MDU Resources (MDU) 10-Q 4 May 18 2018 Q1 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 27, 2018
Entity Information [Line Items]
Entity registrant name	MDU RESOURCES GROUP INC
Entity central index key	67,716
Current fiscal year end date	--12-31
Entity filer category	Large Accelerated Filer
Document type	10-Q
Document period end date	Mar. 31, 2018
Document fiscal year focus	2,018
Document fiscal period focus	Q1
Amendment flag	false
Entity common stock, shares outstanding		195,304,376

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating revenues:
Operating revenues:	$ 976,293	$ 937,925
Operating expenses:
Operation and maintenance:	600,856	558,542
Purchased natural gas sold	181,967	192,948
Depreciation, depletion and amortization	52,729	51,325
Taxes, other than income	48,854	47,438
Electric fuel and purchased power	22,511	21,886
Total operating expenses	906,917	872,139
Operating income	69,376	65,786
Other income	582	2,343
Interest expense	20,447	20,303
Income before income taxes	49,511	47,826
Income taxes	7,551	12,188
Income from continuing operations	41,960	35,638
Income from discontinued operations, net of tax (Note 9)	477	1,687	[1]
Net income	42,437	37,325
Dividends declared on preferred stocks	0	171
Earnings on common stock	$ 42,437	$ 37,154
Earnings per common share - basic:
Earnings before discontinued operations	$ 0.22	$ 0.18
Discontinued operations, net of tax	0	0.01
Earnings per common share - basic	0.22	0.19
Earnings per common share - diluted:
Earnings before discontinued operations	0.22	0.18
Discontinued operations, net of tax	0	0.01
Earnings per common share - diluted	0.22	0.19
Dividends declared per common share	$ 0.1975	$ 0.1925
Weighted average common shares outstanding - basic	195,304	195,304
Weighted average common shares outstanding - diluted	195,982	196,023
Regulated operations [Member]
Operating revenues:
Operating revenues:	$ 424,459	$ 433,614
Operating expenses:
Operation and maintenance:	86,112	79,548
Nonregulated operations [Member]
Operating revenues:
Operating revenues:	551,834	504,311
Operating expenses:
Operation and maintenance:	$ 514,744	$ 478,994
[1]	Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 42,437	$ 37,325
Derivative instruments:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $56 and $56 for the three months ended in 2018 and 2017, respectively	92	91
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $155 and $217 for the three months ended in 2018 and 2017, respectively	418	357
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $(725) for the three months ended in 2018 and 2017, respectively	0	(917)
Postretirement liability adjustment	418	(560)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(1) and $5 for the three months ended in 2018 and 2017, respectively	(2)	9
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(28) and $(15) for the three months ended in 2018 and 2017, respectively	(105)	(27)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $7 and $19 for the three months ended in 2018 and 2017, respectively	30	35
Net unrealized gain (loss) on available-for-sale investments	(75)	8
Other comprehensive income (loss)	433	(452)
Comprehensive income attributable to common stockholders	$ 42,870	$ 36,873

Consolidated Statements of Com4

Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 56	$ 56
Amortization of postretirement liability (gains) losses included in net periodic benefit cost, tax	155	217
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	(725)
Foreign currency translation adjustments recognized during the period, tax	(1)	5
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(28)	(15)
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income (loss), tax	$ 7	$ 19

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Current assets:
Cash and cash equivalents	$ 58,764	$ 34,599	$ 50,735
Receivables, net	664,319	727,030	554,185
Inventories	257,792	226,583	250,609
Prepayments and other current assets	59,481	81,304	79,254
Current assets held for sale	458	479	7,290
Total current assets	1,040,814	1,069,995	942,073
Investments	138,451	137,613	129,009
Property, plant and equipment	6,842,967	6,770,829	6,544,077
Less accumulated depreciation, depletion and amortization	2,725,484	2,691,641	2,609,303
Net property, plant and equipment	4,117,483	4,079,188	3,934,774
Deferred charges and other assets:
Goodwill	631,791	631,791	631,791
Other intangible assets, net	3,465	3,837	5,347
Other	412,456	407,850	409,745
Noncurrent assets held for sale	4,392	4,392	95,719
Total deferred charges and other assets	1,052,104	1,047,870	1,142,602
Total assets	6,348,852	6,334,666	6,148,458
Current liabilities:
Long-term debt due within one year	149,199	148,499	43,499
Accounts payable	267,994	312,327	239,013
Taxes payable	57,354	42,537	74,638
Dividends payable	38,573	38,573	37,767
Accrued compensation	35,087	72,919	32,350
Other accrued liabilities	204,328	186,010	188,373
Current liabilities held for sale	11,726	11,993	2,394
Total current liabilities	764,261	812,858	618,034
Long-term debt	1,630,343	1,566,354	1,659,507
Deferred credits and other liabilities:
Deferred income taxes	346,218	347,271	666,905
Other	1,181,919	1,179,140	890,107
Total deferred credits and other liabilities	1,528,137	1,526,411	1,557,012
Commitments and contingencies
Stockholders' equity:
Preferred stocks	0	0	15,000
Common stockholders' equity:
Authorized - 500,000,000 shares, $1.00 par value Shares issued - 195,843,297 at March 31, 2018 and 2017 and December 31, 2017	195,843	195,843	195,843
Other paid-in capital	1,227,285	1,233,412	1,231,171
Retained earnings	1,051,469	1,040,748	911,702
Accumulated other comprehensive loss	(44,860)	(37,334)	(36,185)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)	(3,626)
Total common stockholders' equity	2,426,111	2,429,043	2,298,905
Total stockholders' equity	2,426,111	2,429,043	2,313,905
Total liabilities and stockholders' equity	$ 6,348,852	$ 6,334,666	$ 6,148,458

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Stockholders' equity:
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1	$ 1
Common stock, shares issued	195,843,297	195,843,297	195,843,297
Treasury shares	538,921	538,921	538,921

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive loss	Treasury stock
Balance (in shares)		150,000
Balance (in shares)			195,843,297
Treasury stock (in shares)							(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					37,325
Net Income	37,325
Other comprehensive income	(452)					(452)
Dividends declared on preferred stocks	(171)				(171)
Dividends declared on common stock	(37,596)				(37,596)
Stock-based compensation	996			1,134	(138)
Repurchase of common stock							(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)
Balance at Mar. 31, 2017	$ 2,313,905	$ 15,000	$ 195,843	1,231,171	911,702	(36,185)	$ (3,626)
Balance (in shares)		150,000
Balance (in shares)	195,843,297		195,843,297
Treasury stock (in shares)	(538,921)						(538,921)
Balance (in shares)	195,843,297		195,843,297
Treasury stock (in shares)	(538,921)						(538,921)
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 at Dec. 31, 2017	2,429,043		195,843	1,233,412	1,040,748	(37,334)	$ (3,626)
Net Income					42,437
Net Income	42,437
Other comprehensive income	433					433
Reclassification of certain prior period tax effects from accumulated other comprehensive loss | Adjustments for new accounting pronouncement	(7,900)				7,959	(7,959)
Dividends declared on common stock	(38,572)				(38,572)
Stock-based compensation	1,090			1,223	(133)
Repurchase of common stock							(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)
Balance at Mar. 31, 2018	$ 2,426,111		$ 195,843	$ 1,227,285	$ 1,051,469	$ (44,860)	$ (3,626)
Balance (in shares)	195,843,297		195,843,297
Treasury stock (in shares)	(538,921)						(538,921)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net income	$ 42,437	$ 37,325
Income from discontinued operations, net of tax	477	1,687	[1]
Income from continuing operations	41,960	35,638
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	52,729	51,325
Deferred income taxes	(2,068)	(332)
Changes in current assets and liabilities, net of acquisitions:
Receivables	62,711	63,684
Inventories	(29,997)	(13,676)
Other current assets	22,506	(31,006)
Accounts payable	(31,864)	(23,380)
Other current liabilities	(5,115)	(1,179)
Other noncurrent changes	(5,302)	2,161
Net cash provided by continuing operations	105,560	83,235
Net cash provided by discontinued operations	231	3,304
Net cash provided by operating activities	105,791	86,539
Investing activities:
Capital expenditures	(105,136)	(72,316)
Net proceeds from sale or disposition of property and other	5,966	117,967
Investments	(1,074)	(116)
Net cash provided by (used in) continuing operations	(100,244)	45,535
Net cash provided by discontinued operations	0	54
Net cash provided by (used in) investing activities	(100,244)	45,589
Financing activities:
Issuance of long-term debt	101,588	59,985
Repayment of long-term debt	(37,047)	(147,277)
Dividends paid	(38,573)	(37,767)
Repurchase of common stock	(5,020)	(1,684)
Tax withholding on stock-based compensation	(2,330)	(757)
Net cash provided by (used in) continuing operations	18,618	(127,500)
Net cash provided by discontinued operations	0	0
Net cash provided by (used in) financing activities	18,618	(127,500)
Increase in cash and cash equivalents	24,165	4,628
Cash and cash equivalents -- beginning of year	34,599	46,107
Cash and cash equivalents -- end of period	$ 58,764	$ 50,735
[1]	Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Basis of presentation

Basis of presentation	3 Months Ended
	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of presentation The accompanying consolidated interim financial statements were prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Interim financial statements do not include all disclosures provided in annual financial statements and, accordingly, these financial statements should be read in conjunction with those appearing in the 2017 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses. Management has also evaluated the impact of events occurring after March 31, 2018 , up to the date of issuance of these consolidated interim financial statements. 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, which resulted in lower income tax expense for the three months ended March 31, 2018 . The Company continues to review the components of the TCJA and the impact on the Company's consolidated financial statements and related disclosures for 2018 and thereafter. While the Company was able to make 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, it may be affected by other analyses related to the TCJA, including, but not limited to, the state tax effect of adjustments to federal temporary differences and the calculation of deemed repatriation of deferred foreign income. The final transition impacts of the TCJA may differ from amounts disclosed, possibly materially, due to, among other things, interpretations, legislative action to address questions, changes in accounting standards for income taxes or related interpretations, or updates or changes to estimates the Company has utilized to calculate the transition impacts. The SEC has issued rules that would allow 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, of which there were no such adjustments in the first quarter of 2018. The Company currently anticipates finalizing and recording any resulting adjustments by December 31, 2018, which will be included in income from continuing operations. Due to the enactment of the TCJA, the regulated jurisdictions in which the Company's regulated businesses provides service have requested the Company furnish plans for the effect of the reduced corporate tax rate, which may impact the Company's tax rates. Therefore, the Company has reserved for such impacts as an offset to revenue in certain jurisdictions. The Company will continue to make changes to reserve balances as required. For more information on the details of each jurisdiction's request, see Note 15 . Effective January 1, 2018, the Company adopted the requirements of the revenue from contracts with customers guidance following the modified retrospective method, as discussed in Notes 6 and 8 . As such, results for reporting periods beginning January 1, 2018, 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 revenue recognition. Based on the Company's analysis, the Company did not identify a significant change in the timing of revenue recognition under the new guidance as compared to the historic accounting for revenue recognition. Certain prior year amounts have been reclassified to conform to the current year presentation in the consolidated financial statements related to the retrospective adoption of the FASB guidance to improve the presentation of net periodic pension and net periodic postretirement benefit costs, which was effective on January 1, 2018. The components of net periodic pension and postretirement costs, other than service costs, were reclassified from operating expenses to other income on the Consolidated Statements of Income, as discussed in Note 6 . 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 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 9

Seasonality of operations

Seasonality of operations	3 Months Ended
	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Seasonality of operations	Seasonality of operations

Accounts receivable and allowan

Accounts receivable and allowance for doubtful accounts	3 Months Ended
	Mar. 31, 2018
Receivables [Abstract]
Accounts receivable and allowance for doubtful accounts	Accounts receivable and allowance for doubtful accounts Accounts receivable consist 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 total balance of receivables past due 90 days or more was $36.3 million , $26.3 million and $34.7 million at March 31, 2018 and 2017 , and December 31, 2017 , 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 March 31, 2018 and 2017 , and December 31, 2017 , was $8.2 million , $10.9 million and $8.1 million

Inventories and natural gas in

Inventories and natural gas in storage	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Inventories and natural gas in storage	Inventories and natural gas in storage Natural gas in storage for the Company's regulated operations is generally carried at lower of cost or net realizable value, or cost using the last-in, first-out method. All other inventories are stated at the lower of cost or net realizable value. The portion of the cost of natural gas in storage expected to be used within one year was included in inventories. Inventories on the Company's Consolidated Balance Sheets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Aggregates held for resale $ 123,053 $ 120,392 $ 115,268 Asphalt oil 61,647 50,538 30,360 Materials and supplies 19,493 22,074 18,650 Merchandise for resale 16,378 16,546 14,905 Natural gas in storage (current) 10,936 11,282 20,950 Other 26,285 29,777 26,450 Total $ 257,792 $ 250,609 $ 226,583 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 $ 47.8 million , $ 49.5 million and $ 49.3 million at March 31, 2018 and 2017 , and December 31, 2017

Earnings per common share

Earnings per common share	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Earnings per common share	Earnings 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 applicable period. 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 applicable period, plus the effect of nonvested performance share awards and restricted stock units. Common stock outstanding includes issued shares less shares held in treasury. Earnings on common stock was the same for both the basic and diluted earnings per share calculations. A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows: Three Months Ended March 31, 2018 2017 (In thousands) Weighted average common shares outstanding - basic 195,304 195,304 Effect of dilutive performance share awards 678 719 Weighted average common shares outstanding - diluted 195,982 196,023 Shares excluded from the calculation of diluted earnings per share — —

New accounting standards

New accounting standards	3 Months Ended
	Mar. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New accounting standards	New accounting standards Recently adopted accounting standards ASU 2014-09 - Revenue from Contracts with Customers In May 2014, the FASB issued guidance on accounting for revenue from contracts with customers. The guidance provides for a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry specific guidance. In August 2015, the FASB issued guidance deferring the effective date of the revenue guidance and allowing entities to early adopt. With this decision, the guidance was effective for the Company on January 1, 2018. Entities had the option of using either a full retrospective or modified retrospective approach to adopting the guidance. Under the modified retrospective approach, an entity recognizes the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings in the period of adoption. The Company adopted the guidance on January 1, 2018, using the modified retrospective approach. The Company elected the practical expedient to not disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied (or partially unsatisfied) as of the end of the reporting period, along with an explanation of when such revenue would be expected to be recognized. This practical expedient was used since the performance obligations are part of contracts with an original duration of one year or less. The Company also 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 one year or less. Upon completion of the Company's evaluation of contracts and methods of revenue recognition under the previous accounting guidance, the Company did not identify any material cumulative effect adjustments to be made to retained earnings. In addition, the Company has expanded revenue disclosures, both quantitatively and qualitatively, related to the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers, as discussed in Note 8 . The Company has reviewed its revenue streams to evaluate the impact of this guidance and did not identify a significant change in the timing of revenue recognition, results of operations, financial position or cash flows. The Company reviewed its internal controls related to revenue recognition and disclosures and concluded that the guidance impacts certain business processes and controls. As such, the Company has developed modifications to its internal controls for certain topics under the guidance as they apply to the Company and such modifications were not deemed to be significant. Results for reporting periods beginning after December 31, 2017, are presented under the new guidance, while prior period amounts are not adjusted and continue to be reported in accordance with historic accounting for revenue recognition. Under the modified retrospective approach, the guidance was applied only to contracts that were not completed as of January 1, 2018. Therefore, the Company recognized the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings at January 1, 2018. In the first quarter of 2018 , there was not a material impact to the financial statements as a result of applying the guidance. The cumulative effect of the changes made to the Consolidated Balance Sheet was as follows: December 31, Adjustments January 1, (In thousands) Liabilities and Stockholders' Equity Current liabilities: Other accrued liabilities $ 186,010 $ 903 $ 186,913 Deferred credits and other liabilities: Deferred income taxes 347,271 (332 ) 346,939 Other 1,179,140 399 1,179,539 Commitments and contingencies Stockholders' equity : Common stockholders' equity: Retained earnings 1,040,748 (970 ) 1,039,778 The cumulative adjustment is related to prepaid natural gas transportation to storage contracts where a separate performance obligation existed and has not yet been satisfied. As such, these contracts were still open and met the criteria for a cumulative effect adjustment. ASU 2016-15 - Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued guidance to clarify the classification of certain cash receipts and payments in the statement of cash flows. The guidance is intended to standardize the presentation and classification of certain transactions, including cash payments for debt prepayment or extinguishment, proceeds from insurance claim settlements and distributions from equity method investments. In addition, the guidance clarifies how to classify transactions that have characteristics of more than one class of cash flows. The Company adopted the guidance on January 1, 2018, on a prospective basis. The guidance did not have a material effect on the Company's statement of cash flows. ASU 2017-01 - Clarifying the Definition of a Business In January 2017, the FASB issued guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions or disposals of assets or businesses. The guidance provides a screen to determine when an integrated set of assets and activities is not a business. The guidance will also affect other aspects of accounting, such as determining reporting units for goodwill testing and whether an entity has acquired or sold a business. The Company adopted the guidance on January 1, 2018, on a prospective basis. The guidance did not have a material effect on the Company's results of operations, financial position, cash flows or disclosures. ASU 2017-07 - Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued guidance to improve the presentation of net periodic pension and net periodic postretirement benefit costs. The guidance requires the service cost component to be presented in the income statement in the same line item or items as other compensation costs arising from services performed during the period. Other components of net periodic benefit cost shall be presented in the income statement separately from the service cost component and outside a subtotal of income from operations. The guidance also only allows the service cost component to be capitalized. The Company adopted the guidance on January 1, 2018, on a retrospective basis. The guidance required the reclassification of all components of net periodic benefit costs, except for the service cost component, from operating expenses to other income on the Company's Consolidated Statements of Income with no impact to earnings. As a result of the retrospective application of this change in accounting guidance, the Company reclassified $1.8 million from operation and maintenance expense to other income on the Consolidated Statements of Income for the three months ended March 31, 2017. The Company also reclassified unrealized gains on investments used to satisfy obligations under the defined benefit plans of $3.1 million for the three months ended March 31, 2017, which were included in operation and maintenance expense, to other income on the Consolidated Statements of Income. The guidance did not have a material effect on the Company's results of operations, cash flows or disclosures. ASU 2018-02 - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued guidance that allows an entity to reclassify the stranded tax effects resulting from the newly enacted federal corporate income tax rate from accumulated other comprehensive income (loss) to retained earnings. The guidance is effective for the Company on January 1, 2019, including interim periods, with early adoption permitted. The guidance can be applied using one of two methods. One method is to record the reclassification of the stranded income taxes at the beginning of the period of adoption. The other method is to apply the guidance retrospectively to each period in which the income tax effects of the TCJA are recognized in accumulated other comprehensive income (loss). The Company early adopted the guidance on January 1, 2018, and elected to reclassify the stranded income taxes at the beginning of the period. During the first quarter of 2018, the Company reclassified $7.9 million of stranded tax expense from accumulated other comprehensive loss to retained earnings. The guidance did not have a material effect on the Company's results of operations, cash flows or disclosures. Recently issued accounting standards not yet adopted ASU 2016-02 - Leases In February 2016, the FASB issued guidance regarding leases. The guidance requires lessees to recognize a lease liability and a right-of-use asset on the balance sheet for operating and financing leases with terms of more than 12 months. The guidance remains 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 requires additional disclosures, both quantitative and qualitative, related to operating and finance leases for the lessee and sales-type, direct financing and operating leases for the lessor. This guidance will be effective for the Company on January 1, 2019, and should be applied using a retrospective approach with early adoption permitted. The Company continues to evaluate the potential impact the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. The Company is planning to adopt the standard on January 1, 2019, utilizing the practical expedient that allows the Company to not reassess whether an expired or existing contract contains a lease, the classification of leases or initial direct costs. 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 is currently evaluating the impact of the practical expedient. In January 2018, the FASB issued a proposed accounting standard update to the guidance that would allow an entity the option to adopt the guidance on a modified retrospective basis. Under the modified retrospective approach, an entity would recognize a cumulative effect adjustment of initially applying the guidance to the opening balance of retained earnings in the period of adoption. The Company is monitoring the status of the proposal. ASU 2017-04 - Simplifying the Test for Goodwill Impairment

Comprehensive income (loss)

Comprehensive income (loss)	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Comprehensive income (loss)	Comprehensive income (loss) The after-tax changes in the components of accumulated other comprehensive loss were as follows: Three Months Ended March 31, 2018 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (1,934 ) $ (35,163 ) $ (155 ) $ (82 ) $ (37,334 ) Other comprehensive loss before reclassifications — — (2 ) (105 ) (107 ) Amounts reclassified from accumulated other comprehensive loss 92 418 — 30 540 Net current-period other comprehensive income (loss) 92 418 (2 ) (75 ) 433 Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss (389 ) (7,520 ) (33 ) (17 ) (7,959 ) Balance at end of period $ (2,231 ) $ (42,265 ) $ (190 ) $ (174 ) $ (44,860 ) Three Months Ended March 31, 2017 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,300 ) $ (33,221 ) $ (149 ) $ (63 ) $ (35,733 ) Other comprehensive income (loss) before reclassifications — — 9 (27 ) (18 ) Amounts reclassified from accumulated other comprehensive loss 91 357 — 35 483 Amounts reclassified to accumulated other comprehensive loss from regulatory asset — (917 ) — — (917 ) Net current-period other comprehensive income (loss) 91 (560 ) 9 8 (452 ) Balance at end of period $ (2,209 ) $ (33,781 ) $ (140 ) $ (55 ) $ (36,185 ) Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Location on Consolidated Statements of Income March 31, 2018 2017 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (148 ) $ (147 ) Interest expense 56 56 Income taxes (92 ) (91 ) Amortization of postretirement liability losses included in net periodic benefit cost (credit) (573 ) (574 ) Other income 155 217 Income taxes (418 ) (357 ) Reclassification adjustment for loss on available-for-sale investments included in net income (37 ) (54 ) Other income 7 19 Income taxes (30 ) (35 ) Total reclassifications $ (540 ) $ (483 )

Revenue from contracts with cus

Revenue from contracts with customers	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue from contracts with customers	Revenue from contracts with customers Revenue is recognized when a performance obligation is satisfied by transferring control over a product or service to a customer. Revenue is measured based on consideration specified in a contract with a customer, and excludes any sales incentives and amounts collected on behalf of third parties. The Company is considered an agent for certain taxes collected from customers. As such, the Company presents revenues net of these taxes at the time of sale to be remitted to governmental authorities, including sales and use taxes. 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 revenue 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 thirty 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 on special projects, 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 twelve days of receipt of the invoice. For gathering contracts not governed by the tariff, amounts are due within twenty days of invoice receipt. For other contracts not governed by the tariff, payment terms are net thirty 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 evaluated and determined to be single performance obligations. 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; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics. The Company estimates variable consideration at the most likely amount expected. 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 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 thirty 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 bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics. The Company estimates variable consideration at the most likely amount expected. 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 thirty 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. 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 13 . Three Months Ended March 31, 2018 Electric Natural gas distribution Pipeline and midstream Construction materials and contracting Construction services Other Total (in thousands) Residential utility sales $ 35,183 $ 192,886 $ — $ — $ — $ — $ 228,069 Commercial utility sales 34,701 116,891 — — — — 151,592 Industrial utility sales 8,770 7,809 — — — — 16,579 Other utility sales 1,836 — — — — — 1,836 Natural gas transportation — 11,179 21,818 — — — 32,997 Natural gas gathering — — 2,270 — — — 2,270 Natural gas storage — — 3,134 — — — 3,134 Contracting services — — — 74,064 — — 74,064 Construction materials — — — 173,591 — — 173,591 Intrasegment eliminations* — — — (34,270 ) — — (34,270 ) Inside specialty contracting — — — — 233,821 — 233,821 Outside specialty contracting — — — — 87,181 — 87,181 Other 8,252 3,999 3,326 — (86 ) 2,696 18,187 Intersegment eliminations — — (21,759 ) (101 ) (11 ) (2,638 ) (24,509 ) Revenues from contracts with customers 88,742 332,764 8,789 213,284 320,905 58 964,542 Revenues out of scope (1,338 ) (100 ) 44 — 13,145 — 11,751 Total external operating revenues $ 87,404 $ 332,664 $ 8,833 $ 213,284 $ 334,050 $ 58 $ 976,293 * 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. 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 change in contract assets and liabilities were as follows: March 31, 2018 December 31, 2017 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 132,610 $ 122,184 $ 10,426 Receivables, net Contract liabilities - current (80,905 ) (71,479 ) (9,426 ) Accounts payable Contract liabilities - noncurrent (399 ) — (399 ) Deferred credits and other liabilities - other Net contract assets (liabilities) $ 51,306 $ 50,705 $ 601 At March 31, 2018 , the Company's net contract assets increased $601,000 compared to December 31, 2017 . Included in the change of total contract assets (liabilities) was an increase in contract assets, due to revenue recognized in excess of billings on contracts, and an increase in contract liabilities, due to billings on contracts in excess of revenues recognized. The Company recognized $52.0 million in revenue for the three months ended March 31, 2018 , which was previously included in contract liabilities at December 31, 2017 . The Company recognized $3.1 million in revenue for the three months ended March 31, 2018

Discontinued operations

Discontinued operations	3 Months Ended
	Mar. 31, 2018
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 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. Dakota Prairie Refining On June 24, 2016, WBI Energy entered into a membership interest purchase agreement with Tesoro to sell all of the outstanding membership interests in Dakota Prairie Refining to Tesoro. WBI Energy and Calumet each previously owned 50 percent of the Dakota Prairie Refining membership interests and were equal members in building and operating Dakota Prairie Refinery. To effectuate the sale, WBI Energy acquired Calumet’s 50 percent membership interest in Dakota Prairie Refining on June 27, 2016. The sale of the membership interests to Tesoro closed on June 27, 2016. The sale of Dakota Prairie Refining reduced the Company’s risk by decreasing exposure to commodity prices. Centennial continues to guarantee certain debt obligations of Dakota Prairie Refining; however, Tesoro has agreed to indemnify Centennial for any losses and litigation expenses arising from the guarantee. For more information related to the guarantee, see Note 16 . The carrying amounts of the major classes of assets and liabilities classified as held for sale, related to the operations of and activity associated with Dakota Prairie Refining, on the Company's Consolidated Balance Sheets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Assets Current assets: Income taxes receivable $ 1,858 (a) $ 11,756 $ 1,778 (a) Total current assets held for sale 1,858 11,756 1,778 Total assets held for sale $ 1,858 $ 11,756 $ 1,778 Liabilities Current liabilities: Accounts payable $ — $ 16 $ — Total current liabilities held for sale — 16 — Noncurrent liabilities: Deferred income taxes (b) 37 55 37 Total noncurrent liabilities held for sale 37 55 37 Total liabilities held for sale $ 37 $ 71 $ 37 (a) On the Company's Consolidated Balance Sheets, these amounts were reclassified to taxes payable and are reflected in current liabilities held for sale. (b) On the Company's Consolidated Balance Sheets, these amounts were reclassified to deferred charges and other assets - deferred income taxes and are reflected in noncurrent assets held for sale. The Company retained certain liabilities of Dakota Prairie Refining. In the first quarter of 2017, the Company recorded a reversal of the previously accrued liability of $7.0 million ( $4.3 million after tax) due to the resolution of a legal matter. As of March 31, 2018 , Dakota Prairie Refining incurred no material exit and disposal costs, and does not expect to incur any material exit and disposal costs. Fidelity In the second quarter of 2015, the Company began the marketing and sale process of Fidelity with an anticipated sale to occur within one year. Between September 2015 and March 2016, the Company entered into purchase and sale agreements to sell substantially all of Fidelity's oil and natural gas assets. The completion of these sales occurred between October 2015 and April 2016. The sale of Fidelity was part of the Company's strategic plan to grow its capital investments in the remaining business segments and to focus on creating a greater long-term value. The carrying amounts of the major classes of assets and liabilities classified as held for sale, related to the operations of Fidelity, on the Company's Consolidated Balance Sheets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Assets Current assets: Receivables, net $ 458 $ 266 $ 479 Total current assets held for sale 458 266 479 Noncurrent assets: Net property, plant and equipment 1,631 4,515 1,631 Deferred income taxes 2,637 91,098 2,637 Other 161 161 161 Total noncurrent assets held for sale 4,429 95,774 4,429 Total assets held for sale $ 4,887 $ 96,040 $ 4,908 Liabilities Current liabilities: Accounts payable $ — $ 67 $ 30 Taxes payable 10,774 4,732 (a) 10,857 Other accrued liabilities 2,810 2,311 2,884 Total current liabilities held for sale 13,584 7,110 13,771 Total liabilities held for sale $ 13,584 $ 7,110 $ 13,771 (a) On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale. The Company's deferred tax assets included in assets held for sale were largely comprised of federal and state net operating loss carryforwards. The Company realized substantially all of the outstanding net operating loss carryforwards in 2017. The Company has incurred $10.5 million of exit and disposal costs to date and incurred no exit and disposal costs for the three months ended March 31, 2018 and 2017, respectively. The Company does not expect to incur any additional material exit and disposal costs. The exit and disposal costs are associated with severance and other related matters. Dakota Prairie Refining and Fidelity The reconciliation of the major classes of income and expense constituting pretax income (loss) from discontinued operations, which includes Dakota Prairie Refining and Fidelity, to the after-tax income from discontinued operations on the Company's Consolidated Statements of Income was as follows: Three Months Ended March 31, 2018 2017 (In thousands) Operating revenues $ 66 $ 105 Operating expenses 174 (6,577 ) Operating income (loss) (108 ) 6,682 Other income (expense) 12 (15 ) Interest expense 575 — Income (loss) from discontinued operations before income taxes (671 ) 6,667 Income taxes (1,148 ) 4,980 * Income from discontinued operations $ 477 $ 1,687 * Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Goodwill and other intangible a

Goodwill and other intangible assets	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and other intangible assets	Goodwill and other intangible assets The changes in the carrying amount of goodwill were as follows: Three Months Ended March 31, 2018 Balance at January 1, 2018 Goodwill Acquired the Year Balance at March 31, 2018 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 — 176,290 Construction services 109,765 — 109,765 Total $ 631,791 $ — $ 631,791 Three Months Ended March 31, 2017 Balance at January 1, 2017 Goodwill Acquired the Year Balance at March 31, 2017 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 — 176,290 Construction services 109,765 — 109,765 Total $ 631,791 $ — $ 631,791 Year Ended December 31, 2017 Balance at January 1, 2017 Goodwill Acquired the Year Balance at December 31, 2017 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 — 176,290 Construction services 109,765 — 109,765 Total $ 631,791 $ — $ 631,791 Other amortizable intangible assets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Customer relationships $ 14,668 $ 15,745 $ 15,248 Less accumulated amortization 13,007 12,910 13,382 1,661 2,835 1,866 Noncompete agreements 2,430 2,430 2,430 Less accumulated amortization 1,842 1,695 1,805 588 735 625 Other 6,458 7,086 6,990 Less accumulated amortization 5,242 5,309 5,644 1,216 1,777 1,346 Total $ 3,465 $ 5,347 $ 3,837 Amortization expense for amortizable intangible assets for the three months ended March 31, 2018 and 2017 , was $400,000 and $600,000 , respectively. Estimated amortization expense for amortizable intangible assets is $1.3 million in 2018 , $1.0 million in 2019 , $500,000 in 2020 , $300,000 in 2021 , $300,000 in 2022 and $500,000

Fair value measurements

Fair value measurements	3 Months Ended
	Mar. 31, 2018
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 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 $76.9 million , $73.8 million and $77.4 million , at March 31, 2018 and 2017 , and December 31, 2017 , respectively, are classified as investments on the Consolidated Balance Sheets. The net unrealized loss on these investments was $500,000 for the three months ended March 31, 2018 . The net unrealized gain on these investments was $2.9 million for the three months ended March 31, 2017 . The change in fair value, which is considered part of the cost of the plan, is classified in other income on the Company's Consolidated Statements of Income. In connection with the adoption of ASU 2017-07, as discussed in Note 6, the Company has elected to reclassify the unrealized gains and losses from operation and maintenance expense to other income on the Company's 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. The available-for-sale securities 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: March 31, 2018 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,282 $ 4 $ 223 $ 10,063 U.S. Treasury securities 466 — 1 465 Total $ 10,748 $ 4 $ 224 $ 10,528 March 31, 2017 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,971 $ 8 $ 94 $ 9,885 U.S. Treasury securities 412 1 — 413 Total $ 10,383 $ 9 $ 94 $ 10,298 December 31, 2017 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,342 $ 4 $ 129 $ 10,217 U.S. Treasury securities 205 — 1 204 Total $ 10,547 $ 4 $ 130 $ 10,421 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 quarter, based on published market quotations on active markets, or using other known sources including pricing from outside sources. The estimated fair value of the Company's 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 three months ended March 31, 2018 and 2017 , there were no transfers between Levels 1 and 2. The Company's assets and liabilities measured at fair value on a recurring basis were as follows: Fair Value Measurements at March 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 March 31, 2018 (In thousands) Assets: Money market funds $ — $ 9,085 $ — $ 9,085 Insurance contract* — 76,941 — 76,941 Available-for-sale securities: Mortgage-backed securities — 10,063 — 10,063 U.S. Treasury securities — 465 — 465 Total assets measured at fair value $ — $ 96,554 $ — $ 96,554 * The insurance contract invests approximately 49 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents. Fair Value Measurements at March 31, 2017, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at March 31, 2017 (In thousands) Assets: Money market funds $ — $ 2,551 $ — $ 2,551 Insurance contract* — 73,775 — 73,775 Available-for-sale securities: Mortgage-backed securities — 9,885 — 9,885 U.S. Treasury securities — 413 — 413 Total assets measured at fair value $ — $ 86,624 $ — $ 86,624 * The insurance contract invests approximately 51 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at December 31, 2017, 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, 2017 (In thousands) Assets: Money market funds $ — $ 6,965 $ — $ 6,965 Insurance contract* — 77,388 — 77,388 Available-for-sale securities: Mortgage-backed securities — 10,217 — 10,217 U.S. Treasury securities — 204 — 204 Total assets measured at fair value $ — $ 94,774 $ — $ 94,774 * The insurance contract invests approximately 49 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 14 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents. 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 based on discounted future cash flows using current market interest rates. The estimated fair value of the Company's Level 2 long-term debt was as follows: Carrying Amount Fair Value (In thousands) Long-term debt at March 31, 2018 $ 1,779,542 $ 1,854,350 Long-term debt at March 31, 2017 $ 1,703,006 $ 1,784,588 Long-term debt at December 31, 2017 $ 1,714,853 $ 1,826,256 The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values

Cash flow information

Cash flow information	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]
Cash flow information	Cash flow information Cash expenditures for interest and income taxes were as follows: Three Months Ended March 31, 2018 2017 (In thousands) Interest, net* $ 17,910 $ 17,546 Income taxes refunded, net** $ (1,056 ) $ (2,762 ) * AFUDC - borrowed was $382,000 and $196,000 for the three months ended March 31, 2018 and 2017, respectively. ** Income taxes paid (refunded), net of discontinued operations, were $ 1.7 million and $ (7.2) million for the three months ended March 31, 2018 and 2017, respectively. Noncash investing transactions were as follows: March 31, 2018 2017 (In thousands) Property, plant and equipment additions in accounts payable $ 16,829 $ 5,212

Business segment data

Business segment data	3 Months Ended
	Mar. 31, 2018
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 operations mine, process and sell construction aggregates (crushed stone, sand and gravel); produce and sell asphalt mix; and supply 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 and Alaska and Hawaii. The construction services segment provides inside and outside specialty contracting 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. Its inside services include design, construction and maintenance of electrical and communication wiring and infrastructure, fire suppression systems, and mechanical piping and services. 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 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. The Other category also includes 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 includes 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 9 . The information below follows the same accounting policies as described in Note 1 of the Company's Notes to Consolidated Financial Statements in the 2017 Annual Report. Information on the Company's segments was as follows: Three Months Ended March 31, 2018 2017 (In thousands) External operating revenues: Regulated operations: Electric $ 87,404 $ 88,225 Natural gas distribution 332,664 342,519 Pipeline and midstream 4,391 2,870 424,459 433,614 Nonregulated operations: Pipeline and midstream 4,442 3,643 Construction materials and contracting 213,284 200,776 Construction services 334,050 299,572 Other 58 320 551,834 504,311 Total external operating revenues $ 976,293 $ 937,925 Intersegment operating revenues: Regulated operations: Electric $ — $ — Natural gas distribution — — Pipeline and midstream 21,735 21,489 21,735 21,489 Nonregulated operations: Pipeline and midstream 24 34 Construction materials and contracting 101 86 Construction services 11 6 Other 2,638 1,743 2,774 1,869 Intersegment eliminations (24,509 ) (23,358 ) Total intersegment operating revenues $ — $ — Three Months Ended March 31, 2018 2017 (In thousands) Earnings on common stock: Regulated operations: Electric $ 13,084 $ 14,333 Natural gas distribution 32,623 27,861 Pipeline and midstream 5,459 4,557 51,166 46,751 Nonregulated operations: Pipeline and midstream (179 ) (628 ) Construction materials and contracting (23,521 ) (19,912 ) Construction services 15,090 7,362 Other (596 ) (279 ) (9,206 ) (13,457 ) Intersegment eliminations* — 2,173 Earnings on common stock before income from discontinued operations 41,960 35,467 Income from discontinued operations, net of tax* 477 1,687 Total earnings on common stock $ 42,437 $ 37,154 * Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans

Employee benefit plans	3 Months Ended
	Mar. 31, 2018
Retirement Benefits [Abstract]
Pension and other postretirement benefit plans	Employee benefit plans Pension and other postretirement plans The Company has qualified defined benefit pension plans and other postretirement benefit plans for certain eligible employees. Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans were as follows: Pension Benefits Other Postretirement Benefits Three Months Ended March 31, 2018 2017 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 407 $ 447 Interest cost 3,807 4,014 778 808 Expected return on assets (4,998 ) (5,029 ) (1,167 ) (1,145 ) Amortization of prior service credit — — (349 ) (343 ) Amortization of net actuarial loss 1,782 1,793 238 336 Net periodic benefit cost (credit), including amount capitalized 591 778 (93 ) 103 Less amount capitalized — 107 40 (39 ) Net periodic benefit cost (credit) $ 591 $ 671 $ (133 ) $ 142 The components of net periodic benefit cost (credit), other than the service cost component, are included in other income on the Company's Consolidated Statements of Income. Nonqualified defined benefit plans In addition to the qualified defined pension benefit plans reflected in the table, 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 Company's net periodic benefit cost for these plans was $1.1 million and $1.2 million for the three months ended March 31, 2018 and 2017

Regulatory matters

Regulatory matters	3 Months Ended
	Mar. 31, 2018
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. The Company's most recent cases by jurisdiction are discussed in the following paragraphs. The jurisdictions in which the Company provides service have requested the Company furnish plans for the effect of the reduced corporate tax rate due to the enactment of the TCJA which may impact the Company's rates. The following paragraphs include additional details on each jurisdiction's request. IPUC On January 17, 2018, the IPUC issued a general order initiating the investigation of the impacts of the TCJA. The order required the tax rate reduction to be deferred as a regulatory liability and for companies to report on the expected impacts of the TCJA by March 30, 2018. On March 12, 2018, the Idaho governor signed into law a decrease in the state corporate income tax rate from 7.4 percent to 6.9 percent retroactive to January 1, 2018. On March 23, 2018, Intermountain filed an application and supporting tariffs incorporating the effects of the decreases in both federal and state income taxes resulting in a decrease in revenues of approximately $5.0 million annually. Until the final order is received, a regulatory liability will be recorded, beginning on January 1, 2018, to capture the customer benefits associated with these lower taxes. MNPUC On December 29, 2017, the MNPUC issued a notice of investigation related to tax changes with the enactment of the TCJA. On January 19, 2018, the MNPUC issued a notice of request for information, commission planning meeting and subsequent comment period. Pursuant to the notice, Great Plains provided preliminary impacts of the TCJA on January 30, 2018. On March 2, 2018, Great Plains submitted its initial filing addressing the impacts of the TCJA and is advocating existing rates are reasonable and a reduction in rates is not warranted. This matter is pending before the MNPUC. MTPSC On September 25, 2017, Montana-Dakota filed an application with the MTPSC for a natural gas rate increase of approximately $2.8 million annually or approximately 4.1 percent above current rates. The requested increase is primarily to recover the increased investment in distribution facilities to enhance system safety and reliability and the depreciation and taxes associated with the increase in investment. Montana-Dakota is also introducing an SSIP and the proposed adjustment mechanism required to fund the SSIP. Montana-Dakota requested an interim increase of approximately $1.6 million or approximately 2.3 percent , subject to refund. On December 27, 2017, the MTPSC requested Montana-Dakota identify a plan for the impacts of the TCJA for the natural gas segment within the existing natural gas rate case. On January 12, 2018, Montana-Dakota filed a revised interim increase of approximately $764,000 , subject to refund, incorporating the estimated impacts of the TCJA reduction in the federal corporate income tax rate. On February 23, 2018, supplemental testimony, regarding the effects of the TCJA, was submitted reducing Montana-Dakota's initial request of $2.8 million to $1.6 million . On March 6, 2018, the interim request for rate relief was denied by the MTPSC. On April 24, 2018, a stipulation was filed with the MTPSC reflecting an annual increase of $975,000 or 1.4 percent . A hearing was held on April 26, 2018. This matter is pending before the MTPSC. On December 27, 2017, the MTPSC requested Montana-Dakota identify a plan for the impacts of the TCJA and to file a proposal for the impacts on the electric segment by March 31, 2018. On April 2, 2018, Montana-Dakota submitted its plan requesting the MTPSC recognize the identified need for additional rate relief and to consider the effects of the TCJA in a general electric rate case to be submitted by September 30, 2018. This matter is pending before the MTPSC. NDPSC On July 21, 2017, Montana-Dakota filed an application with the NDPSC for a natural gas rate increase of approximately $5.9 million annually or approximately 5.4 percent above current rates. The requested increase is primarily to recover the increased investment in distribution facilities to enhance system safety and reliability and the depreciation and taxes associated with the increase in investment. Montana-Dakota is also introducing an SSIP and the proposed adjustment mechanism required to fund the SSIP. Montana-Dakota requested an interim increase of approximately $4.6 million or approximately 4.2 percent , subject to refund. On September 6, 2017, the NDPSC approved the request for interim rates effective with service rendered on or after September 19, 2017. On January 12, 2018, Montana-Dakota requested a delay of the rate case as a result of the enactment of the TCJA to allow the Company time to investigate the implications of the TCJA on the rate case. On February 14, 2018, the NDPSC approved the delay of hearing and scheduled it to begin on May 30, 2018. Also on February 14, 2018, Montana-Dakota filed a revised interim increase request of approximately $2.7 million , subject to refund, incorporating the estimated impacts of the TCJA reduction in the federal corporate income tax rate. On March 1, 2018, the updated interim rates were implemented. The impact of the TCJA was submitted as part of a rebuttal testimony identifying a reduction of the adjusted revenue requirement to $3.6 million . A hearing is scheduled for May 30, 2018. This matter is pending before the NDPSC. On January 10, 2018, the NDPSC issued a general order initiating the investigation into the effects of the TCJA. The order required regulatory deferral accounting on the impacts of the TCJA and for companies to file comments and the expected impacts. On February 15, 2018, Montana-Dakota filed a summary of the primary impacts of the TCJA on the electric and natural gas utilities. On March 9, 2018, Montana-Dakota submitted a request to decrease its electric rates by $7.2 million or 3.9 percent annually. This matter is pending before the NDPSC. OPUC On September 29, 2017, Cascade filed an application with the OPUC for an annual pipeline replacement safety cost recovery mechanism of approximately $784,000 or approximately 1.2 percent of additional revenue. The requested increase includes incremental pipeline replacement investments associated with qualifying pipeline integrity projects. This filing was withdrawn on March 6, 2018, and will be incorporated into an upcoming rate increase filing. On December 29, 2017, Cascade filed a request with the OPUC to use deferral accounting for the 2018 net benefits associated with the implementation of the TCJA. This matter is pending before the OPUC. SDPUC On December 29, 2017, the SDPUC issued an order initiating the investigation into the effects of the TCJA. The order required Montana-Dakota to provide comments by February 1, 2018, regarding the general effects of the TCJA on the cost of service in South Dakota and possible mechanisms for adjusting rates. The order also stated that all rates impacted by the federal income tax shall be adjusted effective January 1, 2018, subject to refund. The Company expects to file detailed plans of the TCJA impact in the second quarter of 2018. WUTC On August 31, 2017, Cascade filed an application with the WUTC for a natural gas rate increase of approximately $5.9 million annually or approximately 2.7 percent above current rates. The requested increase includes costs associated with increased infrastructure investment and the associated operating expenses. Also included in the request is recovery of operation and maintenance costs associated with a maximum allowable operating pressure validation plan. On January 3, 2018, the WUTC filed a bench request requiring Cascade to provide information related to the impacts of the TCJA on Cascade's revenue requirement and a proposed ratemaking treatment of those impacts. On March 23, 2018, Cascade filed its rebuttal testimony revising the revenue requirement to a decrease of $1.7 million annually, which includes the impacts of the TCJA. This matter is pending before the WUTC. WYPSC On December 29, 2017, the WYPSC issued a general order requiring regulatory deferral accounting on the impacts of the TCJA. A technical conference was held on February 6, 2018, to discuss the implications of the TCJA. On March 23, 2018, the WYPSC issued an order requiring all public utilities to submit an initial assessment of the overall effects on the TCJA on their rates by March 30, 2018. On March 30, 2018, Montana-Dakota submitted its initial assessment indicating a rate reduction for its electric rates in the amount of $1.1 million annually or 4.2 percent with a commitment to submit a request to revise electric rates by May 15, 2018. Montana-Dakota reported its natural gas earnings do not support a decrease in rates and requested the WYPSC allow the impacts of the TCJA be addressed in a natural gas rate case to be submitted by June 1, 2019. This matter is pending before the WYPSC. FERC Montana-Dakota and certain MISO Transmission Owners with projected rates submitted a filing to the FERC on February 1, 2018, requesting the FERC to waive certain provisions of the MISO tariff in order for Montana-Dakota and certain MISO Transmission Owners with projected rates to revise their rates to reflect the reduction in the corporate tax rate. Under the MISO tariff, rates are to be changed only on an annual basis with any changes reflected in subsequent true-ups. On March 15, 2018, the FERC approved the waiver request and new rates reflecting the effects of the TCJA were implemented by MISO on March 1, 2018. MISO also retroactively re-billed the January and February 2018 services to reflect the new rates. The total revenue requirement for the Company's multivalue project was reduced to $12.0 million from $13.6 million , which was previously effective on January 1, 2018.

Contingencies

Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries, which may include, but are not limited to, matters involving property damage, personal injury, and environmental, contractual, statutory and regulatory obligations. The Company accrues a liability for those contingencies when the incurrence of a loss is probable and the amount can be reasonably estimated. If a range of amounts can be reasonably estimated and no amount within the range is a better estimate than any other amount, then the minimum of the range is accrued. The Company does not accrue liabilities when the likelihood that the liability has been incurred is probable but the amount cannot be reasonably estimated or when the liability is believed to be only reasonably possible or remote. For contingencies where an unfavorable outcome is probable or reasonably possible and which are material, the Company discloses the nature of the contingency and, in some circumstances, an estimate of the possible loss. Accruals are based on the best information available, but in certain situations management is unable to estimate an amount or range of a reasonably possible loss including, but not limited to when: (1) the damages a re 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. The Company accrued liabilities of $37.4 million , $29.1 million and $35.4 million , which have not been discounted, including liabilities held for sale, for contingencies, including litigation, production taxes, royalty claims and environmental matters at March 31, 2018 and 2017 , and December 31, 2017 , respectively. This includes amounts that may 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 a riverbed site adjacent to a commercial property site acquired by Knife River - Northwest from Georgia-Pacific West, Inc. in 1999. The riverbed site is part of the Portland, Oregon, Harbor Superfund Site. The EPA wants responsible parties to share in the cleanup of sediment contamination in the Willamette River. 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, a group of several entities, which does not include Knife River - Northwest or Georgia-Pacific West, Inc. Investigative costs are indicated to be in excess of $100 million . On January 6, 2017, Region 10 of the EPA issued an ROD with its selected remedy for cleanup of the in-river portion of the site. Implementation of the remedy 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 also received notice in January 2008 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. The Portland Harbor Natural Resource Trustee Council indicates the injury determination is appropriate to facilitate early settlement of damages and restoration for natural resource injuries. It is not possible to estimate the costs of natural resource damages until an assessment is completed and allocations are undertaken. Based upon a review of the Portland Harbor sediment contamination evaluation by the Oregon DEQ and other information available, Knife River - Northwest does not believe it is a responsible party. In addition, Knife River - Northwest 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. By letter in March 2009, LWG 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 There are three claims against Cascade for cleanup of environmental contamination at manufactured gas plant sites operated by Cascade's predecessors. The first claim is for contamination at a site in Eugene, Oregon which was received in 1995. There are PRPs in addition to Cascade that may be liable for cleanup of the contamination. Some of these PRPs have shared in the investigation costs. It is expected that these and other PRPs will share in the cleanup costs. The Oregon DEQ released an ROD in January 2015 that selected a remediation alternative for the site as recommended in an earlier staff report. The total estimated cost for the selected remediation, including long-term maintenance, is approximately $3.5 million of which $400,000 has been incurred. It is not known at this time what share of the cleanup costs will actually be borne by Cascade; however, Cascade has paid 50 percent of the ongoing investigation and design costs and anticipates its proportional share of the final costs could be approximately 50 percent. Cascade has an accrual balance of $1.6 million for remediation of this site. In January 2013, the OPUC approved Cascade's application to defer environmental remediation costs at the Eugene site for a period of 12 months starting November 30, 2012. Cascade received orders reauthorizing the deferred accounting for the 12-month periods starting November 30, 2013, December 1, 2014, December 1, 2015, December 1, 2016 and December 1, 2017. The second claim is 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 $1.8 million has been incurred. Cascade has accrued $5.8 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, which are included in other noncurrent assets, 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. The third claim is 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. The notice indicates that current estimates to complete investigation and cleanup of the site exceed $8.0 million . Other PRPs have reached an agreed order and work plan with the Washington DOE for completion of a remedial investigation and feasibility study for the site. A report documenting the initial phase of the remedial investigation was completed in June 2011. There is currently not enough information available to estimate the potential liability to Cascade associated with this claim although 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, it converted the plant 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 not recorded an accrual for this site. Cascade has received notices from and entered into agreement with certain of its insurance carriers that they will participate in defense of Cascade for these 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 through the OPUC and WUTC of remediation costs in its natural gas rates charged to customers. The accruals related to these matters are reflected in regulatory assets. 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 totaled $ 55.1 million at March 31, 2018 , and are expected to mature in 2023. Tesoro agreed to indemnify Centennial for any losses and litigation expenses arising from the guarantee. The estimated fair values of the indemnity asset and guarantee liability are reflected in deferred charges and other assets - other and deferred credits and other liabilities - other, respectively, on the Consolidated Balance Sheets. Continuation of the guarantee was required as a condition to the sale of Dakota Prairie Refining. In March 2016, a sale agreement was signed to sell Fidelity's assets in the Paradox Basin. In connection with the sale, Centennial agreed to guarantee Fidelity's indemnity obligations associated with the Paradox assets. The guarantee was required by the buyer as a condition to the sale of the Paradox Basin assets. 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 who were the sellers in three purchase and sale agreements for periods ranging up to 10 years from the date of sale. 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 March 31, 2018 , the fixed maximum amounts guaranteed under these agreements aggregated $134.8 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate to $2.7 million in 2018 ; $63.0 million in 2019 ; $62.1 million in 2020 ; $500,000 in 2021 ; $500,000 in 2022 ; $2.0 million thereafter; and $4.0 million , which has no scheduled maturity date. There were no amounts outstanding under the above guarantees at March 31, 2018 . 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 March 31, 2018 , the fixed maximum amounts guaranteed under these letters of credit aggregated $34.0 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these letters of credit aggregate $33.3 million in 2018 and $700,000 in 2019. There were no amounts outstanding under the above letters of credit at March 31, 2018 . In the event of default under these letter of credit obligations, the subsidiary issuing the letter of credit for that particular obligation would be required to make payments under its 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 March 31, 2018 . 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 March 31, 2018 , approximately $738.7 million of surety bonds were outstanding, which were not reflected on the Consolidated Balance Sheet. Variable interest entities The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary. Fuel Contract Coyote Station entered into a coal supply agreement with Coyote Creek that provides for the purchase of coal necessary to supply the coal requirements of the Coyote Station for the period May 2016 through December 2040. Coal purchased under the coal supply agreement is reflected in inventories on the Company's 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 March 31, 2018 , 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 $40.3 million

Basis of presentation (Policies

Basis of presentation (Policies)	3 Months Ended
	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	The accompanying consolidated interim financial statements were prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Interim financial statements do not include all disclosures provided in annual financial statements and, accordingly, these financial statements should be read in conjunction with those appearing in the 2017 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses. Management has also evaluated the impact of events occurring after March 31, 2018

Accounts receivable and allow26

Accounts receivable and allowance for doubtful accounts (Policies)	3 Months Ended
	Mar. 31, 2018
Receivables [Abstract]
Accounts receivable and allowance for doubtful accounts	Accounts receivable consist 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 i27

Inventories and natural gas in storage (Policies)	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Inventories and natural gas in storage	Natural gas in storage for the Company's regulated operations is generally carried at lower of cost or net realizable value, or cost using the last-in, first-out method. All other inventories are stated at the lower of cost or net realizable value.

Earnings per common share (Poli

Earnings per common share (Policies)	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Earnings 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 applicable period. 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 applicable period, plus the effect of nonvested performance share awards and restricted stock units. Common stock outstanding includes issued shares less shares held in treasury.

New accounting standards (Polic

New accounting standards (Policies)	3 Months Ended
	Mar. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New accounting standards	New accounting standards Recently adopted accounting standards ASU 2014-09 - Revenue from Contracts with Customers In May 2014, the FASB issued guidance on accounting for revenue from contracts with customers. The guidance provides for a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry specific guidance. In August 2015, the FASB issued guidance deferring the effective date of the revenue guidance and allowing entities to early adopt. With this decision, the guidance was effective for the Company on January 1, 2018. Entities had the option of using either a full retrospective or modified retrospective approach to adopting the guidance. Under the modified retrospective approach, an entity recognizes the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings in the period of adoption. The Company adopted the guidance on January 1, 2018, using the modified retrospective approach. The Company elected the practical expedient to not disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied (or partially unsatisfied) as of the end of the reporting period, along with an explanation of when such revenue would be expected to be recognized. This practical expedient was used since the performance obligations are part of contracts with an original duration of one year or less. The Company also 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 one year or less. Upon completion of the Company's evaluation of contracts and methods of revenue recognition under the previous accounting guidance, the Company did not identify any material cumulative effect adjustments to be made to retained earnings. In addition, the Company has expanded revenue disclosures, both quantitatively and qualitatively, related to the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers, as discussed in Note 8 . The Company has reviewed its revenue streams to evaluate the impact of this guidance and did not identify a significant change in the timing of revenue recognition, results of operations, financial position or cash flows. The Company reviewed its internal controls related to revenue recognition and disclosures and concluded that the guidance impacts certain business processes and controls. As such, the Company has developed modifications to its internal controls for certain topics under the guidance as they apply to the Company and such modifications were not deemed to be significant. Results for reporting periods beginning after December 31, 2017, are presented under the new guidance, while prior period amounts are not adjusted and continue to be reported in accordance with historic accounting for revenue recognition. Under the modified retrospective approach, the guidance was applied only to contracts that were not completed as of January 1, 2018. Therefore, the Company recognized the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings at January 1, 2018. In the first quarter of 2018 , there was not a material impact to the financial statements as a result of applying the guidance. The cumulative effect of the changes made to the Consolidated Balance Sheet was as follows: December 31, Adjustments January 1, (In thousands) Liabilities and Stockholders' Equity Current liabilities: Other accrued liabilities $ 186,010 $ 903 $ 186,913 Deferred credits and other liabilities: Deferred income taxes 347,271 (332 ) 346,939 Other 1,179,140 399 1,179,539 Commitments and contingencies Stockholders' equity : Common stockholders' equity: Retained earnings 1,040,748 (970 ) 1,039,778 The cumulative adjustment is related to prepaid natural gas transportation to storage contracts where a separate performance obligation existed and has not yet been satisfied. As such, these contracts were still open and met the criteria for a cumulative effect adjustment. ASU 2016-15 - Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued guidance to clarify the classification of certain cash receipts and payments in the statement of cash flows. The guidance is intended to standardize the presentation and classification of certain transactions, including cash payments for debt prepayment or extinguishment, proceeds from insurance claim settlements and distributions from equity method investments. In addition, the guidance clarifies how to classify transactions that have characteristics of more than one class of cash flows. The Company adopted the guidance on January 1, 2018, on a prospective basis. The guidance did not have a material effect on the Company's statement of cash flows. ASU 2017-01 - Clarifying the Definition of a Business In January 2017, the FASB issued guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions or disposals of assets or businesses. The guidance provides a screen to determine when an integrated set of assets and activities is not a business. The guidance will also affect other aspects of accounting, such as determining reporting units for goodwill testing and whether an entity has acquired or sold a business. The Company adopted the guidance on January 1, 2018, on a prospective basis. The guidance did not have a material effect on the Company's results of operations, financial position, cash flows or disclosures. ASU 2017-07 - Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued guidance to improve the presentation of net periodic pension and net periodic postretirement benefit costs. The guidance requires the service cost component to be presented in the income statement in the same line item or items as other compensation costs arising from services performed during the period. Other components of net periodic benefit cost shall be presented in the income statement separately from the service cost component and outside a subtotal of income from operations. The guidance also only allows the service cost component to be capitalized. The Company adopted the guidance on January 1, 2018, on a retrospective basis. The guidance required the reclassification of all components of net periodic benefit costs, except for the service cost component, from operating expenses to other income on the Company's Consolidated Statements of Income with no impact to earnings. As a result of the retrospective application of this change in accounting guidance, the Company reclassified $1.8 million from operation and maintenance expense to other income on the Consolidated Statements of Income for the three months ended March 31, 2017. The Company also reclassified unrealized gains on investments used to satisfy obligations under the defined benefit plans of $3.1 million for the three months ended March 31, 2017, which were included in operation and maintenance expense, to other income on the Consolidated Statements of Income. The guidance did not have a material effect on the Company's results of operations, cash flows or disclosures. ASU 2018-02 - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued guidance that allows an entity to reclassify the stranded tax effects resulting from the newly enacted federal corporate income tax rate from accumulated other comprehensive income (loss) to retained earnings. The guidance is effective for the Company on January 1, 2019, including interim periods, with early adoption permitted. The guidance can be applied using one of two methods. One method is to record the reclassification of the stranded income taxes at the beginning of the period of adoption. The other method is to apply the guidance retrospectively to each period in which the income tax effects of the TCJA are recognized in accumulated other comprehensive income (loss). The Company early adopted the guidance on January 1, 2018, and elected to reclassify the stranded income taxes at the beginning of the period. During the first quarter of 2018, the Company reclassified $7.9 million of stranded tax expense from accumulated other comprehensive loss to retained earnings. The guidance did not have a material effect on the Company's results of operations, cash flows or disclosures. Recently issued accounting standards not yet adopted ASU 2016-02 - Leases In February 2016, the FASB issued guidance regarding leases. The guidance requires lessees to recognize a lease liability and a right-of-use asset on the balance sheet for operating and financing leases with terms of more than 12 months. The guidance remains 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 requires additional disclosures, both quantitative and qualitative, related to operating and finance leases for the lessee and sales-type, direct financing and operating leases for the lessor. This guidance will be effective for the Company on January 1, 2019, and should be applied using a retrospective approach with early adoption permitted. The Company continues to evaluate the potential impact the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. The Company is planning to adopt the standard on January 1, 2019, utilizing the practical expedient that allows the Company to not reassess whether an expired or existing contract contains a lease, the classification of leases or initial direct costs. 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 is currently evaluating the impact of the practical expedient. In January 2018, the FASB issued a proposed accounting standard update to the guidance that would allow an entity the option to adopt the guidance on a modified retrospective basis. Under the modified retrospective approach, an entity would recognize a cumulative effect adjustment of initially applying the guidance to the opening balance of retained earnings in the period of adoption. The Company is monitoring the status of the proposal. ASU 2017-04 - Simplifying the Test for Goodwill Impairment

Revenue from contracts with c30

Revenue from contracts with customers (Policies)	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue from contracts with customers	Revenue is recognized when a performance obligation is satisfied by transferring control over a product or service to a customer. Revenue is measured based on consideration specified in a contract with a customer, and excludes any sales incentives and amounts collected on behalf of third parties. The Company is considered an agent for certain taxes collected from customers. As such, the Company presents revenues net of these taxes at the time of sale to be remitted to governmental authorities, including sales and use taxes. 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 revenue 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 thirty 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 on special projects, 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 twelve days of receipt of the invoice. For gathering contracts not governed by the tariff, amounts are due within twenty days of invoice receipt. For other contracts not governed by the tariff, payment terms are net thirty 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 evaluated and determined to be single performance obligations. 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; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics. The Company estimates variable consideration at the most likely amount expected. 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 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 thirty 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 bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics. The Company estimates variable consideration at the most likely amount expected. 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 thirty days of receipt. There are no material obligations for returns, refunds or other similar obligations.

Fair value disclosures (Policie

Fair value disclosures (Policies)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair value measurements	The Company measures its investments in certain fixed-income and equity securities at fair value with changes in fair value recognized in income.
Investments	The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities. The available-for-sale securities include mortgage-backed securities and U.S. Treasury securities. These available-for-sale securities are recorded at fair value and are classified as investments on the Consolidated Balance Sheets.

Business segment data (Policies

Business segment data (Policies)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Business segment data	The Company's reportable segments are those that are based on the Company's method of internal reporting, which generally segregates the strategic business units due to differences in products, services and regulation. The internal reporting of these operating segments is defined based on the reporting and review process used by the Company's chief executive officer.

Contingencies (Policies)

Contingencies (Policies)	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies	The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries, which may include, but are not limited to, matters involving property damage, personal injury, and environmental, contractual, statutory and regulatory obligations. The Company accrues a liability for those contingencies when the incurrence of a loss is probable and the amount can be reasonably estimated. If a range of amounts can be reasonably estimated and no amount within the range is a better estimate than any other amount, then the minimum of the range is accrued. The Company does not accrue liabilities when the likelihood that the liability has been incurred is probable but the amount cannot be reasonably estimated or when the liability is believed to be only reasonably possible or remote. For contingencies where an unfavorable outcome is probable or reasonably possible and which are material, the Company discloses the nature of the contingency and, in some circumstances, an estimate of the possible loss. Accruals are based on the best information available, but in certain situations management is unable to estimate an amount or range of a reasonably possible loss including, but not limited to when: (1) the damages are unsubstantiated or indeterminate, (2) the proceedings are in the early stages, (3) numerous parties are involved, or (4) the matter involves novel or unsettled legal theories.
Variable interest entity	The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary.

Inventories and natural gas i34

Inventories and natural gas in storage (Tables)	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories on the Company's Consolidated Balance Sheets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Aggregates held for resale $ 123,053 $ 120,392 $ 115,268 Asphalt oil 61,647 50,538 30,360 Materials and supplies 19,493 22,074 18,650 Merchandise for resale 16,378 16,546 14,905 Natural gas in storage (current) 10,936 11,282 20,950 Other 26,285 29,777 26,450 Total $ 257,792 $ 250,609 $ 226,583

Earnings per common share (Tabl

Earnings per common share (Tables)	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Weighted average common shares outstanding	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows: Three Months Ended March 31, 2018 2017 (In thousands) Weighted average common shares outstanding - basic 195,304 195,304 Effect of dilutive performance share awards 678 719 Weighted average common shares outstanding - diluted 195,982 196,023 Shares excluded from the calculation of diluted earnings per share — —

New accounting standards New ac

New accounting standards New accounting standards (Tables)	3 Months Ended
	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The cumulative effect of the changes made to the Consolidated Balance Sheet was as follows: December 31, Adjustments January 1, (In thousands) Liabilities and Stockholders' Equity Current liabilities: Other accrued liabilities $ 186,010 $ 903 $ 186,913 Deferred credits and other liabilities: Deferred income taxes 347,271 (332 ) 346,939 Other 1,179,140 399 1,179,539 Commitments and contingencies Stockholders' equity : Common stockholders' equity: Retained earnings 1,040,748 (970 ) 1,039,778

Comprehensive income (loss) (Ta

Comprehensive income (loss) (Tables)	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Comprehensive income (loss)	The after-tax changes in the components of accumulated other comprehensive loss were as follows: Three Months Ended March 31, 2018 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (1,934 ) $ (35,163 ) $ (155 ) $ (82 ) $ (37,334 ) Other comprehensive loss before reclassifications — — (2 ) (105 ) (107 ) Amounts reclassified from accumulated other comprehensive loss 92 418 — 30 540 Net current-period other comprehensive income (loss) 92 418 (2 ) (75 ) 433 Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss (389 ) (7,520 ) (33 ) (17 ) (7,959 ) Balance at end of period $ (2,231 ) $ (42,265 ) $ (190 ) $ (174 ) $ (44,860 ) Three Months Ended March 31, 2017 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,300 ) $ (33,221 ) $ (149 ) $ (63 ) $ (35,733 ) Other comprehensive income (loss) before reclassifications — — 9 (27 ) (18 ) Amounts reclassified from accumulated other comprehensive loss 91 357 — 35 483 Amounts reclassified to accumulated other comprehensive loss from regulatory asset — (917 ) — — (917 ) Net current-period other comprehensive income (loss) 91 (560 ) 9 8 (452 ) Balance at end of period $ (2,209 ) $ (33,781 ) $ (140 ) $ (55 ) $ (36,185 )
Reclassification out of accumulated other comprehensive income	Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Location on Consolidated Statements of Income March 31, 2018 2017 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (148 ) $ (147 ) Interest expense 56 56 Income taxes (92 ) (91 ) Amortization of postretirement liability losses included in net periodic benefit cost (credit) (573 ) (574 ) Other income 155 217 Income taxes (418 ) (357 ) Reclassification adjustment for loss on available-for-sale investments included in net income (37 ) (54 ) Other income 7 19 Income taxes (30 ) (35 ) Total reclassifications $ (540 ) $ (483 )

Revenue from contracts with c38

Revenue from contracts with customers (Tables)	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of revenue	For more information on the Company's business segments, see Note 13 . Three Months Ended March 31, 2018 Electric Natural gas distribution Pipeline and midstream Construction materials and contracting Construction services Other Total (in thousands) Residential utility sales $ 35,183 $ 192,886 $ — $ — $ — $ — $ 228,069 Commercial utility sales 34,701 116,891 — — — — 151,592 Industrial utility sales 8,770 7,809 — — — — 16,579 Other utility sales 1,836 — — — — — 1,836 Natural gas transportation — 11,179 21,818 — — — 32,997 Natural gas gathering — — 2,270 — — — 2,270 Natural gas storage — — 3,134 — — — 3,134 Contracting services — — — 74,064 — — 74,064 Construction materials — — — 173,591 — — 173,591 Intrasegment eliminations* — — — (34,270 ) — — (34,270 ) Inside specialty contracting — — — — 233,821 — 233,821 Outside specialty contracting — — — — 87,181 — 87,181 Other 8,252 3,999 3,326 — (86 ) 2,696 18,187 Intersegment eliminations — — (21,759 ) (101 ) (11 ) (2,638 ) (24,509 ) Revenues from contracts with customers 88,742 332,764 8,789 213,284 320,905 58 964,542 Revenues out of scope (1,338 ) (100 ) 44 — 13,145 — 11,751 Total external operating revenues $ 87,404 $ 332,664 $ 8,833 $ 213,284 $ 334,050 $ 58 $ 976,293 * 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 change in contract assets and liabilities were as follows: March 31, 2018 December 31, 2017 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 132,610 $ 122,184 $ 10,426 Receivables, net Contract liabilities - current (80,905 ) (71,479 ) (9,426 ) Accounts payable Contract liabilities - noncurrent (399 ) — (399 ) Deferred credits and other liabilities - other Net contract assets (liabilities) $ 51,306 $ 50,705 $ 601

Discontinued operations (Tables

Discontinued operations (Tables)	3 Months Ended
	Mar. 31, 2018
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, related to the operations of Fidelity, on the Company's Consolidated Balance Sheets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Assets Current assets: Receivables, net $ 458 $ 266 $ 479 Total current assets held for sale 458 266 479 Noncurrent assets: Net property, plant and equipment 1,631 4,515 1,631 Deferred income taxes 2,637 91,098 2,637 Other 161 161 161 Total noncurrent assets held for sale 4,429 95,774 4,429 Total assets held for sale $ 4,887 $ 96,040 $ 4,908 Liabilities Current liabilities: Accounts payable $ — $ 67 $ 30 Taxes payable 10,774 4,732 (a) 10,857 Other accrued liabilities 2,810 2,311 2,884 Total current liabilities held for sale 13,584 7,110 13,771 Total liabilities held for sale $ 13,584 $ 7,110 $ 13,771 (a) On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale. March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Assets Current assets: Income taxes receivable $ 1,858 (a) $ 11,756 $ 1,778 (a) Total current assets held for sale 1,858 11,756 1,778 Total assets held for sale $ 1,858 $ 11,756 $ 1,778 Liabilities Current liabilities: Accounts payable $ — $ 16 $ — Total current liabilities held for sale — 16 — Noncurrent liabilities: Deferred income taxes (b) 37 55 37 Total noncurrent liabilities held for sale 37 55 37 Total liabilities held for sale $ 37 $ 71 $ 37 (a) On the Company's Consolidated Balance Sheets, these amounts were reclassified to taxes payable and are reflected in current liabilities held for sale. (b) On the Company's Consolidated Balance Sheets, these amounts were reclassified to deferred charges and other assets - deferred income taxes and are reflected in noncurrent assets held for sale.
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, which includes Dakota Prairie Refining and Fidelity, to the after-tax income from discontinued operations on the Company's Consolidated Statements of Income was as follows: Three Months Ended March 31, 2018 2017 (In thousands) Operating revenues $ 66 $ 105 Operating expenses 174 (6,577 ) Operating income (loss) (108 ) 6,682 Other income (expense) 12 (15 ) Interest expense 575 — Income (loss) from discontinued operations before income taxes (671 ) 6,667 Income taxes (1,148 ) 4,980 * Income from discontinued operations $ 477 $ 1,687 * Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Goodwill and other intangible40

Goodwill and other intangible assets (Tables)	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying amount of goodwill	The changes in the carrying amount of goodwill were as follows: Three Months Ended March 31, 2018 Balance at January 1, 2018 Goodwill Acquired the Year Balance at March 31, 2018 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 — 176,290 Construction services 109,765 — 109,765 Total $ 631,791 $ — $ 631,791 Three Months Ended March 31, 2017 Balance at January 1, 2017 Goodwill Acquired the Year Balance at March 31, 2017 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 — 176,290 Construction services 109,765 — 109,765 Total $ 631,791 $ — $ 631,791 Year Ended December 31, 2017 Balance at January 1, 2017 Goodwill Acquired the Year Balance at December 31, 2017 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 — 176,290 Construction services 109,765 — 109,765 Total $ 631,791 $ — $ 631,791
Other amortizable intangible assets	Other amortizable intangible assets were as follows: March 31, 2018 March 31, 2017 December 31, 2017 (In thousands) Customer relationships $ 14,668 $ 15,745 $ 15,248 Less accumulated amortization 13,007 12,910 13,382 1,661 2,835 1,866 Noncompete agreements 2,430 2,430 2,430 Less accumulated amortization 1,842 1,695 1,805 588 735 625 Other 6,458 7,086 6,990 Less accumulated amortization 5,242 5,309 5,644 1,216 1,777 1,346 Total $ 3,465 $ 5,347 $ 3,837

Fair value measurements (Tables

Fair value measurements (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: March 31, 2018 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,282 $ 4 $ 223 $ 10,063 U.S. Treasury securities 466 — 1 465 Total $ 10,748 $ 4 $ 224 $ 10,528 March 31, 2017 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,971 $ 8 $ 94 $ 9,885 U.S. Treasury securities 412 1 — 413 Total $ 10,383 $ 9 $ 94 $ 10,298 December 31, 2017 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,342 $ 4 $ 129 $ 10,217 U.S. Treasury securities 205 — 1 204 Total $ 10,547 $ 4 $ 130 $ 10,421
Assets and liabilities measured at fair value on a recurring basis	The Company's assets and liabilities measured at fair value on a recurring basis were as follows: Fair Value Measurements at March 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 March 31, 2018 (In thousands) Assets: Money market funds $ — $ 9,085 $ — $ 9,085 Insurance contract* — 76,941 — 76,941 Available-for-sale securities: Mortgage-backed securities — 10,063 — 10,063 U.S. Treasury securities — 465 — 465 Total assets measured at fair value $ — $ 96,554 $ — $ 96,554 * The insurance contract invests approximately 49 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents. Fair Value Measurements at March 31, 2017, Using Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance at March 31, 2017 (In thousands) Assets: Money market funds $ — $ 2,551 $ — $ 2,551 Insurance contract* — 73,775 — 73,775 Available-for-sale securities: Mortgage-backed securities — 9,885 — 9,885 U.S. Treasury securities — 413 — 413 Total assets measured at fair value $ — $ 86,624 $ — $ 86,624 * The insurance contract invests approximately 51 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at December 31, 2017, 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, 2017 (In thousands) Assets: Money market funds $ — $ 6,965 $ — $ 6,965 Insurance contract* — 77,388 — 77,388 Available-for-sale securities: Mortgage-backed securities — 10,217 — 10,217 U.S. Treasury securities — 204 — 204 Total assets measured at fair value $ — $ 94,774 $ — $ 94,774 * The insurance contract invests approximately 49 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 14 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.
Fair value of long term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt was as follows: Carrying Amount Fair Value (In thousands) Long-term debt at March 31, 2018 $ 1,779,542 $ 1,854,350 Long-term debt at March 31, 2017 $ 1,703,006 $ 1,784,588 Long-term debt at December 31, 2017 $ 1,714,853 $ 1,826,256

Cash flow information (Tables)

Cash flow information (Tables)	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]
Cash expenditures for interest and income taxes and noncash investing transactions	Cash expenditures for interest and income taxes were as follows: Three Months Ended March 31, 2018 2017 (In thousands) Interest, net* $ 17,910 $ 17,546 Income taxes refunded, net** $ (1,056 ) $ (2,762 ) * AFUDC - borrowed was $382,000 and $196,000 for the three months ended March 31, 2018 and 2017, respectively. ** Income taxes paid (refunded), net of discontinued operations, were $ 1.7 million and $ (7.2) million for the three months ended March 31, 2018 and 2017, respectively. Noncash investing transactions were as follows: March 31, 2018 2017 (In thousands) Property, plant and equipment additions in accounts payable $ 16,829 $ 5,212

Business segment data (Tables)

Business segment data (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Information on the Company's businesses	Information on the Company's segments was as follows: Three Months Ended March 31, 2018 2017 (In thousands) External operating revenues: Regulated operations: Electric $ 87,404 $ 88,225 Natural gas distribution 332,664 342,519 Pipeline and midstream 4,391 2,870 424,459 433,614 Nonregulated operations: Pipeline and midstream 4,442 3,643 Construction materials and contracting 213,284 200,776 Construction services 334,050 299,572 Other 58 320 551,834 504,311 Total external operating revenues $ 976,293 $ 937,925 Intersegment operating revenues: Regulated operations: Electric $ — $ — Natural gas distribution — — Pipeline and midstream 21,735 21,489 21,735 21,489 Nonregulated operations: Pipeline and midstream 24 34 Construction materials and contracting 101 86 Construction services 11 6 Other 2,638 1,743 2,774 1,869 Intersegment eliminations (24,509 ) (23,358 ) Total intersegment operating revenues $ — $ — Three Months Ended March 31, 2018 2017 (In thousands) Earnings on common stock: Regulated operations: Electric $ 13,084 $ 14,333 Natural gas distribution 32,623 27,861 Pipeline and midstream 5,459 4,557 51,166 46,751 Nonregulated operations: Pipeline and midstream (179 ) (628 ) Construction materials and contracting (23,521 ) (19,912 ) Construction services 15,090 7,362 Other (596 ) (279 ) (9,206 ) (13,457 ) Intersegment eliminations* — 2,173 Earnings on common stock before income from discontinued operations 41,960 35,467 Income from discontinued operations, net of tax* 477 1,687 Total earnings on common stock $ 42,437 $ 37,154 * Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans (Tables)

Employee benefit plans (Tables)	3 Months Ended
	Mar. 31, 2018
Retirement Benefits [Abstract]
Schedule of net benefit costs	Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans were as follows: Pension Benefits Other Postretirement Benefits Three Months Ended March 31, 2018 2017 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 407 $ 447 Interest cost 3,807 4,014 778 808 Expected return on assets (4,998 ) (5,029 ) (1,167 ) (1,145 ) Amortization of prior service credit — — (349 ) (343 ) Amortization of net actuarial loss 1,782 1,793 238 336 Net periodic benefit cost (credit), including amount capitalized 591 778 (93 ) 103 Less amount capitalized — 107 40 (39 ) Net periodic benefit cost (credit) $ 591 $ 671 $ (133 ) $ 142

Accounts receivable and allow45

Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Receivables [Abstract]
Receivables past due 90 days or more	$ 36.3	$ 34.7	$ 26.3
Allowance for doubtful accounts receivable	$ 8.2	$ 8.1	$ 10.9

Inventories and natural gas i46

Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 123,053	$ 115,268	$ 120,392
Asphalt oil	61,647	30,360	50,538
Materials and supplies	19,493	18,650	22,074
Merchandise for resale	16,378	14,905	16,546
Natural gas in storage (current)	10,936	20,950	11,282
Other	26,285	26,450	29,777
Total	257,792	226,583	250,609
Natural gas in storage noncurrent	$ 47,800	$ 49,300	$ 49,500

Earnings per common share (Deta

Earnings per common share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Weighted average common shares outstanding - basic	195,304	195,304
Effect of dilutive performance share awards	678	719
Weighted average common shares outstanding - diluted	195,982	196,023
Shares excluded from the calculation of diluted earnings per share	0	0

New accounting standards ASU 20

New accounting standards ASU 2014-09 (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	$ 204,328		$ 186,010	$ 188,373
Deferred income taxes	346,218		347,271	666,905
Other	1,181,919		1,179,140	890,107
Retained earnings	$ 1,051,469		1,040,748	$ 911,702
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities		$ 186,913
Deferred income taxes		346,939
Other		1,179,539
Retained earnings		1,039,778
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities			186,010
Deferred income taxes			347,271
Other			1,179,140
Retained earnings			$ 1,040,748
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities		903
Deferred income taxes		(332)
Other		399
Retained earnings		$ (970)

New accounting standards ASU 49

New accounting standards ASU 2017-07 (Details 2) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized gains (losses) on investments used to satisfy obligations under nonqualified benefit plans	$ (500,000)	$ 2,900,000
Adjustments for New Accounting Pronouncement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit)		1,800,000
Net unrealized gains (losses) on investments used to satisfy obligations under nonqualified benefit plans		$ 3,100,000

New accounting standards ASU 50

New accounting standards ASU 2018-02 (Details 3) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Adjustments for New Accounting Pronouncement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain prior period tax effects from accumulated other comprehensive loss	$ (7.9)

Comprehensive income (loss) (De

Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,429,043	$ 2,316,244
Amounts reclassified from accumulated other comprehensive loss	(540)	(483)
Net current-period other comprehensive income (loss)	433	(452)
Balance	2,426,111	2,313,905
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,934)	(2,300)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	92	91
Net current-period other comprehensive income (loss)	92	91
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Balance	(2,231)	(2,209)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,163)	(33,221)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	418	357
Net current-period other comprehensive income (loss)	418	(560)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(917)
Balance	(42,265)	(33,781)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(155)	(149)
Other comprehensive income (loss) before reclassifications	(2)	9
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	(2)	9
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Balance	(190)	(140)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(82)	(63)
Other comprehensive income (loss) before reclassifications	(105)	(27)
Amounts reclassified from accumulated other comprehensive loss	30	35
Net current-period other comprehensive income (loss)	(75)	8
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Balance	(174)	(55)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,334)	(35,733)
Other comprehensive income (loss) before reclassifications	(107)	(18)
Amounts reclassified from accumulated other comprehensive loss	540	483
Net current-period other comprehensive income (loss)	433	(452)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(917)
Balance	(44,860)	$ (36,185)
Tax Cuts and Jobs Act | Net unrealized gain (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)
Tax Cuts and Jobs Act | Postretirement 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)
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)
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)
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)

Reclassification out of accumul

Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,447)	$ (20,303)
Income taxes	(7,551)	(12,188)
Other income	582	2,343
Net income	42,437	37,325
Reclassification from accumulated other comprehensive income, current period, net of tax	(540)	(483)
Reclassification adjustment for loss on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	92	91
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive income | Interest rate contract
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	(148)	(147)
Income taxes	56	56
Net income	(92)	(91)
Amortization of postretirement liability losses included in net periodic benefit cost (credit)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	418	357
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	155	217
Other income	(573)	(574)
Net income	(418)	(357)
Reclassification adjustment for loss on available-for-sale investments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	30	35
Reclassification adjustment for loss on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	7	19
Other income	(37)	(54)
Net income	$ (30)	$ (35)

Revenue from contracts with c53

Revenue from contracts with customers Disaggregation of revenue (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues		$ 976,293	$ 937,925
Intersegment operating revenues		0	$ 0
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	(34,270)
Other
Disaggregation of Revenue [Line Items]
Revenues		18,187
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(24,509)
Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		964,542
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		11,751
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		32,997
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		2,270
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		3,134
Contracting services
Disaggregation of Revenue [Line Items]
Revenues		74,064
Construction materials
Disaggregation of Revenue [Line Items]
Revenues		173,591
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		233,821
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		87,181
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		228,069
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		151,592
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		16,579
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		1,836
Electric
Disaggregation of Revenue [Line Items]
Revenues		87,404
Electric | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues		8,252
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		0
Electric | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		88,742
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		(1,338)
Electric | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		35,183
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		34,701
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		8,770
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		1,836
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues		332,664
Natural gas distribution | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues		3,999
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		0
Natural gas distribution | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		332,764
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		(100)
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		11,179
Natural gas distribution | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		192,886
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		116,891
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		7,809
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Revenues		8,833
Pipeline and midstream | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Revenues		3,326
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(21,759)
Pipeline and midstream | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		8,789
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		44
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		21,818
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		2,270
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		3,134
Pipeline and midstream | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues		213,284
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	(34,270)
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(101)
Construction materials and contracting | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		213,284
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		74,064
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		173,591
Construction materials and contracting | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services
Disaggregation of Revenue [Line Items]
Revenues		334,050
Construction services | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues		(86)
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(11)
Construction services | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		320,905
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		13,145
Construction services | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		233,821
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		87,181
Construction services | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other
Disaggregation of Revenue [Line Items]
Revenues		58
Other | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Other | Other
Disaggregation of Revenue [Line Items]
Revenues		2,696
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(2,638)
Other | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		58
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		$ 0
[1]	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.

Revenue from contracts with c54

Revenue from contracts with customers Contract balances (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract assets	$ 132,610	$ 122,184
Change in contract assets	10,426
Contract liabilities - current	(80,905)	(71,479)
Change in contract liabilities - current	(9,426)
Contract liabilities - noncurrent	(399)	0
Change in contract liabilities - noncurrent	(399)
Net contract assets (liabilities)	51,306	$ 50,705
Change in net contract assets (liabilities)	601
Amounts included in contract liability at the beginning of the period	52,000
Amounts from performance obligations satisfied in prior periods	$ 3,100

Discontinued operations Noncont

Discontinued operations Noncontrolling interest (Details) - USD ($) $ in Millions	Jun. 27, 2016	Jun. 24, 2016	Mar. 31, 2017
WBI Energy | Dakota Prairie Refining, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Previous percentage of ownership	50.00%	50.00%
Operating Expense | Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Reversal of an accrual			$ 7
Operating expense - after tax | Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Reversal of an accrual			$ 4.3

Major classes of assets and lia

Major classes of assets and liabilities Refining (Details 2) - USD ($) $ in Thousands		Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Current assets:
Total current assets held for sale		$ 458		$ 479		$ 7,290
Current liabilities:
Total current liabilities held for sale		11,726		11,993		2,394
Refining | Discontinued operations, held-for-sale or disposed of by sale
Current assets:
Income taxes receivable		1,858	[1]	1,778	[1]	11,756
Total current assets held for sale		1,858		1,778		11,756
Total assets held for sale		1,858		1,778		11,756
Current liabilities:
Accounts payable		0		0		16
Total current liabilities held for sale		0		0		16
Noncurrent liabilities:
Deferred income taxes (b)	[2]	37		37		55
Total noncurrent liabilities held for sale		37		37		55
Total liabilities held for sale		$ 37		$ 37		$ 71
[1]	On the Company's Consolidated Balance Sheets, these amounts were reclassified to taxes payable and are reflected in current liabilities held for sale.
[2]	On the Company's Consolidated Balance Sheets, these amounts were reclassified to deferred charges and other assets - deferred income taxes and are reflected in noncurrent assets held for sale.

Major classes of assets and l57

Major classes of assets and liabilities held for sale E&P (Details 3) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Current assets:
Total current assets held for sale	$ 458	$ 479	$ 7,290
Noncurrent assets:
Total noncurrent assets held for sale	4,392	4,392	95,719
Current liabilities:
Total current liabilities held for sale	11,726	11,993	2,394
Exploration and production | Discontinued operations, held-for-sale or disposed of by sale
Current assets:
Receivables, net	458	479	266
Total current assets held for sale	458	479	266
Noncurrent assets:
Net property, plant and equipment	1,631	1,631	4,515
Deferred income taxes	2,637	2,637	91,098
Other	161	161	161
Total noncurrent assets held for sale	4,429	4,429	95,774
Total assets held for sale	4,887	4,908	96,040
Current liabilities:
Accounts payable	0	30	67
Taxes payable	10,774	10,857	4,732	[1]
Other accrued liabilities	2,810	2,884	2,311
Total current liabilities held for sale	13,584	13,771	7,110
Total liabilities held for sale	$ 13,584	$ 13,771	$ 7,110
[1]	On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale.

Business exit costs (Details 4)

Business exit costs (Details 4) - Other restructuring [Member] - Discontinued operations, held-for-sale or disposed of by sale - USD ($)	3 Months Ended	33 Months Ended
	Mar. 31, 2018	Mar. 31, 2018
Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs	$ 0
Exploration and production
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs	$ 0	$ 10,500,000

Reconciliation of income and ex

Reconciliation of income and expenses (Details 5) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations	$ 477	$ 1,687	[1]
Discontinued operations, held-for-sale or disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	66	105
Operating expenses	174	(6,577)
Operating income (loss)	(108)	6,682
Other income	12
Other expense		(15)
Interest expense	575	0
Income (loss) from discontinued operations before income taxes	(671)	6,667
Income taxes	(1,148)	4,980	[2]
Income from discontinued operations	$ 477	$ 1,687
[1]	Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.
[2]	Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Goodwill rollforward (Details)

Goodwill rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Goodwill [Roll Forward]
Balance at beginning of period	$ 631,791	$ 631,791	$ 631,791
Goodwill acquired during the year	0	0	0
Balance at end of period	631,791	631,791	631,791
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736	345,736	345,736
Goodwill acquired during the year	0	0	0
Balance at end of period	345,736	345,736	345,736
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	176,290	176,290	176,290
Goodwill acquired during the year	0	0	0
Balance at end of period	176,290	176,290	176,290
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	109,765	109,765	109,765
Goodwill acquired during the year	0	0	0
Balance at end of period	$ 109,765	$ 109,765	$ 109,765

Other intangible assets (Detail

Other intangible assets (Details 2) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 3,465,000	$ 5,347,000	$ 3,837,000
Amortization of intangible assets	400,000	600,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2,018	1,300,000
2,019	1,000,000
2,020	500,000
2,021	300,000
2,022	300,000
Thereafter	500,000
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	14,668,000	15,745,000	15,248,000
Intangible assets, less accumulated amortization	13,007,000	12,910,000	13,382,000
Intangible assets, net (excluding goodwill)	1,661,000	2,835,000	1,866,000
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	2,430,000	2,430,000	2,430,000
Intangible assets, less accumulated amortization	1,842,000	1,695,000	1,805,000
Intangible assets, net (excluding goodwill)	588,000	735,000	625,000
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	6,458,000	7,086,000	6,990,000
Intangible assets, less accumulated amortization	5,242,000	5,309,000	5,644,000
Intangible assets, net (excluding goodwill)	$ 1,216,000	$ 1,777,000	$ 1,346,000

Fair value measurements Insuran

Fair value measurements Insurance contracts (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 76,900,000	$ 73,800,000	$ 77,400,000
Net unrealized gains (losses) on investments used to satisfy obligations under nonqualified benefit plans	$ (500,000)	$ 2,900,000

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Available-for-sale securities [Abstract]
Cost	$ 10,748	$ 10,547	$ 10,383
Gross unrealized gains	4	4	9
Gross unrealized losses	224	130	94
Fair value	10,528	10,421	10,298
Mortgage backed securities
Available-for-sale securities [Abstract]
Cost	10,282	10,342	9,971
Gross unrealized gains	4	4	8
Gross unrealized losses	223	129	94
Fair value	10,063	10,217	9,885
US Treasury securities
Available-for-sale securities [Abstract]
Cost	466	205	412
Gross unrealized gains	0	0	1
Gross unrealized losses	1	1	0
Fair value	$ 465	$ 204	$ 413

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	49.00%		49.00%		51.00%
Percentage investment in common stock of large-cap companies	22.00%		23.00%		22.00%
Percentage investment in common stock of mid-cap companies	13.00%		14.00%		13.00%
Percentage investment in common stock of small-cap companies	11.00%		11.00%		11.00%
Percentage investment in target date investments	3.00%		2.00%		2.00%
Percentage investment in cash and cash equivalents	2.00%		1.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 96,554		$ 94,774		$ 86,624
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,085		6,965		2,551
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	76,941	[1]	77,388	[2]	73,775	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,063		10,217		9,885
Fair value, measurements, recurring [Member] | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	465		204		413
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	96,554		94,774		86,624
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,085		6,965		2,551
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	76,941	[1]	77,388	[2]	73,775	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,063		10,217		9,885
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	465		204		413
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 0		$ 0		$ 0
[1]	The insurance contract invests approximately 49 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 3 percent in target date investments and 2
[2]	The insurance contract invests approximately 49 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 14 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1
[3]	The insurance contract invests approximately 51 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.

Fair value measurements (Deta65

Fair value measurements (Details 4) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Carrying amount
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 1,779,542	$ 1,714,853	$ 1,703,006
Fair value
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 1,854,350	$ 1,826,256	$ 1,784,588

Cash flow information (Details

Cash flow information (Details 1) - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Interest, net	[1]	$ 17,910,000	$ 17,546,000
Income taxes paid (refunded), net	[2]	(1,056,000)	(2,762,000)
AFUDC borrowed		382,000	196,000
Property, plant and equipment additions in accounts payable		16,829,000	5,212,000
Continuing and discontinued operations
Income taxes paid (refunded), net		$ 1,700,000	$ (7,200,000)
[1]	AFUDC - borrowed was $382,000 and $196,000
[2]	Income taxes paid (refunded), net of discontinued operations, were $ 1.7 million and $ (7.2) million for the three months ended March 31, 2018 and 2017, respectively.

Business segment data (Details)

Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues		$ 976,293	$ 937,925
Intersegment operating revenues		0	0
Earnings on common stock before income from discontinued operations		41,960	35,467
Income from discontinued operations, net of tax		477	1,687	[1]
Earnings on common stock		42,437	37,154
Electric
Segment Reporting Information [Line Items]
Revenues		87,404
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues		332,664
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues		8,833
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues		213,284
Construction services
Segment Reporting Information [Line Items]
Revenues		334,050
Other
Segment Reporting Information [Line Items]
Revenues		58
Intersegment eliminations
Segment Reporting Information [Line Items]
Intersegment operating revenues		(24,509)	(23,358)
Earnings on common stock before income from discontinued operations	[1]	0	2,173
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		424,459	433,614
Intersegment operating revenues		21,735	21,489
Earnings on common stock before income from discontinued operations		51,166	46,751
Regulated operation [Member] | Electric
Segment Reporting Information [Line Items]
Revenues		87,404	88,225
Intersegment operating revenues		0	0
Earnings on common stock before income from discontinued operations		13,084	14,333
Regulated operation [Member] | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues		332,664	342,519
Intersegment operating revenues		0	0
Earnings on common stock before income from discontinued operations		32,623	27,861
Regulated operation [Member] | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues		4,391	2,870
Intersegment operating revenues		21,735	21,489
Earnings on common stock before income from discontinued operations		5,459	4,557
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		551,834	504,311
Intersegment operating revenues		2,774	1,869
Earnings on common stock before income from discontinued operations		(9,206)	(13,457)
Nonregulated operation [Member] | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues		4,442	3,643
Intersegment operating revenues		24	34
Earnings on common stock before income from discontinued operations		(179)	(628)
Nonregulated operation [Member] | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues		213,284	200,776
Intersegment operating revenues		101	86
Earnings on common stock before income from discontinued operations		(23,521)	(19,912)
Nonregulated operation [Member] | Construction services
Segment Reporting Information [Line Items]
Revenues		334,050	299,572
Intersegment operating revenues		11	6
Earnings on common stock before income from discontinued operations		15,090	7,362
Nonregulated operation [Member] | Other
Segment Reporting Information [Line Items]
Revenues		58	320
Intersegment operating revenues		2,638	1,743
Earnings on common stock before income from discontinued operations		$ (596)	$ (279)
[1]	Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans (Details

Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0
Interest cost	3,807	4,014
Expected return on assets	(4,998)	(5,029)
Amortization of prior service credit	0	0
Amortization of net actuarial loss	1,782	1,793
Net periodic benefit cost (credit), including amount capitalized	591	778
Less amount capitalized	0	107
Net periodic benefit cost (credit)	591	671
Qualified plan | Underfunded plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	407	447
Interest cost	778	808
Expected return on assets	(1,167)	(1,145)
Amortization of prior service credit	(349)	(343)
Amortization of net actuarial loss	238	336
Net periodic benefit cost (credit), including amount capitalized	(93)	103
Less amount capitalized	40	(39)
Net periodic benefit cost (credit)	(133)	142
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 1,100	$ 1,200

Idaho Public Utilities Commissi

Idaho Public Utilities Commission (IPUC) (Details) - IPUC - Tax Cuts and Jobs Act - USD ($) $ in Millions	Mar. 23, 2018	Mar. 12, 2018
Public Utilities, General Disclosures [Line Items]
State corporate income tax rate		7.40%
State corporate income tax rate, current		6.90%
Public utilities, requested rate increase (decrease), amount	$ (5)

Montana Public Service Commissi

Montana Public Service Commission (MTPSC) (Details 2) - MTPSC - Natural gas rate proceeding - USD ($)	Apr. 24, 2018	Feb. 23, 2018	Jan. 12, 2018	Sep. 25, 2017
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 2,800,000		$ 2,800,000
Public utilities, requested rate increase (decrease), percentage				4.10%
Public utilities, interim rate increase (decrease), amount				$ 1,600,000
Public utilities, interim rate increase (decrease), percentage				2.30%
Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amended, amount		$ 1,600,000
Public utilities interm rate increase (decrese) revised			$ 764,000
Subsequent Event | Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amended, amount	$ 975,000
Public utilities, requested rate increase (decrease), amended, percentage	1.40%

North Dakota Public Service Com

North Dakota Public Service Commission (NDPSC) (Details 3) - NDPSC - USD ($) $ in Millions	Mar. 09, 2018	Mar. 01, 2018	Feb. 14, 2018	Sep. 06, 2017	Jul. 21, 2017
Natural gas rate proceeding
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount					$ 5.9
Public utilities, requested rate increase (decrease), percentage					5.40%
Public utilities, interim rate increase (decrease), amount				$ 4.6
Public utilities, interim rate increase (decrease), percentage				4.20%
Natural gas rate proceeding | Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities interm rate increase (decrese) revised			$ 2.7
Public utilities, requested rate increase (decrease), amended, amount		$ 3.6
Electric rate proceeding | Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ (7.2)
Public utilities, requested rate increase (decrease), percentage	(3.90%)

Oregon Public Utility Commissio

Oregon Public Utility Commission (OPUC) (Details 4) - OPUC - Pipeline replacement cost recovery	Sep. 29, 2017USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 784,000
Public utilities, requested rate increase (decrease), percentage	1.20%

Washington Utilities and Transp

Washington Utilities and Transportation Commission (WUTC) (Details 5) - WUTC - Natural gas rate proceeding - USD ($) $ in Millions	Mar. 23, 2018	Aug. 31, 2017
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 5.9
Public utilities, requested rate increase (decrease), percentage		2.70%
Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amended, amount	$ (1.7)

Wyoming Public Service Commissi

Wyoming Public Service Commission (WYPSC) (Details) - Electric rate proceeding - WYPSC - Tax Cuts and Jobs Act $ in Millions	Mar. 30, 2018USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ (1.1)
Public utilities, requested rate increase (decrease), percentage	(4.20%)

Midcontinent Independent System

Midcontinent Independent System Operator, Inc. (MISO) (Details 5) - USD ($) $ in Millions	Mar. 01, 2018	Jan. 01, 2018
MISO | Electric transmission rate proceeding | Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 12	$ 13.6

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 37.4	$ 35.4	$ 29.1

Environmental matters (Details

Environmental matters (Details 2) - USD ($)	3 Months Ended
	Mar. 31, 2018	Jan. 06, 2017
Portland Harbor Site
Site Contingency [Line Items]
Environmental matters investigative costs	$ 100,000,000
Environmental matters, estimated costs		$ 1,000,000,000
Eugene, OR manufactured gas plant site
Site Contingency [Line Items]
Total estimated costs for site remediation	3,500,000
Incurred costs for site remediation	$ 400,000
Percent of ongoing costs paid	50.00%
Estimated proportional share of cleanup liability	50.00%
Environmental matters accrual for site remediation	$ 1,600,000
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	1,800,000
Environmental matters accrual of investigative costs	5,800,000
Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	8,000,000
Minimum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	340,000
Maximum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 6,400,000

Guarantees (Details 3)

Guarantees (Details 3)	Mar. 31, 2018USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 134,800,000
Fixed maximum amounts guaranteed by year 2018	2,700,000
Fixed maximum amounts guaranteed by year 2019	63,000,000
Fixed maximum amounts guaranteed by year 2020	62,100,000
Fixed maximum amounts guaranteed by year 2021	500,000
Fixed maximum amounts guaranteed by year 2022	500,000
Fixed maximum amounts guaranteed, thereafter	2,000,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	0
Letters of credit	34,000,000
Letters of credit set to expire - 2018	33,300,000
Letters of credit set to expire - 2019	700,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	738,700,000
Financial guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 55,100,000

Commitment and contingencies va

Commitment and contingencies variable interest entities (Details 4) $ in Millions	Mar. 31, 2018USD ($)
Fuel contract [Member]
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 40.3