31, 2019 , 2018 and 2017 , respectively. Noncash investing and financing transactions at December 31 were as follows: 2019 2018 2017 (In thousands) Property, plant and equipment additions in accounts payable $ 46,119 $ 42,355 $ 29,263 Issuance of common stock in connection with acquisition $ — $ 18,186 $ — Debt assumed in connection with a business combination $ 1,163 $ — $ — Right-of-use assets obtained in exchange for new operating lease liabilities $ 54,880 $ — $ —

Business segment data (Tables)

Business segment data (Tables)	12 Months Ended
	Dec. 31, 2019
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: 2019 2018 2017 (In thousands) External operating revenues: Regulated operations: Electric $ 351,725 $ 335,123 $ 342,805 Natural gas distribution 865,222 823,247 848,388 Pipeline and midstream 62,357 54,857 53,566 1,279,304 1,213,227 1,244,759 Nonregulated operations: Pipeline and midstream 21,835 23,161 19,602 Construction materials and contracting 2,189,651 1,925,185 1,811,964 Construction services 1,845,896 1,369,772 1,366,317 Other 90 207 709 4,057,472 3,318,325 3,198,592 Total external operating revenues $ 5,336,776 $ 4,531,552 $ 4,443,351 Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — Natural gas distribution — — — Pipeline and midstream 56,037 50,580 48,867 56,037 50,580 48,867 Nonregulated operations: Pipeline and midstream 215 325 178 Construction materials and contracting 1,066 669 565 Construction services 3,370 1,681 1,285 Other 16,461 11,052 7,165 21,112 13,727 9,193 Intersegment eliminations (77,149 ) (64,307 ) (58,060 ) Total intersegment operating revenues $ — $ — $ — Depreciation, depletion and amortization: Electric $ 58,721 $ 50,982 $ 47,715 Natural gas distribution 79,564 72,486 69,381 Pipeline and midstream 21,220 17,896 16,788 Construction materials and contracting 77,450 61,158 55,862 Construction services 17,038 15,728 15,739 Other 2,024 1,955 2,001 Total depreciation, depletion and amortization $ 256,017 $ 220,205 $ 207,486 Operating income (loss): Electric $ 64,039 $ 65,148 $ 79,902 Natural gas distribution 69,188 72,336 84,239 Pipeline and midstream 42,796 36,128 36,004 Construction materials and contracting 179,955 141,426 143,230 Construction services 126,426 86,764 81,292 Other (1,184 ) (79 ) (619 ) Total operating income $ 481,220 $ 401,723 $ 424,048 2019 2018 2017 (In thousands) Interest expense: Electric $ 25,334 $ 25,860 $ 25,377 Natural gas distribution 35,488 30,768 31,234 Pipeline and midstream 7,198 5,964 4,990 Construction materials and contracting 23,792 17,290 14,778 Construction services 5,331 3,551 3,742 Other 1,859 2,762 3,564 Intersegment eliminations (415 ) (1,581 ) (897 ) Total interest expense $ 98,587 $ 84,614 $ 82,788 Income taxes: Electric $ (12,650 ) $ (6,482 ) $ 7,699 Natural gas distribution 1,405 4,075 22,756 Pipeline and midstream 7,219 2,677 12,281 Construction materials and contracting 37,389 28,357 5,405 Construction services 29,973 20,000 25,558 Other (57 ) (1,142 ) (1,809 ) Intersegment eliminations — — (6,849 ) Total income taxes $ 63,279 $ 47,485 $ 65,041 Earnings on common stock: Regulated operations: Electric $ 54,763 $ 47,000 $ 49,366 Natural gas distribution 39,517 37,732 32,225 Pipeline and midstream 28,255 26,905 20,620 122,535 111,637 102,211 Nonregulated operations: Pipeline and midstream 1,348 1,554 (127 ) Construction materials and contracting 120,371 92,647 123,398 Construction services 92,998 64,309 53,306 Other (2,086 ) (761 ) (1,422 ) 212,631 157,749 175,155 Intersegment eliminations (a) — — 6,849 Earnings on common stock before income (loss) from discontinued operations 335,166 269,386 284,215 Income (loss) from discontinued operations, net of tax (a) 287 2,932 (3,783 ) Earnings on common stock $ 335,453 $ 272,318 $ 280,432 Capital expenditures: Electric $ 99,449 $ 186,105 $ 109,107 Natural gas distribution 206,799 205,896 146,981 Pipeline and midstream 71,477 70,057 31,054 Construction materials and contracting 190,092 280,396 44,302 Construction services 60,500 25,081 18,630 Other 8,181 1,768 1,850 Total capital expenditures (b) $ 636,498 $ 769,303 $ 351,924 2019 2018 2017 (In thousands) Assets: Electric (c) $ 1,680,194 $ 1,613,822 $ 1,470,922 Natural gas distribution (c) 2,574,965 2,375,871 2,201,081 Pipeline and midstream 677,482 616,959 566,295 Construction materials and contracting 1,684,161 1,508,032 1,238,696 Construction services 761,127 604,798 591,382 Other (d) 303,279 266,111 261,419 Assets held for sale 1,851 2,517 4,871 Total assets $ 7,683,059 $ 6,988,110 $ 6,334,666 Property, plant and equipment: Electric (c) $ 2,227,145 $ 2,148,569 $ 1,982,264 Natural gas distribution (c) 2,688,123 2,499,093 2,319,845 Pipeline and midstream 834,215 764,959 700,284 Construction materials and contracting 1,910,562 1,768,006 1,560,048 Construction services 213,370 188,586 177,265 Other 35,213 28,108 31,123 Less accumulated depreciation, depletion and amortization 2,991,486 2,818,644 2,691,641 Net property, plant and equipment $ 4,917,142 $ 4,578,677 $ 4,079,188 (a) Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations. (b) Capital expenditures for 2019, 2018 and 2017 include noncash transactions such as the issuance of the Company's equity securities in connection with acquisitions, capital expenditure-related accounts payable and AFUDC, totaling $4.8 million , $33.4 million and $10.5 million , respectively. (c) Includes allocations of common utility property. (d) 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, 2019
Retirement Benefits [Abstract]
Schedule of amounts recognized in balance sheet	Changes in benefit obligation and plan assets for the years ended December 31, 2019 and 2018 , and amounts recognized in the Consolidated Balance Sheets at December 31, 2019 and 2018 , were as follows: Pension Benefits Other Postretirement Benefits 2019 2018 2019 2018 (In thousands) Change in benefit obligation: Benefit obligation at beginning of year $ 391,602 $ 445,923 $ 81,201 $ 91,206 Service cost — — 1,142 1,494 Interest cost 15,225 14,591 2,986 2,899 Plan participants' contributions — — 1,040 1,282 Actuarial (gain) loss 40,219 (32,637 ) 2,632 (10,115 ) Benefits paid (25,880 ) (36,275 ) (5,387 ) (5,565 ) Benefit obligation at end of year 421,166 391,602 83,614 81,201 Change in net plan assets: Fair value of plan assets at beginning of year 307,809 354,384 82,516 88,739 Actual gain (loss) on plan assets 58,409 (21,138 ) 15,731 (2,781 ) Employer contribution 24,926 10,838 687 842 Plan participants' contributions — — 1,040 1,281 Benefits paid (25,880 ) (36,275 ) (5,387 ) (5,565 ) Fair value of net plan assets at end of year 365,264 307,809 94,587 82,516 Funded status - over (under) $ (55,902 ) $ (83,793 ) $ 10,973 $ 1,315 Amounts recognized in the Consolidated Balance Sheets at December 31: Deferred charges and other assets - other $ — $ — $ 30,475 20,843 Other accrued liabilities — — 647 660 Deferred credits and other liabilities - other 55,902 83,793 18,855 18,868 Benefit obligation assets (liabilities) - net amount recognized $ (55,902 ) $ (83,793 ) $ 10,973 $ 1,315 Amounts recognized in accumulated other comprehensive loss: Actuarial loss $ 27,748 $ 28,796 $ 6,118 $ 6,372 Prior service credit — — (731 ) (848 ) Total $ 27,748 $ 28,796 $ 5,387 $ 5,524 Amounts recognized in regulatory assets or liabilities: Actuarial (gain) loss $ 155,484 $ 159,939 $ (4,450 ) $ 3,944 Prior service credit — — (8,109 ) (9,390 ) Total $ 155,484 $ 159,939 $ (12,559 ) $ (5,446 )
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: 2019 2018 (In thousands) Projected benefit obligation $ 421,166 $ 391,602 Accumulated benefit obligation $ 421,166 $ 391,602 Fair value of plan assets $ 365,264 $ 307,809 The projected benefit obligation and accumulated benefit obligation for these plans at December 31 were as follows: 2019 2018 (In thousands) Projected benefit obligation $ 99,245 $ 93,988 Accumulated benefit obligation $ 99,245 $ 93,988
Components of net periodic benefit cost	Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2019 2018 2017 2019 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ — $ 1,142 $ 1,494 $ 1,508 Interest cost 15,225 14,591 16,207 2,986 2,899 3,265 Expected return on assets (18,236 ) (20,753 ) (20,528 ) (4,804 ) (4,866 ) (4,641 ) Amortization of prior service credit — — — (1,398 ) (1,394 ) (1,371 ) Recognized net actuarial loss 5,548 7,005 6,355 353 640 857 Net periodic benefit cost (credit), including amount capitalized 2,537 843 2,034 (1,721 ) (1,227 ) (382 ) Less amount capitalized — — 310 113 153 (370 ) Net periodic benefit cost (credit) 2,537 843 1,724 (1,834 ) (1,380 ) (12 ) Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss: Net (gain) loss (144 ) 991 (1,091 ) (127 ) (1,735 ) 1,742 Amortization of actuarial loss (904 ) (1,084 ) (1,040 ) (110 ) (354 ) (289 ) Amortization of prior service (cost) credit — — — 100 (220 ) 161 Total recognized in accumulated other comprehensive loss (1,048 ) (93 ) (2,131 ) (137 ) (2,309 ) 1,614 Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities: Net (gain) loss 189 8,263 (4,736 ) (8,168 ) (732 ) (4,932 ) Amortization of actuarial loss (4,644 ) (5,921 ) (5,315 ) (242 ) (286 ) (568 ) Amortization of prior service credit — — — 1,297 1,614 1,210 Total recognized in regulatory assets or liabilities (4,455 ) 2,342 (10,051 ) (7,113 ) 596 (4,290 ) Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities $ (2,966 ) $ 3,092 $ (10,458 ) $ (9,084 ) $ (3,093 ) $ (2,688 ) Components of net periodic benefit cost for these plans for the years ended December 31 were as follows: 2019 2018 2017 (In thousands) Components of net periodic benefit cost: Service cost $ 109 $ 185 $ 289 Interest cost 3,473 3,157 3,494 Recognized net actuarial loss 764 1,047 883 Net periodic benefit cost $ 4,346 $ 4,389 $ 4,666
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 2019 2018 2019 2018 Discount rate 2.96 % 4.03 % 3.00 % 4.05 % Expected return on plan assets 6.25 % 6.75 % 5.75 % 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 2019 2018 2019 2018 Discount rate 4.03 % 3.38 % 4.05 % 3.41 % Expected return on plan assets 6.25 % 6.75 % 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: 2019 2018 Benefit obligation discount rate 2.73 % 3.86 % Benefit obligation rate of compensation increase N/A N/A Net periodic benefit cost discount rate 3.86 % 3.20 % 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: 2019 2018 Health care trend rate assumed for next year 7.1 % – 7.4 % 7.5 % – 8.1 % Health care cost trend rate - ultimate 4.5 % 4.5 % Year in which ultimate trend rate achieved 2024 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, 2019 : 1 Percentage Point Increase 1 Percentage Point Decrease (In thousands) Effect on total of service and interest cost components $ 245 $ (203 ) Effect on postretirement benefit obligation $ 3,751 $ (3,155 )
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, 2019, are as follows: Years Pension Benefits Other Postretirement Benefits Expected Medicare Part D Subsidy (In thousands) 2020 $ 24,128 $ 5,024 $ 92 2021 24,432 5,073 86 2022 24,642 5,098 80 2023 24,874 5,091 73 2024 24,924 5,000 65 2025-2029 121,205 24,242 222 The amount of future benefit payments for the unfunded, nonqualified defined benefit plans at December 31, 2019 , are expected to aggregate as follows: 2020 2021 2022 2023 2024 2025-2029 (In thousands) Nonqualified benefits $ 7,774 $ 7,795 $ 7,023 $ 7,219 $ 7,597 $ 35,998
The fair value of the pension and postretirement net plan assets by class	The fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices Significant Significant Unobservable 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 Insurance contract* 10 88,486 — 88,496 Total assets measured at fair value $ 2,083 $ 92,504 $ — $ 94,587 * 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. Fair Value Measurements at December 31, 2018, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2018 (In thousands) Assets: Cash equivalents $ — $ 3,866 $ — $ 3,866 Equity securities: U.S. companies 1,767 — — 1,767 International companies — 2 — 2 Insurance contract* 1 76,880 — 76,881 Total assets measured at fair value $ 1,768 $ 80,748 $ — $ 82,516 * The insurance contract invests approximately 51 percent in corporate bonds, 23 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 12 percent in other investments. The fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) 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. Fair Value Measurements at December 31, 2018, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at December 31, 2018 (In thousands) Assets: Cash equivalents $ — $ 4,930 $ — $ 4,930 Equity securities: U.S. companies 11,038 — — 11,038 International companies — 967 — 967 Collective and mutual funds* 145,960 51,600 — 197,560 Corporate bonds — 73,110 — 73,110 Municipal bonds — 10,624 — 10,624 U.S. Government securities 479 5,896 — 6,375 Total assets measured at fair value $ 157,477 $ 147,127 $ — $ 304,604 * Collective and mutual funds invest approximately 27 percent in common stock of international companies, 31 percent in corporate bonds, 18 percent in common stock of large-cap U.S. companies, 5 percent in cash equivalents and 19 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: 2019 2018 (In thousands) Investments Insurance contract* $ 87,009 $ 73,838 Life insurance** 38,659 37,274 Other 8,450 10,818 Total investments $ 134,118 $ 121,930 * 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).

Nonqualified Benefit Plans (Tab

Nonqualified Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Schedule of Accumulated and Projected Benefit Obligations	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: 2019 2018 (In thousands) Projected benefit obligation $ 421,166 $ 391,602 Accumulated benefit obligation $ 421,166 $ 391,602 Fair value of plan assets $ 365,264 $ 307,809 The projected benefit obligation and accumulated benefit obligation for these plans at December 31 were as follows: 2019 2018 (In thousands) Projected benefit obligation $ 99,245 $ 93,988 Accumulated benefit obligation $ 99,245 $ 93,988
Schedule of Net Benefit Costs	Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2019 2018 2017 2019 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ — $ 1,142 $ 1,494 $ 1,508 Interest cost 15,225 14,591 16,207 2,986 2,899 3,265 Expected return on assets (18,236 ) (20,753 ) (20,528 ) (4,804 ) (4,866 ) (4,641 ) Amortization of prior service credit — — — (1,398 ) (1,394 ) (1,371 ) Recognized net actuarial loss 5,548 7,005 6,355 353 640 857 Net periodic benefit cost (credit), including amount capitalized 2,537 843 2,034 (1,721 ) (1,227 ) (382 ) Less amount capitalized — — 310 113 153 (370 ) Net periodic benefit cost (credit) 2,537 843 1,724 (1,834 ) (1,380 ) (12 ) Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss: Net (gain) loss (144 ) 991 (1,091 ) (127 ) (1,735 ) 1,742 Amortization of actuarial loss (904 ) (1,084 ) (1,040 ) (110 ) (354 ) (289 ) Amortization of prior service (cost) credit — — — 100 (220 ) 161 Total recognized in accumulated other comprehensive loss (1,048 ) (93 ) (2,131 ) (137 ) (2,309 ) 1,614 Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities: Net (gain) loss 189 8,263 (4,736 ) (8,168 ) (732 ) (4,932 ) Amortization of actuarial loss (4,644 ) (5,921 ) (5,315 ) (242 ) (286 ) (568 ) Amortization of prior service credit — — — 1,297 1,614 1,210 Total recognized in regulatory assets or liabilities (4,455 ) 2,342 (10,051 ) (7,113 ) 596 (4,290 ) Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities $ (2,966 ) $ 3,092 $ (10,458 ) $ (9,084 ) $ (3,093 ) $ (2,688 ) Components of net periodic benefit cost for these plans for the years ended December 31 were as follows: 2019 2018 2017 (In thousands) Components of net periodic benefit cost: Service cost $ 109 $ 185 $ 289 Interest cost 3,473 3,157 3,494 Recognized net actuarial loss 764 1,047 883 Net periodic benefit cost $ 4,346 $ 4,389 $ 4,666
Schedule of Assumptions Used	Weighted average assumptions used to determine benefit obligations at December 31 were as follows: Pension Benefits Other 2019 2018 2019 2018 Discount rate 2.96 % 4.03 % 3.00 % 4.05 % Expected return on plan assets 6.25 % 6.75 % 5.75 % 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 2019 2018 2019 2018 Discount rate 4.03 % 3.38 % 4.05 % 3.41 % Expected return on plan assets 6.25 % 6.75 % 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: 2019 2018 Benefit obligation discount rate 2.73 % 3.86 % Benefit obligation rate of compensation increase N/A N/A Net periodic benefit cost discount rate 3.86 % 3.20 % Net periodic benefit cost rate of compensation increase N/A N/A
Schedule of Expected Benefit Payments	The following benefit payments, which reflect future service, as appropriate, and expected Medicare Part D subsidies at December 31, 2019, are as follows: Years Pension Benefits Other Postretirement Benefits Expected Medicare Part D Subsidy (In thousands) 2020 $ 24,128 $ 5,024 $ 92 2021 24,432 5,073 86 2022 24,642 5,098 80 2023 24,874 5,091 73 2024 24,924 5,000 65 2025-2029 121,205 24,242 222 The amount of future benefit payments for the unfunded, nonqualified defined benefit plans at December 31, 2019 , are expected to aggregate as follows: 2020 2021 2022 2023 2024 2025-2029 (In thousands) Nonqualified benefits $ 7,774 $ 7,795 $ 7,023 $ 7,219 $ 7,597 $ 35,998
Schedule of Allocation of Plan Assets	The fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices Significant Significant Unobservable 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 Insurance contract* 10 88,486 — 88,496 Total assets measured at fair value $ 2,083 $ 92,504 $ — $ 94,587 * 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. Fair Value Measurements at December 31, 2018, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2018 (In thousands) Assets: Cash equivalents $ — $ 3,866 $ — $ 3,866 Equity securities: U.S. companies 1,767 — — 1,767 International companies — 2 — 2 Insurance contract* 1 76,880 — 76,881 Total assets measured at fair value $ 1,768 $ 80,748 $ — $ 82,516 * The insurance contract invests approximately 51 percent in corporate bonds, 23 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 12 percent in other investments. The fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements at December 31, 2019, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) 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. Fair Value Measurements at December 31, 2018, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at December 31, 2018 (In thousands) Assets: Cash equivalents $ — $ 4,930 $ — $ 4,930 Equity securities: U.S. companies 11,038 — — 11,038 International companies — 967 — 967 Collective and mutual funds* 145,960 51,600 — 197,560 Corporate bonds — 73,110 — 73,110 Municipal bonds — 10,624 — 10,624 U.S. Government securities 479 5,896 — 6,375 Total assets measured at fair value $ 157,477 $ 147,127 $ — $ 304,604 * Collective and mutual funds invest approximately 27 percent in common stock of international companies, 31 percent in corporate bonds, 18 percent in common stock of large-cap U.S. companies, 5 percent in cash equivalents and 19 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: 2019 2018 (In thousands) Investments Insurance contract* $ 87,009 $ 73,838 Life insurance** 38,659 37,274 Other 8,450 10,818 Total investments $ 134,118 $ 121,930 * 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).

Multiemployer Plans (Tables)

Multiemployer Plans (Tables)	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
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 of Collective Bargaining Agreement Pension Fund 2019 2018 2019 2018 2017 (In thousands) Alaska Laborers-Employers Retirement Fund 91-6028298-001 Yellow as of 6/30/2019 Yellow as of 6/30/2018 Implemented $ 815 $ 732 $ 690 No 12/31/2020 Construction Industry and Laborers Joint Pension Trust for So Nevada, Plan A 88-0135695-001 Red Red Implemented 544 346 377 No 6/30/2020 Edison Pension Plan 93-6061681-001 Green Green No 12,252 12,111 12,725 No 12/31/2020 IBEW Local 212 Pension Trust 31-6127280-001 Green as of 4/30/2019 Green as of 4/30/2018 No 1,110 1,341 1,312 No 6/1/2025 IBEW Local 357 Pension Plan A 88-6023284-001 Green Green No 10,162 3,460 3,286 No 5/31/2021 IBEW Local 648 Pension Plan 31-6134845-001 Yellow as of 2/28/2019 Yellow as of 2/28/2018 Implemented 728 2,175 2,254 No 8/29/2021 IBEW Local 82 Pension Plan 31-6127268-001 Green as of 6/30/2019 Green as of 6/30/2018 No 1,662 1,569 1,757 No 12/3/2023 Idaho Plumbers and Pipefitters Pension Plan 82-6010346-001 Green as of 5/31/2019 Green as of 5/31/2018 No 1,307 1,247 1,156 No 3/31/2023 Minnesota Teamsters Construction Division Pension Fund 41-6187751-001 Green as of 11/30/2018 Green as of 11/30/2017 No 673 740 826 No 4/30/2021 National Automatic Sprinkler Industry Pension Fund 52-6054620-001 Red Red Implemented 1,074 738 718 No 3/31/2021- National Electrical Benefit Fund 53-0181657-001 Green Green No 12,679 8,468 8,891 No 8/31/2019- * Pension Trust Fund for Operating Engineers 94-6090764-001 Yellow Yellow Implemented 2,598 2,403 2,391 No 3/31/2020- Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV 95-6052257-001 Yellow Yellow Implemented 2,119 1,774 1,016 No 6/30/2020 Southwest Marine Pension Trust 95-6123404-001 Red Red Implemented 132 81 48 No 1/31/2024 Other funds 24,670 21,537 19,298 Total contributions $ 72,525 $ 58,722 $ 56,745 * 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 of Total Contributions (as of December 31 of the Plan's Year-End) Edison Pension Plan 2018 and 2017 IBEW Local 82 Pension Plan 2018 and 2017 IBEW Local 124 Pension Trust Fund 2018 and 2017 IBEW Local 212 Pension Trust Fund 2018 and 2017 IBEW Local 357 Pension Plan A 2018 and 2017 IBEW Local 648 Pension Plan 2018 and 2017 IBEW Local Union No 226 Open End Pension Fund 2018 Idaho Plumbers and Pipefitters Pension Plan 2018 and 2017 International Union of Operating Engineers Local 701 Pension Trust Fund 2018 and 2017 Minnesota Teamsters Construction Division Pension Fund 2018 and 2017 Pension and Retirement Plan of Plumbers and Pipefitters Local 525 2018 and 2017

Jointly owned facilities (Table

Jointly owned facilities (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 (In thousands) Big Stone Station: 22.7 % Utility plant in service $ 152,836 $ 156,534 Construction work in progress 518 92 Less accumulated depreciation 46,266 49,345 $ 107,088 $ 107,281 BSSE: 50.0 % Utility plant in service $ 105,767 $ — Construction work in progress — 105,846 Less accumulated depreciation 1,232 — $ 104,535 $ 105,846 Coyote Station: 25.0 % Utility plant in service $ 160,235 $ 155,236 Construction work in progress 21 1,920 Less accumulated depreciation 107,638 105,565 $ 52,618 $ 51,591 Wygen III: 25.0 % Utility plant in service $ 67,869 $ 65,382 Construction work in progress 112 220 Less accumulated depreciation 10,482 9,174 $ 57,499 $ 56,428

Commitment and Contingencies Di

Commitment and Contingencies Disclosure (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Purchase commitments	The commitments under these contracts as of December 31, 2019 , were: 2020 2021 2022 2023 2024 Thereafter (In thousands) Purchase commitments $ 405,535 $ 250,266 $ 184,225 $ 123,166 $ 87,297 $ 678,432

Quarterly data (Tables)

Quarterly data (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Quarterly financial data	The following unaudited information shows selected items by quarter for the years 2019 and 2018 : First Quarter Second Quarter Third Quarter Fourth Quarter (In thousands, except per share amounts) 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 2018 Operating revenues $ 976,293 $ 1,064,597 $ 1,280,787 $ 1,209,875 Operating expenses 906,917 990,605 1,140,783 1,091,524 Operating income 69,376 73,992 140,004 118,351 Income from continuing operations 41,960 44,075 107,369 75,982 Income (loss) from discontinued operations, net of tax 477 (273 ) (118 ) 2,846 Net income 42,437 43,802 107,251 78,828 Earnings per share - basic: Income from continuing operations .22 .22 .55 .39 Discontinued operations, net of tax — — — .01 Earnings per share - basic .22 .22 .55 .40 Earnings per share - diluted: Income from continuing operations .22 .22 .55 .39 Discontinued operations, net of tax — — — .01 Earnings per share - diluted .22 .22 .55 .40 Weighted average common shares outstanding: Basic 195,304 195,524 196,018 196,023 Diluted 195,982 196,169 196,265 196,385

Accounts receivable and allowan

Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Receivables past due 90 days or more	$ 46,700	$ 30,000
Allowance for doubtful accounts	8,500	8,900
Accrued unbilled revenue at MDU Energy Capital	100,800	96,200
Short-term retainage	75,590	56,228
Long-term retainage	14,228	4,152
Total retainage	$ 89,818	$ 60,380

Inventories and natural gas in

Inventories and natural gas in storage (Details 2) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Inventory and natural gas in storage [Line Items]
Aggregates held for resale	$ 147,723	$ 139,681
Asphalt oil	41,912	54,741
Materials and supplies	22,512	23,611
Merchandise for resale	22,232	22,552
Natural gas in storage (current)	22,058	22,117
Other	21,970	24,607
Total	278,407	287,309
Natural gas in storage noncurrent	$ 48,400	$ 48,500

Property, plant and equipment (

Property, plant and equipment (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, plant and equipment [Line Items]
AFUDC - borrowed	$ 2,807	$ 2,290	$ 966
AFUDC - equity	698	1,897	909
Property, plant and equipment	7,908,628	7,397,321
Less accumulated depreciation, depletion and amortization	2,991,486	2,818,644	2,691,641
Net property, plant and equipment	4,917,142	4,578,677	4,079,188
Electric
Property, plant and equipment [Line Items]
Property, plant and equipment	2,227,145	2,148,569	1,982,264
Natural gas distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	2,688,123	2,499,093	2,319,845
Pipeline and midstream
Property, plant and equipment [Line Items]
Property, plant and equipment	834,215	764,959	700,284
Construction materials and contracting
Property, plant and equipment [Line Items]
Property, plant and equipment	1,910,562	1,768,006	1,560,048
Construction services
Property, plant and equipment [Line Items]
Property, plant and equipment	213,370	188,586	177,265
Other
Property, plant and equipment [Line Items]
Property, plant and equipment	35,213	28,108	$ 31,123
Regulated: | Electric | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 1,139,059	1,131,484
Weighted average depreciable life in years	48 years
Regulated: | Electric | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 443,780	430,750
Weighted average depreciable life in years	46 years
Regulated: | Electric | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 445,485	302,315
Weighted average depreciable life in years	65 years
Regulated: | Electric | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 66,664	161,893
Regulated: | Electric | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 132,157	122,127
Weighted average depreciable life in years	15 years
Regulated: | Natural gas distribution | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 2,133,249	1,981,356
Weighted average depreciable life in years	47 years
Regulated: | Natural gas distribution | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 39,506	21,028
Regulated: | Natural gas distribution | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 515,368	496,708
Weighted average depreciable life in years	17 years
Regulated: | Pipeline and midstream | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 636,796	585,594
Weighted average depreciable life in years	46 years
Regulated: | Pipeline and midstream | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 35,661	37,829
Weighted average depreciable life in years	20 years
Regulated: | Pipeline and midstream | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 50,001	49,101
Weighted average depreciable life in years	53 years
Regulated: | Pipeline and midstream | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 22,597	5,915
Regulated: | Pipeline and midstream | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 48,340	45,763
Weighted average depreciable life in years	16 years
Nonregulated: | Pipeline and midstream | Gathering and processing
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 31,148	31,094
Weighted average depreciable life in years	19 years
Nonregulated: | Pipeline and midstream | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 154	86
Nonregulated: | Pipeline and midstream | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 9,518	9,577
Weighted average depreciable life in years	10 years
Nonregulated: | Construction materials and contracting | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 127,729	109,541
Nonregulated: | Construction materials and contracting | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 122,064	114,905
Weighted average depreciable life in years	20 years
Nonregulated: | Construction materials and contracting | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 1,180,343	1,090,790
Weighted average depreciable life in years	12 years
Nonregulated: | Construction materials and contracting | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 25,018	22,507
Nonregulated: | Construction materials and contracting | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	455,408	430,263
Nonregulated: | Construction services | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	7,146	5,216
Nonregulated: | Construction services | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 31,735	29,795
Weighted average depreciable life in years	24 years
Nonregulated: | Construction services | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 156,537	145,859
Weighted average depreciable life in years	6 years
Nonregulated: | Construction services | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 17,952	7,716
Weighted average depreciable life in years	2 years
Nonregulated: | Other | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 2,648	2,648
Nonregulated: | Other | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 32,565	$ 25,461
Weighted average depreciable life in years	14 years

Goodwill (Details 4)

Goodwill (Details 4)	3 Months Ended
	Dec. 31, 2019
Risk adjusted capital cost | Minimum
Goodwill [Line Items]
Rates used to value goodwill	4.00%
Risk adjusted capital cost | Maximum
Goodwill [Line Items]
Rates used to value goodwill	9.00%
Long-term growth rate | Minimum
Goodwill [Line Items]
Rates used to value goodwill	2.00%
Long-term growth rate | Maximum
Goodwill [Line Items]
Rates used to value goodwill	3.00%

Natural gas costs recoverable o

Natural gas costs recoverable or refundable through rate adjustments (Details 5) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Natural gas costs refundable	$ 23.8	$ 30
Natural gas costs recoverable	$ 89.2	$ 42.7

Income taxes (Details 6)

Income taxes (Details 6)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Threshold of likelihood of tax positions being realized upon ultimate settlement with a taxing authority	50.00%

Earnings (loss) per common shar

Earnings (loss) per common share (Details 7) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Weighted average common shares outstanding - basic	200,383	199,343	198,270	196,401	196,023	196,018	195,524	195,304	198,612	195,720	195,304
Effect of dilutive performance share awards									14	430	383
Weighted average common shares outstanding - diluted	200,478	199,383	198,287	196,414	196,385	196,265	196,169	195,982	198,626	196,150	195,687
Shares excluded from the calculation of diluted earnings per share									164	10	0

New accounting standards (Detai

New accounting standards (Details 8) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification	$ 112

Accumulated other comprehensive

Accumulated other comprehensive income (loss) (Details 9) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,566,775
Net current-period other comprehensive income (loss)	(3,760)	$ 6,951	$ (1,601)
Balance	2,847,246	2,566,775
Net Unrealized Loss on Derivative Instruments Qualifying as Hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,161)	(1,934)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	731	162
Net current-period other comprehensive income (loss)	731	162
Balance	(1,430)	(2,161)	(1,934)
Post- retirement Liability Adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(36,069)	(35,163)
Other comprehensive income (loss) before reclassifications	(6,151)	4,441
Amounts reclassified from accumulated other comprehensive loss	1,486	2,173
Net current-period other comprehensive income (loss)	(4,665)	6,614
Balance	(40,734)	(36,069)	(35,163)
Foreign Currency Translation Adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	0	(155)
Other comprehensive income (loss) before reclassifications	0	(61)
Amounts reclassified from accumulated other comprehensive loss	0	249
Net current-period other comprehensive income (loss)	0	188
Balance	0	0	(155)
Net Unrealized Gain (Loss) on Available- for-sale Investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(112)	(82)
Other comprehensive income (loss) before reclassifications	134	(144)
Amounts reclassified from accumulated other comprehensive loss	40	131
Net current-period other comprehensive income (loss)	174	(13)
Balance	62	(112)	(82)
Total Accumulated Other Comprehensive Loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(38,342)	(37,334)
Other comprehensive income (loss) before reclassifications	(6,017)	4,236
Amounts reclassified from accumulated other comprehensive loss	2,257	2,715
Net current-period other comprehensive income (loss)	(3,760)	6,951	(1,601)
Balance	$ (42,102)	(38,342)	$ (37,334)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Net Unrealized Loss on Derivative Instruments Qualifying as Hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss		(389)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Post- retirement Liability Adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss		(7,520)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Foreign Currency Translation Adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss		(33)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Net Unrealized Gain (Loss) on Available- for-sale Investments
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss		(17)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Total Accumulated Other Comprehensive Loss
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss		$ (7,959)

Reclassifications out of accumu

Reclassifications out of accumulated other comprehensive (loss) (Details 10) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									$ (98,587)	$ (84,614)	$ (82,788)
Income taxes									(63,279)	(47,485)	(65,041)
Other income									15,812	(238)	8,767
Net income	$ 95,065	$ 137,637	$ 61,825	$ 40,926	$ 78,828	$ 107,251	$ 43,802	$ 42,437	335,453	272,318	$ 281,203
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									(2,257)	(2,715)
Reclassification adjustment for loss on derivative instruments included in net income (loss): | 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									(140)	429
Net income									(731)	(162)
Amortization of postretirement liability losses included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									476	721
Other income									(1,962)	(2,894)
Net income									(1,486)	(2,173)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss) | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									0	75
Other income									0	(324)
Net income									0	(249)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss) | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									10	35
Other income									(50)	(166)
Net income									$ (40)	$ (131)

Disaggregation of revenue (Deta

Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 5,336,776	$ 4,531,552	$ 4,443,351
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(77,149)	(64,307)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(525,749)	(465,969)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									147,114	132,397
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									9,164	9,159
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									11,708	11,543
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									1,088,633	968,755
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,627,833	1,423,068
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,266,196	926,875
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									531,882	392,544
Other
Disaggregation of Revenue [Line Items]
Operating revenues									82,669	76,796
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									5,281,621	4,469,761
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									55,155	61,791
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									608,821	579,436
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									438,431	412,952
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									64,660	58,956
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									7,408	7,556
Electric
Disaggregation of Revenue [Line Items]
Operating revenues									351,725	335,123
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues									35,574	31,568
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									347,712	331,190
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									4,013	3,933
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									125,369	121,477
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									141,596	136,236
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									37,765	34,353
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									7,408	7,556
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues									865,222	823,247
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									45,449	43,238
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues									12,726	14,579
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									865,357	817,095
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									(135)	6,152
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									483,452	457,959
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									296,835	276,716
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									26,895	24,603
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Operating revenues									84,192	78,018
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(56,252)	(50,905)
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									101,665	89,159
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									9,164	9,159
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									11,708	11,543
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Operating revenues									17,687	18,865
Pipeline and midstream | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									83,972	77,821
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									220	197
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues									2,189,651	1,925,185
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(1,066)	(669)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(525,749)	(465,969)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									1,088,633	968,755
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,627,833	1,423,068
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									2,189,651	1,925,185
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues									1,845,896	1,369,772
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(3,370)	(1,681)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,266,196	926,875
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									531,882	392,544
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues									131	525
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									1,794,839	1,318,263
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									51,057	51,509
Other
Disaggregation of Revenue [Line Items]
Operating revenues									90	207
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(16,461)	(11,052)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues									16,551	11,259
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									90	207
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									$ 0	$ 0

Contract balances (Details 2)

Contract balances (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract assets	$ 109,078	$ 104,239	$ 109,540
Change in contract assets	4,839	(5,301)
Contract liabilities - current	(142,768)	(93,901)	(84,123)
Change in contract liabilities - current	(48,867)	(9,778)
Contract liabilities - noncurrent	(19)	(135)	0
Change in contract liabilities - noncurrent	116	(135)
Net contract assets (liabilities)	(33,709)	10,203	$ 25,417
Change in net contract assets	(43,912)	(15,214)
Amounts included in contract liability at the beginning of the period	89,000	78,600
Amounts from performance obligations satisfied in prior periods	$ 44,100	$ 36,700

Revenue from contracts with c_3

Revenue from contracts with customers Remaining performance obligations (Details 3) $ in Millions	Dec. 31, 2019USD ($)
Remaining performance obligation, expected timing of satisfaction, start date: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, amount	$ 1,500
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	229.4
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	259.3
Remaining performance obligation, expected timing of satisfaction, start date: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, amount	$ 2,000

Business Combinations (Details)

Business Combinations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Payments to Acquire Businesses, Gross	$ 56,800,000	$ 168,100,000
Business Combination, Long-Term Debt Assumed	1,200,000
Acquisition Costs	$ 655,000	$ 1,500,000
Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Common Stock Shares Issued		721,610
Common Stock Market Value		$ 20,300,000
Common Stock Discounted Fair Value		$ 18,200,000

Business Combinations - Assets

Business Combinations - Assets Acquired & Liabilities Assumed (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 681,358,000	$ 664,922,000	$ 631,791,000
Measurement Period Adjustments
Goodwill	(6,669,000)	0
2019 Acquisition
Business Acquisition [Line Items]
Total current assets	15,800,000
Property, plant and equipment	16,700,000
Goodwill	23,100,000
Other intangible assets, net	6,700,000
Deferred charges and other assets - other	500,000
Current liabilities	5,900,000
Total deferred credits and other liabilities	100,000
2018 Acquisitions
Business Acquisition [Line Items]
Receivables, net	18,984,000	18,984,000
Inventories	10,101,000	10,329,000
Other current assets	501,000	515,000
Total current assets	29,586,000	29,828,000
Property, plant and equipment	138,435,000	131,766,000
Goodwill	26,462,000	33,131,000
Other intangible assets, net	8,227,000	8,227,000
Deferred charges and other assets - other	927,000	927,000
Total deferred charges and other assets	35,616,000	42,285,000
Total assets acquired	203,637,000	203,879,000
Current liabilities	10,880,000	11,122,000
Asset retirement obligation	914,000	914,000
Deferred income taxes	5,565,000	5,565,000
Total deferred credits and other liabilities	6,479,000	6,479,000
Total liabilities assumed	17,359,000	17,601,000
Total consideration (fair value)	186,278,000	$ 186,278,000
Measurement Period Adjustments
Inventories	(228,000)
Other current assets	(14,000)
Total current assets	(242,000)
Property, plant and equipment	6,669,000
Goodwill	(6,669,000)
Total deferred charges and other assets	(6,669,000)
Total assets acquired	(242,000)
Current liabilities	(242,000)
Total liabilities assumed	$ (242,000)

Taxes and operating loss carryf

Taxes and operating loss carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Discontinued operations, held-for-sale or disposed by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred tax assets, operating loss carryforwards	$ 1.3	$ 1.9

Major classes of assets and lia

Major classes of assets and liabilities held for sale (Details 2) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Total current assets held for sale	$ 425	$ 430
Noncurrent assets:
Total noncurrent assets held for sale	1,426	2,087
Current liabilities:
Total current liabilities held for sale	3,511	4,001
Discontinued operations, held-for-sale or disposed by sale
Current assets:
Receivables, net	425	430
Total current assets held for sale	425	430
Noncurrent assets:
Deferred income taxes	1,265	1,926
Other	161	161
Total noncurrent assets held for sale	1,426	2,087
Total assets held for sale	1,851	2,517
Current liabilities:
Accounts payable	0	80
Taxes payable	1,279	1,451
Other accrued liabilities	2,232	2,470
Total current liabilities held for sale	3,511	4,001
Total liabilities held for sale	$ 3,511	$ 4,001

Reconciliation of income and ex

Reconciliation of income and expenses (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations	$ 261	$ 1,509	$ (1,320)	$ (163)	$ 2,846	$ (118)	$ (273)	$ 477	$ 287	$ 2,932	$ (3,783)
Discontinued operations, held-for-sale or disposed by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues									103	(459)	465
Operating expenses									290	921	(4,607)
Operating income (loss)									(187)	(1,380)	5,072
Other income									0	12
Other expense											(13)
Interest expense									0	575	250
Income (loss) from discontinued operations before income taxes									(187)	(1,943)	4,809
Income taxes									(474)	(4,875)	8,592
Income (loss) from discontinued operations									$ 287	$ 2,932	$ (3,783)

Lease costs (Details)

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

Operating lease liabilities und

Operating lease liabilities undiscounted cash flows maturity schedule (Details 2) $ in Thousands	Dec. 31, 2019USD ($)
Leases [Abstract]
2020	$ 35,156
2021	24,893
2022	16,932
2023	10,227
2024	7,368
Thereafter	47,926
Total	142,502
Less discount	27,096
Total operating lease liabilities	$ 115,406

Leases Operating lease future m

Leases Operating lease future minimum lease payments (Details 3) $ in Thousands	Dec. 31, 2018USD ($)
Operating leases future minimum lease payments due [Abstract]
2019	$ 37,740
2020	26,255
2021	17,868
2022	11,647
2023	7,278
Thereafter	$ 49,098

Lessor accounting (Details 4)

Lessor accounting (Details 4) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating Lease, Lease Income	$ 51.5
Lessor, Operating Lease, Payments to be Received, Next Twelve Months	$ 11.3

Goodwill rollforward (Details)

Goodwill rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Balance at beginning of period	$ 664,922	$ 631,791
Goodwill acquired during the year	23,105	33,131
Goodwill	(6,669)	0
Balance at end of period	681,358	664,922
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736	345,736
Goodwill acquired during the year	0	0
Goodwill	0	0
Balance at end of period	345,736	345,736
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	209,421	176,290
Goodwill acquired during the year	14,482	33,131
Goodwill	(6,669)	0
Balance at end of period	217,234	209,421
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	109,765	109,765
Goodwill acquired during the year	8,623	0
Goodwill	0	0
Balance at end of period	$ 118,388	$ 109,765

Other intangible assets (Detail

Other intangible assets (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 15,246	$ 10,815
Amortization of intangible assets	2,400	1,200	$ 2,000
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	17,958	22,720
Intangible assets, less accumulated amortization	6,268	13,535
Intangible assets, net (excluding goodwill)	11,690	9,185
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	3,439	2,605
Intangible assets, less accumulated amortization	1,957	1,956
Intangible assets, net (excluding goodwill)	1,482	649
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	8,094	6,458
Intangible assets, less accumulated amortization	6,020	5,477
Intangible assets, net (excluding goodwill)	2,074	981
Series of Individually Immaterial Business Acquisitions
Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 6,800	$ 8,200

Goodwill and other intangible_3

Goodwill and other intangible assets Future amortization expense (Details 3) $ in Thousands	Dec. 31, 2019USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2020	$ 3,365
2021	2,016
2022	1,968
2023	1,924
2024	1,610
Thereafter	$ 4,363

Regulatory assets and liabili_3

Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory assets and liabilities
Regulatory assets:	$ 417,397	$ 340,117
Regulatory liabilities:	490,305	521,432
Net regulatory position	(72,908)	(181,315)
Regulatory assets not earning a rate of return	276,500	313,500
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Regulatory liabilities:	249,506	277,833
Plants to be retired
Regulatory assets and liabilities
Regulatory liabilities:	173,722	173,143
Natural gas costs refundable through rate adjustments
Regulatory assets and liabilities
Regulatory liabilities:	23,825	29,995
Pension and postretirement benefits
Regulatory assets and liabilities
Regulatory liabilities:	18,065	15,264
Other regulatory assets/liabilities
Regulatory assets and liabilities
Regulatory liabilities:	$ 25,187	25,197
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	(e)
Regulatory assets:	$ 157,069	165,898
Natural gas costs recoverable through rate adjustments
Regulatory assets and liabilities
Estimated recovery period	Up to 3 years
Regulatory assets:	$ 89,204	42,652
Asset retirement obligation costs
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets:	$ 66,000	60,097
Plants to be retired
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets:	$ 32,931	0
Cost recovery mechanisms (a) (b)
Regulatory assets and liabilities
Estimated recovery period	Up to 3 years
Regulatory assets:	$ 19,396	17,948
Manufactured gas plant sites remediation
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets:	$ 15,347	17,068
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets:	$ 11,486	11,946
Conservation programs (a) (b)
Regulatory assets and liabilities
Estimated recovery period	Up to 3 years
Regulatory assets:	$ 7,405	7,494
Long-term debt refinancing costs
Regulatory assets and liabilities
Estimated recovery period	Up to 18 years
Regulatory assets:	$ 4,286	4,898
Costs related to identifying generation development
Regulatory assets and liabilities
Estimated recovery period	Up to 7 years
Regulatory assets:	$ 2,052	2,508
Other regulatory assets/liabilities
Regulatory assets and liabilities
Estimated recovery period	Up to 19 years
Regulatory assets:	$ 12,221	$ 9,608

Fair value measurements insuran

Fair value measurements insurance contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 87	$ 73.8
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans	$ 13.2	$ (3.6)	$ 9.3

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale securities [Line Items]
Cost	$ 11,032	$ 10,652
Gross Unrealized Gains	88	21
Gross Unrealized Losses	10	162
Fair Value	11,110	10,511
Mortgage-backed securities
Available-for-sale securities [Line Items]
Cost	9,804	10,473
Gross Unrealized Gains	87	21
Gross Unrealized Losses	10	162
Fair Value	9,881	10,332
U.S. Treasury securities
Available-for-sale securities [Line Items]
Cost	1,228	179
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	$ 1,229	$ 179

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Concentration risks, percentage [Abstract]
Percentage in fixed-income investments	51.00%	53.00%
Percentage investment in common stock of large-cap companies	23.00%	21.00%
Percentage investment in common stock of mid-cap companies	12.00%	11.00%
Percentage investment in common stock of small-cap companies	10.00%	10.00%
Percentage investment in target date investments	3.00%	3.00%
Percentage investment in cash and cash equivalents	1.00%	2.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 106,559	$ 95,148
Fair value, measurements, recurring | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	8,440	10,799
Fair value, measurements, recurring | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	87,009	73,838
Fair value, measurements, recurring | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	9,881	10,332
Fair value, measurements, recurring | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	1,229	179
Fair value, measurements, recurring | Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	106,559	95,148
Fair value, measurements, recurring | Fair value, inputs, level 2 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	8,440	10,799
Fair value, measurements, recurring | Fair value, inputs, level 2 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	87,009	73,838
Fair value, measurements, recurring | Fair value, inputs, level 2 | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	9,881	10,332
Fair value, measurements, recurring | Fair value, inputs, level 2 | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 1,229	$ 179

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, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 2,243,107	$ 2,108,695
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	2,243,107	2,108,695
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 2,418,631	$ 2,183,819

Credit facilities (Details)

Credit facilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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	118,600,000	$ 48,500,000
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	104,300,000	289,600,000
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	48,500,000
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	64,600,000	53,800,000
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	24,500,000	$ 56,300,000
Letters of credit at end of period	$ 1,400,000
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]
Option to increase borrowings, maximum amount	$ 700,000,000

Long-term debt outstanding (Det

Long-term debt outstanding (Details 2) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,243,107	$ 2,108,695
Long-term debt due within one year	16,540	251,854
Long-term debt	$ 2,226,567	1,856,841
Senior Notes
Long-term debt outstanding [Line Items]
Weighted average interest rate	4.45%
Long-term debt	$ 1,850,000	1,381,000
Commercial paper
Long-term debt outstanding [Line Items]
Weighted average interest rate	2.04%
Long-term debt	$ 222,900	338,100
Term Loan Agreements
Long-term debt outstanding [Line Items]
Weighted average interest rate	2.00%
Long-term debt	$ 9,100	209,800
Credit Agreements
Long-term debt outstanding [Line Items]
Weighted average interest rate	4.40%
Long-term debt	$ 89,050	110,100
Medium-term notes
Long-term debt outstanding [Line Items]
Weighted average interest rate	6.68%
Long-term debt	$ 50,000	50,000
Other notes
Long-term debt outstanding [Line Items]
Weighted average interest rate	4.48%
Long-term debt	$ 29,117	25,229
Long-term debt
Long-term debt outstanding [Line Items]
Unamortized debt issuance costs	7,010	5,207
Discount	$ 50	$ 327

Long-term borrowings (Details 3

Long-term borrowings (Details 3) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 16, 2019	Dec. 31, 2018
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,243,107		$ 2,108,695
Montana-Dakota Utilities Co. [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	52.00%
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	50.00%
Centennial Energy Holdings, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	34.00%
WBI Energy Transmission, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	40.00%
Senior Notes
Long-term debt outstanding [Line Items]
Long-term debt	$ 1,850,000		$ 1,381,000
Weighted average interest rate	4.45%
Senior Notes | Montana-Dakota Utilities Co. [Member]
Long-term debt outstanding [Line Items]
Long-term debt	$ 200,000
Weighted average interest rate	3.95%
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Senior Notes | Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Long-term debt	$ 75,000
Weighted average interest rate	3.93%
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Senior Notes | Intermountain Gas Company
Long-term debt outstanding [Line Items]
Long-term debt	$ 50,000
Weighted average interest rate	3.92%
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Senior Notes | Centennial Energy Holdings, Inc.
Long-term debt outstanding [Line Items]
Long-term debt	$ 150,000
Weighted average interest rate	4.60%
Ratio of total debt to total capitalization as specified in debt covenants	60.00%
Senior Notes | WBI Energy Transmission, Inc.
Long-term debt outstanding [Line Items]
Long-term debt	$ 170,000	$ 45,000
Weighted average interest rate	4.17%
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	$ 130,000
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%
Facility limit, maximum borrowing capacity	$ 100,000
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%
Facility limit, maximum borrowing capacity	$ 600,000

Schedule of debt maturities (De

Schedule of debt maturities (Details 4) $ in Thousands	Dec. 31, 2019USD ($)
Long-term debt maturities [Line Items]
2020	$ 16,540
2021	1,528
2022	148,021
2023	77,921
2024	373,372
Thereafter	$ 1,632,785

Asset Retirement Obligations (D

Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Asset retirement obligation [Roll Forward]
Balance at beginning of year	$ 375,553	$ 341,969
Liabilities incurred	25,869	13,424
Liabilities acquired	486	1,002
Liabilities settled	(7,097)	(3,699)
Accretion expense	19,789	18,242
Revisions in estimates	2,975	4,615
Balance at end of year	417,575	375,553
Asset retirement obligation costs
Regulatory Assets [Line Items]
Accretion expense, including asset retirement obligations	$ 18,300	$ 16,800

Preferred Stocks (Details)

Preferred Stocks (Details) - USD ($)	Apr. 01, 2017	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Payments for repurchase of redeemable preferred stock	$ 15,600,000
Redeemable preferred stock classified as long term debt	$ 300,000
Preferred stock
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
Preferred stock, series 4.50%
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage	4.50%
Preferred stock, redemption price per share	$ 105
Preferred stock, series 4.70%
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage	4.70%
Preferred stock, redemption price per share	$ 102

Common stock (Details)

Common stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Dividends declared per common share	$ 0.8150	$ 0.7950	$ 0.7750
Dividends declared to common stockholders	$ 162,408	$ 156,453	$ 151,966
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	$ 162,100	$ 155,700	$ 151,500
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Aggregate Authorized Issuance of Debt Securities	$ 1,000,000,000
Common stock, shares authorized	500,000,000	500,000,000
Common Stock, Shares, Issued	200,922,790	196,564,907
Proceeds from Issuance of Common Stock	$ 106,848,000	$ 0	$ 0
At-the-market offering
Class of Stock [Line Items]
Common stock, shares authorized	10,000,000
Common Stock, Shares, Issued	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	7,300,000

Stock based compensation plans

Stock based compensation plans (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Noncash Expense [Abstract]
Share-based compensation arrangement, number of shares available for grant	4.6
Share-based compensation expense, net of tax	$ 6.5	$ 4.6	$ 2.7
Share-based compensation nonvested awards total compensation cost not yet recognized	$ 9.7
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Noncash Expense [Abstract]
Nonemployee director stock compensation shares issued	41,644	38,605	40,572
Fair value of nonemployee director stock compensation shares issued	$ 1.2	$ 1	$ 1.1

Restricted stock awards (Detail

Restricted stock awards (Details 3) - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested at end of period | shares	22,838
Weighted average grant-date fair value | $ / shares	$ 27.48

Performance share awards (Detai

Performance share awards (Details 4) - Performance shares [Member] - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award [Line Items]
Granted	$ 30.66
Fair value of vested shares	$ 9,700,000	$ 0	$ 9,600,000
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Nonvested at beginning of period	668,791
Nonvested at beginning of period	$ 23.03
Granted	327,194
Granted	$ 30.66
Additional performance shares earned	103,159
Additional performance shares earned	$ 14.60
Vested	398,919
Vested	$ 15.52
Forfeited	126,722
Forfeited	$ 24.31
Nonvested at end of period	573,503	668,791
Nonvested at end of period	$ 30.81	$ 23.03
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 2018 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	246,309
Market Condition [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Historical volatility rate	50.00%
Implied volatility rate	50.00%
Granted	$ 35.07	$ 34.55	$ 24.31
Expected volatility rate, minimum	19.50%	17.87%	22.70%
Expected volatility rate, maximum	19.69%	22.14%	25.56%
Risk-free interest rate, minimum	2.46%	1.86%	0.69%
Risk-free interest rate, maximum	2.55%	2.46%	1.61%
Weighted average discounted dividends per share	$ 2.85	$ 2.46	$ 1.70
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Granted	$ 35.07	$ 34.55	$ 24.31
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]
Granted	$ 26.25	27.48
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Granted	$ 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%

Components of income before inc

Components of income before income taxes from continuing operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ 398,532	$ 317,655	$ 350,064
Foreign	(87)	(784)	(37)
Income before income taxes	$ 398,445	$ 316,871	$ 350,027

Income tax expense (benefit) (D

Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ (3,502)	$ (15,901)	$ 74,272
State	3,366	3,651	16,192
Current income taxes	(136)	(12,250)	90,464
Deferred:
Federal	50,218	50,755	(24,497)
State	12,098	7,206	(864)
Investment tax credit - net	1,099	1,774	(62)
Deferred income taxes	63,415	59,735	(25,423)
Total income tax expense	$ 63,279	$ 47,485	$ 65,041

Tax reform (Details 3)

Tax reform (Details 3) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
TCJA revaluation related to accumulated other comprehensive loss balance		$ 39,500	$ 0	$ (42)	$ 7,735
Deferred tax liabilities, regulatory assets			$ 11,904
Other comprehensive income (loss)
TCJA revaluation related to accumulated other comprehensive loss balance		(7,700)
Discontinued operations, held-for-sale
TCJA revaluation related to accumulated other comprehensive loss balance		$ (1,000)
Stranded costs
Deferred tax liabilities, regulatory assets	$ (4,200)

Components of deferred tax asse

Components of deferred tax assets and liabilities (Details 4) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Postretirement	$ 51,075	$ 51,930
Compensation-related	37,330	29,885
Operating lease liabilities	24,459
Asset retirement obligations	7,450	7,083
Customer advances	7,325	7,734
Legal and environmental contingencies	6,601	6,729
Federal renewable energy credit	5,343	8,015
Alternative minimum tax credit carryforward	0	13,404
Other	32,533	37,347
Total deferred tax assets	172,116	162,127
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	511,867	476,832
Postretirement	48,927	44,432
Operating lease right-of-use-assets	24,436
Intangible asset amortization	18,930	17,752
Other	61,385	39,712
Total deferred tax liabilities	665,545	578,728
Valuation allowance	13,154	13,484
Net deferred income tax liability	$ 506,583	$ 430,085

Carryforwards (Details 5)

Carryforwards (Details 5) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
State and local jurisdiction
Carryforwards [Line Items]
Operating loss carryforwards	$ 149.8	$ 153.2
Investment tax credit carryforward, amount	37.4	32.2
Federal and State Taxing Authority
Carryforwards [Line Items]
Tax credit carryforward, amount	$ 43.7	$ 43.5

Deferred tax reconciliation (De

Deferred tax reconciliation (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Change in net deferred income tax liability from the preceding table	$ 76,498
Deferred taxes associated with other comprehensive loss	1,631
Deferred taxes associated with TCJA enactment for regulated activities	(11,904)
Other	(2,810)
Deferred income taxes	$ 63,415	$ 59,735	$ (25,423)

Income tax expense (benefit) st

Income tax expense (benefit) statutory rate versus actual rate (Details 7) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Computed tax at federal statutory rate
Computed tax at federal statutory rate		$ 83,674	$ 66,543	$ 122,509
Federal statutory rate		21.00%	21.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax		$ 14,029	$ 12,190	$ 10,724
State income tax, rate		3.50%	3.80%	3.10%
Federal renewable energy credit		$ (15,843)	$ (11,759)	$ (13,958)
Federal renewable energy credit, rate		(4.00%)	(3.70%)	(4.00%)
Tax compliance and uncertain tax positions		$ (2,739)	$ (2,725)	$ (643)
Tax compliance and uncertain tax positions, rate		(0.70%)	(0.90%)	(0.20%)
Domestic production deduction		$ 0	$ 0	$ (6,849)
Domestic production deduction, rate		0.00%	0.00%	(2.00%)
Excess deferred income tax amortization		$ (11,904)	$ (9,319)	$ (397)
Excess deferred income tax amortization, rate		(3.00%)	(2.90%)	0.00%
TCJA revaluation		$ 0	$ (5,947)	$ (47,242)
TCJA revaluation, rate		0.00%	(1.90%)	(13.50%)
TCJA revaluation related to accumulated other comprehensive loss balance	$ 39,500	$ 0	$ (42)	$ 7,735
TCJA revaluation related to accumulated other comprehensive loss balance, rate		0.00%	0.00%	2.20%
Other		$ (3,938)	$ (1,456)	$ (6,838)
Other, rate		(0.90%)	(0.40%)	(2.00%)
Income taxes		$ 63,279	$ 47,485	$ 65,041
Total income tax expense, rate		15.90%	15.00%	18.60%

Cash flow information (Details)

Cash flow information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest, net	$ 93,414,000	$ 83,009,000	$ 79,638,000
Income taxes paid (refunded), net	(8,475,000)	16,041,000	112,137,000
AFUDC borrowed	2,800,000	2,300,000	966,000
Property, plant and equipment additions in accounts payable	46,119,000	42,355,000	29,263,000
Issuance of common stock in connection with acquisition	0	18,186,000	0
Debt assumed in connection with a business combination	1,163,000
Right-of-use assets obtained in exchange for new operating lease liabilities	54,880,000
Continuing and discontinued operations
Income taxes paid (refunded), net	$ (9,400,000)	$ 5,500,000	$ 9,700,000

Business segment data (Details)

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

Change in benefit obligations a

Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Underfunded Plan | Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 391,602	$ 445,923
Service cost	0	0	$ 0
Interest cost	15,225	14,591	16,207
Plan participants' contributions	0	0
Actuarial (gain) loss	40,219	(32,637)
Benefits paid	(25,880)	(36,275)
Benefit obligation at end of year	421,166	391,602	445,923
Change in net plan assets:
Fair value of plan assets at beginning of year	307,809	354,384
Actual gain (loss) on plan assets	58,409	(21,138)
Employer contribution	24,926	10,838
Plan participants' contributions	0	0
Benefits paid	(25,880)	(36,275)
Fair value of net plan assets at end of year	365,264	307,809	354,384
Funded status - over (under)	(55,902)	(83,793)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Deferred charges and other assets - other	0	0
Other accrued liabilities	0	0
Deferred credits and other liabilities - other	55,902	83,793
Benefit obligation assets (liabilities) - net amount recognized	(55,902)	(83,793)
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	27,748	28,796
Prior service credit	0	0
Total	27,748	28,796
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	155,484	159,939
Prior service credit	0	0
Total	155,484	159,939
Overfunded Plan | Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	81,201	91,206
Service cost	1,142	1,494	1,508
Interest cost	2,986	2,899	3,265
Plan participants' contributions	1,040	1,282
Actuarial (gain) loss	2,632	(10,115)
Benefits paid	(5,387)	(5,565)
Benefit obligation at end of year	83,614	81,201	91,206
Change in net plan assets:
Fair value of plan assets at beginning of year	82,516	88,739
Actual gain (loss) on plan assets	15,731	(2,781)
Employer contribution	687	842
Plan participants' contributions	1,040	1,281
Benefits paid	(5,387)	(5,565)
Fair value of net plan assets at end of year	94,587	82,516	$ 88,739
Funded status - over (under)	10,973	1,315
Amounts recognized in the Consolidated Balance Sheets at December 31:
Deferred charges and other assets - other	30,475	20,843
Other accrued liabilities	647	660
Deferred credits and other liabilities - other	18,855	18,868
Benefit obligation assets (liabilities) - net amount recognized	10,973	1,315
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	6,118	6,372
Prior service credit	(731)	(848)
Total	5,387	5,524
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	(4,450)	3,944
Prior service credit	(8,109)	(9,390)
Total	$ (12,559)	$ (5,446)

Benefit obligations in excess o

Benefit obligations in excess of plan assets (Details 2) - Underfunded Plan - Qualified Plan - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 421,166	$ 391,602
Accumulated benefit obligation	421,166	391,602
Fair value of plan assets	$ 365,264	$ 307,809

Components of net periodic bene

Components of net periodic benefit cost (Details 3) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Underfunded Plan | Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 0	$ 0
Interest cost	15,225	14,591	16,207
Expected return on assets	(18,236)	(20,753)	(20,528)
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	5,548	7,005	6,355
Net periodic benefit cost (credit), including amount capitalized	2,537	843	2,034
Less amount capitalized	0	0	310
Net periodic benefit cost (credit)	2,537	843	1,724
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	(144)	991	(1,091)
Amortization of actuarial loss	(904)	(1,084)	(1,040)
Amortization of prior service (cost) credit	0	0	0
Total recognized in accumulated other comprehensive loss	(1,048)	(93)	(2,131)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	189	8,263	(4,736)
Amortization of actuarial loss	(4,644)	(5,921)	(5,315)
Amortization of prior service credit	0	0	0
Total recognized in regulatory assets or liabilities	(4,455)	2,342	(10,051)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	(2,966)	3,092	(10,458)
Overfunded Plan | Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,142	1,494	1,508
Interest cost	2,986	2,899	3,265
Expected return on assets	(4,804)	(4,866)	(4,641)
Amortization of prior service credit	(1,398)	(1,394)	(1,371)
Recognized net actuarial loss	353	640	857
Net periodic benefit cost (credit), including amount capitalized	(1,721)	(1,227)	(382)
Less amount capitalized	113	153	(370)
Net periodic benefit cost (credit)	(1,834)	(1,380)	(12)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	(127)	(1,735)	1,742
Amortization of actuarial loss	(110)	(354)	(289)
Amortization of prior service (cost) credit	100	(220)	161
Total recognized in accumulated other comprehensive loss	(137)	(2,309)	1,614
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	(8,168)	(732)	(4,932)
Amortization of actuarial loss	(242)	(286)	(568)
Amortization of prior service credit	1,297	1,614	1,210
Total recognized in regulatory assets or liabilities	(7,113)	596	(4,290)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	$ (9,084)	$ (3,093)	$ (2,688)

Estimated net loss and prior se

Estimated net loss and prior service credit (Details 4) - Qualified Plan	Dec. 31, 2019USD ($)
Underfunded Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	$ (7,200,000)
Overfunded Plan | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	(250,000)
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, 2019	Dec. 31, 2018
Underfunded Plan | Pension Benefits
Used to determine benefit obligation
Discount rate	2.96%	4.03%
Expected return on plan assets	6.25%	6.75%
Used to determine benefit cost
Discount rate	4.03%	3.38%
Expected return on plan assets	6.25%	6.75%
Overfunded Plan | Other Postretirement Benefits
Used to determine benefit obligation
Discount rate	3.00%	4.05%
Expected return on plan assets	5.75%	5.75%
Rate of compensation increase	3.00%	3.00%
Used to determine benefit cost
Discount rate	4.05%	3.41%
Expected return on plan assets	5.75%	5.75%
Rate of compensation increase	3.00%	3.00%

Expected rate of return (Detail

Expected rate of return (Details 6) - Qualified Plan	Dec. 31, 2019
Underfunded Plan | Minimum | Equity securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	40.00%
Underfunded Plan | Minimum | Fixed Income Securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	50.00%
Underfunded Plan | Maximum | Equity securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	50.00%
Underfunded Plan | Maximum | Fixed Income Securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	60.00%
Overfunded Plan | Equity securities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	30.00%
Overfunded Plan | Fixed Income Securities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	70.00%

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, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate - ultimate	4.50%	4.50%
Year in which ultimate trend rate achieved	2024	2024
Effect on total of service and interest cost components, one percentage point increase	$ 245
Effect on total of service and interest cost components, one percentage point decrease	(203)
Effect on postretirement benefit obligations, one percentage point increase	3,751
Effect on postretirement benefit obligations, one percentage point decrease	$ (3,155)
Minimum
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year	7.10%	7.50%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year	7.40%	8.10%

Estimated future benefit paymen

Estimated future benefit payments and subsidies (Details 8) - Qualified Plan	12 Months Ended
	Dec. 31, 2019USD ($)
Underfunded Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Additional Contributions by Employer	$ 20,000,000
2020	24,128,000
2021	24,432,000
2022	24,642,000
2023	24,874,000
2024	24,924,000
2025-2029	121,205,000
Overfunded Plan | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	660,000
2020	5,024,000
2020	92,000
2021	5,073,000
2021	86,000
2022	5,098,000
2022	80,000
2023	5,091,000
2023	73,000
2024	5,000,000
2024	65,000
2025-2029	24,242,000
2025-2029	$ 222,000

Fair value - pension (Details 9

Fair value - pension (Details 9) - Qualified Plan - Underfunded Plan - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 365,264	$ 307,809	$ 354,384
Percentage investment in common stock of international companies	29.00%	27.00%
Percentage investment in common stock of large-cap U.S. companies	21.00%	18.00%
Percentage Investment in US Government Securities	18.00%
Percentage investment in corporate bonds	9.00%	31.00%
Percentage investment in cash equivalents	6.00%	5.00%
Percentage investment in other investments	17.00%	19.00%
Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 26,166	$ 4,930
Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	14,457	11,038
Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	938	967
Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	219,800	197,560
Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,768	73,110
Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,828	10,624
U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	9,378	6,375
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	363,335	304,604
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	14,457	11,038
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	160,906	145,960
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	7,296	479
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	182,659	157,477
Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	26,166	4,930
Significant Other Observable Inputs (Level 2) | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	938	967
Significant Other Observable Inputs (Level 2) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	58,894	51,600
Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,768	73,110
Significant Other Observable Inputs (Level 2) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,828	10,624
Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,082	5,896
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	$ 180,676	$ 147,127

Fair value - other postretireme

Fair value - other postretirement (Details 10) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Percentage investment in common stock of small-cap companies	10.00%	10.00%
Other Postretirement Benefits | Qualified Plan | Overfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 94,587	$ 82,516	$ 88,739
Percentage investment in corporate bonds	50.00%	51.00%
Percentage investment in common stock of large-cap U.S. companies	25.00%	23.00%
Percentage Investment in US Government Securities	7.00%	7.00%
Percentage investment in common stock of small-cap companies	7.00%	7.00%
Percentage investment in other investments	11.00%	12.00%
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 4,017	$ 3,866
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,073	1,767
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1	2
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	88,496	76,881
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	94,587	82,516
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,073	1,767
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	10	1
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | 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	2,083	1,768
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	4,017	3,866
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1	2
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	88,486	76,880
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | 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	$ 92,504	$ 80,748

Nonqualified Benefit Plans Bene

Nonqualified Benefit Plans Benefit Obligations (Details 1) - Supplemental Employee Retirement Plan - Nonqualified Plan - Unfunded Plan - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 99,245	$ 93,988
Accumulated benefit obligation	$ 99,245	$ 93,988

Nonqualified Benefit Plans Comp

Nonqualified Benefit Plans Components of NPBC (Details 2) - Supplemental Employee Retirement Plan - Nonqualified Plan - Unfunded Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 109	$ 185	$ 289
Interest cost	3,473	3,157	3,494
Recognized net actuarial loss	764	1,047	883
Net periodic benefit cost	$ 4,346	$ 4,389	$ 4,666

Nonqualified Benefit Plans Weig

Nonqualified Benefit Plans Weighted Average Assumptions (Details 3) - Supplemental Employee Retirement Plan - Nonqualified Plan - Unfunded Plan	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation discount rate	2.73%	3.86%
Net periodic benefit cost discount rate	3.86%	3.20%

Nonqualified Benefit Plans Futu

Nonqualified Benefit Plans Future Benefit Payments (Details 4) - Supplemental Employee Retirement Plan - Nonqualified Plan - Unfunded Plan $ in Thousands	Dec. 31, 2019USD ($)
Defined Benefit Plan Disclosure [Line Items]
2020	$ 7,774
2021	7,795
2022	7,023
2023	7,219
2024	7,597
2025-2029	$ 35,998

Nonqualified Benefit Plans Defi

Nonqualified Benefit Plans Defined Contribution (Details 5) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan
Defined Benefit Plan Disclosure [Line Items]
Nonqualified Defined Contribution Plan, Cost	$ 51,800,000	$ 42,400,000	$ 41,200,000
Nonqualified Plan | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Nonqualified Defined Contribution Plan, Cost	$ 1,600,000	$ 597,000	$ 736,000

Nonqualified Benefit Plans Inve

Nonqualified Benefit Plans Investments (Details 6) - Supplemental Employee Retirement Plan - Unfunded Plan - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	$ 134,118	$ 121,930
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	87,009	73,838
Life insurance carried on plan participants
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	38,659	37,274
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	$ 8,450	$ 10,818

Multiemployer Plans (Details)

Multiemployer Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Multiemployer Plans [Line Items]
Entity tax ID number	30-1133956
Multiemployer Plans, Pension [Member]
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer	$ 72,525	$ 58,722	$ 56,745
Multiemployer Plans, Pension [Member] | Alaska Laborers-Employers Retirement Fund
Multiemployer Plans [Line Items]
Entity tax ID number	91-6028298
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plans, Certified Zone Status, Date	Jun. 30, 2019	Jun. 30, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 815	$ 732	690
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Dec. 31, 2020
Multiemployer Plans, Pension [Member] | Construction Industry and Laborers Joint Pension Trust for So Nevada, Plan A
Multiemployer Plans [Line Items]
Entity tax ID number	88-0135695
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Red	Red
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 544	$ 346	377
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Jun. 30, 2020
Multiemployer Plans, Pension [Member] | Edison Pension Plan
Multiemployer Plans [Line Items]
Entity tax ID number	93-6061681
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 12,252	$ 12,111	12,725
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Dec. 31, 2020
Multiemployer Plans, Pension [Member] | IBEW Local 212 Pension Trust
Multiemployer Plans [Line Items]
Entity tax ID number	31-6127280
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	Apr. 30, 2019	Apr. 30, 2018
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 1,110	$ 1,341	1,312
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Jun. 1, 2025
Multiemployer Plans, Pension [Member] | IBEW Local 357 Pension Plan A
Multiemployer Plans [Line Items]
Entity tax ID number	88-6023284
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 10,162	$ 3,460	3,286
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	May 31, 2021
Multiemployer Plans, Pension [Member] | IBEW Local 648 Pension Plan
Multiemployer Plans [Line Items]
Entity tax ID number	31-6134845
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plans, Certified Zone Status, Date	Feb. 28, 2019	Feb. 28, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 728	$ 2,175	2,254
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Aug. 29, 2021
Multiemployer Plans, Pension [Member] | IBEW Local 82 Pension Plan
Multiemployer Plans [Line Items]
Entity tax ID number	31-6127268
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	Jun. 30, 2019	Jun. 30, 2018
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 1,662	$ 1,569	1,757
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Dec. 3, 2023
Multiemployer Plans, Pension [Member] | Idaho Plumbers and Pipefitters Pension Plan
Multiemployer Plans [Line Items]
Entity tax ID number	82-6010346
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	May 31, 2019	May 31, 2018
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 1,307	$ 1,247	1,156
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Mar. 31, 2023
Multiemployer Plans, Pension [Member] | Minnesota Teamsters Construction Division Pension Fund
Multiemployer Plans [Line Items]
Entity tax ID number	41-6187751
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	Nov. 30, 2018	Nov. 30, 2017
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 673	$ 740	826
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Apr. 30, 2021
Multiemployer Plans, Pension [Member] | National Automatic Sprinkler Industry Pension Fund
Multiemployer Plans [Line Items]
Entity tax ID number	52-6054620
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Red	Red
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 1,074	$ 738	718
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, First	Mar. 31, 2021
Expiration Date of Collective Bargaining Agreement	Jul. 31, 2024
Multiemployer Plans, Pension [Member] | National Electrical Benefit Fund
Multiemployer Plans [Line Items]
Entity tax ID number	53-0181657
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Green	Green
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	No
Multiemployer Plan, Contributions by Employer	$ 12,679	$ 8,468	8,891
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, First	Aug. 31, 2019
Expiration Date of Collective Bargaining Agreement	Jun. 1, 2025
Multiemployer Plans, Pension [Member] | Pension Trust Fund for Operating Engineers
Multiemployer Plans [Line Items]
Entity tax ID number	94-6090764
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 2,598	$ 2,403	2,391
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, First	Mar. 31, 2020
Expiration Date of Collective Bargaining Agreement	Jun. 15, 2022
Multiemployer Plans, Pension [Member] | Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV
Multiemployer Plans [Line Items]
Entity tax ID number	95-6052257
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Yellow	Yellow
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 2,119	$ 1,774	1,016
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Jun. 30, 2020
Multiemployer Plans, Pension [Member] | Southwest Marine Pension Trust
Multiemployer Plans [Line Items]
Entity tax ID number	95-6123404
Multiemployer Plan Number	001
Multiemployer Plans, Certified Zone Status [Fixed List]	Red	Red
Multiemployer Plans, Certified Zone Status, Date	Dec. 31, 2019	Dec. 31, 2018
FIP/RP Status Pending/Implemented	Implemented
Multiemployer Plan, Contributions by Employer	$ 132	$ 81	48
Multiemployer Plans, Surcharge [Fixed List]	No
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date	Jan. 31, 2024
Multiemployer Plans, Pension [Member] | Other funds
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer	$ 24,670	$ 21,537	$ 19,298
Minimum
Multiemployer Plans [Line Items]
Multiemployer plan yellow zone status funded percentage	65.00%
Multiemployer plan green zone status funded percentage	80.00%
Maximum
Multiemployer Plans [Line Items]
Multiemployer plan red zone status funded percentage	65.00%
Mulitiemployer plan yellow status funded percentage	80.00%

Multiemployer Plans (Details 2)

Multiemployer Plans (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Multiemployer Plans, Defined Contribution [Member]
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer	$ 49.2	$ 31.1	$ 32.2
Multiemployer Plans, Postretirement Benefit [Member]
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer	$ 59.5	$ 51.9	$ 50.8

Jointly owned facilities (Detai

Jointly owned facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Jointly owned electricity generation plant | Big Stone Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	22.70%
Utility plant in service	$ 152,836	$ 156,534
Construction work in progress	518	92
Less accumulated depreciation	46,266	49,345
Utility plant in services net	$ 107,088	107,281
Jointly owned electricity generation plant | Coyote Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 160,235	155,236
Construction work in progress	21	1,920
Less accumulated depreciation	107,638	105,565
Utility plant in services net	$ 52,618	51,591
Jointly owned electricity generation plant | Wygen III
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 67,869	65,382
Construction work in progress	112	220
Less accumulated depreciation	10,482	9,174
Utility plant in services net	$ 57,499	56,428
Jointly Owned Electricity Transmission and Distribution System | BSSE
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	50.00%
Utility plant in service	$ 105,767
Construction work in progress	0	105,846
Less accumulated depreciation	1,232
Utility plant in services net	$ 104,535	$ 105,846

MNPUC (Details)

MNPUC (Details) - Great Plains Natural Gas Co. [Member] - Distribution - MNPUC [Member] - Pending Rate Case [Member] - USD ($) $ in Millions	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%
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Interim Rate Increase (Decrease), Amount	$ 2.6
Public Utilities, Interim Rate Increase (Decrease), Percentage	11.00%

NDPSC (Details 2)

NDPSC (Details 2) - Electricity [Member] - Montana-Dakota Utilities Co. [Member] - NDPSC [Member] - USD ($)	Mar. 01, 2020	Oct. 28, 2019
Public Utilities, General Disclosures [Line Items]
Total Transmission Cost Adjustment Rider		$ 9,200,000
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 600,000
Public utilities, approved rate increase (decrease), percentage		7.20%
Transmission Capital Projects Increase (Decrease)		$ 1,500,000
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ (800,000)
Renewable Resource Cost Adjustment Rate Tariff	$ 14,700,000

OPUC (Details 3)

OPUC (Details 3) $ in Millions	12 Months Ended
	Oct. 31, 2020USD ($)
Distribution | Cascade Natural Gas [Member] | Subsequent Event [Member] | Tax Cuts and Jobs Act [Member] | OPUC [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount	$ (1.4)

WUTC (Details 4)

WUTC (Details 4) - Cascade Natural Gas [Member] - WUTC [Member] - USD ($)	Mar. 01, 2020	Nov. 01, 2019	May 31, 2019	Mar. 29, 2019
Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 12,700,000
Public Utilities, Requested Rate Increase (Decrease), Percentage				5.50%
Pipeline Cost Recovery Mechanism [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 1,600,000
Public Utilities, Requested Rate Increase (Decrease), Percentage			0.70%
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 440,000
Public utilities, approved rate increase (decrease), percentage		0.20%
Purchased Gas Adjustment [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 12,800,000
Public utilities, approved rate increase (decrease), percentage		5.70%
Regulatory Asset, Amortization Period		3 years
Subsequent Event [Member] | Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 6,500,000
Public utilities, approved rate increase (decrease), percentage	2.80%

WYPSC (Details 5)

WYPSC (Details 5) - Distribution - Montana-Dakota Utilities Co. [Member] - Pending Rate Case [Member] - WYPSC [Member] - USD ($)	Dec. 17, 2019	May 23, 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 1,100,000
Public Utilities, Requested Rate Increase (Decrease), Percentage		7.00%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount	$ 830,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage	5.50%

FERC (Details 6)

FERC (Details 6) $ in Millions	Jan. 01, 2020USD ($)
Montana-Dakota Utilities Co. [Member] | Electricity [Member] | Subsequent Event [Member] | MISO [Member]
Public Utilities, General Disclosures [Line Items]
Transmission Formula Revenue Requirement	$ 13.1

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 29.1	$ 30.4

Enviromental matters (Details 2

Enviromental matters (Details 2)	12 Months Ended
	Dec. 31, 2019USD ($)
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 accrual for site remediation	375,000
Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Environmental matters accrual for site remediation	6,400,000
Total estimated costs for site remedial investigation and feasibility study	7,600,000
Incurred costs for site remedial investigation and feasibility study	4,400,000
Environmental matters accrual of investigative costs	3,200,000
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	340,000
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	6,400,000
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Purchase commitments due
2020	$ 405,535
2021	250,266
2022	184,225
2023	123,166
2024	87,297
Thereafter	678,432
Amounts purchased under various commitments	$ 686,500	$ 548,000	$ 516,100
Maximum
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Term	41 years

Guarantees (Details 4)

Guarantees (Details 4)	Dec. 31, 2019USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 174,800,000
Fixed maximum amounts guaranteed by year 2020	162,600,000
Fixed maximum amounts guaranteed by year 2021	700,000
Fixed maximum amounts guaranteed by year 2022	400,000
Fixed maximum amounts guaranteed by year 2023	500,000
Fixed maximum amounts guaranteed by year 2024	500,000
Fixed maximum amounts guaranteed, thereafter	1,100,000
No scheduled maturity date	9,000,000
Amount outstanding under guarantees that is reflected on balance sheet	0
Letters of credit	33,200,000
Letters of credit set to expire - 2020	32,700,000
Letters of credit set to expire - 2021	500,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	$ 1,100,000,000

Variable interest entities (Det

Variable interest entities (Details 5) $ in Millions	Dec. 31, 2019USD ($)
Fuel contract
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 36

Condensed Statements of Income

Condensed Statements of Income and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 5,336,776	$ 4,531,552	$ 4,443,351
Operating expenses	1,247,992	1,374,329	1,206,262	1,026,973	1,091,524	1,140,783	990,605	906,917	4,855,556	4,129,829	4,019,303
Operating income	130,221	189,470	97,311	64,218	118,351	140,004	73,992	69,376	481,220	401,723	424,048
Other income									15,812	(238)	8,767
Interest expense									98,587	84,614	82,788
Income before income taxes									398,445	316,871	350,027
Income taxes									63,279	47,485	65,041
Net income from continuing operations	$ 94,804	$ 136,128	$ 63,145	$ 41,089	$ 75,982	$ 107,369	$ 44,075	$ 41,960	335,166	269,386	284,986
Loss on redemption of preferred stock									0	0	600
Earnings on common stock									335,453	272,318	280,432
Comprehensive income									331,693	279,269	279,602
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	628,331	623,693
Operating expenses									0	540,125	520,069
Operating income									0	88,206	103,624
Other income									0	1,504	4,876
Interest expense									0	32,761	31,997
Income before income taxes									0	56,949	76,503
Income taxes									0	(4,259)	13,800
Equity in earnings of subsidiaries from continuing operations									335,166	208,177	222,283
Net income from continuing operations									335,166	269,385	284,986
Equity in earnings (loss) of subsidiaries from discontinued operations									287	2,933	(3,783)
Loss on redemption of preferred stock											600
Dividends declared on preferred stock											171
Earnings on common stock									335,453	272,318	280,432
Comprehensive income									$ 331,693	$ 279,269	$ 279,602

Quarterly financial information

Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 5,336,776	$ 4,531,552	$ 4,443,351
Operating expenses	1,247,992	1,374,329	1,206,262	1,026,973	1,091,524	1,140,783	990,605	906,917	4,855,556	4,129,829	4,019,303
Operating income	130,221	189,470	97,311	64,218	118,351	140,004	73,992	69,376	481,220	401,723	424,048
Income from continuing operations	94,804	136,128	63,145	41,089	75,982	107,369	44,075	41,960	335,166	269,386	284,986
Income (loss) from discontinued operations, net of tax	261	1,509	(1,320)	(163)	2,846	(118)	(273)	477	287	2,932	(3,783)
Net income	$ 95,065	$ 137,637	$ 61,825	$ 40,926	$ 78,828	$ 107,251	$ 43,802	$ 42,437	$ 335,453	$ 272,318	$ 281,203
Earnings per common share - basic:
Income from continuing operations	$ 0.47	$ 0.68	$ 0.32	$ 0.21	$ 0.39	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.38	$ 1.46
Discontinued operations, net of tax	0	0.01	(0.01)	0	0.01	0	0	0	0	0.01	(0.02)
Earnings per share - basic	$ 0.47	$ 0.69	$ 0.31	$ 0.21	$ 0.40	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.39	$ 1.44
Weighted average common shares outstanding - basic	200,383	199,343	198,270	196,401	196,023	196,018	195,524	195,304	198,612	195,720	195,304
Earnings per common share - diluted:
Income from continuing operations	$ 0.47	$ 0.68	$ 0.32	$ 0.21	$ 0.39	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.38	$ 1.45
Discontinued operations, net of tax	0	0.01	(0.01)	0	0.01	0	0	0	0	0.01	(0.02)
Earnings per share - diluted	$ 0.47	$ 0.69	$ 0.31	$ 0.21	$ 0.40	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.39	$ 1.43
Weighted average common shares outstanding - diluted	200,478	199,383	198,287	196,414	196,385	196,265	196,169	195,982	198,626	196,150	195,687

Condensed Balance Sheets (Detai

Condensed Balance Sheets (Details 2) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 66,459	$ 53,948	$ 34,599	$ 46,107
Receivables, net	836,605	722,945
Inventories	278,407	287,309
Prepayments and other current assets	115,805	119,500
Total current assets	1,297,701	1,184,132
Investments	148,656	138,620
Property, plant and equipment	7,908,628	7,397,321
Less accumulated depreciation, depletion and amortization	2,991,486	2,818,644	2,691,641
Net property, plant and equipment	4,917,142	4,578,677	4,079,188
Deferred charges and other assets:
Goodwill	681,358	664,922	631,791
Operating lease right-of-use assets	115,323
Other	506,207	408,857
Total deferred charges and other assets	1,319,560	1,086,681
Total assets	7,683,059	6,988,110	6,334,666
Current liabilities:
Long-term debt due within one year	16,540	251,854
Accounts payable	403,391	358,505
Taxes payable	48,970	41,929
Dividends payable	41,580	39,695
Accrued compensation	99,269	69,007
Current operating lease liabilities	31,664
Other accrued liabilities	221,502	221,059
Total current liabilities	866,427	986,050
Long-term debt	2,226,567	1,856,841
Deferred credits and other liabilities:
Deferred income taxes	506,583	430,085
Noncurrent operating lease liabilities	83,742
Other	1,152,494	1,148,359
Total deferred credits and other liabilities	1,742,819	1,578,444
Commitments and contingencies
Commitments and contingencies
Authorized - 500,000,000 shares, $1.00 par value
Shares issued - 200,922,790 at December 31, 2019 and 196,564,907 at December 31, 2018	200,923	196,565
Other paid-in capital	1,355,404	1,248,576
Retained earnings	1,336,647	1,163,602
Accumulated other comprehensive loss	(42,102)	(38,342)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	2,847,246	2,566,775
Total liabilities and stockholders' equity	$ 7,683,059	$ 6,988,110
Condensed Balance Sheet (Parenthetical)
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1
Common stock, shares issued	200,922,790	196,564,907
Treasury stock, shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 12,326	$ 2,271	$ 843	$ 4,159
Receivables, net	4,727	92,724
Accounts receivable from subsidiaries	49,943	36,015
Inventories	0	13,293
Prepayments and other current assets	501	14,488
Total current assets	67,497	158,791
Investments	46,294	76,202
Investment in subsidiaries	2,842,068	1,790,886
Property, plant and equipment	0	2,846,715
Less accumulated depreciation, depletion and amortization	0	836,735
Net property, plant and equipment	0	2,009,980
Deferred charges and other assets:
Goodwill	0	4,812
Operating lease right-of-use assets	153
Other	34,367	180,473
Total deferred charges and other assets	34,520	185,285
Total assets	2,990,379	4,221,144
Current liabilities:
Long-term debt due within one year	0	200,711
Accounts payable	2,981	50,051
Accounts payable to subsidiaries	4,752	12,438
Taxes payable	1,253	24,704
Dividends payable	41,580	39,695
Accrued compensation	8,812	14,346
Current operating lease liabilities	96
Other accrued liabilities	7,690	54,099
Total current liabilities	67,164	396,044
Long-term debt	0	586,012
Deferred credits and other liabilities:
Deferred income taxes	0	165,122
Noncurrent operating lease liabilities	56
Other	75,913	507,191
Total deferred credits and other liabilities	75,969	672,313
Commitments and contingencies
Commitments and contingencies
Authorized - 500,000,000 shares, $1.00 par value
Shares issued - 200,922,790 at December 31, 2019 and 196,564,907 at December 31, 2018	200,923	196,565
Other paid-in capital	1,355,404	1,248,576
Retained earnings	1,336,647	1,163,602
Accumulated other comprehensive loss	(42,102)	(38,342)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	2,847,246	2,566,775
Total liabilities and stockholders' equity	$ 2,990,379	$ 4,221,144
Condensed Balance Sheet (Parenthetical)
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1
Common stock, shares issued	200,922,790	196,564,907
Treasury stock, 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Operating activities:	$ 542,280	$ 499,881	$ 448,011
Capital expenditures	(576,065)	(568,230)	(341,382)
Net proceeds from sale or disposition of property and other	29,812	26,100	126,588
Investments	(2,011)	(2,321)	(1,608)
Net cash used in investing activities	(603,861)	(710,907)	(214,168)
Issuance of long-term debt	599,455	566,829	140,812
Repayment of long-term debt	(468,917)	(174,520)	(217,394)
Payments of stock issuance costs	0	(10)	0
Proceeds from issuance of common stock	106,848	0	0
Dividends paid	(160,256)	(154,573)	(150,727)
Redemption of preferred stock	0	0	(15,600)
Repurchase of common stock	0	(5,020)	(1,684)
Tax withholding on stock-based compensation	(3,015)	(2,330)	(757)
Net cash used in financing activities	74,092	230,376	(245,350)
Increase (decrease) in cash and cash equivalents	12,511	19,349	(11,508)
Cash and cash equivalents - beginning of year	53,948	34,599	46,107
Cash and cash equivalents - end of year	66,459	53,948	34,599
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating activities:	168,520	294,379	284,075
Capital expenditures	0	(242,692)	(146,370)
Net proceeds from sale or disposition of property and other	0	5,032	(5,665)
Investments in and advances to subsidiaries	(120,000)	(40,000)	(40,000)
Advances from subsidiaries	17,000	70,000	40,000
Investments	(236)	(528)	(468)
Net cash used in investing activities	(103,236)	(208,188)	(152,503)
Issuance of long-term debt	0	199,422	70,080
Repayment of long-term debt	0	(125,961)	(37,569)
Payments of stock issuance costs	0	(10)	0
Proceeds from issuance of common stock	106,848	0	0
Dividends paid	(160,256)	(154,573)	(150,727)
Redemption of preferred stock			(15,600)
Repurchase of common stock	0	(1,920)	(564)
Tax withholding on stock-based compensation	(1,821)	(1,721)	(508)
Net cash used in financing activities	(55,229)	(84,763)	(134,888)
Increase (decrease) in cash and cash equivalents	10,055	1,428	(3,316)
Cash and cash equivalents - beginning of year	2,271	843	4,159
Cash and cash equivalents - end of year	$ 12,326	$ 2,271	$ 843

Notes to Condensed Financial St

Notes to Condensed Financial Statements (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Cash dividends paid to parent company by consolidated subsidiaries	$ 177.1	$ 115.9	$ 116.1

Schedule II - Consolidated Va_2

Schedule II - Consolidated Valuation and Qualifying Accounts (Details 2) - Allowance for doubtful accounts [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts:
Balance, beginning of period	$ 8,850	$ 8,069	$ 10,479
Charged to costs and expenses	7,864	7,532	7,024
Other	980	1,121	989
Deductions	9,197	7,872	10,423
Balance, end of period	$ 8,497	$ 8,850	$ 8,069