MDU Resources (MDU) 10-Q 3 Nov 17 2017 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 27, 2017
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, 2017
Document fiscal year focus	2,017
Document fiscal period focus	Q3
Amendment flag	false
Entity common stock, shares outstanding		195,304,376

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating revenues:
Operating revenues:		$ 1,272,548	$ 1,208,567	$ 3,278,112	$ 3,112,730
Operating expenses:
Operation and maintenance:		972,909	920,540	2,351,053	2,237,486
Electric fuel and purchased power		18,906	16,800	57,544	54,725
Purchased natural gas sold		33,319	34,321	283,936	242,795
Depreciation, depletion and amortization		52,155	54,094	155,138	163,226
Taxes, other than income		38,882	36,128	127,273	116,864
Total operating expenses		1,116,171	1,061,883	2,974,944	2,815,096
Operating income		156,377	146,684	303,168	297,634
Other income		1,011	1,741	2,809	3,662
Interest expense		20,909	22,278	61,978	67,365
Income before income taxes		136,479	126,147	243,999	233,931
Income taxes		46,930	37,761	74,406	67,381
Income from continuing operations		89,549	88,386	169,593	166,550
Loss from discontinued operations, net of tax (Note 8)	[1]	(2,198)	(5,400)	(3,702)	(299,538)
Net income (loss)		87,351	82,986	165,891	(132,988)
Loss from discontinued operations attributable to noncontrolling interest (Note 8)		0	0	0	(131,691)
Loss on redemption of preferred stocks		0	0	600	0
Dividends declared on preferred stocks		0	171	171	514
Earnings (loss) on common stock		$ 87,351	$ 82,815	$ 165,120	$ (1,811)
Earnings (loss) per common share - basic:
Earnings before discontinued operations		$ 0.46	$ 0.45	$ 0.86	$ 0.85
Discontinued operations attributable to the Company, net of tax		(0.01)	(0.03)	(0.01)	(0.86)
Earnings (loss) per common share - basic		0.45	0.42	0.85	(0.01)
Earnings (loss) per common share - diluted:
Earnings before discontinued operations		0.46	0.45	0.86	0.85
Discontinued operations attributable to the Company, net of tax		(0.01)	(0.03)	(0.02)	(0.86)
Earnings (loss) per common share - diluted		0.45	0.42	0.84	(0.01)
Dividends declared per common share		$ 0.1925	$ 0.1875	$ 0.5775	$ 0.5625
Weighted average common shares outstanding - basic		195,304	195,304	195,304	195,298
Weighted average common shares outstanding - diluted		195,783	195,811	195,922	195,794
Regulated operations [Member]
Operating revenues:
Operating revenues:		$ 206,936	$ 192,079	$ 866,035	$ 783,997
Operating expenses:
Operation and maintenance:		79,293	77,662	235,306	229,364
Nonregulated operations [Member]
Operating revenues:
Operating revenues:		1,065,612	1,016,488	2,412,077	2,328,733
Operating expenses:
Operation and maintenance:		$ 893,616	$ 842,878	$ 2,115,747	$ 2,008,122
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income (loss)	$ 87,351	$ 82,986	$ 165,891	$ (132,988)
Net unrealized gain on derivative instruments qualifying as hedges:
Reclassification adjustment for loss on derivative instruments included in net income (loss), net of tax of $56 and $56 for the three months ended and $168 and $170 for the nine months ended in 2017 and 2016, respectively	92	92	275	275
Postretirement liability adjustment:
Amortization of postretirement liability (gains) losses included in net periodic benefit cost (credit), net of tax of $203 and $143 for the three months ended and $609 and $(676) for the nine months ended in 2017 and 2016, respectively	333	236	1,002	(1,111)
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $0 for the three months ended and $(725) and $0 for the nine months ended in 2017 and 2016, respectively	0	0	(917)	0
Postretirement liability adjustment	333	236	85	(1,111)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $9 and $(2) for the three months ended and $5 and $32 for the nine months ended in 2017 and 2016, respectively	15	(4)	9	52
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 $(10) and $(23) for the three months ended and $(38) and $(35) for the nine months ended in 2017 and 2016, respectively	(19)	(42)	(70)	(65)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $14 and $18 for the three months ended and $50 and $57 for the nine months ended in 2017 and 2016, respectively	27	33	93	106
Net unrealized gain (loss) on available-for-sale investments	8	(9)	23	41
Other comprehensive income (loss)	448	315	392	(743)
Comprehensive income (loss)	87,799	83,301	166,283	(133,731)
Loss from discontinued operations attributable to noncontrolling interest	0	0	0	(131,691)
Comprehensive income (loss) attributable to common stockholders	$ 87,799	$ 83,301	$ 166,283	$ (2,040)

Consolidated Statements of Com4

Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 56	$ 56	$ 168	$ 170
Amortization of postretirement liability (gains) losses included in net periodic benefit cost, tax	203	143	609	(676)
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	(725)	0
Foreign currency translation adjustments recognized during the period, tax	9	(2)	5	32
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(10)	(23)	(38)	(35)
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income (loss), tax	$ 14	$ 18	$ 50	$ 57

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Current assets:
Cash and cash equivalents	$ 37,356	$ 46,107	$ 59,868
Receivables, net	739,402	630,243	665,142
Inventories	232,555	238,273	245,790
Prepayments and other current assets	89,625	48,461	49,082
Current assets held for sale	304	14,391	45,867
Total current assets	1,099,242	977,475	1,065,749
Investments	133,895	125,866	126,048
Property, plant and equipment	6,658,891	6,510,229	6,588,445
Less accumulated depreciation, depletion and amortization	2,667,762	2,578,902	2,583,566
Net property, plant and equipment	3,991,129	3,931,327	4,004,879
Deferred charges and other assets:
Goodwill	631,791	631,791	641,527	[1]
Other intangible assets, net	4,209	5,925	6,529
Other	419,846	415,419	360,537
Noncurrent assets held for sale	64,333	196,664	112,440
Total deferred charges and other assets	1,120,179	1,249,799	1,121,033
Total assets	6,344,445	6,284,467	6,317,709
Current liabilities:
Long-term debt due within one year	148,499	43,598	93,598
Accounts payable	304,101	279,962	281,373
Taxes payable	108,946	48,164	59,747
Dividends payable	37,596	37,767	36,791
Accrued compensation	67,097	65,867	58,604
Other accrued liabilities	184,580	184,377	191,904
Current liabilities held for sale	5,749	9,924	18,065
Total current liabilities	856,568	669,659	740,082
Long-term debt	1,592,053	1,746,561	1,808,350
Deferred credits and other liabilities:
Deferred income taxes	652,413	668,226	662,326
Other	889,494	883,777	821,890
Total deferred credits and other liabilities	1,541,907	1,552,003	1,484,216
Commitments and contingencies
Stockholders' equity:
Preferred stocks	0	15,000	15,000
Common stockholders' equity:
Authorized - 500,000,000 shares, $1.00 par value Shares issued - 195,843,297 at September 30, 2017 and 2016 and December 31, 2016	195,843	195,843	195,843
Other paid-in capital	1,232,766	1,232,478	1,231,396
Retained earnings	964,275	912,282	884,339
Accumulated other comprehensive loss	(35,341)	(35,733)	(37,891)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)	(3,626)
Total common stockholders' equity	2,353,917	2,301,244	2,270,061
Total stockholders' equity	2,353,917	2,316,244	2,285,061
Total liabilities and stockholders' equity	$ 6,344,445	$ 6,284,467	$ 6,317,709
[1]	Balance is presented net of accumulated impairment of $12.3 million

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Stockholders' equity:
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1	$ 1
Common stock, shares issued	195,843,297	195,843,297	195,843,297
Treasury shares	538,921	538,921	538,921

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Operating activities:
Net income (loss)		$ 165,891	$ (132,988)
Loss from discontinued operations, net of tax	[1]	(3,702)	(299,538)
Income from continuing operations		169,593	166,550
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization		155,138	163,226
Deferred income taxes		(16,777)	(1,346)
Changes in current assets and liabilities, net of acquisitions:
Receivables		(121,128)	(75,308)
Inventories		2,047	(4,153)
Other current assets		(40,655)	(18,824)
Accounts payable		30,097	15,514
Other current liabilities		66,647	48,973
Other noncurrent changes		(15,081)	(25,284)
Net cash provided by continuing operations		229,881	269,348
Net cash provided by discontinued operations		42,020	7,127
Net cash provided by operating activities		271,901	276,475
Investing activities:
Capital expenditures		(222,084)	(303,873)
Net proceeds from sale or disposition of property and other		121,162	17,583
Investments		(260)	56
Net cash used in continuing operations		(101,182)	(286,234)
Net cash provided by discontinued operations		2,234	31,918
Net cash used in investing activities		(98,948)	(254,316)
Financing activities:
Issuance of long-term debt		133,437	341,777
Repayment of long-term debt		(183,968)	(236,433)
Dividends paid		(113,131)	(110,366)
Redemption of preferred stock		(15,600)	0
Repurchase of common stock		(1,684)	0
Tax withholding on stock-based compensation		(757)	(323)
Net cash used in continuing operations		(181,703)	(5,345)
Net cash used in discontinued operations		0	(40,852)
Net cash used in financing activities		(181,703)	(46,197)
Effect of exchange rate changes on cash and cash equivalents		(1)	3
Decrease in cash and cash equivalents		(8,751)	(24,035)
Cash and cash equivalents -- beginning of year		46,107	83,903
Cash and cash equivalents -- end of period		$ 37,356	$ 59,868
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Basis of presentation

Basis of presentation	9 Months Ended
	Sep. 30, 2017
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 2016 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, 2017 , up to the date of issuance of these consolidated interim financial statements. The assets and liabilities for the Company's discontinued operations have been classified as held for sale and the results of operations are shown in loss from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. At the time the assets were classified as held for sale, depreciation, depletion and amortization expense was no longer recorded. Unless otherwise indicated, the amounts presented in the accompanying notes to the consolidated financial statements relate to the Company's continuing operations. For more information on the Company's discontinued operations, see Note 8

Seasonality of operations

Seasonality of operations	9 Months Ended
	Sep. 30, 2017
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, 2017
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 $27.2 million , $26.3 million and $29.2 million at September 30, 2017 and 2016 , and December 31, 2016 , 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, 2017 and 2016 , and December 31, 2016 , was $9.0 million , $10.2 million and $10.5 million

Inventories and natural gas in

Inventories and natural gas in storage	9 Months Ended
	Sep. 30, 2017
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 is included in inventories. Inventories consisted of: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Aggregates held for resale $ 116,399 $ 119,078 $ 115,471 Natural gas in storage (current) 29,974 35,625 25,761 Asphalt oil 26,682 23,480 29,103 Materials and supplies 20,778 18,584 18,372 Merchandise for resale 15,346 15,672 16,437 Other 23,376 33,351 33,129 Total $ 232,555 $ 245,790 $ 238,273 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 $ 49.5 million , $ 49.1 million and $ 49.5 million at September 30, 2017 and 2016 , and December 31, 2016

Earnings (loss) per common shar

Earnings (loss) per common share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Earnings (loss) per common share	Earnings (loss) per common share Basic earnings (loss) per common share were computed by dividing earnings (loss) on common stock by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings (loss) per common share were computed by dividing earnings (loss) 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 outstanding performance share awards. Common stock outstanding includes issued shares less shares held in treasury. Net income (loss) was the same for both the basic and diluted earnings (loss) per share calculations. A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings (loss) per share calculations was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) Weighted average common shares outstanding - basic 195,304 195,304 195,304 195,298 Effect of dilutive performance share awards 479 507 618 496 Weighted average common shares outstanding - diluted 195,783 195,811 195,922 195,794 Shares excluded from the calculation of diluted earnings per share — — — —

New accounting standards

New accounting standards	9 Months Ended
	Sep. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New accounting standards	New accounting standards Recently adopted accounting standards Balance Sheet Classification of Deferred Taxes In November 2015, the FASB issued guidance regarding the classification of deferred taxes on the balance sheet. The guidance requires all deferred tax assets and liabilities to be classified as noncurrent. These amendments align GAAP with IFRS. The Company adopted the guidance in the fourth quarter of 2016 and applied the retrospective method of adoption. The guidance required a reclassification of current deferred income taxes to noncurrent deferred income taxes on the Consolidated Balance Sheets, but did not impact the Company's results of operations or cash flows. As a result of the retrospective application of this change in accounting principle, the Company reclassified deferred income taxes of $31.4 million from current assets - deferred income taxes to deferred credits and other liabilities - deferred income taxes on its Consolidated Balance Sheet at September 30, 2016. Simplifying the Measurement of Inventory In July 2015, the FASB issued guidance regarding inventory that is measured using the first-in, first-out or average cost method. The guidance does not apply to inventory measured using the last-in, first-out or the retail inventory method. The guidance requires inventory within its scope to be measured at the lower of cost or net realizable value, which is the estimated selling price in the normal course of business less reasonably predictable costs of completion, disposal and transportation. These amendments more closely align GAAP with IFRS. The Company adopted the guidance on January 1, 2017, 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. Improvements to Employee Share-Based Payment Accounting In March 2016, the FASB issued guidance regarding simplification of several aspects of the accounting for share-based payment transactions. The guidance affects the income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and calculation of dilutive shares. The Company adopted the guidance on January 1, 2017. All amendments in the guidance that apply to the Company were adopted on a prospective basis resulting in no adjustments being made to retained earnings. The adoption of the guidance impacted the Consolidated Statement of Income and the Consolidated Balance Sheet in the first quarter of 2017 due to the taxes related to the stock-based compensation award that vested in February 2017 being recognized as income tax expense as compared to a reduction to additional paid-in capital under the previous guidance. Adoption of the guidance also increased the number of shares included in the diluted earnings per share calculation due to the exclusion of tax benefits in the incremental shares calculation. The change in the weighted average common shares outstanding - diluted did not result in a material effect on the earnings per common share - diluted. Recently issued accounting standards not yet adopted 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 will be effective for the Company on January 1, 2018. Entities will have the option of using either a full retrospective or modified retrospective approach to adopting the guidance. The Company plans to adopt the guidance on January 1, 2018, and to use the modified retrospective approach. Under the modified retrospective approach, an entity would recognize the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings in the period of adoption. To date, the Company has not identified any material cumulative effect adjustments to be made to retained earnings. In addition, the guidance will require expanded disclosures, both quantitative and qualitative, related to the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. To date, the Company has reviewed nearly all of its revenue streams, completing the preliminary evaluation of the impact of this guidance. Based on the preliminary evaluation, the Company does not anticipate a significant change in the timing of revenue recognition, results of operations, financial position or cash flows, however the Company will continue to evaluate the impact of this guidance through the date of adoption. Recognition and Measurement of Financial Assets and Financial Liabilities In January 2016, the FASB issued guidance regarding the classification and measurement of financial instruments. The guidance revises the way an entity classifies and measures investments in equity securities, the presentation of certain fair value changes for financial liabilities measured at fair value and amends certain disclosure requirements related to the fair value of financial instruments. This guidance will be effective for the Company on January 1, 2018, with early adoption of certain amendments permitted. The guidance should be applied using a modified retrospective approach with the exception of equity securities without readily determinable fair values which will be applied prospectively. 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. 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. This guidance will be effective for the Company on January 1, 2018, with early adoption permitted. Entities must apply the guidance retrospectively unless it is impracticable to do so, in which case they may apply it prospectively as of the earliest date practicable. The Company plans to adopt the guidance on January 1, 2018. The Company's initial evaluation of the guidance did not identify any changes to the current presentation of the statement of cash flows; therefore, no retrospective adjustments to prior periods will be necessary. Clarifying the Definition of a Business In January 2017, the FASB issued guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions or disposals of assets or businesses. The guidance provides a screen to determine when an integrated set of assets and activities is not a business. The guidance will also affect other aspects of accounting, such as determining reporting units for goodwill testing and whether an entity has acquired or sold a business. The guidance will be effective for the Company on January 1, 2018, and should be applied on a prospective basis with early adoption permitted for transactions that occur before the issuance or effective date of the amendments and only when the transactions have not been reported in the financial statements or made available for issuance. The Company expects to adopt this guidance as required and does not expect the guidance to have a material effect on its results of operations, financial position, cash flows and disclosures. 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 cost and net periodic postretirement benefit cost. The guidance requires the service cost component to be presented in the income statement in the same line item or items as other compensation costs arising from services performed during the period. Other components of net 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 guidance will be effective for the Company on January 1, 2018, including interim periods, with early adoption permitted as of the beginning of an annual period for which the financial statements have not been issued. The guidance shall be applied on a retrospective basis for the financial statement presentation and on a prospective basis for the capitalization of the service cost component. The Company plans to adopt the guidance as required on January 1, 2018, which will include 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. The impact upon adoption of the new guidance will be an increase to operating income and decrease to other income on the Consolidated Statements of Income and no impact to earnings. The guidance will not have a material impact on the Company's disclosures or cash flows. Leases In February 2016, the FASB issued guidance regarding leases. The guidance requires lessees to recognize a lease liability and a right-of-use asset on the balance sheet for operating and financing leases with terms of more than 12 months. The guidance remains largely the same for lessors, although some changes were made to better align lessor accounting with the new lessee accounting and to align with the revenue recognition standard. The guidance also requires additional disclosures, both quantitative and qualitative, related to operating and finance leases for the lessee and sales-type, direct financing and operating leases for the lessor. This guidance will be effective for the Company on January 1, 2019, and should be applied using a modified retrospective approach with early adoption permitted. The Company continues to evaluate the potential impact the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. The Company is planning to adopt the standard on January 1, 2019, utilizing the practical expedient that allows the Company to not reassess whether an expired or existing contract contains a lease, the classification of leases or initial direct costs. 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

Comprehensive income (loss)

Comprehensive income (loss)	9 Months Ended
	Sep. 30, 2017
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, 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 ) Three Months Ended September 30, 2016 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,484 ) $ (35,604 ) $ (144 ) $ 26 $ (38,206 ) Other comprehensive loss before reclassifications — — (4 ) (42 ) (46 ) Amounts reclassified from accumulated other comprehensive loss 92 236 — 33 361 Net current-period other comprehensive income (loss) 92 236 (4 ) (9 ) 315 Balance at end of period $ (2,392 ) $ (35,368 ) $ (148 ) $ 17 $ (37,891 ) 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 a 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 ) Nine Months Ended September 30, 2016 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,667 ) $ (34,257 ) $ (200 ) $ (24 ) $ (37,148 ) Other comprehensive income (loss) before reclassifications — — 52 (65 ) (13 ) Amounts reclassified from accumulated other comprehensive loss 275 (1,111 ) — 106 (730 ) Net current-period other comprehensive income (loss) 275 (1,111 ) 52 41 (743 ) Balance at end of period $ (2,392 ) $ (35,368 ) $ (148 ) $ 17 $ (37,891 ) Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Nine Months Ended Location on Consolidated Statements of Income September 30, September 30, 2017 2016 2017 2016 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income (loss) $ (148 ) $ (148 ) $ (443 ) $ (445 ) Interest expense 56 56 168 170 Income taxes (92 ) (92 ) (275 ) (275 ) Amortization of postretirement liability gains (losses) included in net periodic benefit cost (credit) (536 ) (379 ) (1,611 ) 1,787 (a) 203 143 609 (676 ) Income taxes (333 ) (236 ) (1,002 ) 1,111 Reclassification adjustment for loss on available-for-sale investments included in net income (loss) (41 ) (51 ) (143 ) (163 ) Other income 14 18 50 57 Income taxes (27 ) (33 ) (93 ) (106 ) Total reclassifications $ (452 ) $ (361 ) $ (1,370 ) $ 730 (a) Included in net periodic benefit cost (credit). For more information, see Note 14 .

Assets held for sale and discon

Assets held for sale and discontinued operations	9 Months Ended
	Sep. 30, 2017
Discontinued Operations and Disposal Groups [Abstract]
Assets held for sale and discontinued operations	Assets held for sale and discontinued operations Assets held for sale The assets and liabilities of Pronghorn were classified as held for sale in the fourth quarter of 2016. Pronghorn's results of operations for 2016 were included in the pipeline and midstream segment. Pronghorn On November 21, 2016, WBI Energy Midstream announced it had entered into a purchase and sale agreement to sell its 50 percent non-operating ownership interest in Pronghorn to Tesoro Logistics. The transaction closed on January 1, 2017, which generated approximately $100 million of proceeds for the Company. The sale of Pronghorn further reduces the Company's risk exposure to commodity prices. The carrying amounts of the major classes of assets and liabilities that were classified as held for sale associated with Pronghorn on the Company's Consolidated Balance Sheets were as follows: December 31, 2016 (In thousands) Assets Current assets: Prepayments and other current assets $ 68 Total current assets held for sale 68 Noncurrent assets: Net property, plant and equipment 93,424 Goodwill 9,737 Less allowance for impairment of assets held for sale 2,311 Total noncurrent assets held for sale 100,850 Total assets held for sale $ 100,918 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 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 reduces the Company’s risk by decreasing exposure to commodity prices. In connection with the sale, WBI Energy had cash in an escrow account for RINs obligations, which was included in current assets held for sale on the Consolidated Balance Sheet at September 30, 2016 . The Company retained certain liabilities of Dakota Prairie Refining which were reflected in current liabilities held for sale on the Consolidated Balance Sheets. In October 2016, the RINs liability was paid and the cash was removed from escrow. Also, Centennial continues to guarantee certain debt obligations of Dakota Prairie Refining; however, Tesoro has agreed to indemnify Centennial for any losses and litigation expenses arising from the guarantee. For more information related to the guarantee, see Note 16 . The carrying amounts of the major classes of assets and liabilities that are classified as held for sale related to the operations of and activity associated with Dakota Prairie Refining on the Company's Consolidated Balance Sheets were as follows: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Assets Current assets: Receivables, net $ — $ 13 $ — Income taxes receivable 8,444 (a) 32,388 13,987 Prepayments and other current assets — 7,741 — Total current assets held for sale 8,444 40,142 13,987 Noncurrent assets: Deferred income taxes — 2,984 — Total noncurrent assets held for sale — 2,984 — Total assets held for sale $ 8,444 $ 43,126 $ 13,987 Liabilities Current liabilities: Accounts payable $ — $ 7,063 $ 7,425 Other accrued liabilities — 7,743 — Total current liabilities held for sale — 14,806 7,425 Noncurrent liabilities: Deferred income taxes (b) 55 — 14 Total noncurrent liabilities held for sale 55 — 14 Total liabilities held for sale $ 55 $ 14,806 $ 7,439 (a) On the Company's Consolidated Balance Sheets, this amount was reclassified to income taxes payable and is reflected in current liabilities held for sale. (b) On the Company's Consolidated Balance Sheets, these amounts were reclassified to noncurrent deferred income tax assets and are reflected in noncurrent assets held for sale. In the first quarter of 2017, the Company recorded a reversal of a previously accrued liability of $7.0 million ( $4.3 million after tax) due to the resolution of a legal matter. At September 30, 2017 , Dakota Prairie Refining had not incurred any material exit and disposal costs, and does not expect to incur any material exit and disposal costs. The Company performed a fair value assessment of the assets and liabilities classified as held for sale. In the second quarter of 2016, the fair value assessment was determined using the market approach based on the sale transaction to Tesoro. The fair value assessment indicated an impairment based on the carrying value exceeding the fair value, which resulted in the Company recording an impairment of $251.9 million ( $156.7 million after tax) in the quarter ended June 30, 2016. The impairment was included in operating expenses from discontinued operations. The fair value of Dakota Prairie Refining’s assets has been categorized as Level 3 in the fair value hierarchy. 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 all of Fidelity's oil and natural gas assets. The completion of these sales occurred between October 2015 and April 2016. The sale of Fidelity was part of the Company's strategic plan to grow its capital investments in the remaining business segments and to focus on creating a greater long-term value. The carrying amounts of the major classes of assets and liabilities that are classified as held for sale related to the operations of Fidelity on the Company's Consolidated Balance Sheets were as follows: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Assets Current assets: Receivables, net $ 304 $ 7,930 $ 355 Total current assets held for sale 304 7,930 355 Noncurrent assets: Net property, plant and equipment 2,064 5,507 5,507 Deferred income taxes 62,163 104,726 91,098 Other 161 161 161 Less allowance for impairment of assets held for sale — 938 938 Total noncurrent assets held for sale 64,388 109,456 95,828 Total assets held for sale $ 64,692 $ 117,386 $ 96,183 Liabilities Current liabilities: Accounts payable $ 68 $ 175 $ 141 Taxes payable 11,745 2,205 (a) 19 (a) Other accrued liabilities 2,380 3,084 2,358 Total current liabilities held for sale 14,193 5,464 2,518 Total liabilities held for sale $ 14,193 $ 5,464 $ 2,518 (a) On the Company's Consolidated Balance Sheets, these amounts were reclassified to prepayments and other current assets and are reflected in current assets held for sale. The Company reclassified current income tax assets of $47.5 million and current income tax liabilities of $4.1 million to noncurrent assets - deferred income taxes at September 30, 2016, pursuant to the retrospective application of the adoption of the ASU related to the balance sheet classification of deferred taxes. For more information on this ASU, see Note 6 . The Company performed a fair value assessment of the assets and liabilities classified as held for sale. In the second quarter of 2016, the fair value assessment was determined using the income and market approaches. The income approach was determined by using the present value of future estimated cash flows. The market approach was based on market transactions of similar properties. The estimated carrying value exceeded the fair value and the Company recorded an impairment of $900,000 ( $600,000 after tax) in the second quarter of 2016. In the first quarter of 2016, the fair value assessment was determined using the market approach largely based on a purchase and sale agreement. The estimated fair value exceeded the carrying value and the Company recorded an impairment reversal of $1.4 million ( $900,000 after tax) in the first quarter of 2016. The impairment and impairment reversal were included in operating expenses from discontinued operations. The estimated fair value of Fidelity's assets have been categorized as Level 3 in the fair value hierarchy. The Company incurred transaction costs of approximately $300,000 in the first quarter of 2016. In addition to the transaction costs, and due in part to the change in plans to sell the assets of Fidelity rather than sell Fidelity as a company, Fidelity incurred and expensed approximately $5.6 million of exit and disposal costs for the nine months ended September 30, 2016 , and has incurred $10.5 million of exit and disposal costs to date. Fidelity incurred no exit and disposal costs for the three and nine months ended September 30, 2017 , and the Company does not expect to incur any additional material exit and disposal costs. The exit and disposal costs are associated with severance and other related matters and exclude the office lease expiration discussed in the following paragraph. Fidelity vacated its office space in Denver, Colorado in 2016. The Company incurred lease payments of approximately $900,000 in 2016. A lease termination payment of $3.2 million was made during the second quarter of 2016. Existing office furniture and fixtures were relinquished to the lessor in the second quarter of 2016. 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 loss from discontinued operations on the Company's Consolidated Statements of Income was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) Operating revenues $ 121 $ 162 $ 356 $ 122,894 Operating expenses 384 230 (4,988 ) 513,756 Operating income (loss) (263 ) (68 ) 5,344 (390,862 ) Other income (expense) — 375 (13 ) 762 Interest expense — — 239 1,753 Income (loss) from discontinued operations before income taxes (263 ) 307 5,092 (391,853 ) Income taxes 1,935 5,707 8,794 (92,315 ) Loss from discontinued operations (2,198 ) (5,400 ) (3,702 ) (299,538 ) Loss from discontinued operations attributable to noncontrolling interest — — — (131,691 ) Loss from discontinued operations attributable to the Company $ (2,198 ) $ (5,400 ) $ (3,702 ) $ (167,847 ) The pretax income (loss) from discontinued operations attributable to the Company, related to the operations of and activity associated with Dakota Prairie Refining, were $0 and $935,000 for the three months ended and $6.9 million and $(253.0) million for the nine months ended September 30, 2017 and 2016

Goodwill and other intangible a

Goodwill and other intangible assets	9 Months Ended
	Sep. 30, 2017
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, 2017 Balance at January 1, 2017 Goodwill Acquired 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 Nine Months Ended September 30, 2016 Balance at January 1, 2016 * Goodwill Acquired During the Year Balance at September 30, 2016 * (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Pipeline and midstream 9,737 — 9,737 Construction materials and contracting 176,290 — 176,290 Construction services 103,441 6,323 109,764 Total $ 635,204 $ 6,323 $ 641,527 * Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods. Year Ended December 31, 2016 Balance at January 1, 2016 * Goodwill Acquired During the Year Held for Sale Balance at December 31, 2016 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Pipeline and midstream 9,737 — (9,737 ) — Construction materials and contracting 176,290 — — 176,290 Construction services 103,441 6,324 — 109,765 Total $ 635,204 $ 6,324 $ (9,737 ) $ 631,791 * Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods. Other amortizable intangible assets were as follows: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Customer relationships $ 15,248 $ 17,145 $ 17,145 Less accumulated amortization 13,176 13,524 13,917 2,072 3,621 3,228 Noncompete agreements 2,430 2,430 2,430 Less accumulated amortization 1,769 1,622 1,658 661 808 772 Other 7,020 7,764 7,768 Less accumulated amortization 5,544 5,664 5,843 1,476 2,100 1,925 Total $ 4,209 $ 6,529 $ 5,925 Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2017 , was $500,000 and $1.7 million , respectively. Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2016 , was $600,000 and $1.9 million , respectively. Estimated amortization expense for amortizable intangible assets is $2.2 million in 2017 , $1.2 million in 2018 , $1.0 million in 2019 , $500,000 in 2020 , $200,000 in 2021 and $800,000

Fair value measurements

Fair value measurements	9 Months Ended
	Sep. 30, 2017
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 $75.0 million , $72.8 million and $70.9 million , at September 30, 2017 and 2016 , and December 31, 2016 , respectively, are classified as investments on the Consolidated Balance Sheets. 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 net unrealized gains on these investments were $1.4 million and $5.3 million for the three and nine months ended September 30, 2016 , respectively. The change in fair value, which is considered part of the cost of the plan, is classified in operation and maintenance expense on the Consolidated Statements of Income. The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities, which include mortgage-backed securities and U.S. Treasury securities. These available-for-sale securities are recorded at fair value and are classified as investments on the Consolidated Balance Sheets. Unrealized gains or losses are recorded in accumulated other comprehensive income (loss). Details of available-for-sale securities were as follows: September 30, 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 September 30, 2016 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,882 $ 43 $ (17 ) $ 9,908 Total $ 9,882 $ 43 $ (17 ) $ 9,908 December 31, 2016 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,546 $ 8 $ (105 ) $ 10,449 Total $ 10,546 $ 8 $ (105 ) $ 10,449 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 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, 2017 and 2016 , 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, 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 September 30, 2016, 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, 2016 (In thousands) Assets: Money market funds $ — $ 2,284 $ — $ 2,284 Insurance contract* — 72,818 — 72,818 Available-for-sale securities: Mortgage-backed securities — 9,908 — 9,908 Total assets measured at fair value $ — $ 85,010 $ — $ 85,010 * The insurance contract invests approximately 65 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at December 31, 2016, 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, 2016 (In thousands) Assets: Money market funds $ — $ 1,602 $ — $ 1,602 Insurance contract* — 70,921 — 70,921 Available-for-sale securities: Mortgage-backed securities — 10,449 — 10,449 Total assets measured at fair value $ — $ 82,972 $ — $ 82,972 * The insurance contract invests approximately 52 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 1 percent in target date investments and 2 percent in cash equivalents. For information about fair value assessments of assets and liabilities classified as held for sale, see Note 8 . 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, 2017 $ 1,740,552 $ 1,846,811 Long-term debt at September 30, 2016 $ 1,901,948 $ 2,047,339 Long-term debt at December 31, 2016 $ 1,790,159 $ 1,841,885 The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values

Equity

Equity	9 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Equity	Equity A summary of the changes in equity was as follows: Nine Months Ended September 30, 2017 Total Equity (In thousands) Balance at December 31, 2016 $ 2,316,244 Net income 165,891 Other comprehensive income 392 Dividends declared on preferred stocks (171 ) Dividends declared on common stock (112,788 ) Stock-based compensation 2,390 Repurchase of common stock (1,684 ) Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (757 ) Redemption of preferred stock (15,600 ) Balance at September 30, 2017 $ 2,353,917 Effective April 1, 2017, all outstanding preferred stock, including $300,000 of redeemable preferred stock classified as long-term debt, was redeemed for a repurchase price of approximately $15.9 million . Nine Months Ended September 30, 2016 Total Stockholders' Equity Noncontrolling Interest Total Equity (In thousands) Balance at December 31, 2015 $ 2,396,505 $ 124,043 $ 2,520,548 Net loss (1,297 ) (131,691 ) (132,988 ) Other comprehensive loss (743 ) — (743 ) Dividends declared on preferred stocks (514 ) — (514 ) Dividends declared on common stock (109,858 ) — (109,858 ) Stock-based compensation 2,955 — 2,955 Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (323 ) — (323 ) Net tax deficit on stock-based compensation (1,664 ) — (1,664 ) Contribution from noncontrolling interest — 7,648 7,648 Balance at September 30, 2016 $ 2,285,061 $ — $ 2,285,061

Cash flow information

Cash flow information	9 Months Ended
	Sep. 30, 2017
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, 2017 2016 (In thousands) Interest, net of amount capitalized and AFUDC - borrowed of $676 and $842 in 2017 and 2016, respectively $ 58,119 $ 66,281 Income taxes paid, net* $ 46,430 $ 73,771 * Income taxes paid (refunded), net of discontinued operations, were $ 1.4 million and $ (144,000) for the nine months ended September 30, 2017 and 2016, respectively. Noncash investing transactions were as follows: September 30, 2017 2016 (In thousands) Property, plant and equipment additions in accounts payable $ 16,914 $ 22,560

Business segment data

Business segment data	9 Months Ended
	Sep. 30, 2017
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. For information on the Company's natural gas and oil gathering and processing facility sold on January 1, 2017, see Note 8 . The construction materials and contracting segment mines aggregates and markets crushed stone, sand, gravel and related construction materials, including ready-mixed concrete, cement, asphalt, liquid asphalt and other value-added products. It also performs integrated contracting services. This segment operates in the central, southern and western United States and Alaska and Hawaii. The construction services segment provides construction services specializing in constructing and maintaining electric and communication lines, gas pipelines, fire suppression systems, and external lighting and traffic signalization. This segment also provides utility excavation and inside electrical and mechanical services, and manufactures and distributes transmission line construction equipment and other supplies. 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 deductible layers of the insured companies' 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. Dakota Prairie Refining refined crude oil and produced and sold diesel fuel, naphtha, ATBs and other by-products of the production process. In the second quarter of 2016, the Company sold all of the outstanding membership interests in Dakota Prairie Refining. Fidelity engaged in oil and natural gas development and production activities in the Rocky Mountain and Mid-Continent/Gulf States regions of the United States. Between September 2015 and March 2016, the Company entered into purchase and sale agreements to sell all of Fidelity's oil and natural gas assets. The completion of these sales occurred between October 2015 and April 2016. For more information on discontinued operations, see Note 8 . The information below follows the same accounting policies as described in Note 1 of the Company's Notes to Consolidated Financial Statements in the 2016 Annual Report. Information on the Company's businesses was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) External operating revenues: Regulated operations: Electric $ 91,531 $ 82,156 $ 254,330 $ 238,911 Natural gas distribution 92,253 87,941 566,364 500,106 Pipeline and midstream 23,152 21,982 45,341 44,980 206,936 192,079 866,035 783,997 Nonregulated operations: Pipeline and midstream 5,356 10,732 13,518 29,697 Construction materials and contracting 686,010 724,535 1,388,212 1,475,643 Construction services 374,111 280,801 1,009,693 822,226 Other 135 420 654 1,167 1,065,612 1,016,488 2,412,077 2,328,733 Total external operating revenues $ 1,272,548 $ 1,208,567 $ 3,278,112 $ 3,112,730 Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — $ — Natural gas distribution — — — — Pipeline and midstream 3,081 3,278 30,923 30,969 3,081 3,278 30,923 30,969 Nonregulated operations: Pipeline and midstream 38 41 132 161 Construction materials and contracting 142 155 400 370 Construction services 415 3 715 541 Other 1,910 2,204 5,411 5,542 2,505 2,403 6,658 6,614 Intersegment eliminations (5,586 ) (5,681 ) (37,581 ) (37,583 ) Total intersegment operating revenues $ — $ — $ — $ — Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) Earnings (loss) on common stock: Regulated operations: Electric $ 15,712 $ 12,699 $ 37,904 $ 31,840 Natural gas distribution (10,883 ) (12,524 ) 14,181 4,940 Pipeline and midstream 5,853 5,389 15,901 16,241 10,682 5,564 67,986 53,021 Nonregulated operations: Pipeline and midstream 95 1,304 (770 ) 2,043 Construction materials and contracting 63,221 69,523 64,477 88,747 Construction services 13,144 7,234 32,896 20,198 Other 552 (1,009 ) (1,888 ) (3,572 ) 77,012 77,052 94,715 107,416 Intersegment eliminations* 1,855 5,599 6,121 5,599 Earnings on common stock before loss from discontinued operations 89,549 88,215 168,822 166,036 Loss from discontinued operations, net of tax* (2,198 ) (5,400 ) (3,702 ) (299,538 ) Loss from discontinued operations attributable to noncontrolling interest — — — (131,691 ) Total earnings (loss) on common stock $ 87,351 $ 82,815 $ 165,120 $ (1,811 ) * Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans

Employee benefit plans	9 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
Pension and other postretirement benefit plans	Employee benefit plans Pension and other postretirement plans The Company has noncontributory 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, 2017 2016 2017 2016 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 377 $ 412 Interest cost 4,052 4,305 816 922 Expected return on assets (5,132 ) (5,231 ) (1,160 ) (1,133 ) Amortization of prior service credit — — (343 ) (343 ) Amortization of net actuarial loss 1,589 1,553 213 371 Net periodic benefit cost (credit), including amount capitalized 509 627 (97 ) 229 Less amount capitalized 65 82 (95 ) (34 ) Net periodic benefit cost (credit) $ 444 $ 545 $ (2 ) $ 263 Pension Benefits Other Postretirement Benefits Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 1,130 $ 1,236 Interest cost 12,155 12,915 2,449 2,766 Expected return on assets (15,395 ) (15,693 ) (3,480 ) (3,400 ) Amortization of prior service credit — — (1,029 ) (1,029 ) Amortization of net actuarial loss 4,767 4,660 649 1,118 Net periodic benefit cost (credit), including amount capitalized 1,527 1,882 (281 ) 691 Less amount capitalized 245 284 (248 ) 4 Net periodic benefit cost (credit) $ 1,282 $ 1,598 $ (33 ) $ 687 Nonqualified benefit plans In addition to the qualified plan defined pension benefits reflected in the table, the Company also has unfunded, nonqualified 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, 2017 , was $1.2 million and $3.5 million , respectively. The Company's net periodic benefit cost for these plans for the three and nine months ended September 30, 2016 , was $1.3 million and $600,000 , respectively, which reflects a curtailment gain of $3.3 million

Regulatory matters

Regulatory matters	9 Months Ended
	Sep. 30, 2017
Regulated Operations [Abstract]
Regulatory matters	Regulatory matters On August 12, 2016, Intermountain filed an application with the IPUC for a natural gas rate increase of approximately $10.2 million annually or approximately 4.1 percent above current rates. The request included rate recovery associated with increased investment in facilities and increased operating expenses. On January 17, 2017, Intermountain provided the IPUC with an updated revenue request of approximately $9.4 million . On April 28, 2017, the IPUC issued an order approving an increase of approximately $4.1 million or approximately 1.6 percent above current rates based on a 9.5 percent return on equity effective with service rendered on and after May 1, 2017. On May 18, 2017, Intermountain filed a petition for reconsideration with the IPUC requesting the reconsideration of certain items denied in the order dated April 28, 2017. On June 15, 2017, the IPUC granted the request for reconsideration. On August 17, 2017, Intermountain, the IPUC staff and the interveners of the case filed a stipulation and settlement resolving all issues. The stipulation and settlement reflected an increase of approximately $1.2 million or 1.36 percent more in annual revenue than the amounts approved on April 28, 2017, as well as changes in billing determinants. The total annual increase in revenue of approximately $6.7 million was approved by the IPUC on September 14, 2017, with rates effective October 1, 2017. On December 21, 2016, Great Plains filed an application with the MNPUC requesting authority to implement a natural gas utility infrastructure cost tariff of approximately $456,000 annually. The tariff will allow Great Plains to recover infrastructure investments, not previously included in rates, mandated by federal or state agencies associated with Great Plains' pipeline integrity programs. On October 6, 2017, the MNPUC approved the implementation of the natural gas utility infrastructure cost tariff to collect an annual increase of approximately $456,000 . Great Plains submitted a compliance filing on October 10, 2017, requesting the order to be effective with service rendered on and after November 1, 2017. On May 31, 2017, Cascade filed an application with the WUTC for an annual pipeline replacement cost recovery mechanism of approximately $1.6 million or approximately .75 percent of additional revenue. The requested increase includes incremental pipeline replacement investments associated with qualifying pipeline integrity projects. On October 12, 2017, Cascade filed a required update revising the request to approximately $1.3 million or approximately .61 percent of additional revenue and on October 26, 2017, the WUTC approved the order with rates effective November 1, 2017. On June 30, 2017, Montana-Dakota filed an application for advance determination of prudence and a certificate of public convenience and necessity with the NDPSC to purchase an expansion of the Thunder Spirit Wind farm. The advance determination of prudence would provide Montana-Dakota with assurance that the project is prudent and in the best interest of the public and assists in the recovery of Montana-Dakota's investment upon completion of the project. The expansion is expected to serve customers by the end of 2018 and is estimated to cost approximately $85 million . An informal hearing is scheduled for November 3, 2017. On July 21, 2017, Montana-Dakota filed an application with the NDPSC for a natural gas rate increase of approximately $5.9 million annually or approximately 5.4 percent above current rates. The requested increase is primarily to recover the increased investment in distribution facilities to enhance system safety and reliability and the depreciation and taxes associated with the increase in investment. Montana-Dakota is also introducing an SSIP and the proposed adjustment mechanism required to fund the SSIP. Montana-Dakota requested an interim increase of approximately $4.6 million or approximately 4.2 percent , subject to refund. On September 6, 2017, the NDPSC approved the request for interim rates effective with service rendered on or after September 19, 2017. This matter is pending before the NDPSC. 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. This matter is pending before the WUTC. On September 1, 2017, Montana-Dakota submitted an update to its transmission formula rate under the MISO tariff, which reflects an incremental increase of approximately $2.5 million to include a revenue requirement for the Company's multivalue project, for a total of $13.6 million effective January 1, 2018. On September 25, 2017, Montana-Dakota filed an application with the MTPSC for a natural gas rate increase of approximately $2.8 million annually or approximately 4.1 percent above current rates. The requested increase is primarily to recover the increased investment in distribution facilities to enhance system safety and reliability and the depreciation and taxes associated with the increase in investment. Montana-Dakota is also introducing an SSIP and the proposed adjustment mechanism required to fund the SSIP. Montana-Dakota requested an interim increase of approximately $1.6 million or approximately 2.3 percent , subject to refund. This matter is pending before the MTPSC. On September 29, 2017, Cascade filed an application with the OPUC for an annual pipeline replacement safety cost recovery mechanism of approximately $784,000 or approximately 1.2 percent of additional revenue. The requested increase includes incremental pipeline replacement investments associated with qualifying pipeline integrity projects. If approved, rates will be effective January 1, 2018. This matter is pending before the OPUC. Montana-Dakota previously filed an application with the NDPSC on October 14, 2016, for an electric rate increase which also included a requested return on equity to be used in the determination of applications previously filed by Montana-Dakota for a renewable resource cost adjustment rider, an electric generation resource recovery rider, and a transmission cost adjustment rider, as discussed in the following paragraphs. On April 7, 2017, Montana-Dakota, the NDPSC Advocacy Staff and the interveners in the case filed a settlement agreement resolving all issues in the general rate case. The settlement agreement included a net increase of approximately $7.5 million or 3.7 percent above previously approved final rates and a true-up of the return on equity used in the interim renewable resource cost adjustment, the electric generation resource recovery and transmission cost adjustment riders of 9.45 percent; a return on equity of 9.65 percent for base rates and the renewable resource cost adjustment rider on a go-forward basis; and a return on equity of 9.45 percent through December 31, 2019, for the natural gas-fired internal combustion engines and associated facilities included in the electric generation resource recovery rider. A hearing on the settlement agreement was held on April 10, 2017. On June 16, 2017, the NDPSC approved the settlement agreement. On June 26, 2017, Montana-Dakota submitted a compliance filing and on July 14, 2017, submitted updated tariff sheets and a refund plan. The NDPSC approved the compliance filing and refund plan on July 26, 2017, with final rates effective with service rendered on or after August 7, 2017. The final rates are less than the interim rates currently in effect. Therefore, Montana-Dakota will refund the difference to customers, which is approximately 19 percent of the amount collected from the general rate case interim increase, along with refunds to reflect true-ups for the various riders, as applicable. The background information related to the settlement agreement and related applications are discussed in the following paragraphs. On October 26, 2015, Montana-Dakota filed an application with the NDPSC requesting a renewable resource cost adjustment rider for the recovery of the Thunder Spirit Wind project. On January 5, 2016, the NDPSC approved the rider to be effective January 7, 2016, resulting in an annual increase on an interim basis, subject to refund, of $15.1 million based upon a 10.5 percent return on equity to be finalized upon approval of the electric rate case filed on October 14, 2016. The electric rate case settlement agreement filed on April 7, 2017, included a revised return on equity for the rider. The settlement agreement was approved on June 16, 2017, as previously discussed in this note. On October 26, 2015, Montana-Dakota filed an application with the NDPSC for an update to the electric generation resource recovery rider. On March 9, 2016, the NDPSC approved the rider to be effective with service rendered on and after March 15, 2016, which resulted in interim rates, subject to refund, of $9.7 million based upon a 10.5 percent return on equity to be finalized upon the approval of the electric rate case filed on October 14, 2016. The interim rates include recovery of Montana-Dakota's investment in the 88-MW simple-cycle natural gas turbine and associated facilities near Mandan, North Dakota, and the 19 MW of new generation from natural gas-fired internal combustion engines and associated facilities near Sidney, Montana. The electric rate case settlement agreement filed on April 7, 2017, included the net investment authorized for the natural gas-fired internal combustion engines and the return on equity on both investments. The settlement agreement was approved on June 16, 2017, as previously discussed in this note. On November 25, 2015, Montana-Dakota filed an application with the NDPSC for an update of its transmission cost adjustment rider for recovery of MISO-related charges and two transmission projects in North Dakota. On February 10, 2016, the NDPSC approved the transmission cost adjustment effective with service rendered on and after February 12, 2016, resulting in an annual increase on an interim basis, subject to refund, of $6.8 million based upon a 10.5 percent return on equity to be finalized upon approval of the electric rate case filed on October 14, 2016. The electric rate case settlement agreement filed on April 7, 2017, included a revised return on equity for the rider. The settlement agreement was approved on June 16, 2017, as previously discussed in this note. On October 14, 2016, Montana-Dakota filed an application with the NDPSC for an electric rate increase of approximately $13.4 million annually or 6.6 percent above current rates. The request includes rate recovery associated with increased investment in facilities, along with the related depreciation, operation and maintenance expenses and taxes associated with the increased investment. Montana-Dakota requested an interim increase of approximately $13.0 million or approximately 6.5 percent, subject to refund, to be effective within 60 days of the filing. On November 21, 2016, Montana-Dakota filed and on November 30, 2016, the NDPSC approved a revised interim increase of approximately $11.7 million , based on adjustments accepted by the NDPSC, or approximately 5.8

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2017
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 $34.3 million , $20.0 million and $31.8 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, 2017 and 2016 , and December 31, 2016 , respectively. This includes amounts that may have been accrued for matters discussed in Litigation and 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 can not 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. Litigation Construction Services Capital Electric provided employees in 2012 to perform work for a contractor on a project in Kansas. One of the Capital Electric employees was injured while working on the project and brought a lawsuit against the contractor. Judgment was entered in favor of the employee and his spouse on November 3, 2016, in the amount of $44.8 million following a court determination that the employee’s injuries were caused by the contractor’s negligence. The contractor claims that Capital Electric was contractually required, but failed, to name the contractor as an additional insured under any liability policy in effect at the time of the project and that such failure resulted in the entry of judgment against the contractor. In March 2017, Capital Electric filed a petition for declaratory judgment in the District Court of Wyandotte County, Kansas for a judicial determination that any agreement between Capital Electric and the contractor for the project did not require Capital Electric to include the contractor as an additional insured under any liability policy issued to Capital Electric and that if such an agreement was found to exist, it would be void and unenforceable under Kansas law. The matter is pending before the District Court of Wyandotte County, Kansas and no accrual has been recorded for it. 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 a ROD with its selected remedy for cleanup of the in-river portion of the site. Implementation of the remedy is expected to take up to 13 years with a present value cost estimate of approximately $1 billion . Corrective action will not be taken until remedial design/remedial action plans are approved by the EPA. Knife River - Northwest also received notice in January 2008 that the Portland Harbor Natural Resource Trustee Council intends to perform an injury assessment to natural resources resulting from the release of hazardous substances at the Harbor Superfund Site. The Portland Harbor Natural Resource Trustee Council indicates the injury determination is appropriate to facilitate early settlement of damages and restoration for natural resource injuries. It is not possible to estimate the costs of natural resource damages until an assessment is completed and allocations are undertaken. Based upon a review of the Portland Harbor sediment contamination evaluation by the Oregon DEQ and other information available, Knife River - Northwest does not believe it is a responsible party. In addition, Knife River - Northwest has notified Georgia-Pacific West, Inc., that it intends to seek indemnity for liabilities incurred in relation to the above matters pursuant to the terms of their sale agreement. Knife River - Northwest has entered into an agreement tolling the statute of limitations in connection with the LWG's potential claim for contribution to the costs of the remedial investigation and feasibility study. By letter in March 2009, LWG stated its intent to file suit against Knife River - Northwest and others to recover LWG's investigation costs to the extent Knife River - Northwest cannot demonstrate its non-liability for the contamination or is unwilling to participate in an alternative dispute resolution process that has been established to address the matter. At this time, Knife River - Northwest has agreed to participate in the alternative dispute resolution process. The Company believes it is not probable that it will incur any material environmental remediation costs or damages in relation to the above referenced matter. Manufactured Gas Plant Sites There are three claims against Cascade for cleanup of environmental contamination at manufactured gas plant sites operated by Cascade's predecessors. The first claim is for contamination at a site in Eugene, Oregon which was received in 1995. There are PRPs in addition to Cascade that may be liable for cleanup of the contamination. Some of these PRPs have shared in the investigation costs. It is expected that these and other PRPs will share in the cleanup costs. The Oregon DEQ released a 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 $320,000 has been incurred. It is not known at this time what share of the cleanup costs will actually be borne by Cascade; however, Cascade has paid 50 percent of the ongoing investigation and design costs and anticipates its proportional share of the final costs could be approximately 50 percent. Cascade has an accrual balance of $1.6 million for remediation of this site. In January 2013, the OPUC approved Cascade's application to defer environmental remediation costs at the Eugene site for a period of 12 months starting November 30, 2012. Cascade received orders reauthorizing the deferred accounting for the 12-month periods starting November 30, 2013, December 1, 2014, December 1, 2015 and December 1, 2016. The second claim is for contamination at a 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 $700,000 has been incurred. Cascade has accrued $6.9 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, which are included in other noncurrent assets, incurred in relation to the environmental remediation of this site. The WUTC approved the petition in September 2010, subject to conditions set forth in the order. The third claim is for contamination at a site in Bellingham, Washington. Cascade received notice from a party in May 2008 that Cascade may be a PRP, along with other parties, for contamination from a manufactured gas plant owned by Cascade and its predecessor from about 1946 to 1962. The notice indicates that current estimates to complete investigation and cleanup of the site exceed $8.0 million . Other PRPs have reached an agreed order and work plan with the Washington DOE for completion of a remedial investigation and feasibility study for the site. A report documenting the initial phase of the remedial investigation was completed in June 2011. There is currently not enough information available to estimate the potential liability to Cascade associated with this claim although Cascade believes its proportional share of any liability will be relatively small in comparison to other PRPs. The plant manufactured gas from coal between approximately 1890 and 1946. In 1946, shortly after Cascade's predecessor acquired the plant, it converted the plant to a propane-air gas facility. There are no documented wastes or by-products resulting from the mixing or distribution of propane-air gas. Cascade has not recorded an accrual for this site. Cascade has received notices from and entered into agreement with certain of its insurance carriers that they will participate in defense of Cascade for these contamination claims subject to full and complete reservations of rights and defenses to insurance coverage. To the extent these claims are not covered by insurance, Cascade intends to seek recovery through the OPUC and WUTC of remediation costs in its natural gas rates charged to customers. The accruals related to these matters are reflected in regulatory assets. Guarantees In June 2016, WBI Energy sold all of the outstanding membership interests in Dakota Prairie Refining. In connection with the sale, Centennial agreed to continue to guarantee certain debt obligations of Dakota Prairie Refining which totaled $57.4 million at September 30, 2017 , and are expected to mature by 2023. Tesoro agreed to indemnify Centennial for any losses and litigation expenses arising from the guarantee. The estimated fair values of the indemnity asset and guarantee liability are reflected in deferred charges and other assets - other and deferred credits and other liabilities - other, respectively, on the Consolidated Balance Sheets. Continuation of the guarantee was required as a condition to the sale of Dakota Prairie Refining. In March 2016, a sale agreement was signed to sell Fidelity's assets in the Paradox Basin. In connection with the sale, Centennial agreed to guarantee Fidelity's indemnity obligations associated with the Paradox assets. The guarantee was required by the buyer as a condition to the sale of the Paradox Basin assets. In 2009, multiple sale agreements were signed to sell the Company's ownership interests in the Brazilian Transmission Lines. In connection with the sale, Centennial agreed to guarantee payment of any indemnity obligations of certain of the Company's indirect wholly owned subsidiaries who were the sellers in three purchase and sale agreements for periods ranging up to 10 years from the date of sale. The guarantees were required by the buyers as a condition to the sale of the Brazilian Transmission Lines. Certain subsidiaries of the Company have outstanding guarantees to third parties that guarantee the performance of other subsidiaries of the Company. These guarantees are related to construction contracts, insurance deductibles and loss limits, and certain other guarantees. At September 30, 2017 , the fixed maximum amounts guaranteed under these agreements aggregated $119.4 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate $2.5 million in 2017 ; $21.3 million in 2018 ; $15.8 million in 2019 ; $72.6 million in 2020 ; $500,000 in 2021 ; $2.7 million thereafter; and $4.0 million , which has no scheduled maturity date. There were no amounts outstanding under the above guarantees at September 30, 2017 . 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, 2017 , the fixed maximum amounts guaranteed under these letters of credit aggregated $34.0 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these letters of credit aggregate $29.2 million in 2017 and $4.8 million in 2018. There were no amounts outstanding under the above letters of credit at September 30, 2017 . In the event of default under these letter of credit obligations, the subsidiary issuing the letter of credit for that particular obligation would be required to make payments under its letter of credit. In addition, Centennial, Knife River and MDU Construction Services have issued guarantees to third parties related to the routine purchase of maintenance items, materials and lease obligations for which no fixed maximum amounts have been specified. These guarantees have no scheduled maturity date. In the event a subsidiary of the Company defaults under these obligations, Centennial, Knife River or MDU Construction Services would be required to make payments under these guarantees. Any amounts outstanding by subsidiaries of the Company for these guarantees were reflected on the Consolidated Balance Sheet at September 30, 2017 . 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, 2017 , approximately $556.8 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 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 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, 2017 , 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 $41.4 million

Basis of presentation (Policies

Basis of presentation (Policies)	9 Months Ended
	Sep. 30, 2017
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 2016 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, 2017 , up to the date of issuance of these consolidated interim financial statements.

Accounts receivable and allow25

Accounts receivable and allowance for doubtful accounts (Policies)	9 Months Ended
	Sep. 30, 2017
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 i26

Inventories and natural gas in storage (Policies)	9 Months Ended
	Sep. 30, 2017
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 (loss) per common sh27

Earnings (loss) per common share (Policies)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Earnings (loss) per common share	Basic earnings (loss) per common share were computed by dividing earnings (loss) on common stock by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings (loss) per common share were computed by dividing earnings (loss) 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 outstanding performance share awards. Common stock outstanding includes issued shares less shares held in treasury.

New accounting standards (Polic

New accounting standards (Policies)	9 Months Ended
	Sep. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New accounting standards	New accounting standards Recently adopted accounting standards Balance Sheet Classification of Deferred Taxes In November 2015, the FASB issued guidance regarding the classification of deferred taxes on the balance sheet. The guidance requires all deferred tax assets and liabilities to be classified as noncurrent. These amendments align GAAP with IFRS. The Company adopted the guidance in the fourth quarter of 2016 and applied the retrospective method of adoption. The guidance required a reclassification of current deferred income taxes to noncurrent deferred income taxes on the Consolidated Balance Sheets, but did not impact the Company's results of operations or cash flows. As a result of the retrospective application of this change in accounting principle, the Company reclassified deferred income taxes of $31.4 million from current assets - deferred income taxes to deferred credits and other liabilities - deferred income taxes on its Consolidated Balance Sheet at September 30, 2016. Simplifying the Measurement of Inventory In July 2015, the FASB issued guidance regarding inventory that is measured using the first-in, first-out or average cost method. The guidance does not apply to inventory measured using the last-in, first-out or the retail inventory method. The guidance requires inventory within its scope to be measured at the lower of cost or net realizable value, which is the estimated selling price in the normal course of business less reasonably predictable costs of completion, disposal and transportation. These amendments more closely align GAAP with IFRS. The Company adopted the guidance on January 1, 2017, 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. Improvements to Employee Share-Based Payment Accounting In March 2016, the FASB issued guidance regarding simplification of several aspects of the accounting for share-based payment transactions. The guidance affects the income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and calculation of dilutive shares. The Company adopted the guidance on January 1, 2017. All amendments in the guidance that apply to the Company were adopted on a prospective basis resulting in no adjustments being made to retained earnings. The adoption of the guidance impacted the Consolidated Statement of Income and the Consolidated Balance Sheet in the first quarter of 2017 due to the taxes related to the stock-based compensation award that vested in February 2017 being recognized as income tax expense as compared to a reduction to additional paid-in capital under the previous guidance. Adoption of the guidance also increased the number of shares included in the diluted earnings per share calculation due to the exclusion of tax benefits in the incremental shares calculation. The change in the weighted average common shares outstanding - diluted did not result in a material effect on the earnings per common share - diluted. Recently issued accounting standards not yet adopted 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 will be effective for the Company on January 1, 2018. Entities will have the option of using either a full retrospective or modified retrospective approach to adopting the guidance. The Company plans to adopt the guidance on January 1, 2018, and to use the modified retrospective approach. Under the modified retrospective approach, an entity would recognize the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings in the period of adoption. To date, the Company has not identified any material cumulative effect adjustments to be made to retained earnings. In addition, the guidance will require expanded disclosures, both quantitative and qualitative, related to the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. To date, the Company has reviewed nearly all of its revenue streams, completing the preliminary evaluation of the impact of this guidance. Based on the preliminary evaluation, the Company does not anticipate a significant change in the timing of revenue recognition, results of operations, financial position or cash flows, however the Company will continue to evaluate the impact of this guidance through the date of adoption. Recognition and Measurement of Financial Assets and Financial Liabilities In January 2016, the FASB issued guidance regarding the classification and measurement of financial instruments. The guidance revises the way an entity classifies and measures investments in equity securities, the presentation of certain fair value changes for financial liabilities measured at fair value and amends certain disclosure requirements related to the fair value of financial instruments. This guidance will be effective for the Company on January 1, 2018, with early adoption of certain amendments permitted. The guidance should be applied using a modified retrospective approach with the exception of equity securities without readily determinable fair values which will be applied prospectively. 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. 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. This guidance will be effective for the Company on January 1, 2018, with early adoption permitted. Entities must apply the guidance retrospectively unless it is impracticable to do so, in which case they may apply it prospectively as of the earliest date practicable. The Company plans to adopt the guidance on January 1, 2018. The Company's initial evaluation of the guidance did not identify any changes to the current presentation of the statement of cash flows; therefore, no retrospective adjustments to prior periods will be necessary. Clarifying the Definition of a Business In January 2017, the FASB issued guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions or disposals of assets or businesses. The guidance provides a screen to determine when an integrated set of assets and activities is not a business. The guidance will also affect other aspects of accounting, such as determining reporting units for goodwill testing and whether an entity has acquired or sold a business. The guidance will be effective for the Company on January 1, 2018, and should be applied on a prospective basis with early adoption permitted for transactions that occur before the issuance or effective date of the amendments and only when the transactions have not been reported in the financial statements or made available for issuance. The Company expects to adopt this guidance as required and does not expect the guidance to have a material effect on its results of operations, financial position, cash flows and disclosures. 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 cost and net periodic postretirement benefit cost. The guidance requires the service cost component to be presented in the income statement in the same line item or items as other compensation costs arising from services performed during the period. Other components of net 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 guidance will be effective for the Company on January 1, 2018, including interim periods, with early adoption permitted as of the beginning of an annual period for which the financial statements have not been issued. The guidance shall be applied on a retrospective basis for the financial statement presentation and on a prospective basis for the capitalization of the service cost component. The Company plans to adopt the guidance as required on January 1, 2018, which will include 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. The impact upon adoption of the new guidance will be an increase to operating income and decrease to other income on the Consolidated Statements of Income and no impact to earnings. The guidance will not have a material impact on the Company's disclosures or cash flows. Leases In February 2016, the FASB issued guidance regarding leases. The guidance requires lessees to recognize a lease liability and a right-of-use asset on the balance sheet for operating and financing leases with terms of more than 12 months. The guidance remains largely the same for lessors, although some changes were made to better align lessor accounting with the new lessee accounting and to align with the revenue recognition standard. The guidance also requires additional disclosures, both quantitative and qualitative, related to operating and finance leases for the lessee and sales-type, direct financing and operating leases for the lessor. This guidance will be effective for the Company on January 1, 2019, and should be applied using a modified retrospective approach with early adoption permitted. The Company continues to evaluate the potential impact the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. The Company is planning to adopt the standard on January 1, 2019, utilizing the practical expedient that allows the Company to not reassess whether an expired or existing contract contains a lease, the classification of leases or initial direct costs. 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

Fair value disclosures (Policie

Fair value disclosures (Policies)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair value measurements	The Company measures its investments in certain fixed-income and equity securities at fair value with changes in fair value recognized in income.
Investments	The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities, which include mortgage-backed securities and U.S. Treasury securities. These available-for-sale securities are recorded at fair value and are classified as investments on the Consolidated Balance Sheets.

Business segment data (Policies

Business segment data (Policies)	9 Months Ended
	Sep. 30, 2017
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, 2017
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 i32

Inventories and natural gas in storage (Tables)	9 Months Ended
	Sep. 30, 2017
Inventory Disclosure [Abstract]
Inventories	Inventories consisted of: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Aggregates held for resale $ 116,399 $ 119,078 $ 115,471 Natural gas in storage (current) 29,974 35,625 25,761 Asphalt oil 26,682 23,480 29,103 Materials and supplies 20,778 18,584 18,372 Merchandise for resale 15,346 15,672 16,437 Other 23,376 33,351 33,129 Total $ 232,555 $ 245,790 $ 238,273

Earnings (loss) per common sh33

Earnings (loss) per common share (Tables)	9 Months Ended
	Sep. 30, 2017
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 (loss) per share calculations was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) Weighted average common shares outstanding - basic 195,304 195,304 195,304 195,298 Effect of dilutive performance share awards 479 507 618 496 Weighted average common shares outstanding - diluted 195,783 195,811 195,922 195,794 Shares excluded from the calculation of diluted earnings per share — — — —

Comprehensive income (loss) (Ta

Comprehensive income (loss) (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Comprehensive income (loss)	The after-tax changes in the components of accumulated other comprehensive loss were as follows: 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 ) Three Months Ended September 30, 2016 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,484 ) $ (35,604 ) $ (144 ) $ 26 $ (38,206 ) Other comprehensive loss before reclassifications — — (4 ) (42 ) (46 ) Amounts reclassified from accumulated other comprehensive loss 92 236 — 33 361 Net current-period other comprehensive income (loss) 92 236 (4 ) (9 ) 315 Balance at end of period $ (2,392 ) $ (35,368 ) $ (148 ) $ 17 $ (37,891 ) 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 a 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 ) Nine Months Ended September 30, 2016 Net Unrealized Gain (Loss) on Derivative Postretirement Foreign Net Unrealized Total (In thousands) Balance at beginning of period $ (2,667 ) $ (34,257 ) $ (200 ) $ (24 ) $ (37,148 ) Other comprehensive income (loss) before reclassifications — — 52 (65 ) (13 ) Amounts reclassified from accumulated other comprehensive loss 275 (1,111 ) — 106 (730 ) Net current-period other comprehensive income (loss) 275 (1,111 ) 52 41 (743 ) Balance at end of period $ (2,392 ) $ (35,368 ) $ (148 ) $ 17 $ (37,891 )
Reclassification out of accumulated other comprehensive income	Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Nine Months Ended Location on Consolidated Statements of Income September 30, September 30, 2017 2016 2017 2016 (In thousands) Reclassification adjustment for loss on derivative instruments included in net income (loss) $ (148 ) $ (148 ) $ (443 ) $ (445 ) Interest expense 56 56 168 170 Income taxes (92 ) (92 ) (275 ) (275 ) Amortization of postretirement liability gains (losses) included in net periodic benefit cost (credit) (536 ) (379 ) (1,611 ) 1,787 (a) 203 143 609 (676 ) Income taxes (333 ) (236 ) (1,002 ) 1,111 Reclassification adjustment for loss on available-for-sale investments included in net income (loss) (41 ) (51 ) (143 ) (163 ) Other income 14 18 50 57 Income taxes (27 ) (33 ) (93 ) (106 ) Total reclassifications $ (452 ) $ (361 ) $ (1,370 ) $ 730 (a) Included in net periodic benefit cost (credit). For more information, see Note 14 .

Assets held for sale and disc35

Assets held for sale and discontinued operations (Tables)	9 Months Ended
	Sep. 30, 2017
Discontinued Operations and Disposal Groups [Abstract]
Disclosure of long lived assets held-for-sale	The carrying amounts of the major classes of assets and liabilities that were classified as held for sale associated with Pronghorn on the Company's Consolidated Balance Sheets were as follows: December 31, 2016 (In thousands) Assets Current assets: Prepayments and other current assets $ 68 Total current assets held for sale 68 Noncurrent assets: Net property, plant and equipment 93,424 Goodwill 9,737 Less allowance for impairment of assets held for sale 2,311 Total noncurrent assets held for sale 100,850 Total assets held for sale $ 100,918
Disposal groups, including discontinued operations	The carrying amounts of the major classes of assets and liabilities that are classified as held for sale related to the operations of Fidelity on the Company's Consolidated Balance Sheets were as follows: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Assets Current assets: Receivables, net $ 304 $ 7,930 $ 355 Total current assets held for sale 304 7,930 355 Noncurrent assets: Net property, plant and equipment 2,064 5,507 5,507 Deferred income taxes 62,163 104,726 91,098 Other 161 161 161 Less allowance for impairment of assets held for sale — 938 938 Total noncurrent assets held for sale 64,388 109,456 95,828 Total assets held for sale $ 64,692 $ 117,386 $ 96,183 Liabilities Current liabilities: Accounts payable $ 68 $ 175 $ 141 Taxes payable 11,745 2,205 (a) 19 (a) Other accrued liabilities 2,380 3,084 2,358 Total current liabilities held for sale 14,193 5,464 2,518 Total liabilities held for sale $ 14,193 $ 5,464 $ 2,518 (a) On the Company's Consolidated Balance Sheets, these amounts were reclassified to prepayments and other current assets and are reflected in current assets held for sale. September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Assets Current assets: Receivables, net $ — $ 13 $ — Income taxes receivable 8,444 (a) 32,388 13,987 Prepayments and other current assets — 7,741 — Total current assets held for sale 8,444 40,142 13,987 Noncurrent assets: Deferred income taxes — 2,984 — Total noncurrent assets held for sale — 2,984 — Total assets held for sale $ 8,444 $ 43,126 $ 13,987 Liabilities Current liabilities: Accounts payable $ — $ 7,063 $ 7,425 Other accrued liabilities — 7,743 — Total current liabilities held for sale — 14,806 7,425 Noncurrent liabilities: Deferred income taxes (b) 55 — 14 Total noncurrent liabilities held for sale 55 — 14 Total liabilities held for sale $ 55 $ 14,806 $ 7,439 (a) On the Company's Consolidated Balance Sheets, this amount was reclassified to income taxes payable and is reflected in current liabilities held for sale. (b) On the Company's Consolidated Balance Sheets, these amounts were reclassified to noncurrent deferred income tax assets 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 loss from discontinued operations on the Company's Consolidated Statements of Income was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) Operating revenues $ 121 $ 162 $ 356 $ 122,894 Operating expenses 384 230 (4,988 ) 513,756 Operating income (loss) (263 ) (68 ) 5,344 (390,862 ) Other income (expense) — 375 (13 ) 762 Interest expense — — 239 1,753 Income (loss) from discontinued operations before income taxes (263 ) 307 5,092 (391,853 ) Income taxes 1,935 5,707 8,794 (92,315 ) Loss from discontinued operations (2,198 ) (5,400 ) (3,702 ) (299,538 ) Loss from discontinued operations attributable to noncontrolling interest — — — (131,691 ) Loss from discontinued operations attributable to the Company $ (2,198 ) $ (5,400 ) $ (3,702 ) $ (167,847 )

Goodwill and other intangible36

Goodwill and other intangible assets (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 Balance at January 1, 2017 Goodwill Acquired 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 Nine Months Ended September 30, 2016 Balance at January 1, 2016 * Goodwill Acquired During the Year Balance at September 30, 2016 * (In thousands) Natural gas distribution $ 345,736 $ — $ 345,736 Pipeline and midstream 9,737 — 9,737 Construction materials and contracting 176,290 — 176,290 Construction services 103,441 6,323 109,764 Total $ 635,204 $ 6,323 $ 641,527 * Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods. Year Ended December 31, 2016 Balance at January 1, 2016 * Goodwill Acquired During the Year Held for Sale Balance at December 31, 2016 (In thousands) Natural gas distribution $ 345,736 $ — $ — $ 345,736 Pipeline and midstream 9,737 — (9,737 ) — Construction materials and contracting 176,290 — — 176,290 Construction services 103,441 6,324 — 109,765 Total $ 635,204 $ 6,324 $ (9,737 ) $ 631,791 * Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods.
Other amortizable intangible assets	Other amortizable intangible assets were as follows: September 30, 2017 September 30, 2016 December 31, 2016 (In thousands) Customer relationships $ 15,248 $ 17,145 $ 17,145 Less accumulated amortization 13,176 13,524 13,917 2,072 3,621 3,228 Noncompete agreements 2,430 2,430 2,430 Less accumulated amortization 1,769 1,622 1,658 661 808 772 Other 7,020 7,764 7,768 Less accumulated amortization 5,544 5,664 5,843 1,476 2,100 1,925 Total $ 4,209 $ 6,529 $ 5,925

Fair value measurements (Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: 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 September 30, 2016 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,882 $ 43 $ (17 ) $ 9,908 Total $ 9,882 $ 43 $ (17 ) $ 9,908 December 31, 2016 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 10,546 $ 8 $ (105 ) $ 10,449 Total $ 10,546 $ 8 $ (105 ) $ 10,449
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, 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 September 30, 2016, 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, 2016 (In thousands) Assets: Money market funds $ — $ 2,284 $ — $ 2,284 Insurance contract* — 72,818 — 72,818 Available-for-sale securities: Mortgage-backed securities — 9,908 — 9,908 Total assets measured at fair value $ — $ 85,010 $ — $ 85,010 * The insurance contract invests approximately 65 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents. Fair Value Measurements at December 31, 2016, 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, 2016 (In thousands) Assets: Money market funds $ — $ 1,602 $ — $ 1,602 Insurance contract* — 70,921 — 70,921 Available-for-sale securities: Mortgage-backed securities — 10,449 — 10,449 Total assets measured at fair value $ — $ 82,972 $ — $ 82,972 * The insurance contract invests approximately 52 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 1 percent in target date investments and 2 percent in cash equivalents.
Fair value of long term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt was as follows: Carrying Amount Fair Value (In thousands) Long-term debt at September 30, 2017 $ 1,740,552 $ 1,846,811 Long-term debt at September 30, 2016 $ 1,901,948 $ 2,047,339 Long-term debt at December 31, 2016 $ 1,790,159 $ 1,841,885

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Summary of changes in equity	A summary of the changes in equity was as follows: Nine Months Ended September 30, 2017 Total Equity (In thousands) Balance at December 31, 2016 $ 2,316,244 Net income 165,891 Other comprehensive income 392 Dividends declared on preferred stocks (171 ) Dividends declared on common stock (112,788 ) Stock-based compensation 2,390 Repurchase of common stock (1,684 ) Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (757 ) Redemption of preferred stock (15,600 ) Balance at September 30, 2017 $ 2,353,917 Nine Months Ended September 30, 2016 Total Stockholders' Equity Noncontrolling Interest Total Equity (In thousands) Balance at December 31, 2015 $ 2,396,505 $ 124,043 $ 2,520,548 Net loss (1,297 ) (131,691 ) (132,988 ) Other comprehensive loss (743 ) — (743 ) Dividends declared on preferred stocks (514 ) — (514 ) Dividends declared on common stock (109,858 ) — (109,858 ) Stock-based compensation 2,955 — 2,955 Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (323 ) — (323 ) Net tax deficit on stock-based compensation (1,664 ) — (1,664 ) Contribution from noncontrolling interest — 7,648 7,648 Balance at September 30, 2016 $ 2,285,061 $ — $ 2,285,061

Cash flow information (Tables)

Cash flow information (Tables)	9 Months Ended
	Sep. 30, 2017
Supplemental Cash Flow Information [Abstract]
Supplemental cash flow information	Cash expenditures for interest and income taxes were as follows: Nine Months Ended September 30, 2017 2016 (In thousands) Interest, net of amount capitalized and AFUDC - borrowed of $676 and $842 in 2017 and 2016, respectively $ 58,119 $ 66,281 Income taxes paid, net* $ 46,430 $ 73,771 * Income taxes paid (refunded), net of discontinued operations, were $ 1.4 million and $ (144,000) for the nine months ended September 30, 2017 and 2016, respectively. Noncash investing transactions were as follows: September 30, 2017 2016 (In thousands) Property, plant and equipment additions in accounts payable $ 16,914 $ 22,560

Business segment data (Tables)

Business segment data (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Information on the Company's businesses	Information on the Company's businesses was as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) External operating revenues: Regulated operations: Electric $ 91,531 $ 82,156 $ 254,330 $ 238,911 Natural gas distribution 92,253 87,941 566,364 500,106 Pipeline and midstream 23,152 21,982 45,341 44,980 206,936 192,079 866,035 783,997 Nonregulated operations: Pipeline and midstream 5,356 10,732 13,518 29,697 Construction materials and contracting 686,010 724,535 1,388,212 1,475,643 Construction services 374,111 280,801 1,009,693 822,226 Other 135 420 654 1,167 1,065,612 1,016,488 2,412,077 2,328,733 Total external operating revenues $ 1,272,548 $ 1,208,567 $ 3,278,112 $ 3,112,730 Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — $ — Natural gas distribution — — — — Pipeline and midstream 3,081 3,278 30,923 30,969 3,081 3,278 30,923 30,969 Nonregulated operations: Pipeline and midstream 38 41 132 161 Construction materials and contracting 142 155 400 370 Construction services 415 3 715 541 Other 1,910 2,204 5,411 5,542 2,505 2,403 6,658 6,614 Intersegment eliminations (5,586 ) (5,681 ) (37,581 ) (37,583 ) Total intersegment operating revenues $ — $ — $ — $ — Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 (In thousands) Earnings (loss) on common stock: Regulated operations: Electric $ 15,712 $ 12,699 $ 37,904 $ 31,840 Natural gas distribution (10,883 ) (12,524 ) 14,181 4,940 Pipeline and midstream 5,853 5,389 15,901 16,241 10,682 5,564 67,986 53,021 Nonregulated operations: Pipeline and midstream 95 1,304 (770 ) 2,043 Construction materials and contracting 63,221 69,523 64,477 88,747 Construction services 13,144 7,234 32,896 20,198 Other 552 (1,009 ) (1,888 ) (3,572 ) 77,012 77,052 94,715 107,416 Intersegment eliminations* 1,855 5,599 6,121 5,599 Earnings on common stock before loss from discontinued operations 89,549 88,215 168,822 166,036 Loss from discontinued operations, net of tax* (2,198 ) (5,400 ) (3,702 ) (299,538 ) Loss from discontinued operations attributable to noncontrolling interest — — — (131,691 ) Total earnings (loss) on common stock $ 87,351 $ 82,815 $ 165,120 $ (1,811 ) * Includes 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, 2017
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, 2017 2016 2017 2016 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 377 $ 412 Interest cost 4,052 4,305 816 922 Expected return on assets (5,132 ) (5,231 ) (1,160 ) (1,133 ) Amortization of prior service credit — — (343 ) (343 ) Amortization of net actuarial loss 1,589 1,553 213 371 Net periodic benefit cost (credit), including amount capitalized 509 627 (97 ) 229 Less amount capitalized 65 82 (95 ) (34 ) Net periodic benefit cost (credit) $ 444 $ 545 $ (2 ) $ 263 Pension Benefits Other Postretirement Benefits Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ — $ 1,130 $ 1,236 Interest cost 12,155 12,915 2,449 2,766 Expected return on assets (15,395 ) (15,693 ) (3,480 ) (3,400 ) Amortization of prior service credit — — (1,029 ) (1,029 ) Amortization of net actuarial loss 4,767 4,660 649 1,118 Net periodic benefit cost (credit), including amount capitalized 1,527 1,882 (281 ) 691 Less amount capitalized 245 284 (248 ) 4 Net periodic benefit cost (credit) $ 1,282 $ 1,598 $ (33 ) $ 687

Accounts receivable and allow42

Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Receivables [Abstract]
Receivables past due 90 days or more	$ 27.2	$ 29.2	$ 26.3
Allowance for doubtful accounts receivable	$ 9	$ 10.5	$ 10.2

Inventories and natural gas i43

Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 116,399	$ 115,471	$ 119,078
Natural gas in storage (current)	29,974	25,761	35,625
Asphalt oil	26,682	29,103	23,480
Materials and supplies	20,778	18,372	18,584
Merchandise for resale	15,346	16,437	15,672
Other	23,376	33,129	33,351
Total	232,555	238,273	245,790
Natural gas in storage noncurrent	$ 49,500	$ 49,500	$ 49,100

Earnings (loss) per common sh44

Earnings (loss) per common share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Weighted average common shares outstanding - basic	195,304	195,304	195,304	195,298
Effect of dilutive performance share awards	479	507	618	496
Weighted average common shares outstanding - diluted	195,783	195,811	195,922	195,794
Shares excluded from the calculation of diluted earnings per share	0	0	0	0

New accounting standards New ac

New accounting standards New accounting pronouncements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclass from current assets - deferred income taxes to deferred credits and other liabilities	$ 652,413	$ 668,226	$ 662,326
Accounting Standards Update 2015-17 [Member] | Restatement adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclass from current assets - deferred income taxes to deferred credits and other liabilities			$ 31,400

Comprehensive income (loss) (De

Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated other comprehensive income (loss) [Roll Forward]
Balance				$ 2,520,548
Amounts reclassified from accumulated other comprehensive loss	$ (452)	$ (361)	$ (1,370)	730
Net current-period other comprehensive income (loss)	448	315	392	(743)
Balance		2,285,061		2,285,061
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,117)	(2,484)	(2,300)	(2,667)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	92	92	275	275
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	92	92	275	275
Balance	(2,025)	(2,392)	(2,025)	(2,392)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,469)	(35,604)	(33,221)	(34,257)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	333	236	1,002	(1,111)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			(917)
Net current-period other comprehensive income (loss)	333	236	85	(1,111)
Balance	(33,136)	(35,368)	(33,136)	(35,368)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(155)	(144)	(149)	(200)
Other comprehensive loss before reclassifications	15	(4)	9	52
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	15	(4)	9	52
Balance	(140)	(148)	(140)	(148)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(48)	26	(63)	(24)
Other comprehensive loss before reclassifications	(19)	(42)	(70)	(65)
Amounts reclassified from accumulated other comprehensive loss	27	33	93	106
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	8	(9)	23	41
Balance	(40)	17	(40)	17
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,789)	(38,206)	(35,733)	(37,148)
Other comprehensive loss before reclassifications	(4)	(46)	(61)	(13)
Amounts reclassified from accumulated other comprehensive loss	452	361	1,370	(730)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			(917)
Net current-period other comprehensive income (loss)	448	315	392	(743)
Balance	$ (35,341)	$ (37,891)	$ (35,341)	$ (37,891)

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,909)	$ (22,278)	$ (61,978)	$ (67,365)
Income taxes	(46,930)	(37,761)	(74,406)	(67,381)
Other income	1,011	1,741	2,809	3,662
Net income (loss)	87,351	82,986	165,891	(132,988)
Reclassification from accumulated other comprehensive income, current period, net of tax	(452)	(361)	(1,370)	730
Reclassification adjustment for loss on derivative instruments included in net income (loss)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	92	92	275	275
Reclassification adjustment for loss on derivative instruments included in net income (loss) | Reclassification out of accumulated other comprehensive income [Member] | Interest rate contract [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	(148)	(148)	(443)	(445)
Income taxes	56	56	168	170
Net income (loss)	(92)	(92)	(275)	(275)
Amortization of postretirement liability gains (losses) included in net periodic benefit cost (credit)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	333	236	1,002	(1,111)
Amortization of postretirement liability gains (losses) included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	203	143	609	(676)
Net periodic benefit cost (credit)	(536)	(379)	(1,611)	1,787
Net income (loss)	(333)	(236)	(1,002)	1,111
Reclassification adjustment for loss on available-for-sale investments included in net income (loss)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	27	33	93	106
Reclassification adjustment for loss on available-for-sale investments included in net income (loss) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	14	18	50	57
Other income	(41)	(51)	(143)	(163)
Net income (loss)	$ (27)	$ (33)	$ (93)	$ (106)

Pronghorn (Details)

Pronghorn (Details) - Pronghorn [Member] - Disposal group, held-for-sale, not discontinued operations [Member] - USD ($) $ in Millions	Jan. 01, 2017	Nov. 21, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-operating ownership interest		50.00%
Proceeds	$ 100

Major classes of assets and lia

Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Current assets:
Total current assets held for sale	$ 304	$ 14,391	$ 45,867
Noncurrent assets:
Total noncurrent assets held for sale	$ 64,333	196,664	$ 112,440
Pronghorn [Member] | Disposal group, held-for-sale or disposed of by sale, not discontinued operations [Member]
Current assets:
Prepayments and other current assets		68
Total current assets held for sale		68
Noncurrent assets:
Net property, plant and equipment		93,424
Goodwill		9,737
Less allowance for impairment of assets held for sale		2,311
Total noncurrent assets held for sale		100,850
Total assets held for sale		$ 100,918

Noncontrolling interest (Detail

Noncontrolling interest (Details 3) - USD ($) $ in Millions	Jun. 27, 2016	Jun. 24, 2016	Mar. 31, 2017
Dakota Prairie Refining, LLC [Member] | WBI Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Previous percentage of ownership	50.00%	50.00%
Operating Expense [Member] | Refining [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Reversal of an accrual			$ 7
Operating expense - after tax [Member] | Refining [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Reversal of an accrual			$ 4.3

Major classes of assets and l51

Major classes of assets and liabilities held for sale (Details 4) - USD ($) $ in Thousands		Sep. 30, 2017		Dec. 31, 2016		Sep. 30, 2016
Current assets:
Total current assets held for sale		$ 304		$ 14,391		$ 45,867
Noncurrent assets:
Total noncurrent assets held for sale		64,333		196,664		112,440
Current liabilities:
Total current liabilities held for sale		5,749		9,924		18,065
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net		304		355		7,930
Total current assets held for sale		304		355		7,930
Noncurrent assets:
Net property, plant and equipment		2,064		5,507		5,507
Deferred income taxes		62,163		91,098		104,726
Other		161		161		161
Less allowance for impairment of assets held for sale		0		938		938
Total noncurrent assets held for sale		64,388		95,828		109,456
Total assets held for sale		64,692		96,183		117,386
Current liabilities:
Accounts payable		68		141		175
Taxes payable		11,745		19	[1]	2,205	[1]
Other accrued liabilities		2,380		2,358		3,084
Total current liabilities held for sale		14,193		2,518		5,464
Deferred credits and other liabilities:
Total liabilities held for sale		14,193		2,518		5,464
Refining [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net		0		0		13
Income taxes receivable		8,444	[2]	13,987		32,388
Prepayments and other current assets		0		0		7,741
Total current assets held for sale		8,444		13,987		40,142
Noncurrent assets:
Deferred income taxes		0		0		2,984
Total noncurrent assets held for sale		0		0		2,984
Total assets held for sale		8,444		13,987		43,126
Current liabilities:
Accounts payable		0		7,425		7,063
Other accrued liabilities		0		0		7,743
Total current liabilities held for sale		0		7,425		14,806
Deferred credits and other liabilities:
Deferred income taxes (b)	[3]	55		14		0
Total noncurrent liabilities held for sale		55		14		0
Total liabilities held for sale		$ 55		$ 7,439		$ 14,806
[1]	On the Company's Consolidated Balance Sheets, these amounts were reclassified to prepayments and other current assets and are reflected in current assets held for sale.
[2]	On the Company's Consolidated Balance Sheets, this amount was reclassified to income taxes payable and is reflected in current liabilities held for sale.
[3]	On the Company's Consolidated Balance Sheets, these amounts were reclassified to noncurrent deferred income tax assets and are reflected in noncurrent assets held for sale.

Assets held for sale and disc52

Assets held for sale and discontinued operations Accounting pronouncement reclassification (Details 5) - Restatement adjustment [Member] - Accounting Standards Update 2015-17 [Member] - Exploration and production [Member] - Discontinued operations, held-for-sale or disposed of by sale [Member] $ in Millions	Sep. 30, 2016USD ($)
Other current assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Reclass to deferred tax asset	$ 47.5
Other current liabilities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Reclass to deferred tax asset	$ (4.1)

Impairment fair value (Details

Impairment fair value (Details 6) - Discontinued operations, held-for-sale or disposed of by sale [Member] - USD ($)	3 Months Ended
	Jun. 30, 2016	Mar. 31, 2016
Exploration and production [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value impairment	$ 900,000	$ (1,400,000)
Fair value impairment after tax	600,000	$ (900,000)
Refining [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value impairment	251,900,000
Fair value impairment after tax	$ 156,700,000

Business exit costs (Details 7)

Business exit costs (Details 7) - Discontinued operations, held-for-sale or disposed of by sale [Member] - Exploration and production [Member] - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended	27 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Sep. 30, 2017
Transaction costs [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs		$ 300,000
Other restructuring [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs			$ 0	$ 5,600,000		$ 10,500,000
Facility closing [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs					$ 900,000
Contract Termination [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs	$ 3,200,000

Reconciliation of income and ex

Reconciliation of income and expenses (Details 8) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss from discontinued operations	[1]	$ (2,198,000)	$ (5,400,000)	$ (3,702,000)	$ (299,538,000)
Loss from discontinued operations attributable to noncontrolling interest		0	0	0	(131,691,000)
Discontinued operations, held-for-sale or disposed of by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues		121,000	162,000	356,000	122,894,000
Operating expenses		384,000	230,000	(4,988,000)	513,756,000
Operating income (loss)		(263,000)	(68,000)	5,344,000	(390,862,000)
Other income		0	375,000		762,000
Other expense				(13,000)
Interest expense		0	0	239,000	1,753,000
Income (loss) from discontinued operations before income taxes		(263,000)	307,000	5,092,000	(391,853,000)
Income taxes		1,935,000	5,707,000	8,794,000	(92,315,000)
Loss from discontinued operations		(2,198,000)	(5,400,000)	(3,702,000)	(299,538,000)
Loss from discontinued operations attributable to noncontrolling interest		0	0	0	(131,691,000)
Loss from discontinued operations attributable to the Company		(2,198,000)	(5,400,000)	(3,702,000)	(167,847,000)
Discontinued operations, held-for-sale or disposed of by sale [Member] | Refining [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax income (loss) attributable to the Company		$ 0	$ 935,000	$ 6,900,000	$ (253,000,000)
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Goodwill rollforward (Details)

Goodwill rollforward (Details) - USD ($) $ in Thousands	9 Months Ended			12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Dec. 31, 2016
Goodwill [Roll Forward]
Balance at beginning of period	$ 631,791	$ 635,204	[1]	$ 635,204	[1]
Goodwill acquired during the year	0	6,323		6,324
Held for sale				(9,737)
Balance at end of period	631,791	641,527	[1]	631,791
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance at beginning of period	345,736	345,736		345,736
Goodwill acquired during the year	0	0		0
Held for sale				0
Balance at end of period	345,736	345,736		345,736
Pipeline and midstream [Member]
Goodwill [Roll Forward]
Balance at beginning of period	0	9,737	[1]	9,737	[1]
Goodwill acquired during the year		0		0
Held for sale				(9,737)
Balance at end of period		9,737	[1]	0
Accumulated impairment which occurred in prior periods		12,300		12,300
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance at beginning of period	176,290	176,290		176,290
Goodwill acquired during the year	0	0		0
Held for sale				0
Balance at end of period	176,290	176,290		176,290
Construction services [Member]
Goodwill [Roll Forward]
Balance at beginning of period	109,765	103,441		103,441
Goodwill acquired during the year	0	6,323		6,324
Held for sale				0
Balance at end of period	$ 109,765	$ 109,764		$ 109,765
[1]	Balance is presented net of accumulated impairment of $12.3 million

Other intangible assets (Detail

Other intangible assets (Details 2) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 4,209,000	$ 6,529,000	$ 4,209,000	$ 6,529,000	$ 5,925,000
Amortization of intangible assets	500,000	600,000	1,700,000	1,900,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2,017	2,200,000		2,200,000
2,018	1,200,000		1,200,000
2,019	1,000,000		1,000,000
2,020	500,000		500,000
2,021	200,000		200,000
Thereafter	800,000		800,000
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	15,248,000	17,145,000	15,248,000	17,145,000	17,145,000
Intangible assets, less accumulated amortization	13,176,000	13,524,000	13,176,000	13,524,000	13,917,000
Intangible assets, net (excluding goodwill)	2,072,000	3,621,000	2,072,000	3,621,000	3,228,000
Noncompete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	2,430,000	2,430,000	2,430,000	2,430,000	2,430,000
Intangible assets, less accumulated amortization	1,769,000	1,622,000	1,769,000	1,622,000	1,658,000
Intangible assets, net (excluding goodwill)	661,000	808,000	661,000	808,000	772,000
Other intangible assets [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	7,020,000	7,764,000	7,020,000	7,764,000	7,768,000
Intangible assets, less accumulated amortization	5,544,000	5,664,000	5,544,000	5,664,000	5,843,000
Intangible assets, net (excluding goodwill)	$ 1,476,000	$ 2,100,000	$ 1,476,000	$ 2,100,000	$ 1,925,000

Fair value measurements Insuran

Fair value measurements Insurance contracts (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 75	$ 72.8	$ 75	$ 72.8	$ 70.9
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans	$ 1.9	$ 1.4	$ 6.9	$ 5.3

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Available-for-sale securities [Abstract]
Cost	$ 10,101	$ 10,546	$ 9,882
Gross unrealized gains	11	8	43
Gross unrealized losses	(73)	(105)	(17)
Fair value	10,039	10,449	9,908
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	9,488	10,546	9,882
Gross unrealized gains	11	8	43
Gross unrealized losses	(72)	(105)	(17)
Fair value	9,427	$ 10,449	$ 9,908
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	613
Gross unrealized gains	0
Gross unrealized losses	(1)
Fair value	$ 612

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016		Sep. 30, 2016
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	50.00%		52.00%		65.00%
Percentage investment in common stock of large-cap companies	23.00%		22.00%		18.00%
Percentage investment in common stock of mid-cap companies	13.00%		13.00%		9.00%
Percentage investment in common stock of small-cap companies	11.00%		10.00%		6.00%
Percentage investment in target date investments	2.00%		1.00%		1.00%
Percentage investment in cash and cash equivalents	1.00%		2.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 91,234		$ 82,972		$ 85,010
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	6,204		1,602		2,284
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	74,991	[1]	70,921	[2]	72,818	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	9,427		10,449		9,908
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	612
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	91,234		82,972		85,010
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	6,204		1,602		2,284
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	74,991	[1]	70,921	[2]	72,818	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	9,427		10,449		9,908
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	612
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		$ 0		$ 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 0
[1]	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
[2]	The insurance contract invests approximately 52 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 1 percent in target date investments and 2
[3]	The insurance contract invests approximately 65 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.

Fair value measurements (Deta61

Fair value measurements (Details 4) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Carrying amount [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 1,740,552	$ 1,790,159	$ 1,901,948
Fair value [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 1,846,811	$ 1,841,885	$ 2,047,339

Equity (Details)

Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Increase decrease in stockholders equity roll forward
Total Equity			$ 2,316,244
Net income (loss)	$ 87,351	$ 82,986	165,891	$ (132,988)
Other comprehensive income (loss)	448	315	392	(743)
Dividends declared on preferred stocks			(171)	(514)
Dividends declared on common stock			(112,788)	(109,858)
Stock-based compensation			2,390	2,955
Repurchase of common stock			(1,684)
Total Parent equity	$ 2,353,917	$ 2,285,061	2,353,917	$ 2,285,061
Common stock [Member]
Increase decrease in stockholders equity roll forward
Redemption of preferred stock			(757)
Preferred stock [Member]
Increase decrease in stockholders equity roll forward
Redemption of preferred stock			$ (15,600)

Redemption of preferred stock (

Redemption of preferred stock (Details 2)	Apr. 02, 2017USD ($)
Stockholders' Equity Note [Abstract]
Redeemable preferred stock classified as long term debt	$ 300,000
Payments for repurchase of redeemable preferred stock	$ 15,900,000

Schedule of capitalization, equ

Schedule of capitalization, equity NCI (Details 3) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Capitalization, Equity [Line Items]
Total Equity			$ 2,316,244
Balance				$ 2,520,548
Net income (loss)	$ 87,351	$ 82,986	165,891	(132,988)
Other comprehensive income (loss)	448	315	392	(743)
Dividends declared on preferred stocks			(171)	(514)
Dividends declared on common stock			(112,788)	(109,858)
Stock-based compensation			2,390	2,955
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings				(323)
Net tax deficit on stock-based compensation				(1,664)
Contribution from noncontrolling interest				7,648
Total Parent equity	$ 2,353,917	2,285,061	$ 2,353,917	2,285,061
Balance		2,285,061		2,285,061
Parent [Member]
Schedule of Capitalization, Equity [Line Items]
Total Equity				2,396,505
Net loss				(1,297)
Other comprehensive income (loss)				(743)
Dividends declared on preferred stocks				(514)
Dividends declared on common stock				(109,858)
Stock-based compensation				2,955
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings				(323)
Net tax deficit on stock-based compensation				(1,664)
Contribution from noncontrolling interest				0
Total Parent equity		2,285,061		2,285,061
Noncontrolling interest [Member]
Schedule of Capitalization, Equity [Line Items]
Noncontrolling Interest				124,043
Net loss				(131,691)
Other comprehensive income (loss)				0
Dividends declared on preferred stocks				0
Dividends declared on common stock				0
Stock-based compensation				0
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings				0
Net tax deficit on stock-based compensation				0
Contribution from noncontrolling interest				7,648
Noncontrolling Interest		$ 0		$ 0

Cash flow information (Details

Cash flow information (Details 1) - USD ($)		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Interest, net of amount capitalized and AFUDC - borrowed of $676 and $842 in 2017 and 2016, respectively		$ 58,119,000	$ 66,281,000
Income taxes paid (refunded), net	[1]	46,430,000	73,771,000
Property, plant and equipment additions in accounts payable		16,914,000	22,560,000
Capitalized interest and AFUDC borrowed		676,000	842,000
Continuing and discontinued operations [Member]
Income taxes paid (refunded), net		$ 1,400,000	$ (144,000)
[1]	Income taxes paid (refunded), net of discontinued operations, were $ 1.4 million and $ (144,000) for the nine months ended September 30, 2017 and 2016, respectively.

Business segment data (Details)

Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues		$ 1,272,548	$ 1,208,567	$ 3,278,112	$ 3,112,730
Intersegment operating revenues		0	0	0	0
Earnings on common stock before loss from discontinued operations		89,549	88,215	168,822	166,036
Loss from discontinued operations, net of tax	[1]	(2,198)	(5,400)	(3,702)	(299,538)
Loss from discontinued operations attributable to noncontrolling interest		0	0	0	(131,691)
Earnings (loss) on common stock		87,351	82,815	165,120	(1,811)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues		(5,586)	(5,681)	(37,581)	(37,583)
Earnings on common stock before loss from discontinued operations	[1]	1,855	5,599	6,121	5,599
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		206,936	192,079	866,035	783,997
Intersegment operating revenues		3,081	3,278	30,923	30,969
Earnings on common stock before loss from discontinued operations		10,682	5,564	67,986	53,021
Regulated operation [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Revenues		91,531	82,156	254,330	238,911
Intersegment operating revenues		0	0	0	0
Earnings on common stock before loss from discontinued operations		15,712	12,699	37,904	31,840
Regulated operation [Member] | Natural gas distribution [Member]
Segment Reporting Information [Line Items]
Revenues		92,253	87,941	566,364	500,106
Intersegment operating revenues		0	0	0	0
Earnings on common stock before loss from discontinued operations		(10,883)	(12,524)	14,181	4,940
Regulated operation [Member] | Pipeline and midstream [Member]
Segment Reporting Information [Line Items]
Revenues		23,152	21,982	45,341	44,980
Intersegment operating revenues		3,081	3,278	30,923	30,969
Earnings on common stock before loss from discontinued operations		5,853	5,389	15,901	16,241
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues		1,065,612	1,016,488	2,412,077	2,328,733
Intersegment operating revenues		2,505	2,403	6,658	6,614
Earnings on common stock before loss from discontinued operations		77,012	77,052	94,715	107,416
Nonregulated operation [Member] | Pipeline and midstream [Member]
Segment Reporting Information [Line Items]
Revenues		5,356	10,732	13,518	29,697
Intersegment operating revenues		38	41	132	161
Earnings on common stock before loss from discontinued operations		95	1,304	(770)	2,043
Nonregulated operation [Member] | Construction materials and contracting [Member]
Segment Reporting Information [Line Items]
Revenues		686,010	724,535	1,388,212	1,475,643
Intersegment operating revenues		142	155	400	370
Earnings on common stock before loss from discontinued operations		63,221	69,523	64,477	88,747
Nonregulated operation [Member] | Construction services [Member]
Segment Reporting Information [Line Items]
Revenues		374,111	280,801	1,009,693	822,226
Intersegment operating revenues		415	3	715	541
Earnings on common stock before loss from discontinued operations		13,144	7,234	32,896	20,198
Nonregulated operation [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenues		135	420	654	1,167
Intersegment operating revenues		1,910	2,204	5,411	5,542
Earnings on common stock before loss from discontinued operations		$ 552	$ (1,009)	$ (1,888)	$ (3,572)
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Employee benefit plans (Details

Employee benefit plans (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Qualified plan [Member] | Underfunded plan [Member] | Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0		$ 0	$ 0
Interest cost	4,052,000	4,305,000		12,155,000	12,915,000
Expected return on assets	(5,132,000)	(5,231,000)		(15,395,000)	(15,693,000)
Amortization of prior service credit	0	0		0	0
Amortization of net actuarial loss	1,589,000	1,553,000		4,767,000	4,660,000
Net periodic benefit cost (credit), including amount capitalized	509,000	627,000		1,527,000	1,882,000
Less amount capitalized	65,000	82,000		245,000	284,000
Net periodic benefit cost (credit)	444,000	545,000		1,282,000	1,598,000
Qualified plan [Member] | Underfunded plan [Member] | Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	377,000	412,000		1,130,000	1,236,000
Interest cost	816,000	922,000		2,449,000	2,766,000
Expected return on assets	(1,160,000)	(1,133,000)		(3,480,000)	(3,400,000)
Amortization of prior service credit	(343,000)	(343,000)		(1,029,000)	(1,029,000)
Amortization of net actuarial loss	213,000	371,000		649,000	1,118,000
Net periodic benefit cost (credit), including amount capitalized	(97,000)	229,000		(281,000)	691,000
Less amount capitalized	(95,000)	(34,000)		(248,000)	4,000
Net periodic benefit cost (credit)	(2,000)	263,000		(33,000)	687,000
Nonqualified plan [Member] | Unfunded plan [Member] | Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 1,200,000	$ 1,300,000		$ 3,500,000	$ 600,000
Curtailment gain			$ 3,300,000

Idaho Public Utilities Commissi

Idaho Public Utilities Commission (IPUC) (Details) - IPUC [Member] - Natural gas rate proceeding [Member] - USD ($) $ in Millions	Sep. 14, 2017	Aug. 17, 2017	Apr. 28, 2017	Jan. 17, 2017	Aug. 12, 2016
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 1.2			$ 10.2
Public utilities, requested rate increase (decrease), percentage		1.36%			4.10%
Public utilities, requested rate increase (decrease), amended, amount				$ 9.4
Public utilities, approved rate increase (decrease), amount	$ 6.7		$ 4.1
Public utilities, approved rate increase (decrease), percentage			1.60%
Public utilities, approved return on equity, percentage			9.50%

Minnesota Public Utilities Comm

Minnesota Public Utilities Commission (Details 2) - MNPUC [Member] - Natural gas cost tariff [Member] - USD ($)	Oct. 06, 2017	Dec. 21, 2016
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 456,000
Subsequent event [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 456,000

Washington Utilities and Transp

Washington Utilities and Transportation Commission (WUTC) (Details 3) - WUTC [Member] - USD ($) $ in Millions	Oct. 26, 2017	Aug. 31, 2017	May 31, 2017
Pipeline replacement cost recovery [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount			$ 1.6
Public utilities, requested rate increase (decrease), percentage			0.75%
Natural gas rate proceeding [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 5.9
Public utilities, requested rate increase (decrease), percentage		2.70%
Subsequent event [Member] | Pipeline replacement cost recovery [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 1.3
Public utilities, approved rate increase (decrease), percentage	0.61%

North Dakota Public Service Com

North Dakota Public Service Commission (NDPSC) (Details 4) - NDPSC [Member] - USD ($) $ in Millions	Sep. 06, 2017	Jul. 26, 2017	Jul. 21, 2017	Jun. 30, 2017	Jun. 16, 2017	Nov. 21, 2016	Oct. 14, 2016	Mar. 09, 2016	Feb. 10, 2016	Jan. 05, 2016
Advance determination of prudence [Member]
Public Utilities, General Disclosures [Line Items]
Estimated project cost				$ 85
Natural gas rate proceeding [Member]
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%
Electric rate proceeding [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount							$ 13.4
Public utilities, requested rate increase (decrease), percentage							6.60%
Public utilities, interim rate increase (decrease), amount						$ 11.7	$ 13
Public utilities, interim rate increase (decrease), percentage						5.80%	6.50%
Public utilities, approved rate increase (decrease), amount					$ 7.5
Public utilities, approved rate increase (decrease), percentage					3.70%
Public utilities refund from current rates, percentage		19.00%
Electric rider [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved return on equity, percentage					9.45%
Electric renewable cost rider [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, interim rate increase (decrease), amount										$ 15.1
Public utilities, approved return on equity, percentage					9.65%
Public utilities, requested return on equity, percentage										10.50%
Electric transmission adjustment [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, interim rate increase (decrease), amount									$ 6.8
Public utilities, requested rate increase (decrease), amended, percentage									10.50%
Electric generation rider [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, interim rate increase (decrease), amount								$ 9.7
Public utilities, approved return on equity, percentage					9.45%
Public utilities, requested return on equity, percentage								10.50%

Midcontinent Independent System

Midcontinent Independent System Operator, Inc. (MISO) (Details 5) - MISO [Member] - Electric transmission rate proceeding [Member] $ in Millions	Sep. 01, 2017USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 2.5
Public utilities, approved rate increase (decrease), amount	$ 13.6

Montana Public Service Commissi

Montana Public Service Commission (MTPSC) (Details 6) - MTPSC [Member] - Natural gas rate proceeding [Member] $ in Millions	Sep. 25, 2017USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 2.8
Public utilities, requested rate increase (decrease), percentage	4.10%
Public utilities, interim rate increase (decrease), amount	$ 1.6
Public utilities, interim rate increase (decrease), percentage	2.30%

Oregon Public Utility Commissio

Oregon Public Utility Commission (OPUC) (Details 7) - OPUC [Member] - Pipeline replacement cost recovery [Member]	Sep. 29, 2017USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 784,000
Public utilities, requested rate increase (decrease), percentage	1.20%

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Nov. 03, 2016	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters		$ 34.3	$ 31.8	$ 20
Litigation [Domain]
Loss Contingencies [Line Items]
Loss contingency, damages awarded, value	$ 44.8

Environmental matters (Details

Environmental matters (Details 2) - USD ($)	9 Months Ended
	Sep. 30, 2017	Jan. 06, 2017
Portland Harbor Site [Member]
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 [Member]
Site Contingency [Line Items]
Total estimated costs for site remediation	3,500,000
Incurred costs for site remediation	$ 320,000
Percent of ongoing costs paid	50.00%
Estimated proportional share of cleanup liability	50.00%
Environmental matters accrual for site remediation	$ 1,600,000
Bremerton, WA Manufactured Gas Plant Site [Member]
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	700,000
Environmental matters accrual of investigative costs	6,900,000
Bellingham, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	8,000,000
Minimum [Member] | Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	340,000
Maximum [Member] | Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 6,400,000

Guarantees (Details 3)

Guarantees (Details 3)	Sep. 30, 2017USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 119,400,000
Fixed maximum amounts guaranteed by year 2017	2,500,000
Fixed maximum amounts guaranteed by year 2018	21,300,000
Fixed maximum amounts guaranteed by year 2019	15,800,000
Fixed maximum amounts guaranteed by year 2020	72,600,000
Fixed maximum amounts guaranteed by year 2021.	500,000
Fixed maximum amounts guaranteed, thereafter	2,700,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	0
Letters of credit	34,000,000
Letters of credit set to expire - 2017	29,200,000
Letters of credit set to expire - 2018	4,800,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	556,800,000
Financial guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 57,400,000

Commitment and contingencies va

Commitment and contingencies variable interest entities (Details 4) $ in Millions	Sep. 30, 2017USD ($)
Fuel contract [Member]
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 41.4