MDU Resources (MDU) 10-Q 2 Nov 18 2018 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 26, 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	Sep. 30, 2018
Document fiscal year focus	2,018
Document fiscal period focus	Q3
Amendment flag	false
Entity emerging growth company	false
Entity small business	false
Entity common stock, shares outstanding		196,018,324

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating revenues:
Operating revenues:	$ 1,280,787	$ 1,272,548	$ 3,321,678	$ 3,278,112
Operating expenses:
Operation and maintenance:	996,591	973,966	2,419,531	2,354,456
Purchased natural gas sold	32,123	33,319	270,319	283,936
Depreciation, depletion and amortization	55,016	52,155	161,298	155,138
Taxes, other than income	38,647	38,882	128,257	127,273
Electric fuel and purchased power	18,406	18,906	58,901	57,544
Total operating expenses	1,140,783	1,117,228	3,038,306	2,978,347
Operating income	140,004	155,320	283,372	299,765
Other income	2,683	2,068	4,864	6,212
Interest expense	20,955	20,909	62,202	61,978
Income before income taxes	121,732	136,479	226,034	243,999
Income taxes	14,363	46,930	32,629	74,406
Income from continuing operations	107,369	89,549	193,405	169,593
Income (loss) from discontinued operations, net of tax (Note 10)	(118)	(2,198)	85	(3,702)
Net income	107,251	87,351	193,490	165,891
Loss on redemption of preferred stocks	0	0	0	600
Dividends declared on preferred stocks	0	0	0	171
Earnings on common stock	$ 107,251	$ 87,351	$ 193,490	$ 165,120
Earnings per common share - basic:
Earnings before discontinued operations	$ 0.55	$ 0.46	$ 0.99	$ 0.86
Discontinued operations, net of tax	0	(0.01)	0	(0.01)
Earnings per common share - basic	0.55	0.45	0.99	0.85
Earnings per common share - diluted:
Earnings before discontinued operations	0.55	0.46	0.99	0.86
Discontinued operations, net of tax	0	(0.01)	0	(0.02)
Earnings per common share - diluted	0.55	0.45	0.99	0.84
Dividends declared per common share	$ 0.1975	$ 0.1925	$ 0.5925	$ 0.5775
Weighted average common shares outstanding - basic	196,018	195,304	195,618	195,304
Weighted average common shares outstanding - diluted	196,265	195,783	196,104	195,922
Regulated operation
Operating revenues:
Operating revenues:	$ 200,617	$ 206,936	$ 851,761	$ 866,035
Operating expenses:
Operation and maintenance:	82,920	80,204	252,961	237,881
Nonregulated operation
Operating revenues:
Operating revenues:	1,080,170	1,065,612	2,469,917	2,412,077
Operating expenses:
Operation and maintenance:	$ 913,671	$ 893,762	$ 2,166,570	$ 2,116,575

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 107,251	$ 87,351	$ 193,490	$ 165,891
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $55 and $56 for the three months ended and $164 and $168 for the nine months ended in 2018 and 2017, respectively	92	92	279	275
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $142 and $203 for the three months ended and $442 and $609 for the nine months ended in 2018 and 2017, respectively	442	333	1,309	1,002
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $0 for the three months ended and $0 and $(725) for the nine months ended in 2018 and 2017, respectively	0	0	0	(917)
Postretirement liability adjustment	442	333	1,309	85
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0 and $9 for the three months ended and $(14) and $5 for the nine months ended in 2018 and 2017, respectively	0	15	(61)	9
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0 and $0 for the three months ended and $75 and $0 for the nine months ended in 2018 and 2017, respectively	0	0	249	0
Foreign currency translation adjustment	0	15	188	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 $(13) and $(10) for the three months ended and $(52) and $(38) for the nine months ended in 2018 and 2017, respectively	(51)	(19)	(199)	(70)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $9 and $14 for the three months ended and $26 and $50 for the nine months ended in 2018 and 2017, respectively	33	27	97	93
Net unrealized gain (loss) on available-for-sale investments	(18)	8	(102)	23
Other comprehensive income	516	448	1,674	392
Comprehensive income attributable to common stockholders	$ 107,767	$ 87,799	$ 195,164	$ 166,283

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification adjustment for loss on derivative instruments included in net income, tax	$ 55	$ 56	$ 164	$ 168
Amortization of postretirement liability losses included in net periodic benefit cost (credit), tax	142	203	442	609
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	0	(725)
Foreign currency translation adjustments recognized during the period, tax	0	9	(14)	5
Reclassification of foreign currency translation adjustment	0	0	75	0
Net unrealized loss on available-for-sale investments arising during the period, tax	(13)	(10)	(52)	(38)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	$ 9	$ 14	$ 26	$ 50

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Cash and cash equivalents	$ 67,077	$ 34,599	$ 37,356
Receivables, net	787,344	727,030	739,402
Inventories	270,293	226,583	232,555
Prepayments and other current assets	88,760	81,304	89,625
Current assets held for sale	571	479	304
Total current assets	1,214,045	1,069,995	1,099,242
Investments	143,303	137,613	133,895
Property, plant and equipment	7,102,960	6,770,829	6,658,891
Less accumulated depreciation, depletion and amortization	2,796,649	2,691,641	2,667,762
Net property, plant and equipment	4,306,311	4,079,188	3,991,129
Deferred charges and other assets:
Goodwill	640,203	631,791	631,791
Other intangible assets, net	4,318	3,837	4,209
Other	408,178	407,850	419,846
Noncurrent assets held for sale	1,835	4,392	64,333
Total deferred charges and other assets	1,054,534	1,047,870	1,120,179
Total assets	6,718,193	6,334,666	6,344,445
Current liabilities:
Long-term debt due within one year	3,915	148,499	148,499
Accounts payable	339,713	312,327	304,101
Taxes payable	50,461	42,537	108,946
Dividends payable	38,714	38,573	37,596
Accrued compensation	62,836	72,919	67,097
Other accrued liabilities	221,620	186,010	184,580
Current liabilities held for sale	7,959	11,993	5,749
Total current liabilities	725,218	812,858	856,568
Long-term debt	1,911,555	1,566,354	1,592,053
Deferred credits and other liabilities:
Deferred income taxes	405,761	347,271	652,413
Other	1,154,366	1,179,140	889,494
Total deferred credits and other liabilities	1,560,127	1,526,411	1,541,907
Commitments and contingencies
Stockholders' equity:
Authorized - 500,000,000 shares, $1.00 par value Shares issued - 196,557,245 at September 30, 2018 and 195,843,297 at September 30, 2017 and December 31, 2017	196,557	195,843	195,843
Other paid-in capital	1,247,151	1,233,412	1,232,766
Retained earnings	1,124,830	1,040,748	964,275
Accumulated other comprehensive loss	(43,619)	(37,334)	(35,341)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)	(3,626)
Total stockholders' equity	2,521,293	2,429,043	2,353,917
Total liabilities and stockholders' equity	$ 6,718,193	$ 6,334,666	$ 6,344,445

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Stockholders' equity:
Common stock, shares issued	196,557,245	195,843,297	195,843,297
Treasury shares	538,921	538,921	538,921
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1	$ 1

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Other paid-in capital	Retained earnings	Accumulated other comprehensive loss	Treasury stock
Balance (in shares) at Dec. 31, 2016		150,000
Balance (in shares) at Dec. 31, 2016			195,843,297
Treasury stock (in shares) at Dec. 31, 2016							(538,921)
Balance at Dec. 31, 2016	$ 2,316,244	$ 15,000	$ 195,843	$ 1,232,478	$ 912,282	$ (35,733)	$ (3,626)
Net income	165,891				165,891
Other comprehensive income	392					392
Dividends declared on preferred stocks	(171)				(171)
Dividends declared on common stock	(112,788)				(112,788)
Stock-based compensation	2,390			2,729	(339)
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)
Redemption of preferred stock (shares)		(150,000)
Redemption of preferred stock		$ (15,000)
Redemption of preferred stock	(600)				(600)
Redemption of preferred stock	$ (15,600)
Balance (in shares) at Sep. 30, 2017		0
Balance (in shares) at Sep. 30, 2017	195,843,297		195,843,297
Treasury stock (in shares) at Sep. 30, 2017							(538,921)
Balance at Sep. 30, 2017	$ 2,353,917	$ 0	$ 195,843	1,232,766	964,275	(35,341)	$ (3,626)
Balance at Jun. 30, 2017						(35,789)
Net income	87,351
Other comprehensive income	448					448
Redemption of preferred stock	$ 0
Balance (in shares) at Sep. 30, 2017		0
Balance (in shares) at Sep. 30, 2017	195,843,297		195,843,297
Treasury stock (in shares) at Sep. 30, 2017							(538,921)
Balance at Sep. 30, 2017	$ 2,353,917	$ 0	$ 195,843	1,232,766	964,275	(35,341)	$ (3,626)
Cumulative effect of adoption of ASU 2014-09 | Accounting Standards Update 2014-09	(970)				(970)
Adjusted balance at January 1, 2018	$ 2,428,073		$ 195,843	1,233,412	1,039,778	(37,334)	$ (3,626)
Balance (in shares) at Dec. 31, 2017	195,843,297		195,843,297
Treasury stock (in shares) at Dec. 31, 2017							(538,921)
Balance at Dec. 31, 2017	$ 2,429,043		$ 195,843	1,233,412	1,040,748	(37,334)	$ (3,626)
Net income	193,490				193,490
Other comprehensive income	1,674					1,674
Reclassification of certain prior period tax effects from accumulated other comprehensive loss | Adjustments for new accounting pronouncement	0				7,959	(7,959)
Dividends declared on common stock	(115,998)				(115,998)
Stock-based compensation	3,411			3,810	(399)
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)
Issuance of common stock (shares)			713,948
Issuance of common stock	17,993		$ 714	17,279
Redemption of preferred stock	0
Redemption of preferred stock	$ 0
Balance (in shares) at Sep. 30, 2018	196,557,245		196,557,245
Treasury stock (in shares) at Sep. 30, 2018							(538,921)
Balance at Sep. 30, 2018	$ 2,521,293		$ 196,557	1,247,151	1,124,830	(43,619)	$ (3,626)
Balance at Jun. 30, 2018						(44,135)
Net income	107,251
Other comprehensive income	516					516
Redemption of preferred stock	$ 0
Balance (in shares) at Sep. 30, 2018	196,557,245		196,557,245
Treasury stock (in shares) at Sep. 30, 2018							(538,921)
Balance at Sep. 30, 2018	$ 2,521,293		$ 196,557	$ 1,247,151	$ 1,124,830	$ (43,619)	$ (3,626)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net income	$ 193,490	$ 165,891
Income (loss) from discontinued operations, net of tax	85	(3,702)
Income from continuing operations	193,405	169,593
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	161,298	155,138
Deferred income taxes	42,428	(16,777)
Changes in current assets and liabilities, net of acquisitions:
Receivables	(55,749)	(121,128)
Inventories	(38,785)	2,047
Other current assets	(3,452)	(40,655)
Accounts payable	16,155	30,097
Other current liabilities	31,971	66,647
Other noncurrent changes	(27,170)	(15,081)
Net cash provided by continuing operations	320,101	229,881
Net cash provided by (used in) discontinued operations	(2,720)	42,020
Net cash provided by operating activities	317,381	271,901
Investing activities:
Capital expenditures	(345,599)	(222,084)
Acquisitions, net of cash acquired	(27,858)	0
Net proceeds from sale or disposition of property and other	12,451	121,162
Investments	(1,560)	(260)
Net cash used in continuing operations	(362,566)	(101,182)
Net cash provided by discontinued operations	1,236	2,234
Net cash used in investing activities	(361,330)	(98,948)
Financing activities:
Issuance of long-term debt	356,952	133,437
Repayment of long-term debt	(157,315)	(183,968)
Dividends paid	(115,859)	(113,131)
Redemption of preferred stock	0	(15,600)
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	76,428	(181,703)
Net cash provided by discontinued operations	0	0
Net cash provided by (used in) financing activities	76,428	(181,703)
Effect of exchange rate changes on cash and cash equivalents	(1)	(1)
Increase (decrease) in cash and cash equivalents	32,478	(8,751)
Cash and cash equivalents -- beginning of year	34,599	46,107
Cash and cash equivalents -- end of period	$ 67,077	$ 37,356

Basis of presentation

Basis of presentation	9 Months Ended
	Sep. 30, 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 September 30, 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, reducing the Company's income tax expense for 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, such estimates 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 material adjustments during the nine months ended September 30, 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 provide service have requested the Company furnish plans for the effect of the reduced corporate tax rate, which may impact the Company's rates to customers. 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 new information becomes available. For more information on the details and statuses of the open requests, see Note 16 . 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 income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. At the time the assets were classified as held for sale, depreciation, depletion and amortization expense was no longer recorded. Unless otherwise indicated, the amounts presented in the accompanying notes to the consolidated financial statements relate to the Company's continuing operations. For more information on the Company's discontinued operations, see Note 10

Seasonality of operations

Seasonality of operations	9 Months Ended
	Sep. 30, 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	9 Months Ended
	Sep. 30, 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 $29.2 million , $27.2 million and $34.7 million at September 30, 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 September 30, 2018 and 2017 , and December 31, 2017 , was $7.3 million , $9.0 million and $8.1 million

Inventories and natural gas in

Inventories and natural gas in storage	9 Months Ended
	Sep. 30, 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 Consolidated Balance Sheets were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Aggregates held for resale $ 133,477 $ 116,399 $ 115,268 Asphalt oil 40,781 26,682 30,360 Natural gas in storage (current) 29,084 29,974 20,950 Materials and supplies 23,563 20,778 18,650 Merchandise for resale 15,954 15,346 14,905 Other 27,434 23,376 26,450 Total $ 270,293 $ 232,555 $ 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 September 30, 2018 and 2017 , and December 31, 2017

Earnings per common share

Earnings per common share	9 Months Ended
	Sep. 30, 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 Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) Weighted average common shares outstanding - basic 196,018 195,304 195,618 195,304 Effect of dilutive performance share awards and restricted stock units 247 479 486 618 Weighted average common shares outstanding - diluted 196,265 195,783 196,104 195,922 Shares excluded from the calculation of diluted earnings per share 114 — — —

New accounting standards

New accounting standards	9 Months Ended
	Sep. 30, 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 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 impacted certain business processes and controls. As such, the Company 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. For the nine months ended September 30, 2018 , there were no material impacts to the financial statements as a result of applying the guidance. The cumulative effect of the changes made to the Consolidated Balance Sheet were 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 : Retained earnings 1,040,748 (970 ) 1,039,778 The cumulative effect 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 also affects 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 required 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 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 $1.5 million and $4.5 million from operation and maintenance expense to other income on the Consolidated Statements of Income for the three and nine months ended September 30, 2017, respectively. The Company also reclassified unrealized gains on investments used to satisfy obligations under the defined benefit plans of $2.6 million and $7.9 million for the three and nine months ended September 30, 2017, respectively, 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. 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, with early adoption permitted. In July 2018, the FASB issued ASU 2018-11 - Leases: Targeted Improvements, an accounting standard update to ASU 2016-02. This ASU provides an entity the option to adopt the guidance using one of two modified retrospective approaches. An entity can adopt the guidance using the modified retrospective transition approach beginning in the earliest year presented in the financial statements. This method of adoption would require the restatement of prior periods reported and the presentation of lease disclosures under the new guidance for all periods reported. The additional transition method of adoption introduced by ASU 2018-11, allows entities the option to apply the guidance on the date of adoption by recognizing a cumulative effect adjustment to retained earnings during the period of adoption and does not require prior comparative periods to be restated. 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, as well as the additional transition method of adoption applied on the date of adoption. The Company also plans to adopt a short-term leasing policy as the lessee where leases with a term of 12 months or less will not be included on the Consolidated Balance Sheet. The Company formed a lease implementation team and is currently in the contract review and assessment phase to identify and evaluate contracts containing leases. Additionally, the team is implementing new and revising existing software to meet the reporting and disclosure requirements of the standard. The Company also has started to assess the impact the standard will have on its processes and internal controls and is identifying new and updating current processes. During the assessment phase, the Company is using various analytic methodologies to ensure the completeness of the lease inventory. The Company expects that most of the current operating leases will be subject to the guidance and recognized as operating lease liabilities and right-of-use assets on the Consolidated Balance Sheets upon adoption. During the fourth quarter of 2018, the Company will engage internal auditors to review and assess the completeness of the lease inventory and the effectiveness of the revised and new internal controls. The Company continues to evaluate the impact the new guidance will have on lease contracts where the Company is the lessor and, at this time, does not anticipate a significant impact. 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 planning to adopt this practical expedient. The Company continues to monitor other industry-specific issues as it relates to the regulated businesses but does not expect these issues to have a material impact on the Company's results of operations, financial position or disclosures. ASU 2017-04 - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued guidance on simplifying the test for goodwill impairment by eliminating Step 2, which required an entity to measure the amount of impairment loss by comparing the implied fair value of reporting unit goodwill with the carrying amount of such goodwill. This guidance requires entities to perform a quantitative impairment test, previously Step 1, to identify both the existence of impairment and the amount of impairment loss by comparing the fair value of a reporting unit to its carrying amount. Entities will continue to have the option of performing a qualitative assessment to determine if the quantitative impairment test is necessary. The guidance also requires additional disclosures if an entity has one or more reporting units with zero or negative carrying amounts of net assets. The guidance will be effective for the Company on January 1, 2020, and must be applied on a prospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. ASU 2018-07 - Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued guidance on simplifying the accounting for nonemployee share-based payment transactions by expanding the scope of ASC 718, Compensation - Stock Compensation , to include share-based payment transactions for acquiring goods and services from nonemployees. The guidance will require nonemployee share-based payment awards be measured at the grant-date fair value of the equity instruments at the grant date. The guidance will be effective for the Company on January 1, 2019, including interim periods, and must be applied using a modified retrospective approach with early adoption permitted. The modified retrospective approach requires an entity to record a cumulative-effect adjustment to retained earnings at the beginning of the year of adoption. The Company is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. ASU 2018-13 - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued guidance on modifying the disclosure requirements on fair value measurements as part of the disclosure framework project. The guidance modifies, among other things, the disclosures required for Level 3 fair value measurements, including the range and weighted average of significant unobservable inputs. The guidance removes, among other things, the disclosure requirement to disclose transfers between Levels 1 and 2. The guidance will be effective for the Company on January 1, 2020, including interim periods, with early adoption permitted. Level 3 fair value measurement disclosures should be applied prospectively while all other amendments should be applied retrospectively. The Company is evaluating the effects the adoption of the new guidance will have on its disclosures. ASU 2018-14 - Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued guidance on modifying the disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans as part of the disclosure framework project. The guidance removes disclosures that are no longer considered cost beneficial, clarifies the specific requirements of disclosures and adds disclosure requirements identified as relevant. The guidance adds, among other things, the requirement to include an explanation for significant gains and losses related to changes in benefit obligations for the period. The guidance removes, among other things, the disclosure requirement to disclose the amount of net periodic benefit costs to be amortized over the next fiscal year from accumulated other comprehensive income (loss) and the effects a one percentage point change in assumed health care cost trend rates will have on certain benefit components. The guidance will be effective for the Company on January 1, 2021, and must be applied on a retrospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on the its disclosures. ASU 2018-15 - Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract In August 2018, the FASB issued guidance on the accounting for implementation costs of a hosting arrangement that is a service contract. The guidance aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract similar to the costs incurred to develop or obtain internal-use software and such capitalized costs to be expensed over the term of the hosting arrangement. Costs incurred during the preliminary and postimplementation stages should continue to be expensed as activities are performed. The capitalized costs are required to be presented on the balance sheet in the same line the prepayment for the fees associated with the hosting arrangement would be presented. In addition, the expense related to the capitalized implementation costs should be presented in the same line on the income statement as the fees associated with the hosting element of the arrangements. The guidance will be effective for the Company on January 1, 2020, including interim periods, and may be applied on a retrospective or a prospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on the its results of operations, financial position, cash flows and disclosures. SEC File Number S7-15-16 - Disclosure Update and Simplification In October 2018, the SEC published guidance in the Federal Register on disclosure updates and simplifications. The guidance removes disclosures that are no longer considered cost beneficial, duplicative of GAAP required disclosures, clarifies the specific requirements of disclosures and adds disclosure requirements identified as relevant. The amendments are intended to facilitate disclosure of information to investors and simplify the compliance without significantly altering the total mix of information provided to investors. The guidance will be effective for the Company on November 5, 2018, including interim periods, and must be applied on a prospective basis. The Company is

Comprehensive income (loss)

Comprehensive income (loss)	9 Months Ended
	Sep. 30, 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 September 30, 2018 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,136 ) $ (41,816 ) $ — $ (183 ) $ (44,135 ) Other comprehensive loss before reclassifications — — — (51 ) (51 ) Amounts reclassified from accumulated other comprehensive loss 92 442 — 33 567 Net current-period other comprehensive income (loss) 92 442 — (18 ) 516 Balance at end of period $ (2,044 ) $ (41,374 ) $ — $ (201 ) $ (43,619 ) Three Months Ended September 30, 2017 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,117 ) $ (33,469 ) $ (155 ) $ (48 ) $ (35,789 ) Other comprehensive income (loss) before reclassifications — — 15 (19 ) (4 ) Amounts reclassified from accumulated other comprehensive loss 92 333 — 27 452 Net current-period other comprehensive income 92 333 15 8 448 Balance at end of period $ (2,025 ) $ (33,136 ) $ (140 ) $ (40 ) $ (35,341 ) Nine Months Ended September 30, 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 — — (61 ) (199 ) (260 ) Amounts reclassified from accumulated other comprehensive loss 279 1,309 249 97 1,934 Net current-period other comprehensive income (loss) 279 1,309 188 (102 ) 1,674 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 $ (2,044 ) $ (41,374 ) $ — $ (201 ) $ (43,619 ) Nine Months Ended September 30, 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 (70 ) (61 ) Amounts reclassified from accumulated other comprehensive loss 275 1,002 — 93 1,370 Amounts reclassified to accumulated other comprehensive loss from regulatory asset — (917 ) — — (917 ) Net current-period other comprehensive income 275 85 9 23 392 Balance at end of period $ (2,025 ) $ (33,136 ) $ (140 ) $ (40 ) $ (35,341 ) The following amounts were reclassified out of accumulated other comprehensive loss into net income. The amounts presented in parenthesis indicate a decrease to net income on the Consolidated Statements of Income. The reclassifications were as follows: Three Months Ended Nine Months Ended Location on Consolidated Statements of Income September 30, September 30, 2018 2017 2018 2017 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (147 ) $ (148 ) $ (443 ) $ (443 ) Interest expense 55 56 164 168 Income taxes (92 ) (92 ) (279 ) (275 ) Amortization of postretirement liability losses included in net periodic benefit cost (credit) (584 ) (536 ) (1,751 ) (1,611 ) Other income 142 203 442 609 Income taxes (442 ) (333 ) (1,309 ) (1,002 ) Reclassification adjustment for loss on foreign currency translation adjustment included in net income — — (324 ) — Other income — — 75 — Income taxes — — (249 ) — Reclassification adjustment on available-for-sale investments included in net income (42 ) (41 ) (123 ) (143 ) Other income 9 14 26 50 Income taxes (33 ) (27 ) (97 ) (93 ) Total reclassifications $ (567 ) $ (452 ) $ (1,934 ) $ (1,370 )

Revenue from contracts with cus

Revenue from contracts with customers	9 Months Ended
	Sep. 30, 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 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 evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include liquidated damages; performance bonuses or incentives and penalties; claims; unapproved/unpriced change orders; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using 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 claims, unapproved/unpriced change orders, bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using the input method based on the measurement of progress on a project. The input method is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction equipment and other supplies to third parties and internal customers. The contract for these sales is the use of a sales order or invoice, which includes the pricing and payment terms. All such contracts include a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within 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 14 . Three Months Ended September 30, 2018 Electric Natural gas distribution Pipeline and midstream Construction materials and contracting Construction services Other Total (in thousands) Residential utility sales $ 31,424 $ 43,825 $ — $ — $ — $ — $ 75,249 Commercial utility sales 36,259 28,174 — — — — 64,433 Industrial utility sales 8,738 4,421 — — — — 13,159 Other utility sales 2,056 — — — — — 2,056 Natural gas transportation — 10,841 21,400 — — — 32,241 Natural gas gathering — — 2,320 — — — 2,320 Natural gas storage — — 2,795 — — — 2,795 Contracting services — — — 409,006 — — 409,006 Construction materials — — — 538,962 — — 538,962 Intrasegment eliminations* — — — (204,040 ) — — (204,040 ) Inside specialty contracting — — — — 217,474 — 217,474 Outside specialty contracting — — — — 100,988 — 100,988 Other 6,158 3,208 5,701 — 15 3,084 18,166 Intersegment eliminations — — (3,187 ) (165 ) (782 ) (3,037 ) (7,171 ) Revenues from contracts with customers 84,635 90,469 29,029 743,763 317,695 47 1,265,638 Revenues out of scope 1,445 1,779 42 — 11,883 — 15,149 Total external operating revenues $ 86,080 $ 92,248 $ 29,071 $ 743,763 $ 329,578 $ 47 $ 1,280,787 * 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. Nine Months Ended September 30, 2018 Electric Natural gas distribution Pipeline and midstream Construction materials and contracting Construction services Other Total (in thousands) Residential utility sales $ 93,359 $ 305,399 $ — $ — $ — $ — $ 398,758 Commercial utility sales 103,636 185,885 — — — — 289,521 Industrial utility sales 25,734 17,457 — — — — 43,191 Other utility sales 5,766 — — — — — 5,766 Natural gas transportation — 32,104 64,505 — — — 96,609 Natural gas gathering — — 6,900 — — — 6,900 Natural gas storage — — 8,563 — — — 8,563 Contracting services — — — 730,628 — — 730,628 Construction materials — — — 1,100,185 — — 1,100,185 Intrasegment eliminations* — — — (363,877 ) — — (363,877 ) Inside specialty contracting — — — — 667,664 — 667,664 Outside specialty contracting — — — — 283,432 — 283,432 Other 22,836 10,821 13,353 — 32 8,536 55,578 Intersegment eliminations — — (31,485 ) (501 ) (1,332 ) (8,343 ) (41,661 ) Revenues from contracts with customers 251,331 551,666 61,836 1,466,435 949,796 193 3,281,257 Revenues out of scope 653 2,786 129 — 36,853 — 40,421 Total external operating revenues $ 251,984 $ 554,452 $ 61,965 $ 1,466,435 $ 986,649 $ 193 $ 3,321,678 * Intrasegment revenues are presented within the construction materials and contracting segment to highlight the focus on vertical integration as this segment sells materials to both third parties and internal customers. Due to consolidation requirements, these revenues must be eliminated against construction materials to arrive at the external operating revenue total for the segment. Contract balances The timing of revenue recognition may differ from the timing of invoicing to customers. The timing of invoicing to customers does not necessarily correlate with the timing of revenues being recognized under the cost‐to‐cost method of accounting. Contracts from contracting services are billed as work progresses in accordance with agreed upon contractual terms. Generally, billing to the customer occurs contemporaneous to revenue recognition. A variance in timing of the billings may result in a contract asset or a contract liability. A contract asset occurs when revenues are recognized under the cost-to-cost measure of progress, which exceeds amounts billed on uncompleted contracts. Such amounts will be billed as standard contract terms allow, usually based on various measures of performance or achievement. A contract liability occurs when there are billings in excess of revenues recognized under the cost-to-cost measure of progress on uncompleted contracts. Contract liabilities decrease as revenue is recognized from the satisfaction of the related performance obligation. The changes in contract assets and liabilities were as follows: September 30, 2018 December 31, 2017 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 142,376 $ 109,540 $ 32,836 Receivables, net Contract liabilities - current (98,470 ) (84,123 ) (14,347 ) Accounts payable Contract liabilities - noncurrent (399 ) — (399 ) Deferred credits and other liabilities - other Net contract assets $ 43,507 $ 25,417 $ 18,090 At September 30, 2018 , the Company's net contract assets increased $18.1 million compared to December 31, 2017 . Included in the change of total net contract assets 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 $10.3 million and $79.2 million in revenue for the three and nine months ended September 30, 2018 , respectively, which was previously included in contract liabilities at December 31, 2017 . The Company recognized a net decrease in revenues of $8.7 million and $3.7 million for the three and nine months ended September 30, 2018

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2018
Business Acquisition [Line Items]
Acquisitions	Acquisitions In April 2018, the Company acquired Teevin & Fischer Quarry, LLC, an aggregate producer that provides crushed rock and gravel to construction and retail customers in Oregon. In June 2018, the Company acquired Tri-City Paving, Inc., a general contractor and aggregate, asphalt and ready-mixed concrete supplier in Minnesota. In July 2018, the Company acquired Molalla Redi-Mix and Rock Products, Inc., a producer of ready-mixed concrete in Oregon. As of September 30, 2018, the gross aggregate consideration for these acquisitions was $27.9 million in cash, subject to certain adjustments, and 713,948 shares of common stock with a market value of $20.1 million as of the respective acquisition date. Due to the holding period restriction on the common stock, the share consideration has been discounted to a fair value of approximately $18.0 million , as reflected in the Company's financial statements. The acquisitions are subject to customary adjustments based on, among other things, the amount of cash, debt and working capital in the businesses as of the closing dates. The amounts included in the Consolidated Balance Sheets for these adjustments are considered provisional until final settlement has occurred.

Discontinued operations

Discontinued operations	9 Months Ended
	Sep. 30, 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 (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. 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. As of September 30, 2018 , the Company incurred no material exit and disposal costs related to the sale of Dakota Prairie Refining and does not expect to incur any future material exit and disposal costs. At September 30, 2018, Centennial continued to guarantee certain debt obligations of Dakota Prairie Refining and Tesoro continued to indemnify Centennial for any losses and litigation expenses arising from the guarantee. On October 17, 2018, Centennial was released of any further liabilities or obligations under this guarantee. For more information related to the guarantee, see Note 17 . 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 Consolidated Balance Sheets were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Assets Current assets: Income taxes receivable (a) $ 1,640 $ 8,444 $ 1,778 Total current assets held for sale 1,640 8,444 1,778 Total assets held for sale $ 1,640 $ 8,444 $ 1,778 Liabilities Noncurrent liabilities: Deferred income taxes (b) $ 37 $ 55 $ 37 Total noncurrent liabilities held for sale 37 55 37 Total liabilities held for sale $ 37 $ 55 $ 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. 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. In July 2018, the Company completed the sale of a majority of the remaining property, plant and equipment. 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 Consolidated Balance Sheets were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Assets Current assets: Receivables, net $ 571 $ 304 $ 479 Total current assets held for sale 571 304 479 Noncurrent assets: Net property, plant and equipment — 2,064 1,631 Deferred income taxes 1,711 62,163 2,637 Other 161 161 161 Total noncurrent assets held for sale 1,872 64,388 4,429 Total assets held for sale $ 2,443 $ 64,692 $ 4,908 Liabilities Current liabilities: Accounts payable $ 188 $ 68 $ 30 Taxes payable 7,463 11,745 10,857 Other accrued liabilities 1,948 2,380 2,884 Total current liabilities held for sale 9,599 14,193 13,771 Total liabilities held for sale $ 9,599 $ 14,193 $ 13,771 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. As of September 30, 2018, the Company has incurred no exit and disposal costs for the year. As of September 30, 2017, the Company had incurred no exit and disposal costs for the year. The Company does not expect to incur any additional material exit and disposal costs related to the sale of Fidelity. 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 (loss) from discontinued operations on the Consolidated Statements of Income was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) Operating revenues $ 61 $ 121 $ 201 $ 356 Operating expenses 216 384 825 (4,988 ) Operating income (loss) (155 ) (263 ) (624 ) 5,344 Other income (expense) — — 12 (13 ) Interest expense — — 575 239 Income (loss) from discontinued operations before income taxes (155 ) (263 ) (1,187 ) 5,092 Income taxes (37 ) 1,935 * (1,272 ) 8,794 * Income (loss) from discontinued operations $ (118 ) $ (2,198 ) $ 85 $ (3,702 ) * Includes the eliminations for the presentation of income tax adjustments between continuing and discontinued operations. 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

Goodwill and other intangible a

Goodwill and other intangible assets	9 Months Ended
	Sep. 30, 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: Nine Months Ended September 30, 2018 Balance at January 1, 2018 Goodwill Acquired the Year* Balance at September 30, 2018 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 8,412 184,702 Construction services 109,765 — 109,765 Total $ 631,791 $ 8,412 $ 640,203 * Relates to business acquisitions that occurred during the period, as discussed in Note 9 . Nine Months Ended September 30, 2017 Balance at January 1, 2017 Goodwill Acquired the Year Balance at September 30, 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: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Customer relationships $ 15,920 $ 15,248 $ 15,248 Less accumulated amortization 13,342 13,176 13,382 2,578 2,072 1,866 Noncompete agreements 2,606 2,430 2,430 Less accumulated amortization 1,911 1,769 1,805 695 661 625 Other 6,458 7,020 6,990 Less accumulated amortization 5,413 5,544 5,644 1,045 1,476 1,346 Total $ 4,318 $ 4,209 $ 3,837 Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2018 , was $300,000 and $900,000 , respectively. Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2017 , was $500,000 and $1.7 million , respectively. Estimated amortization expense for amortizable intangible assets is $1.4 million in 2018 , $1.2 million in 2019 , $600,000 in 2020 , $400,000 in 2021 , $400,000 in 2022 and $1.2 million

Fair value measurements

Fair value measurements	9 Months Ended
	Sep. 30, 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 $80.4 million , $75.0 million and $77.4 million , at September 30, 2018 and 2017 , and December 31, 2017 , respectively, are classified as investments on the Consolidated Balance Sheets. The net unrealized gains on these investments were $2.1 million and $3.0 million for the three and nine months ended September 30, 2018 , respectively. The net unrealized gains on these investments were $1.9 million and $6.9 million for the three and nine months ended September 30, 2017 , respectively. The change in fair value, which is considered part of the cost of the plan, is classified in other income on the Consolidated Statements of Income. In connection with the adoption of ASU 2017-07, as discussed in Note 6, the Company has elected to reclassify prior period unrealized gains from operation and maintenance expense to other income on the Consolidated Statements of Income. The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities. 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: September 30, 2018 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,550 $ 3 $ 251 $ 10,302 U.S. Treasury securities 179 — — 179 Total $ 10,729 $ 3 $ 251 $ 10,481 September 30, 2017 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,488 $ 11 $ 72 $ 9,427 U.S. Treasury securities 613 — 1 612 Total $ 10,101 $ 11 $ 73 $ 10,039 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 nine months ended September 30, 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 September 30, 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 September 30, 2018 (In thousands) Assets: Money market funds $ — $ 9,948 $ — $ 9,948 Insurance contract* — 80,358 — 80,358 Available-for-sale securities: Mortgage-backed securities — 10,302 — 10,302 U.S. Treasury securities — 179 — 179 Total assets measured at fair value $ — $ 100,787 $ — $ 100,787 * The insurance contract invests approximately 47 percent in fixed-income investments, 24 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 12 percent in common stock of small-cap companies, 3 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at September 30, 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 September 30, 2017 (In thousands) Assets: Money market funds $ — $ 6,204 $ — $ 6,204 Insurance contract* — 74,991 — 74,991 Available-for-sale securities: Mortgage-backed securities — 9,427 — 9,427 U.S. Treasury securities — 612 — 612 Total assets measured at fair value $ — $ 91,234 $ — $ 91,234 * The insurance contract invests approximately 50 percent in fixed-income investments, 23 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 September 30, 2018 $ 1,915,470 $ 1,987,360 Long-term debt at September 30, 2017 $ 1,740,552 $ 1,846,811 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	9 Months Ended
	Sep. 30, 2018
Supplemental Cash Flow Information [Abstract]
Cash flow information	Cash flow information Cash expenditures for interest and income taxes were as follows: Nine Months Ended September 30, 2018 2017 (In thousands) Interest, net* $ 58,359 $ 58,119 Income taxes paid, net** $ 8,887 $ 46,430 * AFUDC - borrowed was $1.8 million and $676,000 for the nine months ended September 30, 2018 and 2017 , respectively. ** Income taxes paid, net of discontinued operations, were $5.1 million and $1.4 million for the nine months ended September 30, 2018 and 2017 , respectively. Noncash investing and financing transactions were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Property, plant and equipment additions in accounts payable $ 34,594 $ 16,914 $ 29,263 Issuance of common stock in connection with acquisition $ 17,993 $ — $ —

Business segment data

Business segment data	9 Months Ended
	Sep. 30, 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 10 . 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 Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) External operating revenues: Regulated operations: Electric $ 86,080 $ 91,531 $ 251,984 $ 254,330 Natural gas distribution 92,248 92,253 554,452 566,364 Pipeline and midstream 22,289 23,152 45,325 45,341 200,617 206,936 851,761 866,035 Nonregulated operations: Pipeline and midstream 6,782 5,356 16,640 13,518 Construction materials and contracting 743,763 686,010 1,466,435 1,388,212 Construction services 329,578 374,111 986,649 1,009,693 Other 47 135 193 654 1,080,170 1,065,612 2,469,917 2,412,077 Total external operating revenues $ 1,280,787 $ 1,272,548 $ 3,321,678 $ 3,278,112 Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — $ — Natural gas distribution — — — — Pipeline and midstream 3,070 3,081 31,252 30,923 3,070 3,081 31,252 30,923 Nonregulated operations: Pipeline and midstream 117 38 233 132 Construction materials and contracting 165 142 501 400 Construction services 782 415 1,332 715 Other 3,037 1,910 8,343 5,411 4,101 2,505 10,409 6,658 Intersegment eliminations (7,171 ) (5,586 ) (41,661 ) (37,581 ) Total intersegment operating revenues $ — $ — $ — $ — Earnings (loss) on common stock: Regulated operations: Electric $ 15,284 $ 15,712 $ 37,500 $ 37,904 Natural gas distribution (11,887 ) (10,883 ) 13,884 14,181 Pipeline and midstream 10,109 5,853 20,809 15,901 13,506 10,682 72,193 67,986 Nonregulated operations: Pipeline and midstream 848 95 1,136 (770 ) Construction materials and contracting 78,876 63,221 79,691 64,477 Construction services 9,278 13,144 38,457 32,896 Other 4,861 552 1,928 (1,888 ) 93,863 77,012 121,212 94,715 Intersegment eliminations — 1,855 * — 6,121 * Earnings on common stock before income (loss) from discontinued operations 107,369 89,549 193,405 168,822 Income (loss) from discontinued operations, net of tax (118 ) (2,198 ) * 85 (3,702 ) * Total earnings on common stock $ 107,251 $ 87,351 $ 193,490 $ 165,120 * Includes the eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans

Employee benefit plans	9 Months Ended
	Sep. 30, 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 September 30, 2018 2017 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 374 $ 377 Interest cost 3,648 4,052 725 816 Expected return on assets (5,188 ) (5,132 ) (1,217 ) (1,160 ) Amortization of prior service credit — — (349 ) (343 ) Amortization of net actuarial loss 1,751 1,589 160 213 Net periodic benefit cost (credit), including amount capitalized 211 509 (307 ) (97 ) Less amount capitalized — 65 46 (95 ) Net periodic benefit cost (credit) $ 211 $ 444 $ (353 ) $ (2 ) Pension Benefits Other Postretirement Benefits Nine Months Ended September 30, 2018 2017 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 1,121 $ 1,130 Interest cost 10,943 12,155 2,175 2,449 Expected return on assets (15,565 ) (15,395 ) (3,650 ) (3,480 ) Amortization of prior service credit — — (1,046 ) (1,029 ) Amortization of net actuarial loss 5,254 4,767 480 649 Net periodic benefit cost (credit), including amount capitalized 632 1,527 (920 ) (281 ) Less amount capitalized — 245 128 (248 ) Net periodic benefit cost (credit) $ 632 $ 1,282 $ (1,048 ) $ (33 ) In accordance with ASU 2017-07, the components of net periodic benefit cost (credit), other than the service cost component, are included in other income on the Consolidated Statements of Income. Nonqualified defined benefit plans In addition to the qualified defined benefit pension 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 for the three and nine months ended September 30, 2018 , was $1.1 million and $3.3 million , respectively. The Company's net periodic benefit cost for these plans for the three and nine months ended September 30, 2017 , was $1.2 million and $3.5 million

Regulatory matters

Regulatory matters	9 Months Ended
	Sep. 30, 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 and statuses of each open request. 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 advocating existing rates are reasonable and a reduction in rates is not warranted. On August 9, 2018, the MNPUC ruled that Great Plains reduce rates to reflect TCJA impacts and to also provide a one-time refund that captures the TCJA impacts from January 1, 2018 through the implementation date of new rates. To date, an order by the MNPUC has not been issued. This matter is pending before the MNPUC. 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. Montana-Dakota submitted the general electric rate case on September 28, 2018, as discussed below. A hearing is scheduled for December 4, 2018. This matter is pending before the MTPSC. On September 28, 2018, Montana-Dakota filed an application with the MTPSC for an electric rate increase of approximately $11.9 million annually or approximately 18.9 percent above current rates. The requested increase is primarily to recover investments in facilities to enhance safety and reliability and the depreciation and taxes associated with the increase in investment. The increase was offset by tax savings related to the TCJA. Montana-Dakota requested an interim increase of approximately $4.6 million or approximately 7.3 percent, subject to refund. 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 also introduced a 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 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 approximately $3.6 million . A hearing was held May 30, 2018 through June 1, 2018. On July 19, 2018, a settlement was filed reflecting a revised annual revenue increase of approximately $2.5 million or approximately 2.3 percent. The proposed adjustment mechanism to fund the SSIP was not included in the settlement and will be decided on separately by the NDPSC. On September 26, 2018, the NDPSC issued an order approving the settlement as filed but did not approve the SSIP recovery mechanism. On October 5, 2018, Montana-Dakota submitted a compliance filing, which included a refund plan for the interim amount to be refunded to customers. 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. On August 10, 2018, a settlement agreement was filed requesting a decrease in rates of approximately $8.4 million . On September 26, 2018, the NDPSC issued an order approving the settlement along with requiring an additional adjustment to the rates to return 100 percent of the tax-effective 2018 excess deferred income taxes. On October 10, 2018, Montana-Dakota submitted a compliance filing, which included a refund plan for the interim amount to be refunded to customers. This matter is pending before the NDPSC. OPUC 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. On May 31, 2018, Cascade filed a general rate case with the OPUC requesting an overall increase of approximately $2.3 million or approximately 3.5 percent on an annual basis, which incorporates the impact 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. On May 4, 2018 and June 2, 2018, Montana-Dakota submitted detailed plans to address the TCJA impacts on the natural gas and electric utilities, respectively, to the SDPUC staff. On September 28, 2018, a settlement agreement was submitted to the SDPUC reflecting a proposal to refund approximately $600,000 to electric customers and approximately $1.3 million to natural gas customers. These refunds reflect the impact of the TCJA on 2018. On October 23, 2018, an order was issued by the SDPUC approving the settlement agreement. Proposed rate changes to reflect 2018 pro forma results and the TCJA impacts will be submitted by December 1, 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 approximately $1.7 million annually, which includes the impacts of the TCJA. On May 17, 2018, Cascade, the WUTC staff and interveners in the case reached a settlement on all matters in the rate case except the treatment of interim tax benefits for the period January 1, 2018 through July 31, 2018. The settlement included an overall annual rate reduction of approximately $2.9 million or approximately 1.4 percent plus specific tariffs passing excess deferred income taxes back to customers totaling approximately $2.5 million . On July 20, 2018, the WUTC approved the settlement reflecting a decrease in annual revenues, as well as refunding an additional $1.6 million for the interim tax benefits for the period of January 1, 2018 through July 31, 2018. The refund of excess deferred income taxes and interim tax benefits is over a fifteen month period. These changes were effective August 1, 2018. On June 1, 2018, Cascade filed its annual pipeline cost recovery mechanism requesting an increase in annual revenue of $2.3 million or approximately 1.1 percent. On October 11 2018, Cascade filed a revised increase in annual revenue of $2.1 million or approximately 1.0 percent. The increase will go into effect on November 1, 2018. 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 approximately $1.1 million annually or approximately 4.2 percent. Revised electric rates reflecting this reduction were submitted to the WYPSC on June 13, 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. Both matters are 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. On September 4, 2018, Montana-Dakota filed an update to its transmission formula rate under the MISO tariff for the multivalue project for $12.5 million , which is effective January 1, 2019.

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 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 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. The Company accrued liabilities of $30.4 million , $34.3 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 September 30, 2018 and 2017 , and December 31, 2017 , respectively. This includes amounts that have been accrued for matters discussed in Environmental matters within this note. The Company will continue to monitor each matter and adjust accruals as might be warranted based on new information and further developments. Management believes that the outcomes with respect to probable and reasonably possible losses in excess of the amounts accrued, net of insurance recoveries, while uncertain, either cannot be estimated or will not have a material effect upon the Company's financial position, results of operations or cash flows. Unless otherwise required by GAAP, legal costs are expensed as they are incurred. Environmental matters Portland Harbor Site In December 2000, Knife River - Northwest was named by the EPA as a PRP in connection with the cleanup of 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 accruals related to these claims are reflected in regulatory assets. The first claim is for contamination at a site in Eugene, Oregon which was received in 1995. 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. Cascade and other PRPs will share in the cleanup costs with Cascade expecting to pay approximately 50 percent of the remediation and maintenance costs. 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 $2.7 million has been incurred. Cascade has accrued $4.9 million for the remedial investigation and feasibility study, as well as $6.4 million for remediation of this site; however, the accrual for remediation costs will be reviewed and adjusted, if necessary, after completion of the remedial investigation and feasibility study. In April 2010, Cascade filed a petition with the WUTC for authority to defer the costs incurred in relation to the environmental remediation of this site. The WUTC approved the petition in September 2010, subject to conditions set forth in the order. 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. Other PRPs reached an agreed order and work plan with the Washington DOE for completion of a remedial investigation and feasibility study for the site. A feasibility study prepared for one of the PRPs in March 2018 identifies five cleanup action alternatives for the site with estimated costs ranging from $8.0 million to $20.4 million with a selected preferred alternative having an estimated total cost of $9.3 million . 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 recorded an accrual for this site for an amount that is not material. Cascade has received notices from and entered into agreement with certain of its insurance carriers that they will participate in defense of Cascade for certain of the contamination claims subject to full and complete reservations of rights and defenses to insurance coverage. To the extent these claims are not covered by insurance, Cascade intends to seek recovery through the OPUC and WUTC of remediation costs in its natural gas rates charged to customers. 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 $ 49.9 million at September 30, 2018 , and were expected to mature in 2023. Tesoro agreed to indemnify Centennial for any losses and litigation expenses arising from the guarantee. The estimated fair value of the indemnity asset is reflected in deferred charges and other assets - other and the guarantee liability is reflected in other accrued liabilities and deferred credits and other liabilities - other, on the Consolidated Balance Sheets. Continuation of the guarantee was required as a condition to the sale of Dakota Prairie Refining. On October 17, 2018, Centennial was released from this guarantee of certain debt obligations of Dakota Prairie Refining. In 2009, multiple sale agreements were signed to sell the Company's ownership interests in the Brazilian Transmission Lines. In connection with the sale, Centennial agreed to guarantee payment of any indemnity obligations of certain of the Company's indirect wholly owned subsidiaries 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 September 30, 2018 , the fixed maximum amounts guaranteed under these agreements aggregated $197.2 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate to $1.1 million in 2018 ; $114.9 million in 2019 ; $74.2 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 September 30, 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 September 30, 2018 , the fixed maximum amounts guaranteed under these letters of credit aggregated $29.5 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these letters of credit aggregate to $28.0 million in 2018 and $1.5 million in 2019. There were no amounts outstanding under the above letters of credit at September 30, 2018 . In the event of default under these letter of credit obligations, the subsidiary guaranteeing the letter of credit would be obligated for reimbursement of payments made under the letter of credit. In addition, Centennial, Knife River and MDU Construction Services have issued guarantees to third parties related to the routine purchase of maintenance items, materials and lease obligations for which no fixed maximum amounts have been specified. These guarantees have no scheduled maturity date. In the event a subsidiary of the Company defaults under these obligations, Centennial, Knife River or MDU Construction Services would be required to make payments under these guarantees. Any amounts outstanding by subsidiaries of the Company were reflected on the Consolidated Balance Sheet at September 30, 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 September 30, 2018 , approximately $619.6 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 September 30, 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 $39.2 million

Subsequent events

Subsequent events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent events	Subsequent Events On October 4, 2018, the Company acquired Sweetman Construction Company, a provider of aggregates, asphalt and ready-mixed concrete in South Dakota, which will be included in the Company's construction materials and contracting segment. To date, the initial accounting for the acquisition is incomplete. Due to the limited time since the date of the acquisition, it is impracticable for the Company to make business combination disclosures related to the acquisition. The Company is still gathering the necessary information to provide such disclosures in future filings. On October 18, 2018, the Company entered into a $100.0 million

Basis of presentation (Policies

Basis of presentation (Policies)	9 Months Ended
	Sep. 30, 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 September 30, 2018

Accounts receivable and allow_2

Accounts receivable and allowance for doubtful accounts (Policies)	9 Months Ended
	Sep. 30, 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 i_2

Inventories and natural gas in storage (Policies)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 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 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 impacted certain business processes and controls. As such, the Company 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. For the nine months ended September 30, 2018 , there were no material impacts to the financial statements as a result of applying the guidance. The cumulative effect of the changes made to the Consolidated Balance Sheet were 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 : Retained earnings 1,040,748 (970 ) 1,039,778 The cumulative effect 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 also affects 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 required 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 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 $1.5 million and $4.5 million from operation and maintenance expense to other income on the Consolidated Statements of Income for the three and nine months ended September 30, 2017, respectively. The Company also reclassified unrealized gains on investments used to satisfy obligations under the defined benefit plans of $2.6 million and $7.9 million for the three and nine months ended September 30, 2017, respectively, 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. 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, with early adoption permitted. In July 2018, the FASB issued ASU 2018-11 - Leases: Targeted Improvements, an accounting standard update to ASU 2016-02. This ASU provides an entity the option to adopt the guidance using one of two modified retrospective approaches. An entity can adopt the guidance using the modified retrospective transition approach beginning in the earliest year presented in the financial statements. This method of adoption would require the restatement of prior periods reported and the presentation of lease disclosures under the new guidance for all periods reported. The additional transition method of adoption introduced by ASU 2018-11, allows entities the option to apply the guidance on the date of adoption by recognizing a cumulative effect adjustment to retained earnings during the period of adoption and does not require prior comparative periods to be restated. 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, as well as the additional transition method of adoption applied on the date of adoption. The Company also plans to adopt a short-term leasing policy as the lessee where leases with a term of 12 months or less will not be included on the Consolidated Balance Sheet. The Company formed a lease implementation team and is currently in the contract review and assessment phase to identify and evaluate contracts containing leases. Additionally, the team is implementing new and revising existing software to meet the reporting and disclosure requirements of the standard. The Company also has started to assess the impact the standard will have on its processes and internal controls and is identifying new and updating current processes. During the assessment phase, the Company is using various analytic methodologies to ensure the completeness of the lease inventory. The Company expects that most of the current operating leases will be subject to the guidance and recognized as operating lease liabilities and right-of-use assets on the Consolidated Balance Sheets upon adoption. During the fourth quarter of 2018, the Company will engage internal auditors to review and assess the completeness of the lease inventory and the effectiveness of the revised and new internal controls. The Company continues to evaluate the impact the new guidance will have on lease contracts where the Company is the lessor and, at this time, does not anticipate a significant impact. 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 planning to adopt this practical expedient. The Company continues to monitor other industry-specific issues as it relates to the regulated businesses but does not expect these issues to have a material impact on the Company's results of operations, financial position or disclosures. ASU 2017-04 - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued guidance on simplifying the test for goodwill impairment by eliminating Step 2, which required an entity to measure the amount of impairment loss by comparing the implied fair value of reporting unit goodwill with the carrying amount of such goodwill. This guidance requires entities to perform a quantitative impairment test, previously Step 1, to identify both the existence of impairment and the amount of impairment loss by comparing the fair value of a reporting unit to its carrying amount. Entities will continue to have the option of performing a qualitative assessment to determine if the quantitative impairment test is necessary. The guidance also requires additional disclosures if an entity has one or more reporting units with zero or negative carrying amounts of net assets. The guidance will be effective for the Company on January 1, 2020, and must be applied on a prospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. ASU 2018-07 - Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued guidance on simplifying the accounting for nonemployee share-based payment transactions by expanding the scope of ASC 718, Compensation - Stock Compensation , to include share-based payment transactions for acquiring goods and services from nonemployees. The guidance will require nonemployee share-based payment awards be measured at the grant-date fair value of the equity instruments at the grant date. The guidance will be effective for the Company on January 1, 2019, including interim periods, and must be applied using a modified retrospective approach with early adoption permitted. The modified retrospective approach requires an entity to record a cumulative-effect adjustment to retained earnings at the beginning of the year of adoption. The Company is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. ASU 2018-13 - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued guidance on modifying the disclosure requirements on fair value measurements as part of the disclosure framework project. The guidance modifies, among other things, the disclosures required for Level 3 fair value measurements, including the range and weighted average of significant unobservable inputs. The guidance removes, among other things, the disclosure requirement to disclose transfers between Levels 1 and 2. The guidance will be effective for the Company on January 1, 2020, including interim periods, with early adoption permitted. Level 3 fair value measurement disclosures should be applied prospectively while all other amendments should be applied retrospectively. The Company is evaluating the effects the adoption of the new guidance will have on its disclosures. ASU 2018-14 - Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued guidance on modifying the disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans as part of the disclosure framework project. The guidance removes disclosures that are no longer considered cost beneficial, clarifies the specific requirements of disclosures and adds disclosure requirements identified as relevant. The guidance adds, among other things, the requirement to include an explanation for significant gains and losses related to changes in benefit obligations for the period. The guidance removes, among other things, the disclosure requirement to disclose the amount of net periodic benefit costs to be amortized over the next fiscal year from accumulated other comprehensive income (loss) and the effects a one percentage point change in assumed health care cost trend rates will have on certain benefit components. The guidance will be effective for the Company on January 1, 2021, and must be applied on a retrospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on the its disclosures. ASU 2018-15 - Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract In August 2018, the FASB issued guidance on the accounting for implementation costs of a hosting arrangement that is a service contract. The guidance aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract similar to the costs incurred to develop or obtain internal-use software and such capitalized costs to be expensed over the term of the hosting arrangement. Costs incurred during the preliminary and postimplementation stages should continue to be expensed as activities are performed. The capitalized costs are required to be presented on the balance sheet in the same line the prepayment for the fees associated with the hosting arrangement would be presented. In addition, the expense related to the capitalized implementation costs should be presented in the same line on the income statement as the fees associated with the hosting element of the arrangements. The guidance will be effective for the Company on January 1, 2020, including interim periods, and may be applied on a retrospective or a prospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on the its results of operations, financial position, cash flows and disclosures. SEC File Number S7-15-16 - Disclosure Update and Simplification In October 2018, the SEC published guidance in the Federal Register on disclosure updates and simplifications. The guidance removes disclosures that are no longer considered cost beneficial, duplicative of GAAP required disclosures, clarifies the specific requirements of disclosures and adds disclosure requirements identified as relevant. The amendments are intended to facilitate disclosure of information to investors and simplify the compliance without significantly altering the total mix of information provided to investors. The guidance will be effective for the Company on November 5, 2018, including interim periods, and must be applied on a prospective basis. The Company is

Revenue from contracts with c_2

Revenue from contracts with customers (Policies)	9 Months Ended
	Sep. 30, 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 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 evaluates to determine whether a separate performance obligation exists. The transaction price is the original contract price plus any subsequent change orders and variable consideration. Examples of variable consideration that exist in this segment's contracts include liquidated damages; performance bonuses or incentives and penalties; claims; unapproved/unpriced change orders; and index pricing. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using 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 claims, unapproved/unpriced change orders, bonuses, incentives, penalties and liquidated damages. The variable amounts usually arise upon achievement of certain performance metrics or change in project scope. The Company estimates the amount of revenue to be recognized on variable consideration using estimation methods that best predict the most likely amount of consideration the Company expects to be entitled to or expects to incur. The Company includes variable consideration in the estimated transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur or when the uncertainty associated with the variable consideration is resolved. Changes in circumstances could impact management's estimates made in determining the value of variable consideration recorded. The Company updates its estimate of the transaction price each reporting period and the effect of variable consideration on the transaction price is recognized as an adjustment to revenue on a cumulative catch-up basis. Revenue is recognized over time using the input method based on the measurement of progress on a project. The input method is the preferred method of measuring revenue because the costs incurred have been determined to represent the best indication of the overall progress toward the transfer of such goods or services promised to a customer. This segment also sells construction equipment and other supplies to third parties and internal customers. The contract for these sales is the use of a sales order or invoice, which includes the pricing and payment terms. All such contracts include a single performance obligation for the delivery of a single distinct product or a distinct separately identifiable bundle of products and services. Revenue is recognized at a point in time when the performance obligation has been satisfied with the delivery of the products or services. The warranties associated with the sales are those consistent with a standard warranty that the product meets certain specifications for quality or those required by law. For most contracts, amounts billed to customers are due within thirty days of receipt. There are no material obligations for returns, refunds or other similar obligations.

Fair value disclosures (Policie

Fair value disclosures (Policies)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 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 i_3

Inventories and natural gas in storage (Tables)	9 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories on the Consolidated Balance Sheets were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Aggregates held for resale $ 133,477 $ 116,399 $ 115,268 Asphalt oil 40,781 26,682 30,360 Natural gas in storage (current) 29,084 29,974 20,950 Materials and supplies 23,563 20,778 18,650 Merchandise for resale 15,954 15,346 14,905 Other 27,434 23,376 26,450 Total $ 270,293 $ 232,555 $ 226,583

Earnings per common share (Tabl

Earnings per common share (Tables)	9 Months Ended
	Sep. 30, 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 Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) Weighted average common shares outstanding - basic 196,018 195,304 195,618 195,304 Effect of dilutive performance share awards and restricted stock units 247 479 486 618 Weighted average common shares outstanding - diluted 196,265 195,783 196,104 195,922 Shares excluded from the calculation of diluted earnings per share 114 — — —

New accounting standards (Table

New accounting standards (Tables)	9 Months Ended
	Sep. 30, 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 were 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 : Retained earnings 1,040,748 (970 ) 1,039,778

Comprehensive income (loss) (Ta

Comprehensive income (loss) (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Accumulated comprehensive loss	The after-tax changes in the components of accumulated other comprehensive loss were as follows: Three Months Ended September 30, 2018 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,136 ) $ (41,816 ) $ — $ (183 ) $ (44,135 ) Other comprehensive loss before reclassifications — — — (51 ) (51 ) Amounts reclassified from accumulated other comprehensive loss 92 442 — 33 567 Net current-period other comprehensive income (loss) 92 442 — (18 ) 516 Balance at end of period $ (2,044 ) $ (41,374 ) $ — $ (201 ) $ (43,619 ) Three Months Ended September 30, 2017 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,117 ) $ (33,469 ) $ (155 ) $ (48 ) $ (35,789 ) Other comprehensive income (loss) before reclassifications — — 15 (19 ) (4 ) Amounts reclassified from accumulated other comprehensive loss 92 333 — 27 452 Net current-period other comprehensive income 92 333 15 8 448 Balance at end of period $ (2,025 ) $ (33,136 ) $ (140 ) $ (40 ) $ (35,341 ) Nine Months Ended September 30, 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 — — (61 ) (199 ) (260 ) Amounts reclassified from accumulated other comprehensive loss 279 1,309 249 97 1,934 Net current-period other comprehensive income (loss) 279 1,309 188 (102 ) 1,674 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 $ (2,044 ) $ (41,374 ) $ — $ (201 ) $ (43,619 ) Nine Months Ended September 30, 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 (70 ) (61 ) Amounts reclassified from accumulated other comprehensive loss 275 1,002 — 93 1,370 Amounts reclassified to accumulated other comprehensive loss from regulatory asset — (917 ) — — (917 ) Net current-period other comprehensive income 275 85 9 23 392 Balance at end of period $ (2,025 ) $ (33,136 ) $ (140 ) $ (40 ) $ (35,341 )
Reclassification out of accumulated other comprehensive loss	The following amounts were reclassified out of accumulated other comprehensive loss into net income. The amounts presented in parenthesis indicate a decrease to net income on the Consolidated Statements of Income. The reclassifications were as follows: Three Months Ended Nine Months Ended Location on Consolidated Statements of Income September 30, September 30, 2018 2017 2018 2017 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income $ (147 ) $ (148 ) $ (443 ) $ (443 ) Interest expense 55 56 164 168 Income taxes (92 ) (92 ) (279 ) (275 ) Amortization of postretirement liability losses included in net periodic benefit cost (credit) (584 ) (536 ) (1,751 ) (1,611 ) Other income 142 203 442 609 Income taxes (442 ) (333 ) (1,309 ) (1,002 ) Reclassification adjustment for loss on foreign currency translation adjustment included in net income — — (324 ) — Other income — — 75 — Income taxes — — (249 ) — Reclassification adjustment on available-for-sale investments included in net income (42 ) (41 ) (123 ) (143 ) Other income 9 14 26 50 Income taxes (33 ) (27 ) (97 ) (93 ) Total reclassifications $ (567 ) $ (452 ) $ (1,934 ) $ (1,370 )

Revenue from contracts with c_3

Revenue from contracts with customers (Tables)	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of revenue	For more information on the Company's business segments, see Note 14 . Three Months Ended September 30, 2018 Electric Natural gas distribution Pipeline and midstream Construction materials and contracting Construction services Other Total (in thousands) Residential utility sales $ 31,424 $ 43,825 $ — $ — $ — $ — $ 75,249 Commercial utility sales 36,259 28,174 — — — — 64,433 Industrial utility sales 8,738 4,421 — — — — 13,159 Other utility sales 2,056 — — — — — 2,056 Natural gas transportation — 10,841 21,400 — — — 32,241 Natural gas gathering — — 2,320 — — — 2,320 Natural gas storage — — 2,795 — — — 2,795 Contracting services — — — 409,006 — — 409,006 Construction materials — — — 538,962 — — 538,962 Intrasegment eliminations* — — — (204,040 ) — — (204,040 ) Inside specialty contracting — — — — 217,474 — 217,474 Outside specialty contracting — — — — 100,988 — 100,988 Other 6,158 3,208 5,701 — 15 3,084 18,166 Intersegment eliminations — — (3,187 ) (165 ) (782 ) (3,037 ) (7,171 ) Revenues from contracts with customers 84,635 90,469 29,029 743,763 317,695 47 1,265,638 Revenues out of scope 1,445 1,779 42 — 11,883 — 15,149 Total external operating revenues $ 86,080 $ 92,248 $ 29,071 $ 743,763 $ 329,578 $ 47 $ 1,280,787 * 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. Nine Months Ended September 30, 2018 Electric Natural gas distribution Pipeline and midstream Construction materials and contracting Construction services Other Total (in thousands) Residential utility sales $ 93,359 $ 305,399 $ — $ — $ — $ — $ 398,758 Commercial utility sales 103,636 185,885 — — — — 289,521 Industrial utility sales 25,734 17,457 — — — — 43,191 Other utility sales 5,766 — — — — — 5,766 Natural gas transportation — 32,104 64,505 — — — 96,609 Natural gas gathering — — 6,900 — — — 6,900 Natural gas storage — — 8,563 — — — 8,563 Contracting services — — — 730,628 — — 730,628 Construction materials — — — 1,100,185 — — 1,100,185 Intrasegment eliminations* — — — (363,877 ) — — (363,877 ) Inside specialty contracting — — — — 667,664 — 667,664 Outside specialty contracting — — — — 283,432 — 283,432 Other 22,836 10,821 13,353 — 32 8,536 55,578 Intersegment eliminations — — (31,485 ) (501 ) (1,332 ) (8,343 ) (41,661 ) Revenues from contracts with customers 251,331 551,666 61,836 1,466,435 949,796 193 3,281,257 Revenues out of scope 653 2,786 129 — 36,853 — 40,421 Total external operating revenues $ 251,984 $ 554,452 $ 61,965 $ 1,466,435 $ 986,649 $ 193 $ 3,321,678 * Intrasegment revenues are presented within the construction materials and contracting segment to highlight the focus on vertical integration as this segment sells materials to both third parties and internal customers. Due to consolidation requirements, these revenues must be eliminated against construction materials to arrive at the external operating revenue total for the segment.
Contract balances	The changes in contract assets and liabilities were as follows: September 30, 2018 December 31, 2017 Change Location on Consolidated Balance Sheets (In thousands) Contract assets $ 142,376 $ 109,540 $ 32,836 Receivables, net Contract liabilities - current (98,470 ) (84,123 ) (14,347 ) Accounts payable Contract liabilities - noncurrent (399 ) — (399 ) Deferred credits and other liabilities - other Net contract assets $ 43,507 $ 25,417 $ 18,090

Discontinued operations (Tables

Discontinued operations (Tables)	9 Months Ended
	Sep. 30, 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 Consolidated Balance Sheets were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Assets Current assets: Receivables, net $ 571 $ 304 $ 479 Total current assets held for sale 571 304 479 Noncurrent assets: Net property, plant and equipment — 2,064 1,631 Deferred income taxes 1,711 62,163 2,637 Other 161 161 161 Total noncurrent assets held for sale 1,872 64,388 4,429 Total assets held for sale $ 2,443 $ 64,692 $ 4,908 Liabilities Current liabilities: Accounts payable $ 188 $ 68 $ 30 Taxes payable 7,463 11,745 10,857 Other accrued liabilities 1,948 2,380 2,884 Total current liabilities held for sale 9,599 14,193 13,771 Total liabilities held for sale $ 9,599 $ 14,193 $ 13,771 September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Assets Current assets: Income taxes receivable (a) $ 1,640 $ 8,444 $ 1,778 Total current assets held for sale 1,640 8,444 1,778 Total assets held for sale $ 1,640 $ 8,444 $ 1,778 Liabilities Noncurrent liabilities: Deferred income taxes (b) $ 37 $ 55 $ 37 Total noncurrent liabilities held for sale 37 55 37 Total liabilities held for sale $ 37 $ 55 $ 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 (loss) from discontinued operations on the Consolidated Statements of Income was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) Operating revenues $ 61 $ 121 $ 201 $ 356 Operating expenses 216 384 825 (4,988 ) Operating income (loss) (155 ) (263 ) (624 ) 5,344 Other income (expense) — — 12 (13 ) Interest expense — — 575 239 Income (loss) from discontinued operations before income taxes (155 ) (263 ) (1,187 ) 5,092 Income taxes (37 ) 1,935 * (1,272 ) 8,794 * Income (loss) from discontinued operations $ (118 ) $ (2,198 ) $ 85 $ (3,702 ) * Includes the eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Goodwill and other intangible_2

Goodwill and other intangible assets (Tables)	9 Months Ended
	Sep. 30, 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: Nine Months Ended September 30, 2018 Balance at January 1, 2018 Goodwill Acquired the Year* Balance at September 30, 2018 (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Construction materials and contracting 176,290 8,412 184,702 Construction services 109,765 — 109,765 Total $ 631,791 $ 8,412 $ 640,203 * Relates to business acquisitions that occurred during the period, as discussed in Note 9 . Nine Months Ended September 30, 2017 Balance at January 1, 2017 Goodwill Acquired the Year Balance at September 30, 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: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Customer relationships $ 15,920 $ 15,248 $ 15,248 Less accumulated amortization 13,342 13,176 13,382 2,578 2,072 1,866 Noncompete agreements 2,606 2,430 2,430 Less accumulated amortization 1,911 1,769 1,805 695 661 625 Other 6,458 7,020 6,990 Less accumulated amortization 5,413 5,544 5,644 1,045 1,476 1,346 Total $ 4,318 $ 4,209 $ 3,837

Fair value measurements (Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: September 30, 2018 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,550 $ 3 $ 251 $ 10,302 U.S. Treasury securities 179 — — 179 Total $ 10,729 $ 3 $ 251 $ 10,481 September 30, 2017 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,488 $ 11 $ 72 $ 9,427 U.S. Treasury securities 613 — 1 612 Total $ 10,101 $ 11 $ 73 $ 10,039 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 September 30, 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 September 30, 2018 (In thousands) Assets: Money market funds $ — $ 9,948 $ — $ 9,948 Insurance contract* — 80,358 — 80,358 Available-for-sale securities: Mortgage-backed securities — 10,302 — 10,302 U.S. Treasury securities — 179 — 179 Total assets measured at fair value $ — $ 100,787 $ — $ 100,787 * The insurance contract invests approximately 47 percent in fixed-income investments, 24 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 12 percent in common stock of small-cap companies, 3 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at September 30, 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 September 30, 2017 (In thousands) Assets: Money market funds $ — $ 6,204 $ — $ 6,204 Insurance contract* — 74,991 — 74,991 Available-for-sale securities: Mortgage-backed securities — 9,427 — 9,427 U.S. Treasury securities — 612 — 612 Total assets measured at fair value $ — $ 91,234 $ — $ 91,234 * The insurance contract invests approximately 50 percent in fixed-income investments, 23 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 September 30, 2018 $ 1,915,470 $ 1,987,360 Long-term debt at September 30, 2017 $ 1,740,552 $ 1,846,811 Long-term debt at December 31, 2017 $ 1,714,853 $ 1,826,256

Cash flow information (Tables)

Cash flow information (Tables)	9 Months Ended
	Sep. 30, 2018
Supplemental Cash Flow Information [Abstract]
Cash expenditures for interest and income taxes and noncash investing and financing transactions	Cash expenditures for interest and income taxes were as follows: Nine Months Ended September 30, 2018 2017 (In thousands) Interest, net* $ 58,359 $ 58,119 Income taxes paid, net** $ 8,887 $ 46,430 * AFUDC - borrowed was $1.8 million and $676,000 for the nine months ended September 30, 2018 and 2017 , respectively. ** Income taxes paid, net of discontinued operations, were $5.1 million and $1.4 million for the nine months ended September 30, 2018 and 2017 , respectively. Noncash investing and financing transactions were as follows: September 30, 2018 September 30, 2017 December 31, 2017 (In thousands) Property, plant and equipment additions in accounts payable $ 34,594 $ 16,914 $ 29,263 Issuance of common stock in connection with acquisition $ 17,993 $ — $ —

Business segment data (Tables)

Business segment data (Tables)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Information on the Company's businesses	Information on the Company's segments was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) External operating revenues: Regulated operations: Electric $ 86,080 $ 91,531 $ 251,984 $ 254,330 Natural gas distribution 92,248 92,253 554,452 566,364 Pipeline and midstream 22,289 23,152 45,325 45,341 200,617 206,936 851,761 866,035 Nonregulated operations: Pipeline and midstream 6,782 5,356 16,640 13,518 Construction materials and contracting 743,763 686,010 1,466,435 1,388,212 Construction services 329,578 374,111 986,649 1,009,693 Other 47 135 193 654 1,080,170 1,065,612 2,469,917 2,412,077 Total external operating revenues $ 1,280,787 $ 1,272,548 $ 3,321,678 $ 3,278,112 Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (In thousands) Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — $ — Natural gas distribution — — — — Pipeline and midstream 3,070 3,081 31,252 30,923 3,070 3,081 31,252 30,923 Nonregulated operations: Pipeline and midstream 117 38 233 132 Construction materials and contracting 165 142 501 400 Construction services 782 415 1,332 715 Other 3,037 1,910 8,343 5,411 4,101 2,505 10,409 6,658 Intersegment eliminations (7,171 ) (5,586 ) (41,661 ) (37,581 ) Total intersegment operating revenues $ — $ — $ — $ — Earnings (loss) on common stock: Regulated operations: Electric $ 15,284 $ 15,712 $ 37,500 $ 37,904 Natural gas distribution (11,887 ) (10,883 ) 13,884 14,181 Pipeline and midstream 10,109 5,853 20,809 15,901 13,506 10,682 72,193 67,986 Nonregulated operations: Pipeline and midstream 848 95 1,136 (770 ) Construction materials and contracting 78,876 63,221 79,691 64,477 Construction services 9,278 13,144 38,457 32,896 Other 4,861 552 1,928 (1,888 ) 93,863 77,012 121,212 94,715 Intersegment eliminations — 1,855 * — 6,121 * Earnings on common stock before income (loss) from discontinued operations 107,369 89,549 193,405 168,822 Income (loss) from discontinued operations, net of tax (118 ) (2,198 ) * 85 (3,702 ) * Total earnings on common stock $ 107,251 $ 87,351 $ 193,490 $ 165,120 * Includes the eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans (Tables)

Employee benefit plans (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2018 2017 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 374 $ 377 Interest cost 3,648 4,052 725 816 Expected return on assets (5,188 ) (5,132 ) (1,217 ) (1,160 ) Amortization of prior service credit — — (349 ) (343 ) Amortization of net actuarial loss 1,751 1,589 160 213 Net periodic benefit cost (credit), including amount capitalized 211 509 (307 ) (97 ) Less amount capitalized — 65 46 (95 ) Net periodic benefit cost (credit) $ 211 $ 444 $ (353 ) $ (2 ) Pension Benefits Other Postretirement Benefits Nine Months Ended September 30, 2018 2017 2018 2017 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 1,121 $ 1,130 Interest cost 10,943 12,155 2,175 2,449 Expected return on assets (15,565 ) (15,395 ) (3,650 ) (3,480 ) Amortization of prior service credit — — (1,046 ) (1,029 ) Amortization of net actuarial loss 5,254 4,767 480 649 Net periodic benefit cost (credit), including amount capitalized 632 1,527 (920 ) (281 ) Less amount capitalized — 245 128 (248 ) Net periodic benefit cost (credit) $ 632 $ 1,282 $ (1,048 ) $ (33 )

Accounts receivable and allow_3

Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Receivables [Abstract]
Receivables past due 90 days or more	$ 29.2	$ 34.7	$ 27.2
Allowance for doubtful accounts receivable	$ 7.3	$ 8.1	$ 9

Inventories and natural gas i_4

Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 133,477	$ 115,268	$ 116,399
Asphalt oil	40,781	30,360	26,682
Natural gas in storage (current)	29,084	20,950	29,974
Materials and supplies	23,563	18,650	20,778
Merchandise for resale	15,954	14,905	15,346
Other	27,434	26,450	23,376
Total	270,293	226,583	232,555
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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Weighted average common shares outstanding - basic	196,018	195,304	195,618	195,304
Effect of dilutive performance share awards and restricted stock units	247	479	486	618
Weighted average common shares outstanding - diluted	196,265	195,783	196,104	195,922
Shares excluded from the calculation of diluted earnings per share	114	0	0	0

New accounting standards ASU 20

New accounting standards ASU 2014-09 (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	$ 221,620,000		$ 186,010,000	$ 184,580,000
Deferred income taxes	405,761,000		347,271,000	652,413,000
Other	1,154,366,000		1,179,140,000	889,494,000
Retained earnings	1,124,830,000		1,040,748,000	$ 964,275,000
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification	$ 0
Other accrued liabilities		$ 186,913,000
Deferred income taxes		346,939,000
Other		1,179,539,000
Retained earnings		1,039,778,000
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities			186,010,000
Deferred income taxes			347,271,000
Other			1,179,140,000
Retained earnings			$ 1,040,748,000
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		903,000
Deferred income taxes		(332,000)
Other		399,000
Retained earnings		$ (970,000)

New accounting standards ASU _2

New accounting standards ASU 2017-07 (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans	$ 2.1	$ 1.9	$ 3	$ 6.9
Accounting Standards Update 2017-07 [Member] | Other Income [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit)		1.5		4.5
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans		$ 2.6		$ 7.9

New accounting standards ASU _3

New accounting standards ASU 2018-02 (Details 3) - Adjustments for new accounting principle, early adoption $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Retained Earnings Adjustments [Line Items]
Reclassification of certain prior period tax effects from accumulated other comprehensive loss	$ 0
Tax Cuts and Jobs Act
Retained Earnings Adjustments [Line Items]
Reclassification of certain prior period tax effects from accumulated other comprehensive loss	$ 7,900

Comprehensive income (loss) (De

Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 2,429,043	$ 2,316,244
Other comprehensive income	$ 516	$ 448	1,674	392
Balance	2,521,293	2,353,917	2,521,293	2,353,917
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,136)	(2,117)	(1,934)	(2,300)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	92	92	279	275
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income	92	92	279	275
Balance	(2,044)	(2,025)	(2,044)	(2,025)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(41,816)	(33,469)	(35,163)	(33,221)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	442	333	1,309	1,002
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				(917)
Other comprehensive income	442	333	1,309	85
Balance	(41,374)	(33,136)	(41,374)	(33,136)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	0	(155)	(155)	(149)
Other comprehensive loss before reclassifications	0	15	(61)	9
Amounts reclassified from accumulated other comprehensive loss	0	0	249	0
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income	0	15	188	9
Balance	0	(140)	0	(140)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(183)	(48)	(82)	(63)
Other comprehensive loss before reclassifications	(51)	(19)	(199)	(70)
Amounts reclassified from accumulated other comprehensive loss	33	27	97	93
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income	(18)	8	(102)	23
Balance	(201)	(40)	(201)	(40)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(44,135)	(35,789)	(37,334)	(35,733)
Other comprehensive loss before reclassifications	(51)	(4)	(260)	(61)
Amounts reclassified from accumulated other comprehensive loss	567	452	1,934	1,370
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				(917)
Other comprehensive income	516	448	1,674	392
Balance	$ (43,619)	$ (35,341)	(43,619)	$ (35,341)
Adjustments for new accounting principle, early adoption | 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)
Adjustments for new accounting principle, early adoption | 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)
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss			(33)
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss			(17)
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss			$ (7,959)

Reclassification out of accumul

Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,955)	$ (20,909)	$ (62,202)	$ (61,978)
Other income	2,683	2,068	4,864	6,212
Income taxes	(14,363)	(46,930)	(32,629)	(74,406)
Total reclassifications	107,251	87,351	193,490	165,891
Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Total reclassifications	(567)	(452)	(1,934)	(1,370)
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	(147)	(148)	(443)	(443)
Income taxes	55	56	164	168
Total reclassifications	(92)	(92)	(279)	(275)
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]
Other income	(584)	(536)	(1,751)	(1,611)
Income taxes	142	203	442	609
Total reclassifications	(442)	(333)	(1,309)	(1,002)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	0	0	(324)	0
Income taxes	0	0	75	0
Total reclassifications	0	0	(249)	0
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]
Other income	(42)	(41)	(123)	(143)
Income taxes	9	14	26	50
Total reclassifications	$ (33)	$ (27)	$ (97)	$ (93)

Revenue from contracts with c_4

Revenue from contracts with customers Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 1,280,787	$ 1,272,548	$ 3,321,678	$ 3,278,112
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(7,171)	$ (5,586)	(41,661)	$ (37,581)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(204,040)		(363,877)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	32,241		96,609
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,320		6,900
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	2,795		8,563
Contracting services
Disaggregation of Revenue [Line Items]
Revenues	409,006		730,628
Construction materials
Disaggregation of Revenue [Line Items]
Revenues	538,962		1,100,185
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	217,474		667,664
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	100,988		283,432
Other
Disaggregation of Revenue [Line Items]
Revenues	18,166		55,578
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	1,265,638		3,281,257
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	15,149		40,421
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	75,249		398,758
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	64,433		289,521
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	13,159		43,191
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	2,056		5,766
Electric
Disaggregation of Revenue [Line Items]
Revenues	86,080		251,984
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	0		0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	6,158		22,836
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	84,635		251,331
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	1,445		653
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	31,424		93,359
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	36,259		103,636
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	8,738		25,734
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	2,056		5,766
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues	92,248		554,452
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	0		0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	10,841		32,104
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	3,208		10,821
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	90,469		551,666
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	1,779		2,786
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	43,825		305,399
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	28,174		185,885
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	4,421		17,457
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	0		0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Revenues	29,071		61,965
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(3,187)		(31,485)
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	21,400		64,505
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,320		6,900
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	2,795		8,563
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Revenues	5,701		13,353
Pipeline and midstream | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	29,029		61,836
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	42		129
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues	743,763		1,466,435
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(165)		(501)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(204,040)		(363,877)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues	409,006		730,628
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues	538,962		1,100,185
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Revenues	0		0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	743,763		1,466,435
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	0		0
Construction services
Disaggregation of Revenue [Line Items]
Revenues	329,578		986,649
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(782)		(1,332)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	217,474		667,664
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	100,988		283,432
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues	15		32
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	317,695		949,796
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	11,883		36,853
Other
Disaggregation of Revenue [Line Items]
Revenues	47		193
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(3,037)		(8,343)
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	3,084		8,536
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	47		193
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	$ 0		$ 0

Revenue from contracts with c_5

Revenue from contracts with customers Contract balances (Details 2) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract assets	$ 142,376	$ 142,376	$ 109,540
Change in contract assets		32,836
Contract liabilities - current	(98,470)	(98,470)	(84,123)
Change in contract liabilities - current		(14,347)
Contract liabilities - noncurrent	(399)	(399)	0
Change in contract liabilities - noncurrent		(399)
Net contract assets	43,507	43,507	$ 25,417
Change in net contract assets		18,090
Amounts included in contract liability at the beginning of the period	10,300	79,200
Amounts from performance obligations satisfied in prior periods	$ (8,700)	$ (3,700)

Acquisitions (Details)

Acquisitions (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)shares
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Cash consideration, gross	$ 27.9
Common stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Equity issued in business combination, shares | shares	713,948
Equity issued in business combination, value assigned	$ 20.1
Equity issued in business combination, fair value	$ 18

Discontinued operations Noncont

Discontinued operations Noncontrolling interest (Details)	Jun. 27, 2016	Jun. 24, 2016
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%

Major classes of assets and lia

Major classes of assets and liabilities Refining (Details 2) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Total current assets held for sale	$ 571	$ 479	$ 304
Refining | Discontinued operations, held-for-sale or disposed of by sale
Current assets:
Income taxes receivable	1,640	1,778	8,444
Total current assets held for sale	1,640	1,778	8,444
Total assets held for sale	1,640	1,778	8,444
Noncurrent liabilities:
Deferred income taxes (b)	37	37	55
Total noncurrent liabilities held for sale	37	37	55
Total liabilities held for sale	$ 37	$ 37	$ 55

Major classes of assets and l_2

Major classes of assets and liabilities held for sale E&P (Details 3) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Total current assets held for sale	$ 571	$ 479	$ 304
Noncurrent assets:
Total noncurrent assets held for sale	1,835	4,392	64,333
Current liabilities:
Total current liabilities held for sale	7,959	11,993	5,749
Exploration and production | Discontinued operations, held-for-sale or disposed of by sale
Current assets:
Receivables, net	571	479	304
Total current assets held for sale	571	479	304
Noncurrent assets:
Net property, plant and equipment	0	1,631	2,064
Deferred income taxes	1,711	2,637	62,163
Other	161	161	161
Total noncurrent assets held for sale	1,872	4,429	64,388
Total assets held for sale	2,443	4,908	64,692
Current liabilities:
Accounts payable	188	30	68
Taxes payable	7,463	10,857	11,745
Other accrued liabilities	1,948	2,884	2,380
Total current liabilities held for sale	9,599	13,771	14,193
Total liabilities held for sale	$ 9,599	$ 13,771	$ 14,193

Business exit costs (Details 4)

Business exit costs (Details 4) - Other restructuring [Member] - Discontinued operations, held-for-sale or disposed of by sale - USD ($)	9 Months Ended		39 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 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	$ 0	$ 10,500,000

Reconciliation of income and ex

Reconciliation of income and expenses (Details 5) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations	$ (118)	$ (2,198)		$ 85	$ (3,702)
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	61	121		201	356
Operating expenses	216	384		825	(4,988)
Operating income (loss)	(155)	(263)		(624)	5,344
Other income	0	0		12
Other expense					(13)
Interest expense	0	0		575	239
Income (loss) from discontinued operations before income taxes	(155)	(263)		(1,187)	5,092
Income taxes	(37)	1,935		(1,272)	8,794
Income (loss) from discontinued operations	$ (118)	$ (2,198)		$ 85	$ (3,702)
Reversal of accrual | Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating expenses			$ 7,000
Reversal of accrual - after tax | Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating expenses			$ 4,300

Goodwill rollforward (Details)

Goodwill rollforward (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Goodwill [Roll Forward]
Balance at beginning of period	$ 631,791	$ 631,791	$ 631,791
Goodwill acquired during the year	8,412	0	0
Balance at end of period	640,203	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	8,412	0	0
Balance at end of period	184,702	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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 4,318,000	$ 4,209,000	$ 4,318,000	$ 4,209,000	$ 3,837,000
Amortization of intangible assets	300,000	500,000	900,000	1,700,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2,018	1,400,000		1,400,000
2,019	1,200,000		1,200,000
2,020	600,000		600,000
2,021	400,000		400,000
2,022	400,000		400,000
Thereafter	1,200,000		1,200,000
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	15,920,000	15,248,000	15,920,000	15,248,000	15,248,000
Intangible assets, less accumulated amortization	13,342,000	13,176,000	13,342,000	13,176,000	13,382,000
Intangible assets, net (excluding goodwill)	2,578,000	2,072,000	2,578,000	2,072,000	1,866,000
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	2,606,000	2,430,000	2,606,000	2,430,000	2,430,000
Intangible assets, less accumulated amortization	1,911,000	1,769,000	1,911,000	1,769,000	1,805,000
Intangible assets, net (excluding goodwill)	695,000	661,000	695,000	661,000	625,000
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	6,458,000	7,020,000	6,458,000	7,020,000	6,990,000
Intangible assets, less accumulated amortization	5,413,000	5,544,000	5,413,000	5,544,000	5,644,000
Intangible assets, net (excluding goodwill)	$ 1,045,000	$ 1,476,000	$ 1,045,000	$ 1,476,000	$ 1,346,000

Fair value measurements Insuran

Fair value measurements Insurance contracts (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 80.4	$ 75	$ 80.4	$ 75	$ 77.4
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans	$ 2.1	$ 1.9	$ 3	$ 6.9

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Available-for-sale securities [Abstract]
Cost	$ 10,729	$ 10,547	$ 10,101
Gross unrealized gains	3	4	11
Gross unrealized losses	251	130	73
Fair value	10,481	10,421	10,039
Mortgage backed securities
Available-for-sale securities [Abstract]
Cost	10,550	10,342	9,488
Gross unrealized gains	3	4	11
Gross unrealized losses	251	129	72
Fair value	10,302	10,217	9,427
US Treasury securities
Available-for-sale securities [Abstract]
Cost	179	205	613
Gross unrealized gains	0	0	0
Gross unrealized losses	0	1	1
Fair value	$ 179	$ 204	$ 612

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	47.00%	49.00%	50.00%
Percentage investment in common stock of large-cap companies	24.00%	23.00%	23.00%
Percentage investment in common stock of mid-cap companies	13.00%	14.00%	13.00%
Percentage investment in common stock of small-cap companies	12.00%	11.00%	11.00%
Percentage investment in target date investments	3.00%	2.00%	2.00%
Percentage investment in cash and cash equivalents	1.00%	1.00%	1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 100,787	$ 94,774	$ 91,234
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,948	6,965	6,204
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	80,358	77,388	74,991
Fair value, measurements, recurring [Member] | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,302	10,217	9,427
Fair value, measurements, recurring [Member] | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	179	204	612
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	0	0
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	100,787	94,774	91,234
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,948	6,965	6,204
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	80,358	77,388	74,991
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,302	10,217	9,427
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	179	204	612
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	0	0
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

Fair value measurements (Deta_2

Fair value measurements (Details 4) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Carrying amount
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 1,915,470	$ 1,714,853	$ 1,740,552
Fair value
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 1,987,360	$ 1,826,256	$ 1,846,811

Cash flow information (Details)

Cash flow information (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Interest, net	$ 58,359,000	$ 58,119,000
Income taxes paid, net	8,887,000	46,430,000
AFUDC borrowed	1,800,000	676,000
Property, plant and equipment additions in accounts payable	34,594,000	16,914,000	$ 29,263,000
Issuance of common stock in connection with acquisition	17,993,000	0	$ 0
Continuing and discontinued operations
Income taxes paid, net	$ 5,100,000	$ 1,400,000

Business segment data (Details)

Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 1,280,787	$ 1,272,548	$ 3,321,678	$ 3,278,112
Earnings on common stock before income (loss) from discontinued operations	107,369	89,549	193,405	168,822
Income (loss) from discontinued operations, net of tax	(118)	(2,198)	85	(3,702)
Earnings on common stock	107,251	87,351	193,490	165,120
Electric
Segment Reporting Information [Line Items]
Revenues	86,080		251,984
Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	92,248		554,452
Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	29,071		61,965
Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	743,763		1,466,435
Construction services
Segment Reporting Information [Line Items]
Revenues	329,578		986,649
Other
Segment Reporting Information [Line Items]
Revenues	47		193
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	(7,171)	(5,586)	(41,661)	(37,581)
Earnings on common stock before income (loss) from discontinued operations	0	1,855	0	6,121
Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	0		0
Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0		0
Intersegment eliminations | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	(3,187)		(31,485)
Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	(165)		(501)
Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	(782)		(1,332)
Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	(3,037)		(8,343)
Total intersegment operating revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Regulated operation
Segment Reporting Information [Line Items]
Revenues	200,617	206,936	851,761	866,035
Earnings on common stock before income (loss) from discontinued operations	13,506	10,682	72,193	67,986
Regulated operation | Electric
Segment Reporting Information [Line Items]
Revenues	86,080	91,531	251,984	254,330
Earnings on common stock before income (loss) from discontinued operations	15,284	15,712	37,500	37,904
Regulated operation | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	92,248	92,253	554,452	566,364
Earnings on common stock before income (loss) from discontinued operations	(11,887)	(10,883)	13,884	14,181
Regulated operation | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	22,289	23,152	45,325	45,341
Earnings on common stock before income (loss) from discontinued operations	10,109	5,853	20,809	15,901
Regulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	3,070	3,081	31,252	30,923
Regulated operation | Intersegment eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Regulated operation | Intersegment eliminations | Natural gas distribution
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Regulated operation | Intersegment eliminations | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	3,070	3,081	31,252	30,923
Nonregulated operation
Segment Reporting Information [Line Items]
Revenues	1,080,170	1,065,612	2,469,917	2,412,077
Earnings on common stock before income (loss) from discontinued operations	93,863	77,012	121,212	94,715
Nonregulated operation | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	6,782	5,356	16,640	13,518
Earnings on common stock before income (loss) from discontinued operations	848	95	1,136	(770)
Nonregulated operation | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	743,763	686,010	1,466,435	1,388,212
Earnings on common stock before income (loss) from discontinued operations	78,876	63,221	79,691	64,477
Nonregulated operation | Construction services
Segment Reporting Information [Line Items]
Revenues	329,578	374,111	986,649	1,009,693
Earnings on common stock before income (loss) from discontinued operations	9,278	13,144	38,457	32,896
Nonregulated operation | Other
Segment Reporting Information [Line Items]
Revenues	47	135	193	654
Earnings on common stock before income (loss) from discontinued operations	4,861	552	1,928	(1,888)
Nonregulated operation | Intersegment eliminations
Segment Reporting Information [Line Items]
Revenues	4,101	2,505	10,409	6,658
Nonregulated operation | Intersegment eliminations | Pipeline and midstream
Segment Reporting Information [Line Items]
Revenues	117	38	233	132
Nonregulated operation | Intersegment eliminations | Construction materials and contracting
Segment Reporting Information [Line Items]
Revenues	165	142	501	400
Nonregulated operation | Intersegment eliminations | Construction services
Segment Reporting Information [Line Items]
Revenues	782	415	1,332	715
Nonregulated operation | Intersegment eliminations | Other
Segment Reporting Information [Line Items]
Revenues	$ 3,037	$ 1,910	$ 8,343	$ 5,411

Employee benefit plans (Details

Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	3,648	4,052	10,943	12,155
Expected return on assets	(5,188)	(5,132)	(15,565)	(15,395)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial loss	1,751	1,589	5,254	4,767
Net periodic benefit cost (credit), including amount capitalized	211	509	632	1,527
Less amount capitalized	0	65	0	245
Net periodic benefit cost (credit)	211	444	632	1,282
Qualified plan | Underfunded plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	374	377	1,121	1,130
Interest cost	725	816	2,175	2,449
Expected return on assets	(1,217)	(1,160)	(3,650)	(3,480)
Amortization of prior service credit	(349)	(343)	(1,046)	(1,029)
Amortization of net actuarial loss	160	213	480	649
Net periodic benefit cost (credit), including amount capitalized	(307)	(97)	(920)	(281)
Less amount capitalized	46	(95)	128	(248)
Net periodic benefit cost (credit)	(353)	(2)	(1,048)	(33)
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	$ 3,300	$ 3,500

Montana Public Service Commissi

Montana Public Service Commission (MTPSC) (Details) - MTPSC - Electric rate proceeding $ in Millions	Sep. 28, 2018USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 11.9
Public utilities, requested rate increase (decrease), percentage	18.90%
Public utilities, interim rate increase (decrease), amount	$ 4.6
Public utilities, interim rate increase (decrease), percentage	7.30%

North Dakota Public Service Com

North Dakota Public Service Commission (NDPSC) (Details 2) - NDPSC - USD ($) $ in Millions	Sep. 26, 2018	Aug. 10, 2018	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
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 2.5
Public utilities, approved rate increase (decrease), percentage	2.30%
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%)
Public utilities, requested rate increase (decrease), amended, amount		$ (8.4)

Oregon Public Utility Commissio

Oregon Public Utility Commission (OPUC) (Details 3) - OPUC - Tax Cuts and Jobs Act $ in Millions	May 31, 2018USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 2.3
Public utilities, requested rate increase (decrease), percentage	3.50%

South Dakota Public Utilities C

South Dakota Public Utilities Commission (SDPUC) (Details 4) - Subsequent Event - SDPUC	Oct. 23, 2018USD ($)
Electric rate proceeding
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ (600,000)
Natural gas rate proceeding
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ (1,300,000)

Washington Utilities and Transp

Washington Utilities and Transportation Commission (WUTC) (Details 5) - WUTC - USD ($) $ in Millions	Oct. 11, 2018	Jul. 20, 2018	Jun. 01, 2018	Mar. 23, 2018	Aug. 31, 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					2.70%
Natural gas rate proceeding | Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amended, amount				$ (1.7)
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (2.9)
Public utilities, approved rate increase (decrease), percentage		(1.40%)
Public Utilities, Approved Rate Increase (Decrease) For Excess Deferred Income Taxes, Amount		$ (2.5)
Public Utilities, Approved Rate Increase (Decrease) For Tax Benefits, Amount		$ (1.6)
Pipeline replacement cost recovery
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount			$ 2.3
Public utilities, requested rate increase (decrease), percentage			1.10%
Pipeline replacement cost recovery | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 2.1
Public utilities, requested rate increase (decrease), percentage	1.00%

Wyoming Public Service Commissi

Wyoming Public Service Commission (WYPSC) (Details 6) - WYPSC - Electric rate proceeding - 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 7) $ in Millions	Sep. 04, 2018USD ($)
MISO | Electric transmission rate proceeding
Public Utilities, General Disclosures [Line Items]
Public Utilities, Tariff on Capital Project Tracker	$ 12.5

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 30.4	$ 35.4	$ 34.3

Environmental matters (Details

Environmental matters (Details 2) - USD ($)	9 Months Ended
	Sep. 30, 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
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	2,700,000
Environmental matters accrual of investigative costs	4,900,000
Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, preferred alternative estimate	9,300,000
Minimum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	340,000
Minimum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	8,000,000
Maximum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	6,400,000
Maximum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 20,400,000

Guarantees (Details 3)

Guarantees (Details 3)	Sep. 30, 2018USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 197,200,000
Fixed maximum amounts guaranteed by year 2018	1,100,000
Fixed maximum amounts guaranteed by year 2019	114,900,000
Fixed maximum amounts guaranteed by year 2020	74,200,000
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	29,500,000
Letters of credit set to expire - 2018	28,000,000
Letters of credit set to expire - 2019	1,500,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	619,600,000
Financial guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 49,900,000

Variable interest entities (Det

Variable interest entities (Details 4) $ in Millions	Sep. 30, 2018USD ($)
Fuel contract
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 39.2

Subsequent events (Details)

Subsequent events (Details) $ in Millions	Oct. 18, 2018USD ($)
Subsequent Event
Subsequent Event [Line Items]
Long-term debt	$ 100