MDU Resources (MDU) 10-K 24 Feb 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Feb. 16, 2017	Jun. 30, 2016
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 well-known seasoned issuer	No
Entity voluntary filers	No
Entity current reporting status	Yes
Entity filer category	Large Accelerated Filer
Entity public float			$ 4,687,305,024
Entity common stock, shares outstanding		195,304,376
Document fiscal year focus	2,016
Document fiscal period focus	FY
Document type	10-K
Amendment flag	false
Document period end date	Dec. 31, 2016

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues:
Electric, natural gas distribution and regulated pipeline and midstream		$ 1,141,454	$ 1,149,038	$ 1,246,903
Nonregulated pipeline and midstream, construction materials and contracting, construction services and other		2,987,374	2,865,014	2,868,170
Total operating revenues		4,128,828	4,014,052	4,115,073
Operating expenses:
Fuel and purchased power		75,512	86,238	89,312
Purchased natural gas sold		382,753	450,114	558,463
Operation and maintenance:
Electric, natural gas distribution and regulated pipeline and midstream		312,404	278,171	269,175
Nonregulated pipeline and midstream, construction materials and contracting, construction services and other		2,580,895	2,527,052	2,523,039
Depreciation, depletion and amortization		216,318	211,747	203,084
Taxes, other than income		151,826	140,955	144,818
Total operating expenses		3,719,708	3,694,277	3,787,891
Operating income		409,120	319,775	327,182
Other income		4,956	18,457	9,138
Interest expense		87,848	91,179	86,871
Income before income taxes		326,228	247,053	249,449
Income taxes		93,132	70,664	64,422
Income from continuing operations		233,096	176,389	185,027
Income (loss) from discontinued operations, net of tax (Note 2)	[1]	(300,354)	(834,080)	109,311
Net income (loss)		(67,258)	(657,691)	294,338
Loss from discontinued operations attributable to noncontrolling interest (Note 2)		(131,691)	(35,256)	(3,895)
Dividends declared on preferred stocks		685	685	685
Earnings (loss) on common stock		$ 63,748	$ (623,120)	$ 297,548
Earnings (loss) per common share - basic:
Earnings before discontinued operations		$ 1.19	$ 0.90	$ 0.96
Discontinued operations attributable to the Company, net of tax		(0.86)	(4.10)	0.59
Earnings (loss) per common share - basic		0.33	(3.20)	1.55
Earnings (loss) per common share - diluted:
Earnings before discontinued operations		1.19	0.90	0.96
Discontinued operations attributable to the Company, net of tax		(0.86)	(4.10)	0.59
Earnings (loss) per common share - diluted		$ 0.33	$ (3.20)	$ 1.55
Weighted average common shares outstanding - basic		195,299	194,928	192,507
Weighted average common shares outstanding - diluted		195,618	194,986	192,587
[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	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income (loss)	$ (67,258)	$ (657,691)	$ 294,338
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 $226, $233 and $240 in 2016, 2015 and 2014, respectively	367	404	399
Reclassification adjustment for loss on derivative instruments included in income (loss) from discontinued operations, net of tax of $0, $0 and $173 in 2016, 2015 and 2014, respectively	0	0	295
Net unrealized gain on derivative instruments qualifying as hedges	367	404	694
Postretirement liability adjustment:
Postretirement liability losses arising during the period, net of tax of $(836), $(55) and $(7,665) in 2016, 2015 and 2014, respectively	(1,470)	(88)	(12,409)
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $1,425, $1,128 and $492 in 2016, 2015 and 2014, respectively	2,506	1,794	796
Reclassification of postretirement liability adjustment to regulatory asset, net of tax of $0, $1,416 and $4,509 in 2016, 2015 and 2014, respectively	0	2,255	7,202
Postretirement liability adjustment	1,036	3,961	(4,411)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $31, $(105) and $(99) in 2016, 2015 and 2014, respectively	51	(173)	(162)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss), net of tax of $0, $490 and $0 in 2016, 2015 and 2014, respectively	0	802	0
Foreign currency translation adjustment	51	629	(162)
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(98), $(91) and $(83) in 2016, 2015 and 2014, respectively	(182)	(170)	(154)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $77, $70 and $73 in 2016, 2015 and 2014, respectively	143	131	135
Net unrealized loss on available-for-sale investments	(39)	(39)	(19)
Other comprehensive income (loss)	1,415	4,955	(3,898)
Comprehensive income (loss)	(65,843)	(652,736)	290,440
Loss from discontinued operations attributable to noncontrolling interest	(131,691)	(35,256)	(3,895)
Comprehensive income (loss) attributable to common stockholders	$ 65,848	$ (617,480)	$ 294,335

Consolidated Statements of Com4

Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 226	$ 233	$ 240
Reclassification adjustment for (gain) loss on derivative instruments included in income (loss) from discontinued operations, tax	0	0	173
Postretirement liability gains (losses) arising during the period, tax	(836)	(55)	(7,665)
Amortization of postretirement liability (gains) losses included in net periodic benefit cost (credit), tax	1,425	1,128	492
Reclassification of postretirement liability adjustment to regulatory asset, tax	0	1,416	4,509
Foreign currency translation adjustment recognized during the period, tax	31	(105)	(99)
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income (loss), tax	0	490	0
Net unrealized gains (losses) on available-for-sale investments arising during the period, tax	(98)	(91)	(83)
Reclassification adjustments for (gain) loss on available-for-sale investments included in net income (loss), tax	$ 77	$ 70	$ 73

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 46,107	$ 83,903
Receivables, net	630,243	582,475
Inventories	238,273	240,551
Prepayments and other current assets	48,461	29,528
Current assets held for sale	14,391	54,847
Total current assets	977,475	991,304
Investments	125,866	119,704
Property, plant and equipment (Note 1)	6,510,229	6,387,702
Less accumulated depreciation, depletion and amortization	2,578,902	2,489,322
Net property, plant and equipment	3,931,327	3,898,380
Deferred charges and other assets:
Goodwill (Note 3)	631,791	635,204	[1]
Other intangible assets, net (Note 3)	5,925	7,342
Other	415,419	351,603
Noncurrent assets held for sale	196,664	561,617
Total deferred charges and other assets	1,249,799	1,555,766
Total assets	6,284,467	6,565,154
Current liabilities:
Long-term debt due within one year	43,598	238,539
Accounts payable	279,962	286,061
Taxes payable	48,164	46,880
Dividends payable	37,767	36,784
Accrued compensation	65,867	45,192
Other accrued liabilities	184,377	167,322
Current liabilities held for sale	9,924	126,483
Total current liabilities	669,659	947,261
Long-term debt (Note 6)	1,746,561	1,557,624
Deferred credits and other liabilities:
Deferred income taxes	668,226	663,629
Other	883,777	812,342
Noncurrent liabilities held for sale	0	63,750
Total deferred credits and other liabilities	1,552,003	1,539,721
Commitments and contingencies (Notes 14, 16 and 17)
Equity:
Preferred stocks (Note 8)	15,000	15,000
Common stockholders' equity:
Common stock (Note 9) Authorized - 500,000,000 shares, $1.00 par value Issued - 195,843,297 shares in 2016 and 195,804,665 shares in 2015	195,843	195,805
Other paid-in capital	1,232,478	1,230,119
Retained earnings	912,282	996,355
Accumulated other comprehensive loss	(35,733)	(37,148)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total common stockholders' equity	2,301,244	2,381,505
Total stockholders' equity	2,316,244	2,396,505
Noncontrolling interest	0	124,043
Total equity	2,316,244	2,520,548
Total liabilities and equity	$ 6,284,467	$ 6,565,154
[1]	Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Stockholders' equity:
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1
Common stock, shares issued	195,843,297	195,804,665
Treasury stock, shares	538,921	538,921

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Other paid-in capital	Retained earnings	Total accumulated other comprehensive loss	Treasury stock	Noncontrolling interest
Balance (in shares) at Dec. 31, 2014		150,000
Balance (in shares) at Dec. 31, 2013			189,868,780
Treasury stock (in shares) at Dec. 31, 2013							(538,921)
Balance at Dec. 31, 2013	$ 2,855,902	$ 15,000	$ 189,869	$ 1,056,996	$ 1,603,130	$ (38,205)	$ (3,626)	$ 32,738
Net income (loss) attributable to parent					298,233
Net loss attributable to noncontrolling interest								(3,895)
Net income (loss)	294,338
Other comprehensive income (loss)	(3,898)					(3,898)
Dividends declared on preferred stocks	(685)				(685)
Dividends declared on common stock	(137,851)				(137,851)
Stock-based compensation	6,191			6,191
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings			326,122
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(5,564)		$ 326	(5,890)
Excess tax benefit on stock-based compensation	4,729			4,729
Issuance of common stock - shares			4,559,910
Issuance of common stock	149,722		$ 4,560	145,162
Contribution from non-controlling interest	86,900							86,900
Balance (in shares) at Dec. 31, 2013		150,000
Balance (in shares) at Dec. 31, 2014			194,754,812
Treasury stock (in shares) at Dec. 31, 2014							(538,921)
Balance at Dec. 31, 2014	3,249,784	$ 15,000	$ 194,755	1,207,188	1,762,827	(42,103)	$ (3,626)	115,743
Balance (in shares) at Dec. 31, 2015		150,000
Net income (loss) attributable to parent					(622,435)
Net loss attributable to noncontrolling interest								(35,256)
Net income (loss)	(657,691)
Other comprehensive income (loss)	4,955					4,955
Dividends declared on preferred stocks	(685)				(685)
Dividends declared on common stock	(143,352)				(143,352)
Stock-based compensation	3,689			3,689
Net tax deficit on stock-based compensation	(1,606)			(1,606)
Issuance of common stock - shares			1,049,853
Issuance of common stock	21,898		$ 1,050	20,848
Contribution from non-controlling interest	52,000							52,000
Distribution to non-controlling interest	$ (8,444)							(8,444)
Balance (in shares) at Dec. 31, 2014		150,000
Balance (in shares) at Dec. 31, 2015	195,804,665		195,804,665
Treasury stock (in shares) at Dec. 31, 2015	(538,921)						(538,921)
Balance at Dec. 31, 2015	$ 2,520,548	$ 15,000	$ 195,805	1,230,119	996,355	(37,148)	$ (3,626)	124,043
Balance (in shares) at Dec. 31, 2016		150,000
Net income (loss) attributable to parent					64,433
Net loss attributable to noncontrolling interest								(131,691)
Net income (loss)	(67,258)
Other comprehensive income (loss)	1,415					1,415
Dividends declared on preferred stocks	(685)				(685)
Dividends declared on common stock	(147,821)				(147,821)
Stock-based compensation	4,383			4,383
Net tax deficit on stock-based compensation	(1,663)			(1,663)
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings			38,632
Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(323)		$ 38	(361)
Contribution from non-controlling interest	$ 7,648							7,648
Balance (in shares) at Dec. 31, 2015		150,000
Balance (in shares) at Dec. 31, 2016	195,843,297		195,843,297
Treasury stock (in shares) at Dec. 31, 2016	(538,921)						(538,921)
Balance at Dec. 31, 2016	$ 2,316,244	$ 15,000	$ 195,843	$ 1,232,478	$ 912,282	$ (35,733)	$ (3,626)	$ 0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities:
Net income (loss)		$ (67,258)	$ (657,691)	$ 294,338
Income (loss) from discontinued operations, net of tax	[1]	(300,354)	(834,080)	109,311
Income from continuing operations		233,096	176,389	185,027
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization		216,318	211,747	203,084
Deferred income taxes		(2,049)	(25,356)	54,963
Excess tax benefit on stock-based compensation		0	0	(4,729)
Changes in current assets and liabilities, net of acquisitions:
Receivables		(25,641)	4,704	6,652
Inventories		2,433	2,265	(17,484)
Other current assets		(17,925)	60,182	(45,830)
Accounts payable		7,039	37,224	(47,092)
Other current liabilities		36,146	6,864	(17,252)
Other noncurrent changes		(26,459)	(10,240)	(18,144)
Net cash provided by continuing operations		422,958	463,779	299,195
Net cash provided by discontinued operations		39,251	198,053	287,867
Net cash provided by operating activities		462,209	661,832	587,062
Investing activities:
Capital expenditures		(388,183)	(536,832)	(429,336)
Net proceeds from sale or disposition of property and other		44,826	54,569	28,899
Investments		(1,396)	1,515	(1,041)
Net cash used in continuing operations		(344,753)	(480,748)	(401,478)
Net cash provided by (used in) discontinued operations		39,658	98,295	(502,712)
Net cash used in investing activities		(305,095)	(382,453)	(904,190)
Financing activities:
Repayment of short-term borrowings		0	0	(11,500)
Issuance of long-term debt		309,064	345,920	606,168
Repayment of long-term debt		(315,647)	(566,498)	(365,247)
Proceeds from issuance of common stock		0	21,898	150,060
Dividends paid		(147,156)	(142,835)	(136,712)
Excess tax benefit on stock-based compensation		0	0	4,729
Tax withholding on stock-based compensation		(323)	0	(5,564)
Net cash provided by (used in) continuing operations		(154,062)	(341,515)	241,934
Net cash provided by (used in) discontinued operations		(40,852)	85,785	83,262
Net cash provided by (used in) financing activities		(194,914)	(255,730)	325,196
Effect of exchange rate changes on cash and cash equivalents		4	(225)	(155)
Increase (decrease) in cash and cash equivalents		(37,796)	23,424	7,913
Cash and cash equivalents - beginning of year		83,903	60,479	52,566
Cash and cash equivalents - end of year		$ 46,107	$ 83,903	$ 60,479
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of Significant Accounting Policies Basis of presentation The abbreviations and acronyms used throughout are defined following the Notes to Consolidated Financial Statements. The consolidated financial statements of the Company include the accounts of the following businesses: electric, natural gas distribution, pipeline and midstream, construction materials and contracting, construction services and other. The electric and natural gas distribution businesses, as well as a portion of the pipeline and midstream business, are regulated. Construction materials and contracting, construction services and the other businesses, as well as a portion of the pipeline and midstream business, are nonregulated. For further descriptions of the Company's businesses, see Note 13 . Intercompany balances and transactions have been eliminated in consolidation, except for certain transactions related to the Company's regulated operations in accordance with GAAP. The statements also include the ownership interests in the assets, liabilities and expenses of jointly owned electric generating facilities. The Company's regulated businesses are subject to various state and federal agency regulations. The accounting policies followed by these businesses are generally subject to the Uniform System of Accounts of the FERC. These accounting policies differ in some respects from those used by the Company's nonregulated businesses. The Company's regulated businesses account for certain income and expense items under the provisions of regulatory accounting, which requires these businesses to defer as regulatory assets or liabilities certain items that would have otherwise been reflected as expense or income, respectively, based on the expected regulatory treatment in future rates. The expected recovery or flowback of these deferred items generally is based on specific ratemaking decisions or precedent for each item. Regulatory assets and liabilities are being amortized consistently with the regulatory treatment established by the FERC and the applicable state public service commissions. See Note 4 for more information regarding the nature and amounts of these regulatory deferrals. 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 December 31, 2016 , up to the date of issuance of these consolidated financial statements. 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 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 assets and liabilities for the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. The Company's consolidated financial statements and accompanying notes for current and prior periods have been restated. 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 2 . Cash and cash equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. Accounts receivable and allowance for doubtful accounts Accounts receivable consists primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts. For more information, see Percentage-of-completion method in this note. The total balance of receivables past due 90 days or more was $29.2 million and $27.8 million at December 31, 2016 and 2015 , respectively. The allowance for doubtful accounts is determined through a review of past due balances and other specific account data. Account balances are written off when management determines the amounts to be uncollectible. The Company's allowance for doubtful accounts at December 31, 2016 and 2015 , was $10.5 million and $9.8 million , respectively. Inventories and natural gas in storage Natural gas in storage for the Company's regulated operations is generally carried at average cost, or cost using the last-in, first-out method. All other inventories are stated at the lower of average cost or market value. The portion of the cost of natural gas in storage expected to be used within one year was included in inventories. Inventories at December 31 consisted of: 2016 2015 (In thousands) Aggregates held for resale $ 115,471 $ 115,854 Asphalt oil 29,103 36,498 Natural gas in storage (current) 25,761 21,023 Materials and supplies 18,372 16,997 Merchandise for resale 16,437 15,318 Other 33,129 34,861 Total $ 238,273 $ 240,551 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 and $49.1 million at December 31, 2016 and 2015 , respectively. Investments The Company's investments include the cash surrender value of life insurance policies, an insurance contract, mortgage-backed securities and U.S. Treasury securities. The Company measures its investment in the insurance contract at fair value with any unrealized gains and losses recorded on the Consolidated Statements of Income. The Company has not elected the fair value option for its mortgage-backed securities and U.S. Treasury securities and, as a result, the unrealized gains and losses on these investments are recorded in accumulated other comprehensive income (loss). For more information, see Notes 5 and 14 . Property, plant and equipment Additions to property, plant and equipment are recorded at cost. When regulated assets are retired, or otherwise disposed of in the ordinary course of business, the original cost of the asset is charged to accumulated depreciation. With respect to the retirement or disposal of all other assets the resulting gains or losses are recognized as a component of income. The Company is permitted to capitalize AFUDC on regulated construction projects and to include such amounts in rate base when the related facilities are placed in service. In addition, the Company capitalizes interest, when applicable, on certain construction projects associated with its other operations. The amount of AFUDC and interest capitalized for the years ended December 31 were as follows: 2016 2015 2014 (In thousands) Interest capitalized $ — $ 4,381 $ 7,046 AFUDC - borrowed $ 914 $ 4,907 $ 3,023 AFUDC - equity $ 565 $ 7,971 $ 5,803 Generally, property, plant and equipment are depreciated on a straight-line basis over the average useful lives of the assets, except for depletable aggregate reserves, which are depleted based on the units-of-production method. The Company collects removal costs for plant assets in regulated utility rates. These amounts are recorded as regulatory liabilities, which are included in other liabilities. Property, plant and equipment at December 31 was as follows: 2016 2015 Weighted Average Depreciable Life in Years (Dollars in thousands, where applicable) Regulated: Electric: Generation $ 1,036,373 $ 1,003,173 39 Distribution 398,382 375,612 44 Transmission 284,048 255,842 57 Construction in progress 62,212 42,436 - Other 107,598 109,085 14 Natural gas distribution: Distribution 1,718,633 1,624,645 46 Construction in progress 19,934 20,530 - Other 440,846 431,406 18 Pipeline and midstream: Transmission 490,143 460,305 54 Gathering 37,831 37,831 20 Storage 45,350 44,011 62 Construction in progress 16,507 7,549 - Other 40,873 40,168 33 Nonregulated: Pipeline and midstream: Gathering and processing 31,682 158,949 19 Construction in progress 13 89 - Other 9,800 9,827 10 Construction materials and contracting: Land 94,625 95,870 - Buildings and improvements 102,347 96,864 19 Machinery, vehicles and equipment 930,471 937,084 12 Construction in progress 16,181 18,615 - Aggregate reserves 405,751 404,995 * Construction services: Land 5,346 5,025 - Buildings and improvements 26,693 25,259 26 Machinery, vehicles and equipment 132,217 121,940 6 Other 7,105 11,055 4 Other: Land 2,837 2,837 - Other 46,431 46,700 23 Less accumulated depreciation, depletion and amortization 2,578,902 2,489,322 Net property, plant and equipment $ 3,931,327 $ 3,898,380 * Depleted on the units-of-production method based on recoverable aggregate reserves. Impairment of long-lived assets The Company reviews the carrying values of its long-lived assets, excluding goodwill and assets held for sale, whenever events or changes in circumstances indicate that such carrying values may not be recoverable. The determination of whether an impairment has occurred is based on an estimate of undiscounted future cash flows attributable to the assets, compared to the carrying value of the assets. If impairment has occurred, the amount of the impairment recognized is determined by estimating the fair value of the assets and recording a loss if the carrying value is greater than the fair value. In the third quarter of 2015 , the Company recognized an impairment of $14.1 million (before tax), largely related to the sale of certain non-strategic natural gas gathering assets that were written down to their estimated fair value that was determined using the market approach. In the second quarter of 2015 , the Company recognized an impairment of $3.0 million (before tax) related to coalbed natural gas gathering assets located in Wyoming where there had been continued decline in natural gas development and production activity due to low natural gas prices. The coalbed natural gas gathering assets were written down to their estimated fair value that was determined using the income approach. The impairments are recorded in operation and maintenance expense on the Consolidated Statements of Income. For more information on these nonrecurring fair value measurements, see Note 5 . No significant impairment losses were recorded in 2016, other than those related to the Company's assets held for sale and discontinued operations. For more information regarding these impairments, see Note 2 . Unforeseen events and changes in circumstances could require the recognition of impairment losses at some future date. Goodwill Goodwill represents the excess of the purchase price over the fair value of identifiable net tangible and intangible assets acquired in a business combination. Goodwill is required to be tested for impairment annually, which is completed in the fourth quarter, or more frequently if events or changes in circumstances indicate that goodwill may be impaired. The goodwill impairment test is a two-step process performed at the reporting unit level. The Company has determined that the reporting units for its goodwill impairment test are its operating segments, or components of an operating segment, that constitute a business for which discrete financial information is available and for which segment management regularly reviews the operating results. For more information on the Company's operating segments, see Note 13 . The first step of the impairment test involves comparing the fair value of each reporting unit to its carrying value. If the fair value of a reporting unit exceeds its carrying value, the test is complete and no impairment is recorded. If the fair value of a reporting unit is less than its carrying value, step two of the test is performed to determine the amount of impairment loss, if any. The impairment is computed by comparing the implied fair value of the reporting unit's goodwill to the carrying value of that goodwill. If the carrying value is greater than the implied fair value, an impairment loss must be recorded. For the years ended December 31, 2016 , 2015 and 2014 , there were no significant impairment losses recorded. At December 31, 2016 , the fair value substantially exceeded the carrying value at all reporting units. Determining the fair value of a reporting unit requires judgment and the use of significant estimates which include assumptions about the Company's future revenue, profitability and cash flows, amount and timing of estimated capital expenditures, inflation rates, weighted average cost of capital, operational plans, and current and future economic conditions, among others. The fair value of each reporting unit is determined using a weighted combination of income and market approaches. The Company uses a discounted cash flow methodology for its income approach. Under the income approach, the discounted cash flow model determines fair value based on the present value of projected cash flows over a specified period and a residual value related to future cash flows beyond the projection period. Both values are discounted using a rate which reflects the best estimate of the weighted average cost of capital at each reporting unit. The weighted average cost of capital, which varies by reporting unit and is in the range of 5 percent to 9 percent, and a long-term growth rate projection of approximately 3 percent were utilized in the goodwill impairment test performed in the fourth quarter of 2016 . Under the market approach, the Company estimates fair value using multiples derived from comparable sales transactions and enterprise value to EBITDA for comparative peer companies for each respective reporting unit. These multiples are applied to operating data for each reporting unit to arrive at an indication of fair value. In addition, the Company adds a reasonable control premium when calculating the fair value utilizing the peer multiples, which is estimated as the premium that would be received in a sale in an orderly transaction between market participants. The Company believes that the estimates and assumptions used in its impairment assessments are reasonable and based on available market information, but variations in any of the assumptions could result in materially different calculations of fair value and determinations of whether or not an impairment is indicated. Revenue recognition Revenue is recognized when the earnings process is complete, as evidenced by an agreement between the customer and the Company, when delivery has occurred or services have been rendered, when the fee is fixed or determinable and when collection is reasonably assured. The Company recognizes utility revenue each month based on the services provided to all utility customers during the month. Accrued unbilled revenue which is included in receivables, net, represents revenues recognized in excess of amounts billed. Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain was $117.7 million and $102.1 million at December 31, 2016 and 2015 , respectively. The Company recognizes construction contract revenue at its construction businesses using the percentage-of-completion method as discussed later. The Company recognizes all other revenues when services are rendered or goods are delivered. The Company presents revenues net of taxes collected from customers at the time of sale to be remitted to governmental authorities, including sales and use taxes. Percentage-of-completion method The Company recognizes construction contract revenue from fixed-price and modified fixed-price construction contracts at its construction businesses using the percentage-of-completion method, measured by the percentage of costs incurred to date to estimated total costs for each contract. If a loss is anticipated on a contract, the loss is immediately recognized. Costs and estimated earnings in excess of billings on uncompleted contracts represent revenues recognized in excess of amounts billed and were included in receivables, net. Billings in excess of costs and estimated earnings on uncompleted contracts represent billings in excess of revenues recognized and were included in accounts payable. Costs and estimated earnings in excess of billings and billings in excess of costs and estimated earnings on uncompleted contracts at December 31 were as follows: 2016 2015 (In thousands) Costs and estimated earnings in excess of billings on uncompleted contracts $ 64,558 $ 64,369 Billings in excess of costs and estimated earnings on uncompleted contracts $ 64,832 $ 68,048 Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows: 2016 2015 (In thousands) Short-term retainage* $ 45,109 $ 46,207 Long-term retainage** 1,506 1,605 Total retainage $ 46,615 $ 47,812 * Expected to be paid within one year or less and included in receivables, net. ** Included in deferred charges and other assets - other. Asset retirement obligations The Company records the fair value of a liability for an asset retirement obligation in the period in which it is incurred. When the liability is initially recorded, the Company capitalizes a cost by increasing the carrying amount of the related long-lived asset. Over time, the liability is accreted to its present value each period, and the capitalized cost is depreciated over the useful life of the related asset. Upon settlement of the liability, the Company either settles the obligation for the recorded amount or incurs a gain or loss at its nonregulated operations or incurs a regulatory asset or liability at its regulated operations. For more information on asset retirement obligations, see Note 7 . Legal costs The Company expenses external legal fees as they are incurred. Natural gas costs recoverable or refundable through rate adjustments Under the terms of certain orders of the applicable state public service commissions, the Company is deferring natural gas commodity, transportation and storage costs that are greater or less than amounts presently being recovered through its existing rate schedules. Such orders generally provide that these amounts are recoverable or refundable through rate adjustments which are filed annually. Natural gas costs refundable through rate adjustments were $25.6 million and $20.9 million at December 31, 2016 and 2015 , respectively, which is included in other accrued liabilities. Natural gas costs recoverable through rate adjustments were $2.2 million and $547,000 at December 31, 2016 and 2015 , respectively, which is included in prepayments and other current assets. Income taxes The Company provides deferred federal and state income taxes on all temporary differences between the book and tax basis of the Company's assets and liabilities. Excess deferred income tax balances associated with the Company's rate-regulated activities have been recorded as a regulatory liability and are included in other liabilities. These regulatory liabilities are expected to be reflected as a reduction in future rates charged to customers in accordance with applicable regulatory procedures. The Company uses the deferral method of accounting for investment tax credits and amortizes the credits on regulated electric and natural gas distribution plant over various periods that conform to the ratemaking treatment prescribed by the applicable state public service commissions. Tax positions taken or expected to be taken in an income tax return are evaluated for recognition using a more-likely-than-not threshold, and those tax positions requiring recognition are measured as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon ultimate settlement with a taxing authority. The Company recognizes interest and penalties accrued related to unrecognized tax benefits in income taxes. 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 year. 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 year, plus the effect of outstanding performance share awards. In 2016, 2015 and 2014, there were no shares excluded from the calculation of diluted earnings per share. 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 calculation was as follows: 2016 2015 2014 (In thousands) Weighted average common shares outstanding - basic 195,299 194,928 192,507 Effect of dilutive performance share awards 319 58 80 Weighted average common shares outstanding - diluted 195,618 194,986 192,587 Shares excluded from the calculation of diluted earnings per share — — — Use of estimates The preparation of financial statements in conformity with GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities at the date of the financial statements, as well as the reported amounts of revenues and expenses during the reporting period. Estimates are used for items such as impairment testing of assets held for sale, long-lived assets and goodwill; fair values of acquired assets and liabilities under the acquisition method of accounting; aggregate reserves; property depreciable lives; tax provisions; uncollectible accounts; environmental and other loss contingencies; accumulated provision for revenues subject to refund; costs on construction contracts; unbilled revenues; actuarially determined benefit costs; asset retirement obligations; and the valuation of stock-based compensation. As additional information becomes available, or actual amounts are determinable, the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates. New accounting standards 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. This guidance was to be effective for the Company on January 1, 2017. In August 2015, the FASB issued guidance deferring the effective date of the revenue guidance one year 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. Under the modified 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. In addition, the modified approach will require additional disclosures. The Company is planning to adopt the guidance using the modified retrospective approach and continues to evaluate the effects it will have on its results of operations, financial position, cash flows and disclosures. Simplifying the Presentation of Debt Issuance Costs In April 2015, the FASB issued guidance on simplifying the presentation of debt issuance costs in the financial statements. This guidance requires entities to present debt issuance costs as a direct deduction to the related debt liability. The amortization of these costs will be reported as interest expense. The guidance was effective for the Company on January 1, 2016, and was to be applied retrospectively. Early adoption of this guidance was permitted, however the Company did not elect to do so. The guidance required a reclassification of the debt issuance costs 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 debt issuance costs of $100,000 from prepayments and other current assets and $6.0 million from deferred charges and other assets - other to long-term debt on its Consolidated Balance Sheets at December 31, 2015. Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or its Equivalent) In May 2015, the FASB issued guidance on fair value measurement and disclosure requirements removing the requirement to include investments in the fair value hierarchy for which fair value is measured using the net asset value per share practical expedient. The new guidance also removes the requirement to make certain disclosures for all investments that are eligible to be measured at net asset value using the practical expedient, and rather limits those disclosures to investments for which the practical expedient has been elected. This guidance was effective for the Company on January 1, 2016, with early adoption permitted. The application of this guidance affected the Company's disclosures; however, it did not impact the Company's results of operations, financial position or cash flows. 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. This guidance was effective for the Company on January 1, 2017, on a prospective basis. The Company does not anticipate the guidance will have a material effect on its results of operations, financial position or cash flows. 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 will require all deferred tax assets and liabilities to be classified as noncurrent. These amendments will align GAAP with IFRS. Entities had the option to apply the guidance prospectively, for all deferred tax assets and liabilities, or retrospectively. 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 $33.1 million from current assets - deferred income taxes to deferred credits and other liabilities - deferred income taxes on its Consolidated Balance Sheets at December 31, 2015. 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. Leases In February 2016, the FASB issued guidance regarding leases. The guidance requires lessees to recognize a liability to make lease payments and a right-of-use asset representing its right to use the underlying asset for the lease term on the statement of financial position for leases with terms of more than 12 months. This guidance also requires additional disclosures. 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 is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and 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 will affect the income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and calculation of dilutive shares. Certain amendments of this guidance are to be applied retrospectively and others prospectively. 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 Company anticipates the guidance will impact the Consolidated Statements of Income and the Consolidated Balance Sheets, as well as the dilutive earnings per share calculation, on a prospective basis with all taxes related to share-based payments recognized as income tax expense or benefit and no longer recognized in additional paid-in capital. The Company anticipates the guidance will not have a material impact on its cash flows. 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. An entity that elects early adoption must adopt all the amendments in the same period and apply any adjustments as of the beginning of the fiscal year. 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 is evaluating the effects the adoption of the new guidance will have on its cash flows and disclosures. Clarifying the Definition of a Business In January 2017, the FASB issued guidance to assist entities with evaluating whether transactions should be accounted for

Discontinued operations

Discontinued operations	12 Months Ended
	Dec. 31, 2016
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 have been classified as held for sale. Pronghorn's results of operations are included in the pipeline and midstream segment. The Company's consolidated financial statements and accompanying notes for the current period reflect Pronghorn classified as held for sale. 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. 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 are classified as held for sale associated with Pronghorn on the Company's Consolidated Balance Sheets at December 31 were as follows: 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 The Company performed a fair value assessment of the assets and liabilities classified as held for sale. In the fourth quarter of 2016, the fair value assessment was determined using the market approach based on the purchase and sale agreement with Tesoro Logistics. 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 $2.3 million ( $1.4 million after tax) in the quarter ended December 31, 2016. The fair value of Pronghorn's assets has been categorized as Level 3 in the fair value hierarchy. The impairment was recorded in operation and maintenance expense on the Consolidated Statement of Income. Discontinued operations The assets and liabilities of the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. The Company's consolidated financial statements and accompanying notes for current and prior periods have been restated. 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. The Company retained certain liabilities of Dakota Prairie Refining which are reflected in current liabilities held for sale on the Consolidated Balance Sheet at December 31, 2016. 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 for the guarantee. For more information related to the guarantee, see Note 17 . 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 at December 31 were as follows: 2016 2015 (In thousands) Assets Current assets: Cash and cash equivalents $ — $ 688 Receivables, net — 7,693 Inventories — 13,176 Income taxes receivable 13,987 2,495 Prepayments and other current assets — 6,214 Total current assets held for sale 13,987 30,266 Noncurrent assets: Net property, plant and equipment — 412,717 Other — 9,627 Total noncurrent assets held for sale — 422,344 Total assets held for sale $ 13,987 $ 452,610 Liabilities Current liabilities: Short-term borrowings $ — $ 45,500 Long-term debt due within one year — 5,250 Accounts payable 7,425 24,468 Taxes payable — 1,391 Accrued compensation — 938 Other accrued liabilities — 4,953 Total current liabilities held for sale 7,425 82,500 Noncurrent liabilities: Long-term debt — 63,750 Deferred income taxes 14 (a) 23,841 (a) Total noncurrent liabilities held for sale 14 87,591 Total liabilities held for sale $ 7,439 $ 170,091 (a) 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. 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 have been categorized as Level 3 in the fair value hierarchy. At December 31, 2016, the Company has not incurred any material exit and disposal costs related to Dakota Prairie Refining, and does not expect to incur any material exit and disposal costs. Fidelity In the second quarter of 2015, the Company began the marketing and sale process of Fidelity with an anticipated sale to occur within one year. Between September 2015 and March 2016, the Company entered into purchase and sale agreements to sell all of Fidelity's oil and natural gas assets. The completion of the majority 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 at December 31 were as follows: 2016 2015 (In thousands) Assets Current assets: Receivables, net $ 355 $ 13,387 Inventories — 1,308 Income taxes receivable — 9,665 Prepayments and other current assets — 221 Total current assets held for sale 355 24,581 Noncurrent assets: Investments — 37 Net property, plant and equipment 5,507 793,422 Deferred income taxes 91,098 124,035 Other 161 161 Less allowance for impairment of assets held for sale 938 754,541 Total noncurrent assets held for sale 95,828 163,114 Total assets held for sale $ 96,183 $ 187,695 Liabilities Current liabilities: Accounts payable $ 141 $ 25,013 Taxes payable 19 (a) 1,052 Accrued compensation — 13,080 Other accrued liabilities 2,358 4,838 Total current liabilities held for sale 2,518 43,983 Total liabilities held for sale $ 2,518 $ 43,983 (a) On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale. At December 31, 2016 and 2015, the Company’s deferred tax assets included in assets held for sale were largely comprised of $89.3 million and $78.9 million , respectively, of federal and state net operating loss carryforwards. The Company had federal income tax net operating loss carryforwards of $297.2 million and $208.2 million at December 31, 2016 and 2015, respectively. At December 31, 2016 and 2015, the Company had various state income tax net operating loss carryforwards of $189.1 million and $201.4 million , respectively. The federal net operating loss carryforwards expire in 2036 and 2037 if not utilized. The state net operating loss carryforwards are due to expire between 2023 and 2037. It is likely a portion of the benefit from the state carryforwards will not be realized; therefore, valuation allowances of $500,000 and $300,000 have been provided in 2016 and 2015, respectively. 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. In the second quarter of 2015, the estimated fair value was determined using the income and the market approaches. The income approach was determined by using the present value of future estimated cash flows. The income approach considered management’s views on current operating measures as well as assumptions pertaining to market forces in the oil and gas industry including estimated reserves, estimated prices, market differentials, estimates of well operating and future development costs and timing of operations. The estimated cash flows were discounted using a rate believed to be consistent with those used by principal market participants. The market approach was provided by a third party and based on market transactions involving similar interests in oil and natural gas properties. The fair value assessment indicated an impairment based on the carrying value exceeding the estimated fair value, which resulted in the Company writing down Fidelity’s assets at June 30, 2015, and recording an impairment of $400.0 million ( $252.0 million after tax) during the second quarter of 2015. In the third quarter of 2015, the estimated fair value of Fidelity was determined by agreed upon pricing in the purchase and sale agreements for the assets subject to the agreements, the majority of which closed during the fourth quarter of 2015, including customary purchase price adjustments. The values received in the bid proposals were lower than originally anticipated due to lower commodity prices than those projected in the second quarter of 2015. For those assets for which a purchase and sale agreement had not been entered into at that time, the fair value was based on the market approach utilizing multiples based on similar interests in oil and natural gas properties. The fair value assessment indicated an impairment based on the carrying value exceeding the estimated fair value, which resulted in the Company writing down Fidelity’s assets at September 30, 2015, and recording an impairment of $356.1 million ( $224.4 million after tax). In the fourth quarter of 2015, the fair value assessment was determined using the market approach based on purchase and sale agreements. The estimated fair value exceeded the carrying value and the Company recorded an impairment reversal of $1.6 million ( $1.0 million after tax) in the fourth quarter of 2015. The impairments 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 and $2.5 million in 2015. 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 in 2016, and has incurred $10.5 million of exit and disposal costs to date. 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. The Company incurred lease payments of approximately $900,000 in 2016. Lease termination payments of $3.2 million and $3.3 million were made during the second quarter of 2016 and fourth quarter of 2015, respectively. Existing office furniture and fixtures were relinquished to the lessor in the second quarter of 2016. Historically, the Company used the full-cost method of accounting for its oil and natural gas production activities. Under this method, all costs incurred in the acquisition, exploration and development of oil and natural gas properties are capitalized and amortized on the units-of-production method based on total proved reserves. Prior to the oil and natural gas properties being classified as held for sale, capitalized costs were subject to a "ceiling test" that limits such costs to the aggregate of the present value of future net cash flows from proved reserves discounted at 10 percent, as mandated under the rules of the SEC, plus the cost of unproved properties not subject to amortization, plus the effects of cash flow hedges, less applicable income taxes. Proved reserves and associated future cash flows are determined based on SEC Defined Prices and exclude cash outflows associated with asset retirement obligations that have been accrued on the balance sheet. If capitalized costs, less accumulated amortization and related deferred income taxes, exceed the full-cost ceiling at the end of any quarter, a permanent noncash write-down is required to be charged to earnings in that quarter regardless of subsequent price changes. The Company's capitalized cost under the full-cost method of accounting exceeded the full-cost ceiling at March 31, 2015. SEC Defined Prices, adjusted for market differentials, were used to calculate the ceiling test. Accordingly, the Company was required to write down its oil and natural gas producing properties. The Company recorded a $500.4 million ( $315.3 million after tax) noncash write-down in operating expenses from discontinued operations in the first quarter of 2015. Fidelity previously held commodity derivatives that were not designated as hedging instruments. The amount of gain (loss) recognized in discontinued operations, before tax, was $(18.3) million and $23.4 million in the years ended December 31, 2015 and 2014, respectively. On February 10, 2014, the Company entered into agreements to purchase working interests and leasehold positions in oil and natural gas production assets in the southern Powder River Basin of Wyoming. The effective date of the acquisition was October 1, 2013, and the closing occurred on March 6, 2014. The total purchase price, including purchase price adjustments, for acquisitions in 2014 was approximately $209.2 million , including the above acquisition which is reflected in discontinued operations. Pro forma financial amounts reflecting the effects of the acquisitions are not presented, as such acquisitions were not material to the Company's financial position or results of operations. CEM In 2007, Centennial Resources sold CEM to Bicent. In connection with the sale, Centennial Resources agreed to indemnify Bicent and its affiliates from certain third party claims arising out of or in connection with Centennial Resources' ownership or operation of CEM prior to the sale. In addition, Centennial had previously guaranteed CEM's obligations under a construction contract. The Company incurred legal expenses and had a benefit related to the resolution of this matter in the second quarter of 2014, which are reflected in discontinued operations in the consolidated financial statements and accompanying notes. Dakota Prairie Refining, Fidelity and CEM The reconciliation of the major classes of income and expense constituting pretax income (loss) from discontinued operations, which includes Dakota Prairie Refining, Fidelity and CEM, to the after-tax net income (loss) from discontinued operations on the Company's Consolidated Statements of Income at December 31 were as follows: 2016 2015 2014 (In thousands) Operating revenues $ 123,024 $ 363,115 $ 547,571 Operating expenses 513,813 1,666,941 386,651 Operating income (loss) (390,789 ) (1,303,826 ) 160,920 Other income 306 3,149 1,898 Interest expense 1,753 2,124 145 Income (loss) from discontinued operations before income taxes (392,236 ) (1,302,801 ) 162,673 Income taxes (91,882 ) (468,721 ) 53,362 Income (loss) from discontinued operations (300,354 ) (834,080 ) 109,311 Loss from discontinued operations attributable to noncontrolling interest (131,691 ) (35,256 ) (3,895 ) Income (loss) from discontinued operations attributable to the Company $ (168,663 ) $ (798,824 ) $ 113,206 The pretax loss from discontinued operations attributable to the Company, related to the operations of and activity associated with Dakota Prairie Refining, was $253.5 million , $31.5 million and $3.2 million for the years ended December 31, 2016, 2015 and 2014, respectively.

Goodwill and other intangible a

Goodwill and other intangible assets	12 Months Ended
	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and other intangible assets	Goodwill and Other Intangible Assets The changes in the carrying amount of goodwill for the year ended December 31, 2016 , were as follows: 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. The changes in the carrying amount of goodwill for the year ended December 31, 2015 , were as follows: Balance at January 1, 2015 * Goodwill Acquired During the Year Balance at December 31, 2015 * (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 — 103,441 Total $ 635,204 $ — $ 635,204 * 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 at December 31 were as follows: 2016 2015 (In thousands) Customer relationships $ 17,145 $ 20,975 Less accumulated amortization 13,917 16,845 3,228 4,130 Noncompete agreements 2,430 4,409 Less accumulated amortization 1,658 3,655 772 754 Other 7,768 8,304 Less accumulated amortization 5,843 5,846 1,925 2,458 Total $ 5,925 $ 7,342 Amortization expense for amortizable intangible assets for the years ended December 31, 2016 , 2015 and 2014 , was $2.5 million , $2.5 million and $3.2 million , respectively. Estimated amortization expense for 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 thereafter.

Regulatory assets and liabiliti

Regulatory assets and liabilities	12 Months Ended
	Dec. 31, 2016
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets and liabilities	Regulatory Assets and Liabilities The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31: Estimated Recovery Period * 2016 2015 (In thousands) Regulatory assets: Pension and postretirement benefits (a) (e) $ 176,025 $ 185,832 Taxes recoverable from customers (a) Over plant lives 28,278 27,682 Manufactured gas plant sites remediation (a) — 18,259 18,617 Asset retirement obligations (a) — 42,580 8,000 Natural gas costs recoverable through rate adjustments (b) Up to 1 year 2,242 547 Long-term debt refinancing costs (a) Up to 21 years 6,248 7,031 Costs related to identifying generation development (a) Up to 10 years 3,407 3,808 Other (a) (b) Largely within 1- 4 years 30,281 11,741 Total regulatory assets 307,320 263,258 Regulatory liabilities: Plant removal and decommissioning costs (c) 176,972 182,981 Taxes refundable to customers (c) 11,010 17,060 Pension and postretirement benefits (c) 9,099 4,764 Natural gas costs refundable through rate adjustments (d) 25,580 20,884 Other (c) (d) 19,191 17,429 Total regulatory liabilities 241,852 243,118 Net regulatory position $ 65,468 $ 20,140 * Estimated recovery period for regulatory assets currently being recovered in rates charged to customers. (a) Included in deferred charges and other assets - other on the Consolidated Balance Sheets. (b) Included in prepayments and other current assets on the Consolidated Balance Sheets. (c) Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets. (d) Included in other accrued liabilities on the Consolidated Balance Sheets. (e) Recovered as expense is incurred or cash contributions are made. The regulatory assets are expected to be recovered in rates charged to customers. A portion of the Company's regulatory assets are not earning a return; however, these regulatory assets are expected to be recovered from customers in future rates. As of December 31, 2016 and 2015 , approximately $255.4 million and $224.7 million , respectively, of regulatory assets were not earning a rate of return. If, for any reason, the Company's regulated businesses cease to meet the criteria for application of regulatory accounting for all or part of their operations, the regulatory assets and liabilities relating to those portions ceasing to meet such criteria would be removed from the balance sheet and included in the statement of income or accumulated other comprehensive income (loss) in the period in which the discontinuance of regulatory accounting occurs.

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 2016
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 $70.9 million and $67.5 million at December 31, 2016 and 2015 , respectively, are classified as investments on the Consolidated Balance Sheets. The net unrealized gains on these investments for the years ended December 31, 2016 , 2015 and 2014 , were $3.4 million , $1.7 million and $3.4 million , 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: 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 December 31, 2015 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,128 $ 19 $ (49 ) $ 9,098 U.S. Treasury securities 1,315 — (6 ) 1,309 Total $ 10,443 $ 19 $ (55 ) $ 10,407 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 period, based on published market quotations on active markets, or using other known sources including pricing from outside sources. The estimated fair value of the Company's Level 2 mortgage-backed securities and U.S. Treasury securities are based on comparable market transactions, other observable inputs or other sources, including pricing from outside sources. The estimated fair value of the Company's Level 2 insurance contract is based on contractual cash surrender values that are determined primarily by investments in managed separate accounts of the insurer. These amounts approximate fair value. The managed separate accounts are valued based on other observable inputs or corroborated market data. Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value. For the years ended December 31, 2016 and 2015 , 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 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 Measurements at December 31, 2015, 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, 2015 (In thousands) Assets: Money market funds $ — $ 1,420 $ — $ 1,420 Insurance contract* — 67,459 — 67,459 Available-for-sale securities: Mortgage-backed securities — 9,098 — 9,098 U.S. Treasury securities — 1,309 — 1,309 Total assets measured at fair value $ — $ 79,286 $ — $ 79,286 * The insurance contract invests approximately 63 percent in fixed-income investments, 19 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents. The Company applies the provisions of the fair value measurement standard to its nonrecurring, non-financial measurements, including long-lived asset impairments. These assets are not measured at fair value on an ongoing basis but are subject to fair value adjustments only in certain circumstances. The Company reviews the carrying value of its long-lived assets, excluding goodwill, whenever events or changes in circumstances indicate that such carrying amounts may not be recoverable. During the second quarter of 2015, coalbed natural gas gathering assets at the pipeline and midstream segment were reviewed for impairment and found to be impaired and were written down to their estimated fair value using the income approach. Under this approach, fair value is determined by using the present value of future estimated cash flows. The factors used to determine the estimated future cash flows include, but are not limited to, internal estimates of gathering revenue, future commodity prices and operating costs and equipment salvage values. The estimated cash flows are discounted using a rate that approximates the weighted average cost of capital of a market participant. These fair value inputs are not typically observable. At June 30, 2015, natural gas gathering assets were written down to the nonrecurring fair value measurement of $1.1 million . During the third quarter of 2015, the Company was negotiating the sale of certain non-strategic natural gas gathering assets at the pipeline and midstream segment and as a result these assets were found to be impaired and were written down to their estimated fair value using the market approach. The estimated fair value of natural gas gathering assets that were impaired at September 30, 2015, was largely determined by agreed upon pricing in a purchase and sale agreement that the Company was negotiating, and these assets were sold in the fourth quarter of 2015. At September 30, 2015, natural gas gathering assets were written down to the nonrecurring fair value measurement of $10.8 million . The fair value of these natural gas gathering assets have been categorized as Level 3 in the fair value hierarchy. The Company performed a fair value assessment of the assets and liabilities classified as held for sale. For more information on these Level 3 nonrecurring fair value measurements, see Note 2 . 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 at December 31 was as follows: 2016 2015 Carrying Amount Fair Value Carrying Amount Fair Value (In thousands) Long-term debt $ 1,790,159 $ 1,841,885 $ 1,796,163 $ 1,819,828 The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values.

Debt

Debt	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Debt	Debt Certain debt instruments of the Company and its subsidiaries, including those discussed later, contain restrictive covenants and cross-default provisions. In order to borrow under the respective credit agreements, the Company and its subsidiaries must be in compliance with the applicable covenants and certain other conditions. In the event the Company and its subsidiaries do not comply with the applicable covenants and other conditions, alternative sources of funding may need to be pursued. The following table summarizes the outstanding revolving credit facilities of the Company and its subsidiaries: Company Facility Facility Limit Amount Outstanding at December 31, 2016 Amount Outstanding at December 31, 2015 Letters of Credit at December 31, 2016 Expiration Date (In millions) MDU Resources Group, Inc. Commercial paper/Revolving credit agreement (a) $ 175.0 $ 111.0 (b) $ 44.5 (b) $ — 5/8/19 Cascade Natural Gas Corporation Revolving credit agreement $ 50.0 (c) $ — $ — $ 2.2 (d) 7/9/18 Intermountain Gas Company Revolving credit agreement $ 65.0 (e) $ 20.9 $ 47.9 $ — 7/13/18 Centennial Energy Holdings, Inc. Commercial paper/Revolving credit agreement (f) $ 500.0 $ 151.0 (b) $ 18.0 (b) $ — 9/23/21 (a) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of the Company on stated conditions, up to a maximum of $225.0 million ). There were no amounts outstanding under the credit agreement. (b) Amount outstanding under commercial paper program. (c) Certain provisions allow for increased borrowings, up to a maximum of $75.0 million . (d) Outstanding letter(s) of credit reduce the amount available under the credit agreement. (e) Certain provisions allow for increased borrowings, up to a maximum of $90.0 million . (f) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Centennial on stated conditions, up to a maximum of $600.0 million ). There were no amounts outstanding under the credit agreement. The Company's and Centennial's respective commercial paper programs are supported by revolving credit agreements. While the amount of commercial paper outstanding does not reduce available capacity under the respective revolving credit agreements, the Company and Centennial do not issue commercial paper in an aggregate amount exceeding the available capacity under their credit agreements. The commercial paper borrowings may vary during the period, largely the result of fluctuations in working capital requirements due to the seasonality of the construction businesses. The following includes information related to the preceding table. Long-term debt MDU Resources Group, Inc. The Company's revolving credit agreement supports its commercial paper program. Commercial paper borrowings under this agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued commercial paper borrowings. The credit agreement contains customary covenants and provisions, including covenants of the Company not to permit, as of the end of any fiscal quarter, (A) the ratio of funded debt to total capitalization (determined on a consolidated basis) to be greater than 65 percent or (B) the ratio of funded debt to capitalization (determined with respect to the Company alone, excluding its subsidiaries) to be greater than 65 percent. Other covenants include limitations on the sale of certain assets and on the making of certain loans and investments. There are no credit facilities that contain cross-default provisions between the Company and any of its subsidiaries. On November 21, 2016, the Company entered into a $100.0 million note purchase agreement. The Company issued $40.0 million of Senior Notes under the agreement on November 21, 2016, with a due date of November 21, 2046, at an interest rate of 4.15 percent . The Company contracted to issue an additional $60.0 million of Senior Notes under the agreement on March 21, 2017, with due dates ranging from March 2032 to March 2037 at a weighted average interest rate of 3.61 percent . MDU Energy Capital, LLC The ability to request additional borrowings under the master shelf agreement expired; however, there is debt outstanding that is reflected in the following table of long-term debt outstanding. The master shelf agreement contains customary covenants and provisions, including covenants of MDU Energy Capital not to permit (A) the ratio of its total debt (on a consolidated basis) to adjusted total capitalization to be greater than 70 percent, or (B) the ratio of subsidiary debt to subsidiary capitalization to be greater than 65 percent, or (C) the ratio of Intermountain’s total debt (determined on a consolidated basis) to total capitalization to be greater than 65 percent. The agreement also includes a covenant requiring the ratio of MDU Energy Capital earnings before interest and taxes to interest expense (on a consolidated basis), for the 12-month period ended each fiscal quarter, to be greater than 1.5 to 1. In addition, payment obligations under the master shelf agreement may be accelerated upon the occurrence of an event of default (as described in the agreement). Cascade Natural Gas Corporation Any borrowings under the revolving credit agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued borrowings. The credit agreement contains customary covenants and provisions, including a covenant of Cascade not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. Other covenants include restrictions on the sale of certain assets, limitations on indebtedness and the making of certain investments. Cascade's credit agreement also contains cross-default provisions. These provisions state that if Cascade fails to make any payment with respect to any indebtedness or contingent obligation, in excess of a specified amount, under any agreement that causes such indebtedness to be due prior to its stated maturity or the contingent obligation to become payable, Cascade will be in default under the revolving credit agreement. Intermountain Gas Company Any borrowings under the revolving credit agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued borrowings. The credit agreement contains customary covenants and provisions, including a covenant of Intermountain not to permit, at any time, the ratio of total debt to total capitalization to be greater than 65 percent. Other covenants include restrictions on the sale of certain assets, limitations on indebtedness and the making of certain investments. Intermountain's credit agreement also contains cross-default provisions. These provisions state that if Intermountain fails to make any payment with respect to any indebtedness or contingent obligation, in excess of a specified amount, under any agreement that causes such indebtedness to be due prior to its stated maturity or the contingent obligation to become payable, or certain conditions result in an early termination date under any swap contract that is in excess of a specified amount, then Intermountain will be in default under the revolving credit agreement. On November 9, 2016, Intermountain issued $30.0 million of Senior Notes with a due date of November 9, 2046, at an interest rate of 4.0 percent. Centennial Energy Holdings, Inc. On September 23, 2016, Centennial amended its revolving credit agreement to decrease the borrowing limit by $150.0 million to $500.0 million and extend the termination date to September 23, 2021. Centennial's revolving credit agreement supports its commercial paper program. Commercial paper borrowings under this agreement are classified as long-term debt as they are intended to be refinanced on a long-term basis through continued commercial paper borrowings. Centennial's revolving credit agreement and certain debt outstanding under an expired uncommitted long-term master shelf agreement contain customary covenants and provisions, including a covenant of Centennial, not to permit, as of the end of any fiscal quarter, the ratio of total consolidated debt to total consolidated capitalization to be greater than 65 percent (for the revolving credit agreement) and a covenant of Centennial and certain of its subsidiaries, not to permit, as of the end of any fiscal quarter, the ratio of total debt to total capitalization to be greater than 60 percent (for the master shelf agreement). The master shelf agreement also includes a covenant that does not permit the ratio of Centennial's EBITDA to interest expense, for the 12-month period ended each fiscal quarter, to be less than 1.75 to 1. Other covenants include restricted payments, restrictions on the sale of certain assets, limitations on subsidiary indebtedness, minimum consolidated net worth, limitations on priority debt and the making of certain loans and investments. Certain of Centennial's financing agreements contain cross-default provisions. These provisions state that if Centennial or any subsidiary of Centennial fails to make any payment with respect to any indebtedness or contingent obligation, in excess of a specified amount, under any agreement that causes such indebtedness to be due prior to its stated maturity or the contingent obligation to become payable, the applicable agreements will be in default. WBI Energy Transmission, Inc. On May 17, 2016, WBI Energy Transmission entered into an amendment to its amended and restated uncommitted note purchase and private shelf agreement to increase the aggregate issuance capacity from $175.0 million to $200.0 million and extend the issuance period to May 16, 2019. WBI Energy Transmission had $100.0 million of notes outstanding at December 31, 2016 , which reduced the remaining capacity under this uncommitted private shelf agreement to $100.0 million . This agreement contains customary covenants and provisions, including a covenant of WBI Energy Transmission not to permit, as of the end of any fiscal quarter, the ratio of total debt to total capitalization to be greater than 55 percent. Other covenants include a limitation on priority debt and restrictions on the sale of certain assets and the making of certain investments. Long-term Debt Outstanding Long-term debt outstanding at December 31 was as follows: 2016 2015 (In thousands) Senior Notes at a weighted average rate of 4.87%, due on dates ranging from August 31, 2017 to January 15, 2055 $ 1,437,831 $ 1,616,246 Commercial paper at a weighted average rate of 1.27%, supported by revolving credit agreements 262,000 62,500 Medium-Term Notes at a weighted average rate of 6.68%, due on dates ranging from September 1, 2020 to March 16, 2029 50,000 50,000 Other notes at a weighted average rate of 5.25%, due on February 1, 2035 24,471 24,589 Credit agreements at a weighted average rate of 3.14%, due on dates ranging from July 13, 2018 to November 30, 2038 21,793 48,906 Unamortized debt issuance costs (5,832 ) (6,069 ) Discount (104 ) (9 ) Total long-term debt 1,790,159 1,796,163 Less current maturities 43,598 238,539 Net long-term debt $ 1,746,561 $ 1,557,624 Schedule of Debt Maturities Long-term debt maturities for the five years and thereafter following December 31, 2016 , were as follows: 2017 2018 2019 2020 2021 Thereafter (In thousands) Long-term debt maturities $ 43,598 $ 169,449 $ 162,154 $ 15,021 $ 151,013 $ 1,254,860

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2016
Asset Retirement Obligation [Abstract]
Asset retirement obligations	Asset Retirement Obligations The Company records obligations related to retirement costs of natural gas distribution mains and lines, natural gas transmission lines, storage facilities, decommissioning of certain electric generating facilities, reclamation of certain aggregate properties, special handling and disposal of hazardous materials at certain electric generating facilities, natural gas distribution facilities and buildings, and certain other obligations as asset retirement obligations. A reconciliation of the Company's liability, which is included in other accrued liabilities and deferred credits and other liabilities - other on the Consolidated Balance Sheets, for the years ended December 31 was as follows: 2016 2015 (In thousands) Balance at beginning of year $ 242,224 $ 27,211 Liabilities incurred 15,114 2,751 Liabilities settled (4,338 ) (1,708 ) Accretion expense 13,918 2,134 Revisions in estimates 48,052 211,836 Balance at end of year $ 314,970 $ 242,224 The 2016 revisions in estimates consist principally of updated asset retirement obligation costs associated with natural gas transmission lines and storage facilities at the pipeline and midstream segment. The 2015 revisions in estimates consist principally of updated asset retirement obligation costs associated with natural gas distribution mains and lines at the natural gas distribution segment. The Company believes that largely all expenses related to asset retirement obligations at the Company's regulated operations will be recovered in rates over time and, accordingly, defers such expenses as regulatory assets. For more information on the Company's regulatory assets and liabilities, see Note 4 .

Preferred Stocks

Preferred Stocks	12 Months Ended
	Dec. 31, 2016
Class of Stock Disclosures [Abstract]
Preferred stocks	Preferred Stocks Preferred stocks at December 31 were as follows: 2016 2015 (In thousands, except shares and per share amounts) Authorized: Preferred - 500,000 shares, cumulative, par value $100, issuable in series Preferred stock A - 1,000,000 shares, cumulative, without par value, issuable in series (none outstanding) Preference - 500,000 shares, cumulative, without par value, issuable in series (none outstanding) Outstanding: 4.50% Series - 100,000 shares $ 10,000 $ 10,000 4.70% Series - 50,000 shares 5,000 5,000 Total preferred stocks $ 15,000 $ 15,000 For the years 2016 , 2015 and 2014 , dividends declared on the 4.50% Series and 4.70% Series preferred stocks were $4.50 and $4.70 per share, respectively. The 4.50% Series and 4.70% Series preferred stocks outstanding are subject to redemption, in whole or in part, at the option of the Company with certain limitations on 30 days notice on any quarterly dividend date at a redemption price, plus accrued dividends, of $105 per share and $102 per share, respectively. In the event of a voluntary or involuntary liquidation, all preferred stock series holders are entitled to $100 per share, plus accrued dividends. The affirmative vote of two-thirds of a series of the Company's outstanding preferred stock is necessary for amendments to the Company's charter or bylaws that adversely affect that series; creation of or increase in the amount of authorized stock ranking senior to that series (or an affirmative majority vote where the authorization relates to a new class of stock that ranks on parity with such series); a voluntary liquidation or sale of substantially all of the Company's assets; a merger or consolidation, with certain exceptions; or the partial retirement of that series of preferred stock when all dividends on that series of preferred stock have not been paid. The consent of the holders of a particular series is not required for such corporate actions if the equivalent vote of all outstanding series of preferred stock voting together has consented to the given action and no particular series is affected differently than any other series. Subject to the foregoing, the holders of common stock exclusively possess all voting power. However, if cumulative dividends on preferred stock are in arrears, in whole or in part, for one year, the holders of preferred stock would obtain the right to one vote per share until all dividends in arrears have been paid and current dividends have been declared and set aside.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2016
Class of Stock Disclosures [Abstract]
Common Stock	Common Stock For the years 2016 , 2015 and 2014 , dividends declared on common stock were $.7550 , $.7350 and $.7150 per common share, respectively. The Stock Purchase Plan provided interested investors the opportunity to make optional cash investments and to reinvest all or a percentage of their cash dividends in shares of the Company's common stock. The K-Plan provides participants the option to invest in the Company's common stock. From January 2014 through August 2015, the Stock Purchase Plan and K-Plan, with respect to Company stock, purchased shares of authorized but unissued common stock from the Company. From September 2015 through December 2016, the K-Plan purchased shares of common stock on the open market. At December 31, 2016 , there were 7.8 million shares of common stock reserved for original issuance under the K-Plan. From September 2015 through December 4, 2016, the Stock Purchase Plan purchased shares of common stock on the open market. On December 5, 2016, the Stock Purchase Plan was terminated and all remaining shares reserved for original issuance under the plan have been de-registered. The Company depends on earnings from its divisions and dividends from its subsidiaries to pay dividends on common stock. The declaration and payment of dividends is at the sole discretion of the board of directors, subject to limitations imposed by the Company's credit agreements, federal and state laws, and applicable regulatory limitations. In addition, the Company and Centennial are generally restricted to paying dividends out of capital accounts or net assets. The following discusses the most restrictive limitations. Pursuant to a covenant under a credit agreement, Centennial may only declare or pay distributions if as of the last day of any fiscal quarter, the ratio of Centennial's average consolidated indebtedness as of the last day of such fiscal quarter and each of the preceding three fiscal quarters to Centennial's Consolidated EBITDA does not exceed 3 to 1; and after giving effect to such distribution, all distributions made during the 12-month period ending on the last day of the fiscal quarter in which such distribution is made will not exceed the remainder of Centennial's Consolidated EBITDA minus Centennial's capital expenditures less the net cash proceeds from all sales of capital assets from continuing operations, for the immediately preceding 12-month period. Intermountain and Cascade have regulatory limitations on the amount of dividends each can pay. Based on these limitations, approximately $1.3 billion of the net assets of the Company's subsidiaries were restricted from being used to transfer funds to the Company at December 31, 2016 . In addition, the Company's credit agreement also contains restrictions on dividend payments. The most restrictive limitation requires the Company not to permit the ratio of funded debt to capitalization (determined with respect to the Company alone, excluding its subsidiaries) to be greater than 65 percent. Based on this limitation, approximately $351 million of the Company's (excluding its subsidiaries) net assets, which represents common stockholders' equity including retained earnings, would be restricted from use for dividend payments at December 31, 2016 . In addition, state regulatory commissions may require the Company to maintain certain capitalization ratios. These requirements are not expected to affect the Company's ability to pay dividends in the near term.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Share-based Compensation [Abstract]
Stock-based compensation	Stock-Based Compensation The Company has several stock-based compensation plans under which it is currently authorized to grant restricted stock and stock. As of December 31, 2016 , there are 5.5 million remaining shares available to grant under these plans. The Company generally issues new shares of common stock to satisfy employee performance share awards and purchases shares on the open market for nonemployee director stock awards. Total stock-based compensation expense (after tax) was $3.3 million , $2.9 million and $4.4 million in 2016 , 2015 and 2014 , respectively. As of December 31, 2016 , total remaining unrecognized compensation expense related to stock-based compensation was approximately $4.9 million (before income taxes) which will be amortized over a weighted average period of 1.5 years. Stock awards Nonemployee directors may receive shares of common stock instead of cash in payment for directors' fees under the nonemployee director stock compensation plan. There were 37,218 shares with a fair value of $1.1 million , 58,181 shares with a fair value of $1.1 million and 43,088 shares with a fair value of $1.1 million issued under this plan during the years ended December 31, 2016 , 2015 and 2014 , respectively. Performance share awards Since 2003, key employees of the Company have been awarded performance share awards each year. Entitlement to performance shares is based on the Company's total shareholder return over designated performance periods as measured against a selected peer group. Target grants of performance shares outstanding at December 31, 2016 , were as follows: Grant Date Performance Period Target Grant of Shares February 2014 2014-2016 136,901 February 2015 2015-2017 200,112 June 2015 2015-2017 14,441 February 2016 2016-2018 310,583 March 2016 2016-2018 2,151 Participants may earn from zero to 200 percent of the target grant of shares based on the Company's total shareholder return relative to that of the selected peer group. Compensation expense is based on the grant-date fair value as determined by Monte Carlo simulation. The blended volatility term structure ranges are comprised of 50 percent historical volatility and 50 percent implied volatility. Risk-free interest rates were based on U.S. Treasury security rates in effect as of the grant date. Assumptions used for grants of performance shares issued in 2016 , 2015 and 2014 were: 2016 2015 2014 Weighted average grant-date fair value $14.60 $18.98 $41.13 Blended volatility range 29.25 % – 32.51 % 22.86 % – 24.61 % 18.94 % – 20.43 % Risk-free interest rate range .47 % – .92 % .05 % – 1.07 % .03 % – .74 % Weighted average discounted dividends per share $1.56 $1.57 $2.15 The fair value of the performance shares that vested during the years ended December 31, 2016 and 2014, was $953,000 and $16.6 million , respectively. There were no performance shares that vested in 2015. A summary of the status of the performance share awards for the year ended December 31, 2016 , was as follows: Number of Shares Weighted Average Grant-Date Fair Value Nonvested at beginning of period 565,896 $ 27.90 Granted 324,205 14.60 Less: Vested 58,401 29.01 Forfeited 167,512 27.30 Nonvested at end of period 664,188 $ 21.47

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income taxes	Income Taxes The components of income before income taxes from continuing operations for each of the years ended December 31 were as follows: 2016 2015 2014 (In thousands) United States $ 326,252 $ 248,379 $ 249,501 Foreign (24 ) (1,326 ) (52 ) Income before income taxes from continuing operations $ 326,228 $ 247,053 $ 249,449 Income tax expense from continuing operations for the years ended December 31 was as follows: 2016 2015 2014 (In thousands) Current: Federal $ 81,989 $ 85,897 $ 8,837 State 13,190 10,093 622 Foreign 2 30 — 95,181 96,020 9,459 Deferred: Income taxes: Federal (2,102 ) (19,632 ) 52,041 State 1,184 (5,304 ) 1,913 Investment tax credit - net (1,131 ) (420 ) 1,009 (2,049 ) (25,356 ) 54,963 Total income tax expense $ 93,132 $ 70,664 $ 64,422 Components of deferred tax assets and deferred tax liabilities at December 31 were as follows: 2016 2015 (In thousands) Deferred tax assets: Postretirement $ 87,872 $ 97,666 Compensation-related 44,995 33,714 Alternative minimum tax credit carryforward 29,338 28,169 Federal renewable energy credit 16,944 3,400 Customer advances 13,524 12,623 Legal and environmental contingencies 9,895 6,377 Asset retirement obligations 8,867 8,694 Other 46,957 43,306 Total deferred tax assets 258,392 233,949 Deferred tax liabilities: Depreciation and basis differences on property, plant and equipment 774,838 756,444 Postretirement 70,670 71,835 Intangible asset amortization 26,413 23,950 Other 45,580 36,359 Total deferred tax liabilities 917,501 888,588 Valuation allowance 9,117 8,990 Net deferred income tax liability $ 668,226 $ 663,629 As of December 31, 2016 and 2015 , the Company had various state income tax net operating loss carryforwards of $114.7 million and $116.2 million , respectively, and federal and state income tax credit carryforwards, excluding alternative minimum tax credit carryforwards, of $43.3 million and $21.3 million , respectively. Included in the state credits are various regulatory investment tax credits of approximately $20.7 million and $13.9 million at December 31, 2016 and 2015, respectively. The federal income tax credit carryforwards expire in 2036 and 2037 if not utilized and state income tax credit carryforwards are due to expire between 2019 and 2042. It is likely that a portion of the benefit from the state carryforwards will not be realized; therefore, valuation allowances have been provided. Changes in tax regulations or assumptions regarding current and future taxable income could require additional valuation allowances in the future. The alternative minimum tax credit carryforwards do not expire. For information regarding net operating loss carryforwards and valuation allowances related to discontinued operations, see Note 2 . The following table reconciles the change in the net deferred income tax liability from December 31, 2015 , to December 31, 2016 , to deferred income tax expense: 2016 (In thousands) Change in net deferred income tax liability from the preceding table $ 4,597 Deferred taxes associated with other comprehensive income (825 ) Other (5,821 ) Deferred income tax benefit for the period $ (2,049 ) Total income tax expense differs from the amount computed by applying the statutory federal income tax rate to income before taxes. The reasons for this difference were as follows: Years ended December 31, 2016 2015 2014 Amount % Amount % Amount % (Dollars in thousands) Computed tax at federal statutory rate $ 114,179 35.0 $ 86,468 35.0 $ 87,308 35.0 Increases (reductions) resulting from: State income taxes, net of federal income tax 9,027 2.8 8,208 3.3 7,019 2.8 Federal renewable energy credit (13,544 ) (4.2 ) (3,400 ) (1.4 ) (3,655 ) (1.5 ) Tax compliance and uncertain tax positions (3,028 ) (.9 ) (2,607 ) (1.0 ) (8,568 ) (3.4 ) Domestic production activities (6,251 ) (1.9 ) (6,842 ) (2.8 ) (3,993 ) (1.6 ) Other (7,251 ) (2.3 ) (11,163 ) (4.5 ) (13,689 ) (5.5 ) Total income tax expense $ 93,132 28.5 $ 70,664 28.6 $ 64,422 25.8 Deferred income taxes have been accrued with respect to temporary differences related to the Company's foreign operations. The amount of cumulative undistributed earnings for which there are temporary differences is approximately $2.4 million at December 31, 2016 . The amount of deferred tax liability, net of allowable foreign tax credits, associated with the undistributed earnings at December 31, 2016 , was approximately $889,000 . The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction, and various state, local and foreign jurisdictions. The Company is no longer subject to U.S. federal or non-U.S. income tax examinations by tax authorities for years ending prior to 2012. With few exceptions, as of December 31, 2016 , the Company is no longer subject to state and local income tax examinations by tax authorities for years ending prior to 2011. A reconciliation of the unrecognized tax benefits (excluding interest) for the years ended December 31 was as follows: 2016 2015 2014 (In thousands) Balance at beginning of year $ — $ 105 $ 7,845 Settlements — — (7,740 ) Lapse of statute of limitations — (105 ) — Balance at end of year $ — $ — $ 105 Included in income tax expense is interest on uncertain tax positions. For the years ended December 31, 2016 , 2015 and 2014 , the Company recognized approximately $(92,000) , $122,000 and $387,000 , respectively, of interest (income) expense in income tax expense. At December 31, 2016 and 2015 , the Company had accrued receivables of approximately $54,000 and interest payable of $94,000 , respectively, for the receipt or payment of interest.

Cash flow information

Cash flow information	12 Months Ended
	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash flow information	Cash Flow Information Cash expenditures for interest and income taxes for the years ended December 31 were as follows: 2016 2015 2014 (In thousands) Interest, net of amount capitalized and AFUDC - borrowed of $914, $9,288 and $10,069 in 2016, 2015 and 2014, respectively $ 87,920 $ 88,775 $ 81,195 Income taxes paid, net* $ 105,908 $ 61,405 $ 80,090 * Income taxes paid, net of discontinued operations, were $1.3 million , $2.4 million and $69.8 million for the years ended December 31, 2016, 2015 and 2014, respectively. Noncash investing transactions at December 31 were as follows: 2016 2015 2014 (In thousands) Property, plant and equipment additions in accounts payable $ 22,712 $ 39,754 $ 12,791

Business segment data

Business segment data	12 Months Ended
	Dec. 31, 2016
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, gathering and processing services, as well as oil gathering, through regulated and nonregulated pipeline systems and processing facilities primarily in the Rocky Mountain and northern Great Plains regions of the United States. This segment also provides cathodic protection and other energy-related services. The construction materials and contracting segment mines 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 utility 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 and pollution liability 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 the Brazilian Transmission Lines. Discontinued operations includes the results 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. Discontinued operations also includes legal expenses and a benefit related to the vacation of an arbitration award in 2014 related to Centennial Resources. For more information on discontinued operations, see Note 2 . The information below follows the same accounting policies as described in the Summary of Significant Accounting Policies. Information on the Company's businesses as of December 31 and for the years then ended was as follows: 2016 2015 2014 (In thousands) External operating revenues: Regulated operations: Electric $ 322,356 $ 280,615 $ 277,874 Natural gas distribution 766,115 817,419 921,986 Pipeline and midstream 52,983 51,004 47,043 1,141,454 1,149,038 1,246,903 Nonregulated operations: Pipeline and midstream 39,602 54,281 64,494 Construction materials and contracting 1,873,696 1,901,530 1,740,089 Construction services 1,072,663 907,767 1,062,055 Other 1,413 1,436 1,532 2,987,374 2,865,014 2,868,170 Total external operating revenues $ 4,128,828 $ 4,014,052 $ 4,115,073 Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — Natural gas distribution — — — Pipeline and midstream 48,794 49,065 45,013 48,794 49,065 45,013 Nonregulated operations: Pipeline and midstream 223 554 742 Construction materials and contracting 574 2,752 25,241 Construction services 609 18,660 57,474 Other 7,230 7,755 7,832 8,636 29,721 91,289 Intersegment eliminations (57,430 ) (78,786 ) (136,302 ) Total intersegment operating revenues $ — $ — $ — Depreciation, depletion and amortization: Electric $ 50,220 $ 37,583 $ 35,008 Natural gas distribution 65,426 64,756 54,700 Pipeline and midstream 24,885 27,981 29,749 Construction materials and contracting 58,413 65,937 68,557 Construction services 15,307 13,420 12,874 Other 2,067 2,070 2,196 Total depreciation, depletion and amortization $ 216,318 $ 211,747 $ 203,084 Interest expense: Electric $ 24,982 $ 17,421 $ 15,595 Natural gas distribution 30,405 29,471 27,217 Pipeline and midstream 7,903 9,895 9,946 Construction materials and contracting 15,265 15,183 16,368 Construction services 4,059 3,959 4,176 Other 5,854 15,853 13,823 Intersegment eliminations (620 ) (603 ) (254 ) Total interest expense $ 87,848 $ 91,179 $ 86,871 2016 2015 2014 (In thousands) Income taxes: Electric $ 1,449 $ 11,523 $ 12,442 Natural gas distribution 9,181 11,377 11,350 Pipeline and midstream 12,408 7,505 12,232 Construction materials and contracting 60,625 41,619 18,586 Construction services 17,748 16,432 24,753 Other (2,028 ) (9,834 ) (11,136 ) Intersegment eliminations (6,251 ) (7,958 ) (3,805 ) Total income taxes $ 93,132 $ 70,664 $ 64,422 Earnings (loss) on common stock: Regulated operations: Electric $ 42,222 $ 35,914 $ 36,731 Natural gas distribution 27,102 23,607 30,484 Pipeline and midstream 22,060 20,680 15,440 91,384 80,201 82,655 Nonregulated operations: Pipeline and midstream 1,375 (7,430 ) 9,226 Construction materials and contracting 102,687 89,096 51,510 Construction services 33,945 23,762 54,432 Other (3,231 ) (14,941 ) (7,386 ) 134,776 90,487 107,782 Intersegment eliminations (a) 6,251 5,016 (6,095 ) Earnings on common stock before income (loss) from discontinued operations 232,411 175,704 184,342 Income (loss) from discontinued operations, net of tax (a) (300,354 ) (834,080 ) 109,311 Loss from discontinued operations attributable to noncontrolling interest (131,691 ) (35,256 ) (3,895 ) Total earnings (loss) on common stock $ 63,748 $ (623,120 ) $ 297,548 Capital expenditures: Electric $ 111,134 $ 332,876 $ 185,121 Natural gas distribution 126,272 130,793 120,613 Pipeline and midstream 34,467 18,315 61,754 Construction materials and contracting 37,845 48,126 37,896 Construction services 60,344 38,269 26,942 Other 2,358 3,755 2,131 Total capital expenditures (b) $ 372,420 $ 572,134 $ 434,457 Assets: Electric (c) $ 1,406,694 $ 1,325,858 $ 1,028,001 Natural gas distribution (c) 2,099,296 2,038,433 1,935,271 Pipeline and midstream 550,615 591,651 651,925 Construction materials and contracting 1,220,459 1,261,963 1,260,534 Construction services 513,093 442,845 437,322 Other (d) 283,255 287,940 315,495 Assets held for sale 211,055 616,464 2,176,857 Total assets $ 6,284,467 $ 6,565,154 $ 7,805,405 2016 2015 2014 (In thousands) Property, plant and equipment: Electric (c) $ 1,888,613 $ 1,786,148 $ 1,457,101 Natural gas distribution (c) 2,179,413 2,076,581 1,904,759 Pipeline and midstream 672,199 758,729 818,388 Construction materials and contracting 1,549,375 1,553,428 1,529,942 Construction services 171,361 163,279 144,395 Other 49,268 49,537 50,937 Less accumulated depreciation, depletion and amortization 2,578,902 2,489,322 2,385,202 Net property, plant and equipment $ 3,931,327 $ 3,898,380 $ 3,520,320 (a) Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations. (b) Capital expenditures for 2016, 2015 and 2014 include noncash capital expenditure-related accounts payable and AFUDC, totaling $(15.8) million , $35.3 million and $5.1 million , respectively. (c) Includes allocations of common utility property. (d) Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets).

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
Employee Benefit Plans	Employee Benefit Plans Pension and other postretirement benefit plans The Company has noncontributory defined benefit pension plans and other postretirement benefit plans for certain eligible employees. The Company uses a measurement date of December 31 for all of its pension and postretirement benefit plans. Prior to 2013, defined pension plan benefits and accruals for all nonunion and certain union plans were frozen. On June 30, 2015, an additional union plan was frozen. As of June 30, 2015, all of the Company's defined pension plans were frozen. These employees were eligible to receive additional defined contribution plan benefits. Effective January 1, 2010, eligibility to receive retiree medical benefits was modified at certain of the Company's businesses. Employees who had attained age 55 with 10 years of continuous service by December 31, 2010, will be provided the current retiree medical insurance benefits or can elect the new benefit, if desired, regardless of when they retire. All other current employees must meet the new eligibility criteria of age 60 and 10 years of continuous service at the time they retire. These employees will be eligible for a specified company funded Retiree Reimbursement Account. Employees hired after December 31, 2009, will not be eligible for retiree medical benefits at certain of the Company's businesses. In 2012, the Company modified health care coverage for certain retirees. Effective January 1, 2013, post-65 coverage was replaced by a fixed-dollar subsidy for retirees and spouses to be used to purchase individual insurance through an exchange. Changes in benefit obligation and plan assets for the years ended December 31, 2016 and 2015 , and amounts recognized in the Consolidated Balance Sheets at December 31, 2016 and 2015 , were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2016 2015 (In thousands) Change in benefit obligation: Benefit obligation at beginning of year $ 442,960 $ 475,337 $ 92,734 $ 99,012 Service cost — 86 1,647 1,816 Interest cost 17,218 17,141 3,688 3,607 Plan participants' contributions — — 1,405 1,408 Actuarial (gain) loss 1,882 (24,875 ) (3,872 ) (5,873 ) Benefits paid (25,753 ) (24,729 ) (6,298 ) (7,236 ) Benefit obligation at end of year 436,307 442,960 89,304 92,734 Change in net plan assets: Fair value of plan assets at beginning of year 332,667 354,363 82,593 87,586 Actual gain (loss) on plan assets 26,595 (10,879 ) 4,184 258 Employer contribution — 13,912 962 577 Plan participants' contributions — — 1,405 1,408 Benefits paid (25,753 ) (24,729 ) (6,298 ) (7,236 ) Fair value of net plan assets at end of year 333,509 332,667 82,846 82,593 Funded status - under $ (102,798 ) $ (110,293 ) $ (6,458 ) $ (10,141 ) Amounts recognized in the Consolidated Balance Sheets at December 31: Other assets (noncurrent) $ — $ — $ 13,131 $ 5,095 Other accrued liabilities (current) — — (538 ) (421 ) Other liabilities (noncurrent) (102,798 ) (110,293 ) (19,051 ) (14,815 ) Net amount recognized $ (102,798 ) $ (110,293 ) $ (6,458 ) $ (10,141 ) Amounts recognized in accumulated other comprehensive (income) loss consist of: Actuarial loss $ 198,668 $ 208,671 $ 17,470 $ 22,484 Prior service cost (credit) — — (13,003 ) (14,374 ) Total $ 198,668 $ 208,671 $ 4,467 $ 8,110 Employer contributions and benefits paid in the preceding table include only those amounts contributed directly to, or paid directly from, plan assets. Accumulated other comprehensive (income) loss in the above table includes amounts related to regulated operations, which are recorded as regulatory assets (liabilities) and are expected to be reflected in rates charged to customers over time. For more information on regulatory assets (liabilities), see Note 4 . Unrecognized pension actuarial losses in excess of 10 percent of the greater of the projected benefit obligation or the market-related value of assets are amortized over the average life expectancy of plan participants for frozen plans. The market-related value of assets is determined using a five-year average of assets. The pension plans all have accumulated benefit obligations in excess of plan assets. The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for these plans at December 31 were as follows: 2016 2015 (In thousands) Projected benefit obligation $ 436,307 $ 442,960 Accumulated benefit obligation $ 436,307 $ 442,960 Fair value of plan assets $ 333,509 $ 332,667 Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2014 2016 2015 2014 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ 86 $ 129 $ 1,647 $ 1,816 $ 1,518 Interest cost 17,218 17,141 17,682 3,688 3,607 3,521 Expected return on assets (20,924 ) (22,254 ) (21,218 ) (4,533 ) (4,795 ) (4,617 ) Amortization of prior service cost (credit) — 36 71 (1,371 ) (1,371 ) (1,393 ) Recognized net actuarial loss 6,215 7,016 4,869 1,491 1,960 649 Curtailment loss — 258 — — — — Net periodic benefit cost (credit), including amount capitalized 2,509 2,283 1,533 922 1,217 (322 ) Less amount capitalized 381 316 388 (52 ) 120 (21 ) Net periodic benefit cost (credit) 2,128 1,967 1,145 974 1,097 (301 ) Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss: Net (gain) loss (3,789 ) 8,257 77,238 (3,523 ) (1,336 ) 15,114 Amortization of actuarial loss (6,215 ) (7,016 ) (4,869 ) (1,491 ) (1,960 ) (649 ) Amortization of prior service (cost) credit — (294 ) (71 ) 1,371 1,371 1,393 Total recognized in accumulated other comprehensive (income) loss (10,004 ) 947 72,298 (3,643 ) (1,925 ) 15,858 Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss $ (7,876 ) $ 2,914 $ 73,443 $ (2,669 ) $ (828 ) $ 15,557 The estimated net loss for the defined benefit pension plans that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2017 is $6.4 million . The estimated net loss and prior service credit for the other postretirement benefit plans that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2017 are $900,000 and $1.4 million , respectively. Prior service cost is amortized on a straight line basis over the average remaining service period of active participants. Weighted average assumptions used to determine benefit obligations at December 31 were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2016 2015 Discount rate 3.83 % 4.00 % 3.86 % 4.06 % Expected return on plan assets 6.75 % 6.75 % 5.75 % 5.75 % Rate of compensation increase N/A N/A 3.00 % 3.00 % Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2016 2015 Discount rate 4.00 % 3.70 % 4.06 % 3.74 % Expected return on plan assets 6.75 % 7.00 % 5.75 % 6.00 % Rate of compensation increase N/A N/A 3.00 % 3.00 % The expected rate of return on pension plan assets is based on a targeted asset allocation range determined by the funded ratio of the plan. As of December 31, 2016, the expected rate of return on pension plan assets is based on the targeted asset allocation range of 40 percent to 50 percent equity securities and 50 percent to 60 percent fixed-income securities and the expected rate of return from these asset categories. The expected rate of return on other postretirement plan assets is based on the targeted asset allocation range of 30 percent to 40 percent equity securities and 60 percent to 70 percent fixed-income securities and the expected rate of return from these asset categories. The expected return on plan assets for other postretirement benefits reflects insurance-related investment costs. Health care rate assumptions for the Company's other postretirement benefit plans as of December 31 were as follows: 2016 2015 Health care trend rate assumed for next year 8.6 % – 10.7 % 4.0 % – 8.0 % Health care cost trend rate - ultimate 4.5 % 5.0 % – 6.0 % Year in which ultimate trend rate achieved 2024 2021 The Company's other postretirement benefit plans include health care and life insurance benefits for certain retirees. The plans underlying these benefits may require contributions by the retiree depending on such retiree's age and years of service at retirement or the date of retirement. The accounting for the health care plans anticipates future cost-sharing changes that are consistent with the Company's expressed intent to generally increase retiree contributions each year by the excess of the expected health care cost trend rate over six percent. Assumed health care cost trend rates may have a significant effect on the amounts reported for the health care plans. A one percentage point change in the assumed health care cost trend rates would have had the following effects at December 31, 2016 : 1 Percentage Point Increase 1 Percentage Point Decrease (In thousands) Effect on total of service and interest cost components $ 255 $ (210 ) Effect on postretirement benefit obligation $ 5,741 $ (4,834 ) Outside investment managers manage the Company's pension and postretirement assets. The Company's investment policy with respect to pension and other postretirement assets is to make investments solely in the interest of the participants and beneficiaries of the plans and for the exclusive purpose of providing benefits accrued and defraying the reasonable expenses of administration. The Company strives to maintain investment diversification to assist in minimizing the risk of large losses. The Company's policy guidelines allow for investment of funds in cash equivalents, fixed-income securities and equity securities. The guidelines prohibit investment in commodities and futures contracts, equity private placement, employer securities, leveraged or derivative securities, options, direct real estate investments, precious metals, venture capital and limited partnerships. The guidelines also prohibit short selling and margin transactions. The Company's practice is to periodically review and rebalance asset categories based on its targeted asset allocation percentage policy. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the measurement date. The ASC establishes a hierarchy for grouping assets and liabilities, based on the significance of inputs. The estimated fair values of the Company's pension plans' assets are determined using the market approach. The carrying value of the pension plans' Level 2 cash equivalents approximates fair value and is determined using observable inputs in active markets or the net asset value of shares held at year end, which is determined using other observable inputs including pricing from outside sources. The estimated fair value of the pension plans' Level 1 equity securities is based on the closing price reported on the active market on which the individual securities are traded. The estimated fair value of the pension plans' Level 1 and Level 2 collective and mutual funds are based on the net asset value of shares held at year end, based on either published market quotations on active markets or other known sources including pricing from outside sources. The estimated fair value of the pension plans' Level 2 corporate and municipal bonds is determined using other observable inputs, including benchmark yields, reported trades, broker/dealer quotes, bids, offers, future cash flows and other reference data. The estimated fair value of the pension plans' Level 1 U.S. Government securities are valued based on quoted prices on an active market. The estimated fair value of the pension plans' Level 2 U.S. Government securities are valued mainly using other observable inputs, including benchmark yields, reported trades, broker/dealer quotes, bids, offers, to be announced prices, future cash flows and other reference data. Some of these securities are valued using pricing from outside sources. Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value. For the years ended December 31, 2016 and 2015 , there were no transfers between Levels 1 and 2. The fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements at December 31, 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: Cash equivalents $ — $ 6,347 $ — $ 6,347 Equity securities: U.S. companies 11,348 — — 11,348 International companies 1,584 — — 1,584 Collective and mutual funds* 162,055 64,052 — 226,107 Corporate bonds — 68,677 — 68,677 Municipal bonds — 11,002 — 11,002 U.S. Government securities 4,352 2,044 — 6,396 Total assets measured at fair value $ 179,339 $ 152,122 $ — $ 331,461 * Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in corporate bonds, 20 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 7 percent in U.S. Government securities and 15 percent in other investments. Fair Value Measurements at December 31, 2015, 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, 2015 (In thousands) Assets: Cash equivalents $ — $ 8,379 $ — $ 8,379 Equity securities: U.S. companies 15,135 — — 15,135 International companies 2,332 — — 2,332 Collective and mutual funds* 154,400 63,568 — 217,968 Corporate bonds — 62,145 — 62,145 Municipal bonds — 11,680 — 11,680 U.S. Government securities 5,288 6,823 — 12,111 Total assets measured at fair value $ 177,155 $ 152,595 $ — $ 329,750 * Collective and mutual funds invest approximately 29 percent in common stock of international companies, 19 percent in common stock of large-cap U.S. companies, 16 percent in corporate bonds, 16 percent in cash equivalents, 6 percent in common stock of mid-cap U.S. companies and 14 percent in other investments. The estimated fair values of the Company's other postretirement benefit plans' assets are determined using the market approach. The estimated fair value of the other postretirement benefit plans' Level 2 cash equivalents is valued at the net asset value of shares held at year end, based on published market quotations on active markets, or using other known sources including pricing from outside sources. The estimated fair value of the other postretirement benefit plans' Level 1 equity securities is based on the closing price reported on the active market on which the individual securities are traded. The estimated fair value of the other postretirement benefit plans' Level 2 insurance contract is based on contractual cash surrender values that are determined primarily by investments in managed separate accounts of the insurer. These amounts approximate fair value. The managed separate accounts are valued based on other observable inputs or corroborated market data. Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value. For the years ended December 31, 2016 and 2015 , there were no transfers between Levels 1 and 2. The fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements at December 31, 2016, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2016 (In thousands) Assets: Cash equivalents $ — $ 250 $ — $ 250 Equity securities: U.S. companies 2,328 — — 2,328 International companies 5 — — 5 Insurance contract* — 80,263 — 80,263 Total assets measured at fair value $ 2,333 $ 80,513 $ — $ 82,846 * The insurance contract invests approximately 38 percent in corporate bonds, 25 percent in common stock of large-cap U.S. companies, 20 percent in U.S. Government securities, 9 percent in mortgage-backed securities and 8 percent in other investments. Fair Value Measurements at December 31, 2015, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2015 (In thousands) Assets: Cash equivalents $ — $ 3,261 $ — $ 3,261 Equity securities: U.S. companies 2,274 — — 2,274 International companies 9 — — 9 Insurance contract* — 77,044 — 77,044 Total assets measured at fair value $ 2,283 $ 80,305 $ — $ 82,588 * The insurance contract invests approximately 36 percent in corporate bonds, 22 percent in U.S. Government securities, 19 percent in common stock of large-cap U.S. companies, 10 percent in mortgage-backed securities and 13 percent in other investments. The Company expects to contribute approximately $2.0 million to its defined benefit pension plans and approximately $900,000 to its postretirement benefit plans in 2017 . The following benefit payments, which reflect future service, as appropriate, and expected Medicare Part D subsidies are as follows: Years Pension Benefits Other Postretirement Benefits Expected Medicare Part D Subsidy (In thousands) 2017 $ 24,798 $ 5,410 $ 168 2018 25,054 5,573 165 2019 25,271 5,603 160 2020 25,616 5,500 154 2021 25,987 5,511 146 2022 - 2026 132,224 27,956 568 Nonqualified benefit plans In addition to the qualified plan defined pension benefits reflected in the table at the beginning of this note, 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 was $1.8 million , $7.1 million and $6.6 million in 2016 , 2015 and 2014 , respectively, which reflects a curtailment gain of $3.3 million in the first quarter of 2016 . The total projected benefit obligation for these plans was $101.8 million and $110.8 million at December 31, 2016 and 2015 , respectively. The accumulated benefit obligation for these plans was $101.8 million and $104.6 million at December 31, 2016 and 2015 , respectively. A weighted average discount rate of 3.56 percent and 3.77 percent at December 31, 2016 and 2015 , respectively, and a rate of compensation increase of 4.00 percent at December 31, 2015 , were used to determine benefit obligations. No rate of compensation increase was used to determine the benefit obligation at December 31, 2016 , due to the plans being froze. A discount rate of 3.77 percent and 3.51 percent for the years ended December 31, 2016 and 2015 , respectively, and a rate of compensation increase of 4.00 percent and 4.00 percent for the years ended December 31, 2016 and 2015 , respectively, were used to determine net periodic benefit cost. The amount of benefit payments for the unfunded, nonqualified benefit plans are expected to aggregate $6.7 million in 2017 ; $7.1 million in 2018 ; $7.3 million in 2019 ; $7.7 million in 2020 ; $7.7 million in 2021 and $36.4 million for the years 2022 through 2026. In 2012, the Company established a nonqualified defined contribution plan for certain key management employees. Expenses incurred under this plan for 2016 , 2015 and 2014 were $395,000 , $207,000 and $104,000 , respectively. The Company had investments of $111.0 million and $105.2 million at December 31, 2016 and 2015 , respectively, consisting of equity securities of $62.5 million and $54.2 million , respectively, life insurance carried on plan participants (payable upon the employee's death) of $35.5 million and $34.3 million , respectively, and other investments of $13.0 million and $16.7 million , respectively. The Company anticipates using these investments to satisfy obligations under these plans. Defined contribution plans The Company sponsors various defined contribution plans for eligible employees and the costs incurred under these plans were $40.9 million in 2016 , $36.8 million in 2015 and $34.4 million in 2014 . Multiemployer plans The Company contributes to a number of multiemployer defined benefit pension plans under the terms of collective-bargaining agreements that cover its union-represented employees. The risks of participating in these multiemployer plans are different from single-employer plans in the following aspects: • Assets contributed to the MEPP by one employer may be used to provide benefits to employees of other participating employers • If a participating employer stops contributing to the plan, the unfunded obligations of the plan may be borne by the remaining participating employers • If the Company chooses to stop participating in some of its MEPPs, the Company may be required to pay those plans an amount based on the underfunded status of the plan, referred to as a withdrawal liability The Company's participation in these plans is outlined in the following table. Unless otherwise noted, the most recent Pension Protection Act zone status available in 2016 and 2015 is for the plan's year-end at December 31, 2015 , and December 31, 2014 , respectively. The zone status is based on information that the Company received from the plan and is certified by the plan's actuary. Among other factors, plans in the red zone are generally less than 65 percent funded, plans in the yellow zone are between 65 percent and 80 percent funded, and plans in the green zone are at least 80 percent funded. EIN/Pension Plan Number Pension Protection Act Zone Status FIP/RP Status Pending/Implemented Contributions Surcharge Imposed Expiration Date of Collective Bargaining Agreement Pension Fund 2016 2015 2016 2015 2014 (In thousands) Alaska Laborers-Employers Retirement Fund 91-6028298-001 Yellow as of 6/30/2016 Yellow as of 6/30/2015 Implemented $ 766 $ 917 $ 666 No 12/31/2016 Edison Pension Plan 93-6061681-001 Green as of 12/31/2016 Green as of 12/31/2015 No 6,242 5,517 9,061 No 12/31/2017 IBEW Local No. 82 Pension Plan 31-6127268-001 Green as of 6/30/2016 Red as of 6/30/2015 Implemented 2,560 2,252 1,392 No 12/1/2019 IBEW Local No. 357 Pension Plan A 88-6023284-001 Green Green No 3,016 1,896 3,575 No 5/31/2018 IBEW Local 648 Pension Plan 31-6134845-001 Red as of 2/29/2016 Red as of 2/28/2015 Implemented 773 745 1,110 No 9/2/2018 Idaho Plumbers and Pipefitters Pension Plan 82-6010346-001 Green as of 5/31/2016 Green as of 5/31/2015 No 1,221 1,169 1,125 No 9/30/2019 Local Union 212 IBEW Pension Trust Fund 31-6127280-001 Yellow as of 4/30/2016 Yellow as of 4/30/2015 Implemented 1,146 937 568 No 6/2/2019 National Automatic Sprinkler Industry Pension Fund 52-6054620-001 Red as of 12/31/2016 Red as of 12/31/2015 Implemented 775 677 608 No 7/31/2018- National Electrical Benefit Fund 53-0181657-001 Green Green No 6,366 5,271 6,476 No 1/1/2017- Operating Engineers Local 800 & WY Contractors Association, Inc. Pension Plan for Wyoming** 83-6011320-001 Red as of 12/31/2016 Red as of 12/31/2015 Implemented — — 68 No 10/31/2005* Sheet Metal Workers' Pension Plan of Southern CA, AZ and NV 95-6052257-001 Red as of 12/31/2016 Red as of 12/31/2015 Implemented 1,087 714 676 No 6/30/2017 Southwest Marine Pension Trust 95-6123404-001 Red Red Implemented 50 26 31 No 1/31/2019 Other funds 20,525 18,991 17,461 Total contributions $ 44,527 $ 39,112 $ 42,817 * Plan includes collective bargaining agreements which have expired. The agreements contain provisions that automatically renew the existing contracts in lieu of a new negotiated collective bargaining agreement. ** The Company withdrew from the plan as of October 26, 2014, as discussed later. The Company was listed in the plans' Forms 5500 as providing more than 5 percent of the total contributions for the following plans and plan years: Pension Fund Year Contributions to Plan Exceeded More Than 5 Percent of Total Contributions (as of December 31 of the Plan's Year-End) Edison Pension Plan 2015 and 2014 IBEW Local No. 82 Pension Plan 2015 and 2014 Local Union No. 124 IBEW Pension Trust Fund 2015 and 2014 Local Union 212 IBEW Pension Trust Fund 2015 and 2014 IBEW Local Union No. 357 Pension Plan A 2015 and 2014 IBEW Local 573 Pension Plan 2014 IBEW Local 648 Pension Plan 2015 and 2014 Idaho Plumbers and Pipefitters Pension Plan 2015 and 2014 Minnesota Teamsters Construction Division Pension Fund 2015 and 2014 Operating Engineers Local 800 & WY Contractors Association, Inc. Pension Plan for Wyoming* 2014 Pension and Retirement Plan of Plumbers and Pipefitters Union Local No. 525 2015 and 2014 * The Company withdrew from the plan as of October 26, 2014, as discussed later. On September 24, 2014, Knife River provided notice to the Operating Engineers Local 800 & WY Contractors Association, Inc. Pension Plan for Wyoming that it was withdrawing from the plan effective October 26, 2014. In the fourth quarter of 2016, Knife River and the plan entered into a settlement agreement whereby the plan administrator assessed Knife River’s final withdrawal liability with quarterly payments of approximately $42,000 until all benefits are satisfied. Knife River discounted the expected future payments. Based on this calculation, Knife River adjusted its liability accrual from $16.4 million to $5.2 million . The Company also contributes to a number of multiemployer other postretirement plans under the terms of collective-bargaining agreements that cover its union-represented employees. These plans provide benefits such as health insurance, disability insurance and life insurance to retired union employees. Many of the multiemployer other postretirement plans are combined with active multiemployer health and welfare plans. The Company's total contributions to its multiemployer other postretirement plans, which also includes contributions to active multiemployer health and welfare plans, were $36.1 million , $31.4 million and $34.6 million for the years ended December 31, 2016 , 2015 and 2014 , respectively. Amounts contributed in 2016 , 2015 and 2014 to defined contribution multiemployer plans were $23.8 million , $19.5 million and $22.0 million , respectively.

Jointly owned facilities

Jointly owned facilities	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Jointly owned facilities	Jointly Owned Facilities The consolidated financial statements include the Company's ownership interests in the assets, liabilities and expenses of the Big Stone Station, Coyote Station and Wygen III. Each owner of the stations is responsible for financing its investment in the jointly owned facilities. The Company's share of the stations operating expenses was reflected in the appropriate categories of operating expenses (fuel, operation and maintenance and taxes, other than income) in the Consolidated Statements of Income. At December 31, the Company's share of the cost of utility plant in service and related accumulated depreciation for the stations was as follows: 2016 2015 (In thousands) Big Stone Station: Utility plant in service $ 157,144 $ 157,761 Less accumulated depreciation 49,568 48,242 $ 107,576 $ 109,519 Coyote Station: Utility plant in service $ 156,334 $ 140,895 Less accumulated depreciation 105,928 94,755 $ 50,406 $ 46,140 Wygen III: Utility plant in service $ 66,251 $ 65,023 Less accumulated depreciation 7,550 6,788 $ 58,701 $ 58,235

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Regulatory matters	Regulatory Matters On September 30, 2015, Great Plains filed an application for a natural gas rate increase with the MNPUC. Great Plains requested a total increase of approximately $1.6 million annually or approximately 6.4 percent above current rates to recover increased operating expenses along with increased investment in facilities, including the related depreciation expense and taxes. An interim increase of approximately $1.5 million or approximately 6.4 percent, subject to refund, was effective with service rendered on and after January 1, 2016. The MNPUC issued an order on September 6, 2016, authorizing an increase of approximately $1.1 million annually or approximately 5.2 percent with the requirement that Great Plains submit a compliance filing within 30 days. On September 22, 2016, Great Plains submitted the required compliance filing which included a refund plan to return the amount of interim revenues collected above the final rates. On December 22, 2016, the MNPUC issued an order approving the rates to be implemented January 1, 2017. Great Plains will issue refunds for the difference with interest to customers no later than March 1, 2017. 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. The interim rate is pending the determination of the return on equity in the general rate case application filed on October 14, 2016, as 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. 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 net investment authorized for the natural gas-fired internal combustion engines and the return on equity on both investments are pending in the general rate case application filed October 14, 2016, as 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. The interim rate is pending the determination of the return on equity in the general rate case application filed October 14, 2016, as discussed in this note. On April 29, 2016, Cascade filed an application with the OPUC for a natural gas rate increase of approximately $1.9 million annually or approximately 2.8 percent above current rates. The request includes rate recovery associated with pipeline replacement and improvement projects to ensure the integrity of Cascade's system. On October 6, 2016, Cascade, staff of the OPUC and the interveners in the case filed a stipulation and settlement agreement reflecting an annual increase of approximately $754,000 to be effective March 1, 2017. The OPUC issued an order approving the stipulation and settlement agreement on December 12, 2016. On June 1, 2016, Cascade filed an application with the WUTC for an annual pipeline replacement cost recovery mechanism of $4.6 million annually or approximately 2.0 percent of additional revenue. The requested increase includes $2.4 million associated with incremental pipeline replacement investments and $2.2 million for an alternative recovery request of incremental operation and maintenance costs associated with a maximum allowable operating pressure validation plan. On October 17, 2016, Cascade filed an update to the application that reduced the incremental pipeline replacement investment to $1.9 million and removed the operation and maintenance costs associated with a maximum allowable operating pressure validation plan. On October 27, 2016, the WUTC allowed the pipeline replacement cost recovery mechanism which was effective November 1, 2016. On June 1, 2016, Cascade filed an accounting order to defer the costs related to the maximum allowable operating pressure validation plan and on November 10, 2016, the WUTC granted the order. On June 10, 2016, Montana-Dakota filed an application for an increase in electric rates with the WYPSC. Montana-Dakota requested an increase of approximately $3.2 million annually or approximately 13.1 percent above current rates to recover Montana-Dakota's increased investment in facilities along with additional depreciation, operation and maintenance expenses including increased fuel costs, and taxes associated with the increases in investment. On December 28, 2016, Montana-Dakota and the interveners of the case filed a stipulation and agreement reflecting an increase of approximately $2.7 million annually or approximately 11.1 percent above current rates effective for service rendered on and after March 1, 2017. The WYPSC rendered a bench decision approving the stipulation and agreement on January 18, 2017. 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 includes rate recovery associated with increased investment in facilities and increased operating expenses. On November 23, 2016, Intermountain provided the IPUC with an updated revenue request of approximately $9.6 million . A hearing has been scheduled for March 1-2, 2017. This matter is pending before the IPUC. On September 1, 2016, and as amended on January 10, 2017, Montana-Dakota submitted an update to its transmission formula rate under the MISO tariff including a revenue requirement for the Company's multivalue project along with a true-up of prior year expenditures of $11.1 million , which was effective January 1, 2017. 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 a revised interim increase of approximately $11.7 million , based on adjustments accepted by the NDPSC, or approximately 5.8 percent above current rates, subject to refund. The NDPSC approved the revised interim rates effective with service rendered on or after December 13, 2016. A technical hearing is scheduled for April 10, 2017. This matter is pending before the NDPSC. On December 2, 2016, Montana-Dakota filed an application with the MTPSC requesting authority to implement gas and electric tax tracking adjustments for Montana state and local taxes and fees that reflect the changes in state and local property taxes applicable to gas and electric utilities pursuant to Montana law. The requested tax tracking adjustments would result in an increase in revenues of approximately $814,000 . On January 17, 2017, the MTPSC issued an order on the tax tracking adjustments. The gas tracking adjustment was approved as an increase to revenues of approximately $474,000 effective January 1, 2017. The electric tax tracking adjustment was approved as an increase to revenues of approximately $251,000 effective May 15, 2017. Montana-Dakota filed a motion for reconsideration of the electric tax tracking adjustment on January 27, 2017. The motion for reconsideration is pending before the MTPSC. On December 21, 2016, Great Plains filed an application with the MNPUC requesting authority to implement a gas utility infrastructure cost tariff of approximately $456,000 annually effective beginning with service rendered May 20, 2017. 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. This matter is pending before the MNPUC.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries. 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. The Company had accrued liabilities of $31.8 million and $19.5 million , which include liabilities held for sale, for contingencies, including litigation, production taxes, royalty claims and environmental matters at December 31, 2016 and 2015 , respectively, including amounts that may have been accrued for matters discussed in Litigation and Environmental matters within this note. Litigation Natural Gas Gathering Operations Omimex filed a complaint against WBI Energy Midstream in Montana Seventeenth Judicial District Court in July 2010 alleging WBI Energy Midstream breached a gathering contract with Omimex as a result of the increased operating pressures demanded by a third party on a natural gas gathering system in Montana. In December 2011, Omimex filed an amended complaint alleging WBI Energy Midstream breached obligations to operate its gathering system as a common carrier under United States and Montana law. WBI Energy Midstream removed the action to the United States District Court for the District of Montana. The parties subsequently settled the breach of contract claim and, subject to final determination on liability, stipulated to the damages on the common carrier claim, for amounts that are not material. A trial on the common carrier claim was held during July 2013. On December 9, 2014, the United States District Court for the District of Montana issued an order determining WBI Energy Midstream breached its obligations as a common carrier and ordered judgment in favor of Omimex for the amount of the stipulated damages. WBI Energy Midstream filed an appeal from the United States District Court for the District of Montana's order and judgment. The Company also is subject to other litigation, and actual and potential claims in the ordinary course of its business which may include, but are not limited to, matters involving property damage, personal injury, and environmental, contractual, statutory and regulatory obligations. Accruals are based on the best information available but actual losses in future periods are affected by various factors making them uncertain. After taking into account liabilities accrued for the foregoing matters, management believes that the outcomes with respect to the above issues and other probable and reasonably possible losses in excess of the amounts accrued, while uncertain, will not have a material effect upon the Company's financial position, results of operations or cash flows. 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. Several alternatives for cleanup have been identified, with preliminary cost estimates ranging from approximately $500,000 to $11.0 million . The Oregon DEQ released a ROD in January 2015 that selected a remediation alternative for the site as recommended in an earlier staff report. It is not known at this time what share of the cleanup costs will actually be borne by Cascade; however, Cascade anticipates its proportional share could be approximately 50 percent. Cascade has accrued $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 and December 1, 2015. Cascade has requested authority to defer accounting for the 12-month period starting December 1, 2016, which is pending before the OPUC. 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. Cascade has accrued $12.5 million for the remedial investigation, feasibility study and remediation of this site. 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 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 will 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. For more information, see Note 4 . Operating leases The Company leases certain equipment, facilities and land under operating lease agreements. The amounts of annual minimum lease payments due under these leases as of December 31, 2016 , were $51.7 million in 2017 , $43.3 million in 2018 , $33.9 million in 2019 , $23.2 million in 2020 , $9.4 million in 2021 and $42.0 million thereafter. Rent expense was $65.0 million , $53.9 million and $46.9 million for the years ended December 31, 2016 , 2015 and 2014 , respectively. Purchase commitments The Company has entered into various commitments, largely construction, natural gas and coal supply, purchased power, natural gas transportation and storage, and service, shipping and construction materials supply contracts, some of which are subject to variability in volume and price. These commitments range from one to 44 years . The commitments under these contracts as of December 31, 2016 , were $367.7 million in 2017 , $215.7 million in 2018 , $189.4 million in 2019 , $138.0 million in 2020 , $130.6 million in 2021 and $859.5 million thereafter. These commitments were not reflected in the Company's consolidated financial statements. Amounts purchased under various commitments for the years ended December 31, 2016 , 2015 and 2014 , were $539.3 million , $842.1 million and $759.0 million , respectively. Guarantees In June 2016, WBI Energy sold all of the outstanding membership interests in Dakota Prairie Refining. In connection with the sale, Centennial agreed to continue to guarantee certain debt obligations of Dakota Prairie Refining which totaled $63.8 million at December 31, 2016 , 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 Basin assets. The guarantee was required by the buyer as a condition to the sale of the Paradox Basin assets. In 2009, multiple sales agreements were signed to sell the Company's ownership interests in the Brazilian Transmission Lines. In connection with the sale, Centennial has agreed to guarantee payment of any indemnity obligations of certain of the Company's indirect wholly owned subsidiaries who are 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 December 31, 2016 , the fixed maximum amounts guaranteed under these agreements aggregated $98.7 million . The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate $33.6 million in 2017 ; $4.5 million in 2018 ; $56.6 million in 2019 ; and $4.0 million , which has no scheduled maturity date. There were no amounts outstanding under the above guarantees at December 31, 2016 . In the event of default under these guarantee obligations, the subsidiary issuing the guarantee for that particular obligation would be required to make payments under its guarantee. Certain subsidiaries have outstanding letters of credit to third parties related to insurance policies and other agreements, some of which are guaranteed by other subsidiaries of the Company. At December 31, 2016 , the fixed maximum amounts guaranteed under these letters of credit aggregated $30.8 million , all of which expire in 2017. There were no amounts outstanding under the above letters of credit at December 31, 2016 . 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 and 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 December 31, 2016 . 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. As of December 31, 2016 , approximately $516.1 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. For more information, see Note 1 . Dakota Prairie Refining, LLC On February 7, 2013, WBI Energy and Calumet formed a limited liability company, Dakota Prairie Refining, and entered into an operating agreement to develop, build and operate Dakota Prairie Refinery in southwestern North Dakota. WBI Energy and Calumet each had a 50 percent ownership interest in Dakota Prairie Refining. WBI Energy's and Calumet's capital commitments, based on a total project cost of $300 million , under the agreement were $150 million and $75 million , respectively. Capital commitments in excess of $300 million were shared equally between WBI Energy and Calumet. Dakota Prairie Refining entered into a term loan for project debt financing of $75 million on April 22, 2013. The operating agreement provided for allocation of profits and losses consistent with ownership interests; however, deductions attributable to project financing debt was allocated to Calumet. Calumet's cash distributions from Dakota Prairie Refining were decreased by the principal and interest paid on the project debt, while the cash distributions to WBI Energy were not decreased. Pursuant to the operating agreement, Centennial agreed to guarantee Dakota Prairie Refining's obligation under the term loan. The net loss attributable to noncontrolling interest on the Consolidated Statements of Income is pretax as Dakota Prairie Refining was a limited liability company. For more information related to the guarantee, see Guarantees in this note. Dakota Prairie Refining was determined to be a VIE, and the Company had determined that it was the primary beneficiary as it had an obligation to absorb losses that could have been potentially significant to the VIE through WBI Energy's equity investment and Centennial's guarantee of the third-party term loan. Accordingly, the Company consolidated Dakota Prairie Refining in its financial statements and recorded a noncontrolling interest for Calumet's ownership interest. 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. 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. For more information on the Company's discontinued operations, see Note 2 . Dakota Prairie Refinery commenced operations in May 2015. The assets of Dakota Prairie Refining were used solely for the benefit of Dakota Prairie Refining. The total assets and liabilities of Dakota Prairie Refining at December 31 were as follows: 2015 (In thousands) Assets Current assets: Cash and cash equivalents $ 851 Accounts receivable 7,693 Inventories 13,176 Other current assets 6,215 Total current assets 27,935 Net property, plant and equipment 425,123 Deferred charges and other assets: Other 9,626 Total deferred charges and other assets 9,626 Total assets $ 462,684 Liabilities Current liabilities: Short-term borrowings $ 45,500 Long-term debt due within one year 5,250 Accounts payable 24,766 Taxes payable 1,391 Accrued compensation 938 Other accrued liabilities 4,953 Total current liabilities 82,798 Long-term debt 63,750 Total liabilities $ 146,548 Fuel Contract Coyote Station entered into a coal supply agreement with Coyote Creek that provides for the purchase of coal necessary to supply the coal requirements of the Coyote Station for the period May 2016 through December 2040. Coal purchased under the coal supply agreement is reflected in inventories on the Company's Consolidated Balance Sheets and is recovered from customers as a component of fuel and purchased power. The coal supply agreement creates a variable interest in Coyote Creek due to the transfer of all operating and economic risk to the Coyote Station owners, as the agreement is structured so that the price of the coal will cover all costs of operations as well as future reclamation costs. The Coyote Station owners are also providing a guarantee of the value of the assets of Coyote Creek as they would be required to buy the assets at book value should they terminate the contract prior to the end of the contract term and are providing a guarantee of the value of the equity of Coyote Creek in that they are required to buy the entity at the end of the contract term at equity value. Although the Company has determined that Coyote Creek is a VIE, the Company has concluded that it is not the primary beneficiary of Coyote Creek because the authority to direct the activities of the entity is shared by the four unrelated owners of the Coyote Station, with no primary beneficiary existing. As a result, Coyote Creek is not required to be consolidated in the Company's financial statements. At December 31, 2016 , 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 $43.3 million .

Quarterly Data (Unaudited)

Quarterly Data (Unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Supplementary financial information-quarterly data (Unaudited)	Supplementary Financial Information Quarterly Data (Unaudited) The following unaudited information shows selected items by quarter for the years 2016 and 2015 : First Quarter Second Quarter Third Quarter Fourth Quarter (In thousands, except per share amounts) 2016 Operating revenues $ 860,214 $ 1,043,948 $ 1,208,567 $ 1,016,099 Operating expenses 798,229 954,983 1,061,883 904,613 Operating income 61,985 88,965 146,684 111,486 Income from continuing operations 31,865 46,298 88,386 66,547 Loss from discontinued operations attributable to the Company, net of tax (6,996 ) (155,451 ) (5,400 ) (816 ) Net income (loss) attributable to the Company 24,869 (109,153 ) 82,986 65,731 Earnings (loss) per common share - basic: Earnings before discontinued operations .16 .24 .45 .34 Discontinued operations attributable to the Company, net of tax (.03 ) (.80 ) (.03 ) — Earnings (loss) per common share - basic .13 (.56 ) .42 .34 Earnings (loss) per common share - diluted: Earnings before discontinued operations .16 .24 .45 .33 Discontinued operations attributable to the Company, net of tax (.03 ) (.80 ) (.03 ) — Earnings (loss) per common share - diluted .13 (.56 ) .42 .33 Weighted average common shares outstanding: Basic 195,284 195,304 195,304 195,304 Diluted 195,284 195,699 195,811 195,889 2015 Operating revenues $ 860,845 $ 938,039 $ 1,198,342 $ 1,016,826 Operating expenses 810,537 878,330 1,070,514 934,896 Operating income 50,308 59,709 127,828 81,930 Income from continuing operations 20,540 26,061 73,886 55,902 Loss from discontinued operations attributable to the Company, net of tax (326,457 ) (255,665 ) (213,334 ) (3,368 ) Net income (loss) attributable to the Company (305,917 ) (229,604 ) (139,448 ) 52,534 Earnings (loss) per common share - basic: Earnings before discontinued operations .10 .13 .38 .29 Discontinued operations attributable to the Company, net of tax (1.67 ) (1.31 ) (1.10 ) (.02 ) Earnings (loss) per common share - basic (1.57 ) (1.18 ) (.72 ) .27 Earnings (loss) per common share - diluted: Earnings before discontinued operations .10 .13 .38 .29 Discontinued operations attributable to the Company, net of tax (1.67 ) (1.31 ) (1.10 ) (.02 ) Earnings (loss) per common share - diluted (1.57 ) (1.18 ) (.72 ) .27 Weighted average common shares outstanding: Basic 194,479 194,805 195,151 195,266 Diluted 194,566 194,838 195,169 195,324 Notes: • Fourth quarter 2016 reflects a reduction to a previously recorded MEPP withdrawal liability of $11.1 million (before tax). For more information, see Note 14 . • 2015 and first quarter 2016 have been recast to present the results of operations of Dakota Prairie Refining as discontinued operations, other than certain general and administrative costs and interest expense which were previously allocated to the former refining segment and do not meet the criteria for income (loss) from discontinued operations. • First quarter 2015 has been recast to present the results of operations of Fidelity as discontinued operations, other than certain general and administrative costs and interest expense which were previously allocated to the former exploration and production segment and do not meet the criteria for income (loss) from discontinued operations. • First quarter 2015 reflects a MEPP withdrawal liability of $2.4 million (before tax). For more information, see Note 14 . • Second quarter 2015 reflects an impairment of coalbed natural gas gathering assets of $3.0 million (before tax). For more information, see Note 1 . • Third quarter 2015 reflects an impairment of coalbed natural gas gathering assets of $14.1 million (before tax). For more information, see Note 1 . Certain Company operations are highly seasonal and revenues from and certain expenses for such operations may fluctuate significantly among quarterly periods. Accordingly, quarterly financial information may not be indicative of results for a full year.

Exploration and Production Acti

Exploration and Production Activities (Unaudited) (Notes)	12 Months Ended
	Dec. 31, 2016
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Exploration and production activities	Exploration and Production Activities (Unaudited) 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. At the time the Company committed to a plan to sell Fidelity, the Company stopped the use of the full-cost method of accounting for its oil and natural gas production activities. The assets and liabilities have been classified as held for sale and the results of operations included in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. Prior to the asset sales, Fidelity was significantly involved in the development and production of oil and natural gas resources. For more information, see Note 2 . Previously, Fidelity shared revenues and expenses from the development of specified properties in proportion to its ownership interests. The information that follows includes Fidelity's proportionate share of all its previously owned oil and natural gas interests. The following table sets forth capitalized costs and accumulated depreciation, depletion and amortization related to oil and natural gas producing activities, prior to Fidelity's assets being held for sale, at December 31: 2014 (In thousands) Subject to amortization $ 3,205,036 Not subject to amortization 132,141 Total capitalized costs 3,337,177 Less accumulated depreciation, depletion and amortization 1,752,566 Net capitalized costs $ 1,584,611 Capital expenditures, including those not subject to amortization, related to oil and natural gas producing activities prior to Fidelity's assets being held for sale, excluding the years ended December 31, 2016 and 2015, due to no wells being drilled during that time, were as follows: Year ended December 31, 2014 * (In thousands) Acquisitions: Proved properties $ 87,919 Unproved properties 138,683 Exploration 16,879 Development 331,400 Total capital expenditures $ 574,881 * Excludes net reductions to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of oil and natural gas wells of $9.0 million for the year ended December 31, 2014. Estimates of proved reserves were prepared in accordance with guidelines established by the industry and the SEC. The estimates are arrived at using actual historical wellhead production trends and/or standard reservoir engineering methods utilizing available geological, geophysical, engineering and economic data. The proved reserve estimates as of December 31, 2015 and 2014, were calculated using SEC Defined Prices. Other factors used in the proved reserve estimates were current estimates of well operating and future development costs (which include asset retirement costs), taxes, timing of operations, and the interests owned by the Company in the properties. These estimates are refined as new information becomes available. The reserve estimates were prepared by internal engineers assigned to an asset team by geographic area. Senior management reviewed and approved the reserve estimates to ensure they were materially accurate. Estimates of economically recoverable oil, NGL and natural gas reserves and future net revenues therefrom are based upon a number of variable factors and assumptions. For these reasons, estimates of economically recoverable reserves and future net revenues may vary from actual results. The changes in the Company's estimated quantities of proved oil, NGL and natural gas reserves for the year ended December 31, 2016, were as follows: Oil (MBbls) NGL (MBbls) Natural Gas (MMcf) Total (MBOE) Proved developed and undeveloped reserves: Balance at beginning of year 12,687 211 2,531 13,321 Production — — — — Extensions and discoveries — — — — Improved recovery — — — — Purchases of proved reserves — — — — Sales of proved reserves (12,687 ) (211 ) (2,531 ) (13,321 ) Revisions of previous estimates — — — — Balance at end of year — — — — Significant changes in proved reserves for the year ended December 31, 2016, include: • Sales of proved reserves of (13.3) MMBOE, due to the Company's decision to sell Fidelity and exit the exploration and production business The changes in the Company's estimated quantities of proved oil, NGL and natural gas reserves for the year ended December 31, 2015, were as follows: Oil (MBbls) NGL (MBbls) Natural Gas (MMcf) Total (MBOE) Proved developed and undeveloped reserves: Balance at beginning of year 43,918 7,187 245,011 91,940 Production (3,286 ) (393 ) (16,747 ) (6,471 ) Extensions and discoveries 744 29 681 888 Improved recovery — — — — Purchases of proved reserves — — — — Sales of proved reserves (16,474 ) (6,864 ) (202,560 ) (57,097 ) Revisions of previous estimates (12,215 ) 252 (23,854 ) (15,939 ) Balance at end of year 12,687 211 2,531 13,321 Significant changes in proved reserves for the year ended December 31, 2015, include: • Sales of proved reserves of (57.1) MMBOE, primarily due to the Company's decision to sell Fidelity and exit the exploration and production business • Revisions of previous estimates of (15.9) MMBOE, largely the result of lower commodity prices The changes in the Company's estimated quantities of proved oil, NGL and natural gas reserves for the year ended December 31, 2014, were as follows: Oil (MBbls) NGL (MBbls) Natural Gas (MMcf) Total (MBOE) Proved developed and undeveloped reserves: Balance at beginning of year 41,019 6,602 198,445 80,695 Production (4,919 ) (609 ) (20,822 ) (8,998 ) Extensions and discoveries 9,654 3,634 64,420 24,025 Improved recovery — — — — Purchases of proved reserves 5,463 — 7,711 6,748 Sales of proved reserves (4,945 ) (3,109 ) (40,451 ) (14,796 ) Revisions of previous estimates (2,354 ) 669 35,708 4,266 Balance at end of year 43,918 7,187 245,011 91,940 Significant changes in proved reserves for the year ended December 31, 2014, include: • Extensions and discoveries of 24.0 MMBOE, primarily due to drilling activity at the Company's East Texas, Bakken and Powder River Basin properties • Purchases of proved reserves of 6.7 MMBOE, primarily due to the purchase of working interests and leasehold positions in the Powder River Basin • Sales of proved reserves of (14.8) MMBOE, primarily at the Company's South Texas and Bakken properties • Revisions of previous estimates of 4.3 MMBOE, largely the result of higher natural gas prices and well performance revisions The following table summarizes the breakdown of the Company's proved reserves between proved developed and PUD reserves at December 31: 2016 2015 2014 Proved developed reserves: Oil (MBbls) — 11,380 30,130 NGL (MBbls) — 144 4,217 Natural Gas (MMcf) — 2,033 184,437 Total (MBOE) — 11,865 65,086 PUD reserves: Oil (MBbls) — 1,307 13,788 NGL (MBbls) — 67 2,970 Natural Gas (MMcf) — 498 60,574 Total (MBOE) — 1,456 26,854 Total proved reserves: Oil (MBbls) — 12,687 43,918 NGL (MBbls) — 211 7,187 Natural Gas (MMcf) — 2,531 245,011 Total (MBOE) — 13,321 91,940 As of December 31, 2016 , the Company had no PUD reserves, which is a decrease of 1.5 MMBOE from December 31, 2015 . The decrease relates to the asset sales during 2016.

Summary of significant accoun28

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The abbreviations and acronyms used throughout are defined following the Notes to Consolidated Financial Statements. The consolidated financial statements of the Company include the accounts of the following businesses: electric, natural gas distribution, pipeline and midstream, construction materials and contracting, construction services and other. The electric and natural gas distribution businesses, as well as a portion of the pipeline and midstream business, are regulated. Construction materials and contracting, construction services and the other businesses, as well as a portion of the pipeline and midstream business, are nonregulated. For further descriptions of the Company's businesses, see Note 13 . Intercompany balances and transactions have been eliminated in consolidation, except for certain transactions related to the Company's regulated operations in accordance with GAAP. The statements also include the ownership interests in the assets, liabilities and expenses of jointly owned electric generating facilities. The Company's regulated businesses are subject to various state and federal agency regulations. The accounting policies followed by these businesses are generally subject to the Uniform System of Accounts of the FERC. These accounting policies differ in some respects from those used by the Company's nonregulated businesses. The Company's regulated businesses account for certain income and expense items under the provisions of regulatory accounting, which requires these businesses to defer as regulatory assets or liabilities certain items that would have otherwise been reflected as expense or income, respectively, based on the expected regulatory treatment in future rates. The expected recovery or flowback of these deferred items generally is based on specific ratemaking decisions or precedent for each item. Regulatory assets and liabilities are being amortized consistently with the regulatory treatment established by the FERC and the applicable state public service commissions. See Note 4 for more information regarding the nature and amounts of these regulatory deferrals. 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 December 31, 2016 , up to the date of issuance of these consolidated financial statements. 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 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 assets and liabilities for the Company's discontinued operations have been classified as held for sale and the results of operations are shown in income (loss) from discontinued operations, other than certain general and administrative costs and interest expense which do not meet the criteria for income (loss) from discontinued operations. The Company's consolidated financial statements and accompanying notes for current and prior periods have been restated. 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 2 .
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents.
Accounts receivable and allowance for doubtful accounts	Accounts receivable and allowance for doubtful accounts Accounts receivable consists primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts.
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 average cost, or cost using the last-in, first-out method. All other inventories are stated at the lower of average cost or market value.
Investments	Investments The Company's investments include the cash surrender value of life insurance policies, an insurance contract, mortgage-backed securities and U.S. Treasury securities. The Company measures its investment in the insurance contract at fair value with any unrealized gains and losses recorded on the Consolidated Statements of Income. The Company has not elected the fair value option for its mortgage-backed securities and U.S. Treasury securities and, as a result, the unrealized gains and losses on these investments are recorded in accumulated other comprehensive income (loss).
Property, plant and equipment	Property, plant and equipment Additions to property, plant and equipment are recorded at cost. When regulated assets are retired, or otherwise disposed of in the ordinary course of business, the original cost of the asset is charged to accumulated depreciation. With respect to the retirement or disposal of all other assets the resulting gains or losses are recognized as a component of income. The Company is permitted to capitalize AFUDC on regulated construction projects and to include such amounts in rate base when the related facilities are placed in service. In addition, the Company capitalizes interest, when applicable, on certain construction projects associated with its other operations.
Impairment of long-lived assets	Impairment of long-lived assets The Company reviews the carrying values of its long-lived assets, excluding goodwill and assets held for sale, whenever events or changes in circumstances indicate that such carrying values may not be recoverable. The determination of whether an impairment has occurred is based on an estimate of undiscounted future cash flows attributable to the assets, compared to the carrying value of the assets. If impairment has occurred, the amount of the impairment recognized is determined by estimating the fair value of the assets and recording a loss if the carrying value is greater than the fair value.
Goodwill	Goodwill Goodwill represents the excess of the purchase price over the fair value of identifiable net tangible and intangible assets acquired in a business combination. Goodwill is required to be tested for impairment annually, which is completed in the fourth quarter, or more frequently if events or changes in circumstances indicate that goodwill may be impaired.
Revenue recognition	Revenue recognition Revenue is recognized when the earnings process is complete, as evidenced by an agreement between the customer and the Company, when delivery has occurred or services have been rendered, when the fee is fixed or determinable and when collection is reasonably assured. The Company recognizes utility revenue each month based on the services provided to all utility customers during the month. Accrued unbilled revenue which is included in receivables, net, represents revenues recognized in excess of amounts billed.
Percentage-of-completion method	Percentage-of-completion method The Company recognizes construction contract revenue from fixed-price and modified fixed-price construction contracts at its construction businesses using the percentage-of-completion method, measured by the percentage of costs incurred to date to estimated total costs for each contract. If a loss is anticipated on a contract, the loss is immediately recognized. Costs and estimated earnings in excess of billings on uncompleted contracts represent revenues recognized in excess of amounts billed and were included in receivables, net. Billings in excess of costs and estimated earnings on uncompleted contracts represent billings in excess of revenues recognized and were included in accounts payable.
Asset retirement obligations	Asset retirement obligations The Company records the fair value of a liability for an asset retirement obligation in the period in which it is incurred. When the liability is initially recorded, the Company capitalizes a cost by increasing the carrying amount of the related long-lived asset. Over time, the liability is accreted to its present value each period, and the capitalized cost is depreciated over the useful life of the related asset. Upon settlement of the liability, the Company either settles the obligation for the recorded amount or incurs a gain or loss at its nonregulated operations or incurs a regulatory asset or liability at its regulated operations.
Legal costs	Legal costs The Company expenses external legal fees as they are incurred.
Natural gas costs recoverable or refundable through rate adjustments	Natural gas costs recoverable or refundable through rate adjustments Under the terms of certain orders of the applicable state public service commissions, the Company is deferring natural gas commodity, transportation and storage costs that are greater or less than amounts presently being recovered through its existing rate schedules. Such orders generally provide that these amounts are recoverable or refundable through rate adjustments which are filed annually.
Income taxes	Income taxes The Company provides deferred federal and state income taxes on all temporary differences between the book and tax basis of the Company's assets and liabilities. Excess deferred income tax balances associated with the Company's rate-regulated activities have been recorded as a regulatory liability and are included in other liabilities. These regulatory liabilities are expected to be reflected as a reduction in future rates charged to customers in accordance with applicable regulatory procedures. The Company uses the deferral method of accounting for investment tax credits and amortizes the credits on regulated electric and natural gas distribution plant over various periods that conform to the ratemaking treatment prescribed by the applicable state public service commissions. Tax positions taken or expected to be taken in an income tax return are evaluated for recognition using a more-likely-than-not threshold, and those tax positions requiring recognition are measured as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon ultimate settlement with a taxing authority. The Company recognizes interest and penalties accrued related to unrecognized tax benefits in income taxes.
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 year. 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 year, plus the effect of outstanding performance share awards.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities at the date of the financial statements, as well as the reported amounts of revenues and expenses during the reporting period. Estimates are used for items such as impairment testing of assets held for sale, long-lived assets and goodwill; fair values of acquired assets and liabilities under the acquisition method of accounting; aggregate reserves; property depreciable lives; tax provisions; uncollectible accounts; environmental and other loss contingencies; accumulated provision for revenues subject to refund; costs on construction contracts; unbilled revenues; actuarially determined benefit costs; asset retirement obligations; and the valuation of stock-based compensation. As additional information becomes available, or actual amounts are determinable, the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates.
New accounting standards	New accounting standards 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. This guidance was to be effective for the Company on January 1, 2017. In August 2015, the FASB issued guidance deferring the effective date of the revenue guidance one year 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. Under the modified 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. In addition, the modified approach will require additional disclosures. The Company is planning to adopt the guidance using the modified retrospective approach and continues to evaluate the effects it will have on its results of operations, financial position, cash flows and disclosures. Simplifying the Presentation of Debt Issuance Costs In April 2015, the FASB issued guidance on simplifying the presentation of debt issuance costs in the financial statements. This guidance requires entities to present debt issuance costs as a direct deduction to the related debt liability. The amortization of these costs will be reported as interest expense. The guidance was effective for the Company on January 1, 2016, and was to be applied retrospectively. Early adoption of this guidance was permitted, however the Company did not elect to do so. The guidance required a reclassification of the debt issuance costs 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 debt issuance costs of $100,000 from prepayments and other current assets and $6.0 million from deferred charges and other assets - other to long-term debt on its Consolidated Balance Sheets at December 31, 2015. Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or its Equivalent) In May 2015, the FASB issued guidance on fair value measurement and disclosure requirements removing the requirement to include investments in the fair value hierarchy for which fair value is measured using the net asset value per share practical expedient. The new guidance also removes the requirement to make certain disclosures for all investments that are eligible to be measured at net asset value using the practical expedient, and rather limits those disclosures to investments for which the practical expedient has been elected. This guidance was effective for the Company on January 1, 2016, with early adoption permitted. The application of this guidance affected the Company's disclosures; however, it did not impact the Company's results of operations, financial position or cash flows. 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. This guidance was effective for the Company on January 1, 2017, on a prospective basis. The Company does not anticipate the guidance will have a material effect on its results of operations, financial position or cash flows. 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 will require all deferred tax assets and liabilities to be classified as noncurrent. These amendments will align GAAP with IFRS. Entities had the option to apply the guidance prospectively, for all deferred tax assets and liabilities, or retrospectively. 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 $33.1 million from current assets - deferred income taxes to deferred credits and other liabilities - deferred income taxes on its Consolidated Balance Sheets at December 31, 2015. 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. Leases In February 2016, the FASB issued guidance regarding leases. The guidance requires lessees to recognize a liability to make lease payments and a right-of-use asset representing its right to use the underlying asset for the lease term on the statement of financial position for leases with terms of more than 12 months. This guidance also requires additional disclosures. 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 is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and 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 will affect the income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and calculation of dilutive shares. Certain amendments of this guidance are to be applied retrospectively and others prospectively. 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 Company anticipates the guidance will impact the Consolidated Statements of Income and the Consolidated Balance Sheets, as well as the dilutive earnings per share calculation, on a prospective basis with all taxes related to share-based payments recognized as income tax expense or benefit and no longer recognized in additional paid-in capital. The Company anticipates the guidance will not have a material impact on its cash flows. 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. An entity that elects early adoption must adopt all the amendments in the same period and apply any adjustments as of the beginning of the fiscal year. 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 is evaluating the effects the adoption of the new guidance will have on its cash flows and disclosures. 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. 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 is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures. Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued guidance on simplifying the test for goodwill impairment by eliminating Step 2, which required an entity to measure the amount of impairment loss by comparing the implied fair value of reporting unit goodwill with the carrying amount of such goodwill. This guidance requires entities to perform a quantitative impairment test, previously Step 1, to identify both the existence of impairment and the amount of impairment loss by comparing the fair value of a reporting unit to its carrying amount. Entities will continue to have the option of performing a qualitative assessment to determine if the quantitative impairment test is necessary. The guidance also requires additional disclosures if an entity has one or more reporting units with zero or negative carrying amounts of net assets. The guidance will be effective for the Company on January 1, 2020, and should be applied on a prospective basis with early adoption permitted. The Company is evaluating the effects the adoption of the new guidance will have on its results of operations, financial position, cash flows and disclosures.
Variable interest entities	Variable interest entities The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary. GAAP provides a framework for identifying VIEs and determining when a company should include the assets, liabilities, noncontrolling interest and results of activities of a VIE in its consolidated financial statements. A VIE should be consolidated if a party with an ownership, contractual or other financial interest in the VIE (a variable interest holder) has the power to direct the VIE's most significant activities and the obligation to absorb losses or right to receive benefits of the VIE that could be significant to the VIE. A variable interest holder that consolidates the VIE is called the primary beneficiary. Upon consolidation, the primary beneficiary generally must initially record all of the VIE's assets, liabilities and noncontrolling interests at fair value and subsequently account for the VIE as if it were consolidated. The Company's evaluation of whether it qualifies as the primary beneficiary of a VIE involves significant judgments, estimates and assumptions and includes a qualitative analysis of the activities that most significantly impact the VIE's economic performance and whether the Company has the power to direct those activities, the design of the entity, the rights of the parties and the purpose of the arrangement.
Comprehensive income (loss)	Comprehensive income (loss) Comprehensive income (loss) is the sum of net income (loss) as reported and other comprehensive income (loss). The Company's other comprehensive income (loss) resulted from gains (losses) on derivative instruments qualifying as hedges, postretirement liability adjustments, foreign currency translation adjustments and gains (losses) on available-for-sale investments.

Summary of significant accoun29

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Inventories	Inventories at December 31 consisted of: 2016 2015 (In thousands) Aggregates held for resale $ 115,471 $ 115,854 Asphalt oil 29,103 36,498 Natural gas in storage (current) 25,761 21,023 Materials and supplies 18,372 16,997 Merchandise for resale 16,437 15,318 Other 33,129 34,861 Total $ 238,273 $ 240,551
AFUDC and interest capitailized	The amount of AFUDC and interest capitalized for the years ended December 31 were as follows: 2016 2015 2014 (In thousands) Interest capitalized $ — $ 4,381 $ 7,046 AFUDC - borrowed $ 914 $ 4,907 $ 3,023 AFUDC - equity $ 565 $ 7,971 $ 5,803
Property, plant and equipment	Property, plant and equipment at December 31 was as follows: 2016 2015 Weighted Average Depreciable Life in Years (Dollars in thousands, where applicable) Regulated: Electric: Generation $ 1,036,373 $ 1,003,173 39 Distribution 398,382 375,612 44 Transmission 284,048 255,842 57 Construction in progress 62,212 42,436 - Other 107,598 109,085 14 Natural gas distribution: Distribution 1,718,633 1,624,645 46 Construction in progress 19,934 20,530 - Other 440,846 431,406 18 Pipeline and midstream: Transmission 490,143 460,305 54 Gathering 37,831 37,831 20 Storage 45,350 44,011 62 Construction in progress 16,507 7,549 - Other 40,873 40,168 33 Nonregulated: Pipeline and midstream: Gathering and processing 31,682 158,949 19 Construction in progress 13 89 - Other 9,800 9,827 10 Construction materials and contracting: Land 94,625 95,870 - Buildings and improvements 102,347 96,864 19 Machinery, vehicles and equipment 930,471 937,084 12 Construction in progress 16,181 18,615 - Aggregate reserves 405,751 404,995 * Construction services: Land 5,346 5,025 - Buildings and improvements 26,693 25,259 26 Machinery, vehicles and equipment 132,217 121,940 6 Other 7,105 11,055 4 Other: Land 2,837 2,837 - Other 46,431 46,700 23 Less accumulated depreciation, depletion and amortization 2,578,902 2,489,322 Net property, plant and equipment $ 3,931,327 $ 3,898,380 * Depleted on the units-of-production method based on recoverable aggregate reserves.
Percentage-of-completion method	Costs and estimated earnings in excess of billings and billings in excess of costs and estimated earnings on uncompleted contracts at December 31 were as follows: 2016 2015 (In thousands) Costs and estimated earnings in excess of billings on uncompleted contracts $ 64,558 $ 64,369 Billings in excess of costs and estimated earnings on uncompleted contracts $ 64,832 $ 68,048 Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows: 2016 2015 (In thousands) Short-term retainage* $ 45,109 $ 46,207 Long-term retainage** 1,506 1,605 Total retainage $ 46,615 $ 47,812 * Expected to be paid within one year or less and included in receivables, net. ** Included in deferred charges and other assets - other.
Schedule of earnings per share reconciliation	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings (loss) per share calculation was as follows: 2016 2015 2014 (In thousands) Weighted average common shares outstanding - basic 195,299 194,928 192,507 Effect of dilutive performance share awards 319 58 80 Weighted average common shares outstanding - diluted 195,618 194,986 192,587 Shares excluded from the calculation of diluted earnings per share — — —
After-tax changes in the components of accumulated other comprehensive income (loss)	The after-tax changes in the components of accumulated other comprehensive loss as of December 31, 2016 , 2015 and 2014 , were as follows: Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges Post- retirement Liability Adjustment Foreign Currency Translation Adjustment Net Unrealized Gain (Loss) on Available- for-sale Investments Total Accumulated Other Comprehensive Loss (In thousands) Balance at December 31, 2014 $ (3,071 ) $ (38,218 ) $ (829 ) $ 15 $ (42,103 ) Other comprehensive income (loss) before reclassifications — (88 ) (173 ) (170 ) (431 ) Amounts reclassified from accumulated other comprehensive loss 404 1,794 802 131 3,131 Amounts reclassified from accumulated other comprehensive loss to a regulatory asset — 2,255 — — 2,255 Net current-period other comprehensive income (loss) 404 3,961 629 (39 ) 4,955 Balance at December 31, 2015 (2,667 ) (34,257 ) (200 ) (24 ) (37,148 ) Other comprehensive income (loss) before reclassifications — (1,470 ) 51 (182 ) (1,601 ) Amounts reclassified from accumulated other comprehensive loss 367 2,506 — 143 3,016 Net current-period other comprehensive income (loss) 367 1,036 51 (39 ) 1,415 Balance at December 31, 2016 $ (2,300 ) $ (33,221 ) $ (149 ) $ (63 ) $ (35,733 )
Reclassification out of accumulated other comprehensive income (loss)	Reclassifications out of accumulated other comprehensive loss for the years ended December 31 were as follows: 2016 2015 Location on Consolidated Statements of Income (In thousands) Reclassification adjustment for loss on derivative instruments included in net income (loss): Interest rate derivative instruments $ (593 ) $ (637 ) Interest expense 226 233 Income taxes (367 ) (404 ) Amortization of postretirement liability losses included in net periodic benefit cost (credit) (3,931 ) (2,922 ) (a) 1,425 1,128 Income taxes (2,506 ) (1,794 ) Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss) — (1,292 ) Other income — 490 Income taxes — (802 ) Reclassification adjustment for loss on available-for-sale investments included in net income (loss) (220 ) (201 ) Other income 77 70 Income taxes (143 ) (131 ) Total reclassifications $ (3,016 ) $ (3,131 ) (a) Included in net periodic benefit cost (credit). For more information, see Note 14 .

Discontinued operations (Tables

Discontinued operations (Tables)	12 Months Ended
	Dec. 31, 2016
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 are classified as held for sale associated with Pronghorn on the Company's Consolidated Balance Sheets at December 31 were as follows: 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 at December 31 were as follows: 2016 2015 (In thousands) Assets Current assets: Receivables, net $ 355 $ 13,387 Inventories — 1,308 Income taxes receivable — 9,665 Prepayments and other current assets — 221 Total current assets held for sale 355 24,581 Noncurrent assets: Investments — 37 Net property, plant and equipment 5,507 793,422 Deferred income taxes 91,098 124,035 Other 161 161 Less allowance for impairment of assets held for sale 938 754,541 Total noncurrent assets held for sale 95,828 163,114 Total assets held for sale $ 96,183 $ 187,695 Liabilities Current liabilities: Accounts payable $ 141 $ 25,013 Taxes payable 19 (a) 1,052 Accrued compensation — 13,080 Other accrued liabilities 2,358 4,838 Total current liabilities held for sale 2,518 43,983 Total liabilities held for sale $ 2,518 $ 43,983 (a) On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale. The 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 at December 31 were as follows: 2016 2015 (In thousands) Assets Current assets: Cash and cash equivalents $ — $ 688 Receivables, net — 7,693 Inventories — 13,176 Income taxes receivable 13,987 2,495 Prepayments and other current assets — 6,214 Total current assets held for sale 13,987 30,266 Noncurrent assets: Net property, plant and equipment — 412,717 Other — 9,627 Total noncurrent assets held for sale — 422,344 Total assets held for sale $ 13,987 $ 452,610 Liabilities Current liabilities: Short-term borrowings $ — $ 45,500 Long-term debt due within one year — 5,250 Accounts payable 7,425 24,468 Taxes payable — 1,391 Accrued compensation — 938 Other accrued liabilities — 4,953 Total current liabilities held for sale 7,425 82,500 Noncurrent liabilities: Long-term debt — 63,750 Deferred income taxes 14 (a) 23,841 (a) Total noncurrent liabilities held for sale 14 87,591 Total liabilities held for sale $ 7,439 $ 170,091 (a) 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, Fidelity and CEM, to the after-tax net income (loss) from discontinued operations on the Company's Consolidated Statements of Income at December 31 were as follows: 2016 2015 2014 (In thousands) Operating revenues $ 123,024 $ 363,115 $ 547,571 Operating expenses 513,813 1,666,941 386,651 Operating income (loss) (390,789 ) (1,303,826 ) 160,920 Other income 306 3,149 1,898 Interest expense 1,753 2,124 145 Income (loss) from discontinued operations before income taxes (392,236 ) (1,302,801 ) 162,673 Income taxes (91,882 ) (468,721 ) 53,362 Income (loss) from discontinued operations (300,354 ) (834,080 ) 109,311 Loss from discontinued operations attributable to noncontrolling interest (131,691 ) (35,256 ) (3,895 ) Income (loss) from discontinued operations attributable to the Company $ (168,663 ) $ (798,824 ) $ 113,206

Goodwill and other intangible31

Goodwill and other intangible assets (Tables)	12 Months Ended
	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying amount of goodwill	The changes in the carrying amount of goodwill for the year ended December 31, 2016 , were as follows: 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. The changes in the carrying amount of goodwill for the year ended December 31, 2015 , were as follows: Balance at January 1, 2015 * Goodwill Acquired During the Year Balance at December 31, 2015 * (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 — 103,441 Total $ 635,204 $ — $ 635,204 * 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 at December 31 were as follows: 2016 2015 (In thousands) Customer relationships $ 17,145 $ 20,975 Less accumulated amortization 13,917 16,845 3,228 4,130 Noncompete agreements 2,430 4,409 Less accumulated amortization 1,658 3,655 772 754 Other 7,768 8,304 Less accumulated amortization 5,843 5,846 1,925 2,458 Total $ 5,925 $ 7,342

Regulatory assets and liabili32

Regulatory assets and liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory assets	The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31: Estimated Recovery Period * 2016 2015 (In thousands) Regulatory assets: Pension and postretirement benefits (a) (e) $ 176,025 $ 185,832 Taxes recoverable from customers (a) Over plant lives 28,278 27,682 Manufactured gas plant sites remediation (a) — 18,259 18,617 Asset retirement obligations (a) — 42,580 8,000 Natural gas costs recoverable through rate adjustments (b) Up to 1 year 2,242 547 Long-term debt refinancing costs (a) Up to 21 years 6,248 7,031 Costs related to identifying generation development (a) Up to 10 years 3,407 3,808 Other (a) (b) Largely within 1- 4 years 30,281 11,741 Total regulatory assets 307,320 263,258 Regulatory liabilities: Plant removal and decommissioning costs (c) 176,972 182,981 Taxes refundable to customers (c) 11,010 17,060 Pension and postretirement benefits (c) 9,099 4,764 Natural gas costs refundable through rate adjustments (d) 25,580 20,884 Other (c) (d) 19,191 17,429 Total regulatory liabilities 241,852 243,118 Net regulatory position $ 65,468 $ 20,140 * Estimated recovery period for regulatory assets currently being recovered in rates charged to customers. (a) Included in deferred charges and other assets - other on the Consolidated Balance Sheets. (b) Included in prepayments and other current assets on the Consolidated Balance Sheets. (c) Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets. (d) Included in other accrued liabilities on the Consolidated Balance Sheets. (e) Recovered as expense is incurred or cash contributions are made.
Regulatory liabilities	The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31: Estimated Recovery Period * 2016 2015 (In thousands) Regulatory assets: Pension and postretirement benefits (a) (e) $ 176,025 $ 185,832 Taxes recoverable from customers (a) Over plant lives 28,278 27,682 Manufactured gas plant sites remediation (a) — 18,259 18,617 Asset retirement obligations (a) — 42,580 8,000 Natural gas costs recoverable through rate adjustments (b) Up to 1 year 2,242 547 Long-term debt refinancing costs (a) Up to 21 years 6,248 7,031 Costs related to identifying generation development (a) Up to 10 years 3,407 3,808 Other (a) (b) Largely within 1- 4 years 30,281 11,741 Total regulatory assets 307,320 263,258 Regulatory liabilities: Plant removal and decommissioning costs (c) 176,972 182,981 Taxes refundable to customers (c) 11,010 17,060 Pension and postretirement benefits (c) 9,099 4,764 Natural gas costs refundable through rate adjustments (d) 25,580 20,884 Other (c) (d) 19,191 17,429 Total regulatory liabilities 241,852 243,118 Net regulatory position $ 65,468 $ 20,140 * Estimated recovery period for regulatory assets currently being recovered in rates charged to customers. (a) Included in deferred charges and other assets - other on the Consolidated Balance Sheets. (b) Included in prepayments and other current assets on the Consolidated Balance Sheets. (c) Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets. (d) Included in other accrued liabilities on the Consolidated Balance Sheets. (e) Recovered as expense is incurred or cash contributions are made.

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows: 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 December 31, 2015 Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (In thousands) Mortgage-backed securities $ 9,128 $ 19 $ (49 ) $ 9,098 U.S. Treasury securities 1,315 — (6 ) 1,309 Total $ 10,443 $ 19 $ (55 ) $ 10,407
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 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 Measurements at December 31, 2015, 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, 2015 (In thousands) Assets: Money market funds $ — $ 1,420 $ — $ 1,420 Insurance contract* — 67,459 — 67,459 Available-for-sale securities: Mortgage-backed securities — 9,098 — 9,098 U.S. Treasury securities — 1,309 — 1,309 Total assets measured at fair value $ — $ 79,286 $ — $ 79,286 * The insurance contract invests approximately 63 percent in fixed-income investments, 19 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.
Fair value of long-term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt at December 31 was as follows: 2016 2015 Carrying Amount Fair Value Carrying Amount Fair Value (In thousands) Long-term debt $ 1,790,159 $ 1,841,885 $ 1,796,163 $ 1,819,828

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Outstanding credit facilities	The following table summarizes the outstanding revolving credit facilities of the Company and its subsidiaries: Company Facility Facility Limit Amount Outstanding at December 31, 2016 Amount Outstanding at December 31, 2015 Letters of Credit at December 31, 2016 Expiration Date (In millions) MDU Resources Group, Inc. Commercial paper/Revolving credit agreement (a) $ 175.0 $ 111.0 (b) $ 44.5 (b) $ — 5/8/19 Cascade Natural Gas Corporation Revolving credit agreement $ 50.0 (c) $ — $ — $ 2.2 (d) 7/9/18 Intermountain Gas Company Revolving credit agreement $ 65.0 (e) $ 20.9 $ 47.9 $ — 7/13/18 Centennial Energy Holdings, Inc. Commercial paper/Revolving credit agreement (f) $ 500.0 $ 151.0 (b) $ 18.0 (b) $ — 9/23/21 (a) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of the Company on stated conditions, up to a maximum of $225.0 million ). There were no amounts outstanding under the credit agreement. (b) Amount outstanding under commercial paper program. (c) Certain provisions allow for increased borrowings, up to a maximum of $75.0 million . (d) Outstanding letter(s) of credit reduce the amount available under the credit agreement. (e) Certain provisions allow for increased borrowings, up to a maximum of $90.0 million . (f) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Centennial on stated conditions, up to a maximum of $600.0 million ). There were no amounts outstanding under the credit agreement.
Long term debt outstanding	Long-term debt outstanding at December 31 was as follows: 2016 2015 (In thousands) Senior Notes at a weighted average rate of 4.87%, due on dates ranging from August 31, 2017 to January 15, 2055 $ 1,437,831 $ 1,616,246 Commercial paper at a weighted average rate of 1.27%, supported by revolving credit agreements 262,000 62,500 Medium-Term Notes at a weighted average rate of 6.68%, due on dates ranging from September 1, 2020 to March 16, 2029 50,000 50,000 Other notes at a weighted average rate of 5.25%, due on February 1, 2035 24,471 24,589 Credit agreements at a weighted average rate of 3.14%, due on dates ranging from July 13, 2018 to November 30, 2038 21,793 48,906 Unamortized debt issuance costs (5,832 ) (6,069 ) Discount (104 ) (9 ) Total long-term debt 1,790,159 1,796,163 Less current maturities 43,598 238,539 Net long-term debt $ 1,746,561 $ 1,557,624
Schedule of debt maturities	Schedule of Debt Maturities Long-term debt maturities for the five years and thereafter following December 31, 2016 , were as follows: 2017 2018 2019 2020 2021 Thereafter (In thousands) Long-term debt maturities $ 43,598 $ 169,449 $ 162,154 $ 15,021 $ 151,013 $ 1,254,860

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2016
Asset Retirement Obligation [Abstract]
Reconciliation of the company's asset retirement obligation	A reconciliation of the Company's liability, which is included in other accrued liabilities and deferred credits and other liabilities - other on the Consolidated Balance Sheets, for the years ended December 31 was as follows: 2016 2015 (In thousands) Balance at beginning of year $ 242,224 $ 27,211 Liabilities incurred 15,114 2,751 Liabilities settled (4,338 ) (1,708 ) Accretion expense 13,918 2,134 Revisions in estimates 48,052 211,836 Balance at end of year $ 314,970 $ 242,224

Preferred Stocks (Tables)

Preferred Stocks (Tables)	12 Months Ended
	Dec. 31, 2016
Class of Stock Disclosures [Abstract]
Preferred stock	Preferred stocks at December 31 were as follows: 2016 2015 (In thousands, except shares and per share amounts) Authorized: Preferred - 500,000 shares, cumulative, par value $100, issuable in series Preferred stock A - 1,000,000 shares, cumulative, without par value, issuable in series (none outstanding) Preference - 500,000 shares, cumulative, without par value, issuable in series (none outstanding) Outstanding: 4.50% Series - 100,000 shares $ 10,000 $ 10,000 4.70% Series - 50,000 shares 5,000 5,000 Total preferred stocks $ 15,000 $ 15,000

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Share-based Compensation [Abstract]
Target grants performance share	Target grants of performance shares outstanding at December 31, 2016 , were as follows: Grant Date Performance Period Target Grant of Shares February 2014 2014-2016 136,901 February 2015 2015-2017 200,112 June 2015 2015-2017 14,441 February 2016 2016-2018 310,583 March 2016 2016-2018 2,151
Schedule of share-based payment award, performance shares, valuation assumptions	Assumptions used for grants of performance shares issued in 2016 , 2015 and 2014 were: 2016 2015 2014 Weighted average grant-date fair value $14.60 $18.98 $41.13 Blended volatility range 29.25 % – 32.51 % 22.86 % – 24.61 % 18.94 % – 20.43 % Risk-free interest rate range .47 % – .92 % .05 % – 1.07 % .03 % – .74 % Weighted average discounted dividends per share $1.56 $1.57 $2.15
Summary of the status of the performance share awards	A summary of the status of the performance share awards for the year ended December 31, 2016 , was as follows: Number of Shares Weighted Average Grant-Date Fair Value Nonvested at beginning of period 565,896 $ 27.90 Granted 324,205 14.60 Less: Vested 58,401 29.01 Forfeited 167,512 27.30 Nonvested at end of period 664,188 $ 21.47

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Components of income before income taxes	The components of income before income taxes from continuing operations for each of the years ended December 31 were as follows: 2016 2015 2014 (In thousands) United States $ 326,252 $ 248,379 $ 249,501 Foreign (24 ) (1,326 ) (52 ) Income before income taxes from continuing operations $ 326,228 $ 247,053 $ 249,449
Income tax expense	Income tax expense from continuing operations for the years ended December 31 was as follows: 2016 2015 2014 (In thousands) Current: Federal $ 81,989 $ 85,897 $ 8,837 State 13,190 10,093 622 Foreign 2 30 — 95,181 96,020 9,459 Deferred: Income taxes: Federal (2,102 ) (19,632 ) 52,041 State 1,184 (5,304 ) 1,913 Investment tax credit - net (1,131 ) (420 ) 1,009 (2,049 ) (25,356 ) 54,963 Total income tax expense $ 93,132 $ 70,664 $ 64,422
Deferred tax assets and deferred tax liabilities	Components of deferred tax assets and deferred tax liabilities at December 31 were as follows: 2016 2015 (In thousands) Deferred tax assets: Postretirement $ 87,872 $ 97,666 Compensation-related 44,995 33,714 Alternative minimum tax credit carryforward 29,338 28,169 Federal renewable energy credit 16,944 3,400 Customer advances 13,524 12,623 Legal and environmental contingencies 9,895 6,377 Asset retirement obligations 8,867 8,694 Other 46,957 43,306 Total deferred tax assets 258,392 233,949 Deferred tax liabilities: Depreciation and basis differences on property, plant and equipment 774,838 756,444 Postretirement 70,670 71,835 Intangible asset amortization 26,413 23,950 Other 45,580 36,359 Total deferred tax liabilities 917,501 888,588 Valuation allowance 9,117 8,990 Net deferred income tax liability $ 668,226 $ 663,629
Schedule of change in net deferred income tax liability reconciliation	The following table reconciles the change in the net deferred income tax liability from December 31, 2015 , to December 31, 2016 , to deferred income tax expense: 2016 (In thousands) Change in net deferred income tax liability from the preceding table $ 4,597 Deferred taxes associated with other comprehensive income (825 ) Other (5,821 ) Deferred income tax benefit for the period $ (2,049 )
Reconciliation of income tax expense (benefit) at statutory federal rate versus actual rate	Total income tax expense differs from the amount computed by applying the statutory federal income tax rate to income before taxes. The reasons for this difference were as follows: Years ended December 31, 2016 2015 2014 Amount % Amount % Amount % (Dollars in thousands) Computed tax at federal statutory rate $ 114,179 35.0 $ 86,468 35.0 $ 87,308 35.0 Increases (reductions) resulting from: State income taxes, net of federal income tax 9,027 2.8 8,208 3.3 7,019 2.8 Federal renewable energy credit (13,544 ) (4.2 ) (3,400 ) (1.4 ) (3,655 ) (1.5 ) Tax compliance and uncertain tax positions (3,028 ) (.9 ) (2,607 ) (1.0 ) (8,568 ) (3.4 ) Domestic production activities (6,251 ) (1.9 ) (6,842 ) (2.8 ) (3,993 ) (1.6 ) Other (7,251 ) (2.3 ) (11,163 ) (4.5 ) (13,689 ) (5.5 ) Total income tax expense $ 93,132 28.5 $ 70,664 28.6 $ 64,422 25.8
Reconciliation of unrecognized tax benefits (excluding interest)	A reconciliation of the unrecognized tax benefits (excluding interest) for the years ended December 31 was as follows: 2016 2015 2014 (In thousands) Balance at beginning of year $ — $ 105 $ 7,845 Settlements — — (7,740 ) Lapse of statute of limitations — (105 ) — Balance at end of year $ — $ — $ 105

Cash flow information (Tables)

Cash flow information (Tables)	12 Months Ended
	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash expenditures for interest and income taxes and noncash investing transactions	Cash expenditures for interest and income taxes for the years ended December 31 were as follows: 2016 2015 2014 (In thousands) Interest, net of amount capitalized and AFUDC - borrowed of $914, $9,288 and $10,069 in 2016, 2015 and 2014, respectively $ 87,920 $ 88,775 $ 81,195 Income taxes paid, net* $ 105,908 $ 61,405 $ 80,090 * Income taxes paid, net of discontinued operations, were $1.3 million , $2.4 million and $69.8 million for the years ended December 31, 2016, 2015 and 2014, respectively. Noncash investing transactions at December 31 were as follows: 2016 2015 2014 (In thousands) Property, plant and equipment additions in accounts payable $ 22,712 $ 39,754 $ 12,791

Business segment data (Tables)

Business segment data (Tables)	12 Months Ended
	Dec. 31, 2016
Segment Reporting [Abstract]
Information on the Company's businesses	The information below follows the same accounting policies as described in the Summary of Significant Accounting Policies. Information on the Company's businesses as of December 31 and for the years then ended was as follows: 2016 2015 2014 (In thousands) External operating revenues: Regulated operations: Electric $ 322,356 $ 280,615 $ 277,874 Natural gas distribution 766,115 817,419 921,986 Pipeline and midstream 52,983 51,004 47,043 1,141,454 1,149,038 1,246,903 Nonregulated operations: Pipeline and midstream 39,602 54,281 64,494 Construction materials and contracting 1,873,696 1,901,530 1,740,089 Construction services 1,072,663 907,767 1,062,055 Other 1,413 1,436 1,532 2,987,374 2,865,014 2,868,170 Total external operating revenues $ 4,128,828 $ 4,014,052 $ 4,115,073 Intersegment operating revenues: Regulated operations: Electric $ — $ — $ — Natural gas distribution — — — Pipeline and midstream 48,794 49,065 45,013 48,794 49,065 45,013 Nonregulated operations: Pipeline and midstream 223 554 742 Construction materials and contracting 574 2,752 25,241 Construction services 609 18,660 57,474 Other 7,230 7,755 7,832 8,636 29,721 91,289 Intersegment eliminations (57,430 ) (78,786 ) (136,302 ) Total intersegment operating revenues $ — $ — $ — Depreciation, depletion and amortization: Electric $ 50,220 $ 37,583 $ 35,008 Natural gas distribution 65,426 64,756 54,700 Pipeline and midstream 24,885 27,981 29,749 Construction materials and contracting 58,413 65,937 68,557 Construction services 15,307 13,420 12,874 Other 2,067 2,070 2,196 Total depreciation, depletion and amortization $ 216,318 $ 211,747 $ 203,084 Interest expense: Electric $ 24,982 $ 17,421 $ 15,595 Natural gas distribution 30,405 29,471 27,217 Pipeline and midstream 7,903 9,895 9,946 Construction materials and contracting 15,265 15,183 16,368 Construction services 4,059 3,959 4,176 Other 5,854 15,853 13,823 Intersegment eliminations (620 ) (603 ) (254 ) Total interest expense $ 87,848 $ 91,179 $ 86,871 2016 2015 2014 (In thousands) Income taxes: Electric $ 1,449 $ 11,523 $ 12,442 Natural gas distribution 9,181 11,377 11,350 Pipeline and midstream 12,408 7,505 12,232 Construction materials and contracting 60,625 41,619 18,586 Construction services 17,748 16,432 24,753 Other (2,028 ) (9,834 ) (11,136 ) Intersegment eliminations (6,251 ) (7,958 ) (3,805 ) Total income taxes $ 93,132 $ 70,664 $ 64,422 Earnings (loss) on common stock: Regulated operations: Electric $ 42,222 $ 35,914 $ 36,731 Natural gas distribution 27,102 23,607 30,484 Pipeline and midstream 22,060 20,680 15,440 91,384 80,201 82,655 Nonregulated operations: Pipeline and midstream 1,375 (7,430 ) 9,226 Construction materials and contracting 102,687 89,096 51,510 Construction services 33,945 23,762 54,432 Other (3,231 ) (14,941 ) (7,386 ) 134,776 90,487 107,782 Intersegment eliminations (a) 6,251 5,016 (6,095 ) Earnings on common stock before income (loss) from discontinued operations 232,411 175,704 184,342 Income (loss) from discontinued operations, net of tax (a) (300,354 ) (834,080 ) 109,311 Loss from discontinued operations attributable to noncontrolling interest (131,691 ) (35,256 ) (3,895 ) Total earnings (loss) on common stock $ 63,748 $ (623,120 ) $ 297,548 Capital expenditures: Electric $ 111,134 $ 332,876 $ 185,121 Natural gas distribution 126,272 130,793 120,613 Pipeline and midstream 34,467 18,315 61,754 Construction materials and contracting 37,845 48,126 37,896 Construction services 60,344 38,269 26,942 Other 2,358 3,755 2,131 Total capital expenditures (b) $ 372,420 $ 572,134 $ 434,457 Assets: Electric (c) $ 1,406,694 $ 1,325,858 $ 1,028,001 Natural gas distribution (c) 2,099,296 2,038,433 1,935,271 Pipeline and midstream 550,615 591,651 651,925 Construction materials and contracting 1,220,459 1,261,963 1,260,534 Construction services 513,093 442,845 437,322 Other (d) 283,255 287,940 315,495 Assets held for sale 211,055 616,464 2,176,857 Total assets $ 6,284,467 $ 6,565,154 $ 7,805,405 2016 2015 2014 (In thousands) Property, plant and equipment: Electric (c) $ 1,888,613 $ 1,786,148 $ 1,457,101 Natural gas distribution (c) 2,179,413 2,076,581 1,904,759 Pipeline and midstream 672,199 758,729 818,388 Construction materials and contracting 1,549,375 1,553,428 1,529,942 Construction services 171,361 163,279 144,395 Other 49,268 49,537 50,937 Less accumulated depreciation, depletion and amortization 2,578,902 2,489,322 2,385,202 Net property, plant and equipment $ 3,931,327 $ 3,898,380 $ 3,520,320 (a) Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations. (b) Capital expenditures for 2016, 2015 and 2014 include noncash capital expenditure-related accounts payable and AFUDC, totaling $(15.8) million , $35.3 million and $5.1 million , respectively. (c) Includes allocations of common utility property. (d) Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets).

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
Schedule of amounts recognized in balance sheet	Changes in benefit obligation and plan assets for the years ended December 31, 2016 and 2015 , and amounts recognized in the Consolidated Balance Sheets at December 31, 2016 and 2015 , were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2016 2015 (In thousands) Change in benefit obligation: Benefit obligation at beginning of year $ 442,960 $ 475,337 $ 92,734 $ 99,012 Service cost — 86 1,647 1,816 Interest cost 17,218 17,141 3,688 3,607 Plan participants' contributions — — 1,405 1,408 Actuarial (gain) loss 1,882 (24,875 ) (3,872 ) (5,873 ) Benefits paid (25,753 ) (24,729 ) (6,298 ) (7,236 ) Benefit obligation at end of year 436,307 442,960 89,304 92,734 Change in net plan assets: Fair value of plan assets at beginning of year 332,667 354,363 82,593 87,586 Actual gain (loss) on plan assets 26,595 (10,879 ) 4,184 258 Employer contribution — 13,912 962 577 Plan participants' contributions — — 1,405 1,408 Benefits paid (25,753 ) (24,729 ) (6,298 ) (7,236 ) Fair value of net plan assets at end of year 333,509 332,667 82,846 82,593 Funded status - under $ (102,798 ) $ (110,293 ) $ (6,458 ) $ (10,141 ) Amounts recognized in the Consolidated Balance Sheets at December 31: Other assets (noncurrent) $ — $ — $ 13,131 $ 5,095 Other accrued liabilities (current) — — (538 ) (421 ) Other liabilities (noncurrent) (102,798 ) (110,293 ) (19,051 ) (14,815 ) Net amount recognized $ (102,798 ) $ (110,293 ) $ (6,458 ) $ (10,141 ) Amounts recognized in accumulated other comprehensive (income) loss consist of: Actuarial loss $ 198,668 $ 208,671 $ 17,470 $ 22,484 Prior service cost (credit) — — (13,003 ) (14,374 ) Total $ 198,668 $ 208,671 $ 4,467 $ 8,110
The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets	The pension plans all have accumulated benefit obligations in excess of plan assets. The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for these plans at December 31 were as follows: 2016 2015 (In thousands) Projected benefit obligation $ 436,307 $ 442,960 Accumulated benefit obligation $ 436,307 $ 442,960 Fair value of plan assets $ 333,509 $ 332,667
Components of net periodic benefit cost	Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2014 2016 2015 2014 (In thousands) Components of net periodic benefit cost (credit): Service cost $ — $ 86 $ 129 $ 1,647 $ 1,816 $ 1,518 Interest cost 17,218 17,141 17,682 3,688 3,607 3,521 Expected return on assets (20,924 ) (22,254 ) (21,218 ) (4,533 ) (4,795 ) (4,617 ) Amortization of prior service cost (credit) — 36 71 (1,371 ) (1,371 ) (1,393 ) Recognized net actuarial loss 6,215 7,016 4,869 1,491 1,960 649 Curtailment loss — 258 — — — — Net periodic benefit cost (credit), including amount capitalized 2,509 2,283 1,533 922 1,217 (322 ) Less amount capitalized 381 316 388 (52 ) 120 (21 ) Net periodic benefit cost (credit) 2,128 1,967 1,145 974 1,097 (301 ) Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss: Net (gain) loss (3,789 ) 8,257 77,238 (3,523 ) (1,336 ) 15,114 Amortization of actuarial loss (6,215 ) (7,016 ) (4,869 ) (1,491 ) (1,960 ) (649 ) Amortization of prior service (cost) credit — (294 ) (71 ) 1,371 1,371 1,393 Total recognized in accumulated other comprehensive (income) loss (10,004 ) 947 72,298 (3,643 ) (1,925 ) 15,858 Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss $ (7,876 ) $ 2,914 $ 73,443 $ (2,669 ) $ (828 ) $ 15,557
Weighted average assumptions used to determine benefit obligations and net periodic benefit costs	Weighted average assumptions used to determine benefit obligations at December 31 were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2016 2015 Discount rate 3.83 % 4.00 % 3.86 % 4.06 % Expected return on plan assets 6.75 % 6.75 % 5.75 % 5.75 % Rate of compensation increase N/A N/A 3.00 % 3.00 % Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31 were as follows: Pension Benefits Other Postretirement Benefits 2016 2015 2016 2015 Discount rate 4.00 % 3.70 % 4.06 % 3.74 % Expected return on plan assets 6.75 % 7.00 % 5.75 % 6.00 % Rate of compensation increase N/A N/A 3.00 % 3.00 %
Health care rate assumptions for the Company's other postretirement benefit plans	Health care rate assumptions for the Company's other postretirement benefit plans as of December 31 were as follows: 2016 2015 Health care trend rate assumed for next year 8.6 % – 10.7 % 4.0 % – 8.0 % Health care cost trend rate - ultimate 4.5 % 5.0 % – 6.0 % Year in which ultimate trend rate achieved 2024 2021
Assumed health care cost trend rates	Assumed health care cost trend rates may have a significant effect on the amounts reported for the health care plans. A one percentage point change in the assumed health care cost trend rates would have had the following effects at December 31, 2016 : 1 Percentage Point Increase 1 Percentage Point Decrease (In thousands) Effect on total of service and interest cost components $ 255 $ (210 ) Effect on postretirement benefit obligation $ 5,741 $ (4,834 )
The fair value of the pension and postretirement net plan assets by class	The fair value of the Company's pension plans' assets (excluding cash) by class were as follows: Fair Value Measurements 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: Cash equivalents $ — $ 6,347 $ — $ 6,347 Equity securities: U.S. companies 11,348 — — 11,348 International companies 1,584 — — 1,584 Collective and mutual funds* 162,055 64,052 — 226,107 Corporate bonds — 68,677 — 68,677 Municipal bonds — 11,002 — 11,002 U.S. Government securities 4,352 2,044 — 6,396 Total assets measured at fair value $ 179,339 $ 152,122 $ — $ 331,461 * Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in corporate bonds, 20 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 7 percent in U.S. Government securities and 15 percent in other investments. Fair Value Measurements at December 31, 2015, 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, 2015 (In thousands) Assets: Cash equivalents $ — $ 8,379 $ — $ 8,379 Equity securities: U.S. companies 15,135 — — 15,135 International companies 2,332 — — 2,332 Collective and mutual funds* 154,400 63,568 — 217,968 Corporate bonds — 62,145 — 62,145 Municipal bonds — 11,680 — 11,680 U.S. Government securities 5,288 6,823 — 12,111 Total assets measured at fair value $ 177,155 $ 152,595 $ — $ 329,750 * Collective and mutual funds invest approximately 29 percent in common stock of international companies, 19 percent in common stock of large-cap U.S. companies, 16 percent in corporate bonds, 16 percent in cash equivalents, 6 percent in common stock of mid-cap U.S. companies and 14 percent in other investments. The fair value of the Company's other postretirement benefit plans' assets (excluding cash) by asset class were as follows: Fair Value Measurements at December 31, 2016, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2016 (In thousands) Assets: Cash equivalents $ — $ 250 $ — $ 250 Equity securities: U.S. companies 2,328 — — 2,328 International companies 5 — — 5 Insurance contract* — 80,263 — 80,263 Total assets measured at fair value $ 2,333 $ 80,513 $ — $ 82,846 * The insurance contract invests approximately 38 percent in corporate bonds, 25 percent in common stock of large-cap U.S. companies, 20 percent in U.S. Government securities, 9 percent in mortgage-backed securities and 8 percent in other investments. Fair Value Measurements at December 31, 2015, Using Quoted Prices Significant Significant Unobservable Balance at December 31, 2015 (In thousands) Assets: Cash equivalents $ — $ 3,261 $ — $ 3,261 Equity securities: U.S. companies 2,274 — — 2,274 International companies 9 — — 9 Insurance contract* — 77,044 — 77,044 Total assets measured at fair value $ 2,283 $ 80,305 $ — $ 82,588 * The insurance contract invests approximately 36 percent in corporate bonds, 22 percent in U.S. Government securities, 19 percent in common stock of large-cap U.S. companies, 10 percent in mortgage-backed securities and 13 percent in other investments.
Benefit payments expected to be paid	The following benefit payments, which reflect future service, as appropriate, and expected Medicare Part D subsidies are as follows: Years Pension Benefits Other Postretirement Benefits Expected Medicare Part D Subsidy (In thousands) 2017 $ 24,798 $ 5,410 $ 168 2018 25,054 5,573 165 2019 25,271 5,603 160 2020 25,616 5,500 154 2021 25,987 5,511 146 2022 - 2026 132,224 27,956 568
Multiemployer plans	EIN/Pension Plan Number Pension Protection Act Zone Status FIP/RP Status Pending/Implemented Contributions Surcharge Imposed Expiration Date of Collective Bargaining Agreement Pension Fund 2016 2015 2016 2015 2014 (In thousands) Alaska Laborers-Employers Retirement Fund 91-6028298-001 Yellow as of 6/30/2016 Yellow as of 6/30/2015 Implemented $ 766 $ 917 $ 666 No 12/31/2016 Edison Pension Plan 93-6061681-001 Green as of 12/31/2016 Green as of 12/31/2015 No 6,242 5,517 9,061 No 12/31/2017 IBEW Local No. 82 Pension Plan 31-6127268-001 Green as of 6/30/2016 Red as of 6/30/2015 Implemented 2,560 2,252 1,392 No 12/1/2019 IBEW Local No. 357 Pension Plan A 88-6023284-001 Green Green No 3,016 1,896 3,575 No 5/31/2018 IBEW Local 648 Pension Plan 31-6134845-001 Red as of 2/29/2016 Red as of 2/28/2015 Implemented 773 745 1,110 No 9/2/2018 Idaho Plumbers and Pipefitters Pension Plan 82-6010346-001 Green as of 5/31/2016 Green as of 5/31/2015 No 1,221 1,169 1,125 No 9/30/2019 Local Union 212 IBEW Pension Trust Fund 31-6127280-001 Yellow as of 4/30/2016 Yellow as of 4/30/2015 Implemented 1,146 937 568 No 6/2/2019 National Automatic Sprinkler Industry Pension Fund 52-6054620-001 Red as of 12/31/2016 Red as of 12/31/2015 Implemented 775 677 608 No 7/31/2018- National Electrical Benefit Fund 53-0181657-001 Green Green No 6,366 5,271 6,476 No 1/1/2017- Operating Engineers Local 800 & WY Contractors Association, Inc. Pension Plan for Wyoming** 83-6011320-001 Red as of 12/31/2016 Red as of 12/31/2015 Implemented — — 68 No 10/31/2005* Sheet Metal Workers' Pension Plan of Southern CA, AZ and NV 95-6052257-001 Red as of 12/31/2016 Red as of 12/31/2015 Implemented 1,087 714 676 No 6/30/2017 Southwest Marine Pension Trust 95-6123404-001 Red Red Implemented 50 26 31 No 1/31/2019 Other funds 20,525 18,991 17,461 Total contributions $ 44,527 $ 39,112 $ 42,817 * Plan includes collective bargaining agreements which have expired. The agreements contain provisions that automatically renew the existing contracts in lieu of a new negotiated collective bargaining agreement. ** The Company withdrew from the plan as of October 26, 2014, as discussed later. The Company was listed in the plans' Forms 5500 as providing more than 5 percent of the total contributions for the following plans and plan years: Pension Fund Year Contributions to Plan Exceeded More Than 5 Percent of Total Contributions (as of December 31 of the Plan's Year-End) Edison Pension Plan 2015 and 2014 IBEW Local No. 82 Pension Plan 2015 and 2014 Local Union No. 124 IBEW Pension Trust Fund 2015 and 2014 Local Union 212 IBEW Pension Trust Fund 2015 and 2014 IBEW Local Union No. 357 Pension Plan A 2015 and 2014 IBEW Local 573 Pension Plan 2014 IBEW Local 648 Pension Plan 2015 and 2014 Idaho Plumbers and Pipefitters Pension Plan 2015 and 2014 Minnesota Teamsters Construction Division Pension Fund 2015 and 2014 Operating Engineers Local 800 & WY Contractors Association, Inc. Pension Plan for Wyoming* 2014 Pension and Retirement Plan of Plumbers and Pipefitters Union Local No. 525 2015 and 2014 * The Company withdrew from the plan as of October 26, 2014, as discussed later.

Jointly owned facilities (Table

Jointly owned facilities (Tables)	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Company's share of the cost of utility plant in service and related accumulated depreciation for the stations	At December 31, the Company's share of the cost of utility plant in service and related accumulated depreciation for the stations was as follows: 2016 2015 (In thousands) Big Stone Station: Utility plant in service $ 157,144 $ 157,761 Less accumulated depreciation 49,568 48,242 $ 107,576 $ 109,519 Coyote Station: Utility plant in service $ 156,334 $ 140,895 Less accumulated depreciation 105,928 94,755 $ 50,406 $ 46,140 Wygen III: Utility plant in service $ 66,251 $ 65,023 Less accumulated depreciation 7,550 6,788 $ 58,701 $ 58,235

Commitment and Contingencies Di

Commitment and Contingencies Disclosure (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Limited liability project reflected on consolidated balance sheet	The total assets and liabilities of Dakota Prairie Refining at December 31 were as follows: 2015 (In thousands) Assets Current assets: Cash and cash equivalents $ 851 Accounts receivable 7,693 Inventories 13,176 Other current assets 6,215 Total current assets 27,935 Net property, plant and equipment 425,123 Deferred charges and other assets: Other 9,626 Total deferred charges and other assets 9,626 Total assets $ 462,684 Liabilities Current liabilities: Short-term borrowings $ 45,500 Long-term debt due within one year 5,250 Accounts payable 24,766 Taxes payable 1,391 Accrued compensation 938 Other accrued liabilities 4,953 Total current liabilities 82,798 Long-term debt 63,750 Total liabilities $ 146,548

Quarterly Financial Information

Quarterly Financial Information Disclosure (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly financial data	The following unaudited information shows selected items by quarter for the years 2016 and 2015 : First Quarter Second Quarter Third Quarter Fourth Quarter (In thousands, except per share amounts) 2016 Operating revenues $ 860,214 $ 1,043,948 $ 1,208,567 $ 1,016,099 Operating expenses 798,229 954,983 1,061,883 904,613 Operating income 61,985 88,965 146,684 111,486 Income from continuing operations 31,865 46,298 88,386 66,547 Loss from discontinued operations attributable to the Company, net of tax (6,996 ) (155,451 ) (5,400 ) (816 ) Net income (loss) attributable to the Company 24,869 (109,153 ) 82,986 65,731 Earnings (loss) per common share - basic: Earnings before discontinued operations .16 .24 .45 .34 Discontinued operations attributable to the Company, net of tax (.03 ) (.80 ) (.03 ) — Earnings (loss) per common share - basic .13 (.56 ) .42 .34 Earnings (loss) per common share - diluted: Earnings before discontinued operations .16 .24 .45 .33 Discontinued operations attributable to the Company, net of tax (.03 ) (.80 ) (.03 ) — Earnings (loss) per common share - diluted .13 (.56 ) .42 .33 Weighted average common shares outstanding: Basic 195,284 195,304 195,304 195,304 Diluted 195,284 195,699 195,811 195,889 2015 Operating revenues $ 860,845 $ 938,039 $ 1,198,342 $ 1,016,826 Operating expenses 810,537 878,330 1,070,514 934,896 Operating income 50,308 59,709 127,828 81,930 Income from continuing operations 20,540 26,061 73,886 55,902 Loss from discontinued operations attributable to the Company, net of tax (326,457 ) (255,665 ) (213,334 ) (3,368 ) Net income (loss) attributable to the Company (305,917 ) (229,604 ) (139,448 ) 52,534 Earnings (loss) per common share - basic: Earnings before discontinued operations .10 .13 .38 .29 Discontinued operations attributable to the Company, net of tax (1.67 ) (1.31 ) (1.10 ) (.02 ) Earnings (loss) per common share - basic (1.57 ) (1.18 ) (.72 ) .27 Earnings (loss) per common share - diluted: Earnings before discontinued operations .10 .13 .38 .29 Discontinued operations attributable to the Company, net of tax (1.67 ) (1.31 ) (1.10 ) (.02 ) Earnings (loss) per common share - diluted (1.57 ) (1.18 ) (.72 ) .27 Weighted average common shares outstanding: Basic 194,479 194,805 195,151 195,266 Diluted 194,566 194,838 195,169 195,324 Notes: • Fourth quarter 2016 reflects a reduction to a previously recorded MEPP withdrawal liability of $11.1 million (before tax). For more information, see Note 14 . • 2015 and first quarter 2016 have been recast to present the results of operations of Dakota Prairie Refining as discontinued operations, other than certain general and administrative costs and interest expense which were previously allocated to the former refining segment and do not meet the criteria for income (loss) from discontinued operations. • First quarter 2015 has been recast to present the results of operations of Fidelity as discontinued operations, other than certain general and administrative costs and interest expense which were previously allocated to the former exploration and production segment and do not meet the criteria for income (loss) from discontinued operations. • First quarter 2015 reflects a MEPP withdrawal liability of $2.4 million (before tax). For more information, see Note 14 . • Second quarter 2015 reflects an impairment of coalbed natural gas gathering assets of $3.0 million (before tax). For more information, see Note 1 . • Third quarter 2015 reflects an impairment of coalbed natural gas gathering assets of $14.1 million (before tax). For more information, see Note 1 .

Exploration and Production Ac45

Exploration and Production Activities (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Capitalized costs relating to oil and gas producing activities disclosure	The following table sets forth capitalized costs and accumulated depreciation, depletion and amortization related to oil and natural gas producing activities, prior to Fidelity's assets being held for sale, at December 31: 2014 (In thousands) Subject to amortization $ 3,205,036 Not subject to amortization 132,141 Total capitalized costs 3,337,177 Less accumulated depreciation, depletion and amortization 1,752,566 Net capitalized costs $ 1,584,611
Cost incurred in oil and gas property acquisition, exploration, and development activities disclosure	Capital expenditures, including those not subject to amortization, related to oil and natural gas producing activities prior to Fidelity's assets being held for sale, excluding the years ended December 31, 2016 and 2015, due to no wells being drilled during that time, were as follows: Year ended December 31, 2014 * (In thousands) Acquisitions: Proved properties $ 87,919 Unproved properties 138,683 Exploration 16,879 Development 331,400 Total capital expenditures $ 574,881 * Excludes net reductions to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of oil and natural gas wells of $9.0 million for the year ended December 31, 2014.
Schedule of proved developed and undeveloped oil and gas reserve quantities	The changes in the Company's estimated quantities of proved oil, NGL and natural gas reserves for the year ended December 31, 2016, were as follows: Oil (MBbls) NGL (MBbls) Natural Gas (MMcf) Total (MBOE) Proved developed and undeveloped reserves: Balance at beginning of year 12,687 211 2,531 13,321 Production — — — — Extensions and discoveries — — — — Improved recovery — — — — Purchases of proved reserves — — — — Sales of proved reserves (12,687 ) (211 ) (2,531 ) (13,321 ) Revisions of previous estimates — — — — Balance at end of year — — — — Significant changes in proved reserves for the year ended December 31, 2016, include: • Sales of proved reserves of (13.3) MMBOE, due to the Company's decision to sell Fidelity and exit the exploration and production business The changes in the Company's estimated quantities of proved oil, NGL and natural gas reserves for the year ended December 31, 2015, were as follows: Oil (MBbls) NGL (MBbls) Natural Gas (MMcf) Total (MBOE) Proved developed and undeveloped reserves: Balance at beginning of year 43,918 7,187 245,011 91,940 Production (3,286 ) (393 ) (16,747 ) (6,471 ) Extensions and discoveries 744 29 681 888 Improved recovery — — — — Purchases of proved reserves — — — — Sales of proved reserves (16,474 ) (6,864 ) (202,560 ) (57,097 ) Revisions of previous estimates (12,215 ) 252 (23,854 ) (15,939 ) Balance at end of year 12,687 211 2,531 13,321 Significant changes in proved reserves for the year ended December 31, 2015, include: • Sales of proved reserves of (57.1) MMBOE, primarily due to the Company's decision to sell Fidelity and exit the exploration and production business • Revisions of previous estimates of (15.9) MMBOE, largely the result of lower commodity prices The changes in the Company's estimated quantities of proved oil, NGL and natural gas reserves for the year ended December 31, 2014, were as follows: Oil (MBbls) NGL (MBbls) Natural Gas (MMcf) Total (MBOE) Proved developed and undeveloped reserves: Balance at beginning of year 41,019 6,602 198,445 80,695 Production (4,919 ) (609 ) (20,822 ) (8,998 ) Extensions and discoveries 9,654 3,634 64,420 24,025 Improved recovery — — — — Purchases of proved reserves 5,463 — 7,711 6,748 Sales of proved reserves (4,945 ) (3,109 ) (40,451 ) (14,796 ) Revisions of previous estimates (2,354 ) 669 35,708 4,266 Balance at end of year 43,918 7,187 245,011 91,940 Significant changes in proved reserves for the year ended December 31, 2014, include: • Extensions and discoveries of 24.0 MMBOE, primarily due to drilling activity at the Company's East Texas, Bakken and Powder River Basin properties • Purchases of proved reserves of 6.7 MMBOE, primarily due to the purchase of working interests and leasehold positions in the Powder River Basin • Sales of proved reserves of (14.8) MMBOE, primarily at the Company's South Texas and Bakken properties • Revisions of previous estimates of 4.3 MMBOE, largely the result of higher natural gas prices and well performance revisions The following table summarizes the breakdown of the Company's proved reserves between proved developed and PUD reserves at December 31: 2016 2015 2014 Proved developed reserves: Oil (MBbls) — 11,380 30,130 NGL (MBbls) — 144 4,217 Natural Gas (MMcf) — 2,033 184,437 Total (MBOE) — 11,865 65,086 PUD reserves: Oil (MBbls) — 1,307 13,788 NGL (MBbls) — 67 2,970 Natural Gas (MMcf) — 498 60,574 Total (MBOE) — 1,456 26,854 Total proved reserves: Oil (MBbls) — 12,687 43,918 NGL (MBbls) — 211 7,187 Natural Gas (MMcf) — 2,531 245,011 Total (MBOE) — 13,321 91,940

Summary of significant accoun46

Summary of significant accounting policies Percentage of assets sold (Details)	Jun. 27, 2016	Feb. 07, 2013
Calumet [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Previous percentage of ownership		50.00%
WBI Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Previous percentage of ownership		50.00%
Percentage of ownership acquired	50.00%

Accounts receivable and allowan

Accounts receivable and allowance for doubtful accounts (Details 2) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Receivables past due 90 days or more	$ 29.2	$ 27.8
Allowance for doubtful accounts	$ 10.5	$ 9.8

Inventories and natural gas in

Inventories and natural gas in storage (Details 3) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Inventory and natural gas in storage [Line Items]
Aggregates held for resale	$ 115,471	$ 115,854
Asphalt oil	29,103	36,498
Natural gas in storage (current)	25,761	21,023
Materials and supplies	18,372	16,997
Merchandise for resale	16,437	15,318
Other	33,129	34,861
Total	238,273	240,551
Natural gas in storage noncurrent	$ 49,500	$ 49,100

Property, plant and equipment (

Property, plant and equipment (Details 4) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, plant and equipment [Line Items]
Interest capitalized		$ 0	$ 4,381	$ 7,046
AFUDC - borrowed		914	4,907	3,023
AFUDC - equity		565	7,971	5,803
Property, plant and equipment		6,510,229	6,387,702
Less accumulated depreciation, depletion and amortization		2,578,902	2,489,322	2,385,202
Net property, plant and equipment		3,931,327	3,898,380	3,520,320
Electric:
Property, plant and equipment [Line Items]
Property, plant and equipment	[1]	1,888,613	1,786,148	1,457,101
Natural gas distribution:
Property, plant and equipment [Line Items]
Property, plant and equipment	[1]	2,179,413	2,076,581	1,904,759
Pipeline and midstream:
Property, plant and equipment [Line Items]
Property, plant and equipment		672,199	758,729	818,388
Construction materials and contracting:
Property, plant and equipment [Line Items]
Property, plant and equipment		1,549,375	1,553,428	1,529,942
Construction services:
Property, plant and equipment [Line Items]
Property, plant and equipment		171,361	163,279	144,395
Other:
Property, plant and equipment [Line Items]
Property, plant and equipment		49,268	49,537	$ 50,937
Regulated: | Electric: | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 1,036,373	1,003,173
Weighted average depreciable life in years		39 years
Regulated: | Electric: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 398,382	375,612
Weighted average depreciable life in years		44 years
Regulated: | Electric: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 284,048	255,842
Weighted average depreciable life in years		57 years
Regulated: | Electric: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 62,212	42,436
Regulated: | Electric: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 107,598	109,085
Weighted average depreciable life in years		14 years
Regulated: | Natural gas distribution: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 1,718,633	1,624,645
Weighted average depreciable life in years		46 years
Regulated: | Natural gas distribution: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 19,934	20,530
Regulated: | Natural gas distribution: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 440,846	431,406
Weighted average depreciable life in years		18 years
Regulated: | Pipeline and midstream: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 490,143	460,305
Weighted average depreciable life in years		54 years
Regulated: | Pipeline and midstream: | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 37,831	37,831
Weighted average depreciable life in years		20 years
Regulated: | Pipeline and midstream: | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 45,350	44,011
Weighted average depreciable life in years		62 years
Regulated: | Pipeline and midstream: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 16,507	7,549
Regulated: | Pipeline and midstream: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 40,873	40,168
Weighted average depreciable life in years		33 years
Nonregulated: | Pipeline and midstream: | Gathering and processing
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 31,682	158,949
Weighted average depreciable life in years		19 years
Nonregulated: | Pipeline and midstream: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 13	89
Nonregulated: | Pipeline and midstream: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 9,800	9,827
Weighted average depreciable life in years		10 years
Nonregulated: | Construction materials and contracting: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 94,625	95,870
Nonregulated: | Construction materials and contracting: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 102,347	96,864
Weighted average depreciable life in years		19 years
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 930,471	937,084
Weighted average depreciable life in years		12 years
Nonregulated: | Construction materials and contracting: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 16,181	18,615
Nonregulated: | Construction materials and contracting: | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	[2]	405,751	404,995
Nonregulated: | Construction services: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		5,346	5,025
Nonregulated: | Construction services: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 26,693	25,259
Weighted average depreciable life in years		26 years
Nonregulated: | Construction services: | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 132,217	121,940
Weighted average depreciable life in years		6 years
Nonregulated: | Construction services: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 7,105	11,055
Weighted average depreciable life in years		4 years
Nonregulated: | Other: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 2,837	2,837
Nonregulated: | Other: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 46,431	$ 46,700
Weighted average depreciable life in years		23 years
[1]	Includes allocations of common utility property
[2]	Depleted on the units-of-production method based on recoverable aggregate reserves.

Impairment of long-lived assets

Impairment of long-lived assets (Details 5) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2015	Jun. 30, 2015
Accounting Policies [Abstract]
Asset impairment charges	$ 14.1	$ 3

Goodwill (Details 6)

Goodwill (Details 6)	3 Months Ended
	Dec. 31, 2016
Goodwill [Line Items]
Long-term growth rate projections	3.00%
Minimum [Member]
Goodwill [Line Items]
Fair value inputs discount rate, range	5.00%
Maximum [Member]
Goodwill [Line Items]
Fair value inputs discount rate, range	9.00%

Revenue recognition (Details 7)

Revenue recognition (Details 7) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain	$ 117.7	$ 102.1

Percentage-of-completion method

Percentage-of-completion method (Details 8) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Costs and estimated earnings in excess of billings on uncompleted contracts		$ 64,558	$ 64,369
Billings in excess of costs and estimated earnings on uncompleted contracts		64,832	68,048
Short-term retainage	[1]	45,109	46,207
Long-term retainage	[2]	1,506	1,605
Total retainage		$ 46,615	$ 47,812
[1]	Expected to be paid within one year or less and included in receivables, net.
[2]	Included in deferred charges and other assets - other.

Natural gas costs recoverable o

Natural gas costs recoverable or refundable through rate adjustments (Details 9) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Natural gas costs refundable	$ 25,600,000	$ 20,900,000
Natural gas costs recoverable	$ 2,200,000	$ 547,000

Income taxes (Details 10)

Income taxes (Details 10)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Threshold of likelihood of tax positions being realized upon ultimate settlement with a taxing authority	50.00%

Earnings (loss) percommon share

Earnings (loss) percommon share (Details 11) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Weighted average common shares outstanding - basic	195,304	195,304	195,304	195,284	195,266	195,151	194,805	194,479	195,299	194,928	192,507
Effect of dilutive performance share awards									319	58	80
Weighted average common shares outstanding - diluted	195,889	195,811	195,699	195,284	195,324	195,169	194,838	194,566	195,618	194,986	192,587
Shares excluded from the calculation of diluted earnings per share									0	0	0

New accounting standards (Detai

New accounting standards (Details 12) - Restatement Adjustment [Member]	Dec. 31, 2015USD ($)
Accounting Standards Update 2015-03 debt issuance costs [Member] | Prepayments and other current assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prior period reclassification adjustment	$ 100,000
Accounting Standards Update 2015-03 debt issuance costs [Member] | Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prior period reclassification adjustment	6,000,000
Accounting Standards Update 2015-17 deferred taxes [Member] | Other current assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prior period reclassification adjustment	$ 33,100,000

Comprehensive income (loss) (De

Comprehensive income (loss) (Details 13) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,520,548	$ 3,249,784	$ 2,855,902
Amounts reclassified from accumulated other comprehensive loss	(3,016)	(3,131)
Net current-period other comprehensive income (loss)	1,415	4,955	(3,898)
Balance	2,316,244	2,520,548	3,249,784
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,667)	(3,071)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	367	404
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		0
Net current-period other comprehensive income (loss)	367	404
Balance	(2,300)	(2,667)	(3,071)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(34,257)	(38,218)
Other comprehensive income (loss) before reclassifications	(1,470)	(88)
Amounts reclassified from accumulated other comprehensive loss	2,506	1,794
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		2,255
Net current-period other comprehensive income (loss)	1,036	3,961
Balance	(33,221)	(34,257)	(38,218)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(200)	(829)
Other comprehensive income (loss) before reclassifications	51	(173)
Amounts reclassified from accumulated other comprehensive loss	0	802
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		0
Net current-period other comprehensive income (loss)	51	629
Balance	(149)	(200)	(829)
Net unrealizaed gasin (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(24)	15
Other comprehensive income (loss) before reclassifications	(182)	(170)
Amounts reclassified from accumulated other comprehensive loss	143	131
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		0
Net current-period other comprehensive income (loss)	(39)	(39)
Balance	(63)	(24)	15
Accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,148)	(42,103)	(38,205)
Other comprehensive income (loss) before reclassifications	(1,601)	(431)
Amounts reclassified from accumulated other comprehensive loss	3,016	3,131
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		2,255
Net current-period other comprehensive income (loss)	1,415	4,955	(3,898)
Balance	$ (35,733)	$ (37,148)	$ (42,103)

Reclassifications out of accumu

Reclassifications out of accumulated other comprehensive (loss) (Details 14) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ (87,848)	$ (91,179)	$ (86,871)
Income taxes	(93,132)	(70,664)	(64,422)
Other income	4,956	18,457	9,138
Net income (loss)	(67,258)	(657,691)	$ 294,338
Reclassification from accumulated other comprehensive income, current period, net of tax	(3,016)	(3,131)
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	367	404
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	(593)	(637)
Income taxes	226	233
Net income (loss)	(367)	(404)
Amortization of postretirement liability losses included in net periodic benefit cost (credit)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	2,506	1,794
Amortization of postretirement liability 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	1,425	1,128
Net periodic benefit cost	(3,931)	(2,922)
Net income (loss)	(2,506)	(1,794)
Reclassification adjustment for loss on foreign currency translation adjustment 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	0	802
Reclassification adjustment for loss on foreign currency translation adjustment 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	0	490
Other income	0	(1,292)
Net income (loss)	0	(802)
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	143	131
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	77	70
Other income	(220)	(201)
Net income (loss)	$ (143)	$ (131)

Percentage of assets sold (Deta

Percentage of assets sold (Details)	Nov. 21, 2016
Pronghorn assets held for sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percent of ownership to sell	50.00%

Major classes of assets and lia

Major classes of assets and liabilities held for sale (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Total current assets held for sale	$ 14,391	$ 54,847
Noncurrent assets:
Total noncurrent assets held for sale	196,664	$ 561,617
Pronghorn assets held for sale [Member] | Disposal group, held-for-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
Fair value impairment	2,300
Fair value impairment, net of tax	$ 1,400

Noncontrolling interest (Detail

Noncontrolling interest (Details 3)	Jun. 27, 2016	Feb. 07, 2013
Calumet [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Previous percentage of ownership		50.00%
WBI Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Previous percentage of ownership		50.00%
Percentage of ownership acquired	50.00%

Major classes of assets and l63

Major classes of assets and liabilities (Details 4) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015
Current assets:
Total current assets held for sale		$ 14,391		$ 54,847
Noncurrent assets:
Total noncurrent assets held for sale		196,664		561,617
Current liabilities:
Total current liabilities held for sale		9,924		126,483
Noncurrent liabilities:
Total noncurrent liabilities held for sale		0		63,750
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Current assets:
Receivables, net		355		13,387
Inventories		0		1,308
Income taxes receivable		0		9,665
Prepayments and other current assets		0		221
Total current assets held for sale		355		24,581
Noncurrent assets:
Investments		0		37
Net property, plant and equipment		5,507		793,422
Deferred income taxes		91,098		124,035
Other		161		161
Less allowance for impairment of assets held for sale		938		754,541
Total noncurrent assets held for sale		95,828		163,114
Total assets held for sale		96,183		187,695
Current liabilities:
Accounts payable		141		25,013
Taxes payable		19	[1]	1,052
Accrued compensation		0		13,080
Other accrued liabilities		2,358		4,838
Total current liabilities held for sale		2,518		43,983
Noncurrent liabilities:
Total liabilities held for sale		2,518		43,983
Refining [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Current assets:
Cash and cash equivalents		0		688
Receivables, net		0		7,693
Inventories		0		13,176
Income taxes receivable		13,987		2,495
Prepayments and other current assets		0		6,214
Total current assets held for sale		13,987		30,266
Noncurrent assets:
Net property, plant and equipment		0		412,717
Other		0		9,627
Total noncurrent assets held for sale		0		422,344
Total assets held for sale		13,987		452,610
Current liabilities:
Short-term borrowings		0		45,500
Long-term debt due within one year		0		5,250
Accounts payable		7,425		24,468
Taxes payable		0		1,391
Accrued compensation		0		938
Other accrued liabilities		0		4,953
Total current liabilities held for sale		7,425		82,500
Noncurrent liabilities:
Long-term debt		0		63,750
Deferred income taxes	[2]	14		23,841
Total noncurrent liabilities held for sale		14		87,591
Total liabilities held for sale		$ 7,439		$ 170,091
[1]	On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale.
[2]	On the Company's Consolidated Balance Sheets, these amounts were reclassified to noncurrent deferred income tax assetsand are reflected in noncurrent assets held for sale.

Taxes and operating loss carryf

Taxes and operating loss carryforwards (Details 5) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Valuation allowance	$ 9,117,000	$ 8,990,000
State and local jurisdiction [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating loss carryforwards	114,700,000	116,200,000
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred tax assets, operating loss carryforwards	89,300,000	78,900,000
Exploration and production [Member] | Domestic tax authority [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating loss carryforwards	297,200,000	208,200,000
Exploration and production [Member] | State and local jurisdiction [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating loss carryforwards	189,100,000	201,400,000
Valuation allowance	$ 500,000	$ 300,000

Impairment fair value, ceiling

Impairment fair value, ceiling test and transactions costs (Details 6) - USD ($)	3 Months Ended						12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
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)	$ (1,600,000)	$ 356,100,000	$ 400,000,000
Fair value impairment after tax	600,000	$ (900,000)	$ (1,000,000)	$ 224,400,000	$ 252,000,000
Impairment of oil and gas properties, after tax						$ 315,300,000
Impairment of oil and gas properties						$ 500,400,000
Business combination, consideration transferred							$ 209,200,000
Refining [Member] | Discontinued operations, held-for-sale or disposed by sale [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) - Exploration and production [Member] - USD ($)	3 Months Ended			12 Months Ended		18 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Other restructuring [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs		$ 300,000		$ 5,600,000	$ 2,500,000	$ 10,500,000
Contract termination [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs	$ 3,200,000		$ 3,300,000	$ 900,000

Derivative instruments gains an

Derivative instruments gains and losses on derivative instruments (Details 8) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Commodity derivatives [Member] | Not designated as hedging instrument [Member] | Exploration and production [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of gain (loss) recognized in discontinued operations, before tax	$ (18.3)	$ 23.4

Reconciliation of income and ex

Reconciliation of income and expenses (Details 9) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax	[1]									$ (300,354)	$ (834,080)	$ 109,311
Loss from discontinued operations attributable to noncontrolling interest										(131,691)	(35,256)	(3,895)
Income (loss) from discontinued operations attributable to the Company		$ (816)	$ (5,400)	$ (155,451)	$ (6,996)	$ (3,368)	$ (213,334)	$ (255,665)	$ (326,457)
Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues										123,024	363,115	547,571
Operating expenses										513,813	1,666,941	386,651
Operating income (loss)										(390,789)	(1,303,826)	160,920
Other income										306	3,149	1,898
Interest expense										1,753	2,124	145
Income (loss) from discontinued operations before income taxes										(392,236)	(1,302,801)	162,673
Income taxes										(91,882)	(468,721)	53,362
Income (loss) from discontinued operations, net of tax										(300,354)	(834,080)	109,311
Loss from discontinued operations attributable to noncontrolling interest										(131,691)	(35,256)	(3,895)
Income (loss) from discontinued operations attributable to the Company										(168,663)	(798,824)	113,206
Refining [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax loss attributable to the company										$ (253,500)	$ (31,500)	$ (3,200)
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

Goodwill and other intangible69

Goodwill and other intangible assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	$ 635,204	$ 635,204
Goodwill acquired during the year		6,324	0
Held for sale		(9,737)
Balance as of end of period		631,791	635,204	[1]
Natural gas distribution:
Goodwill [Roll Forward]
Balance as of beginning of period		345,736	345,736
Goodwill acquired during the year		0	0
Held for sale		0
Balance as of end of period		345,736	345,736
Pipeline and midstream:
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	9,737	9,737
Goodwill acquired during the year		0	0
Held for sale		(9,737)
Balance as of end of period		0	9,737	[1]
Accumulated impairment which occurred in prior periods		12,300	12,300
Construction materials and contracting:
Goodwill [Roll Forward]
Balance as of beginning of period		176,290	176,290
Goodwill acquired during the year		0	0
Held for sale		0
Balance as of end of period		176,290	176,290
Construction services:
Goodwill [Roll Forward]
Balance as of beginning of period		103,441	103,441
Goodwill acquired during the year		6,324	0
Held for sale		0
Balance as of end of period		$ 109,765	$ 103,441
[1]	Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods.

Goodwill and other intangible70

Goodwill and other intangible assets (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-lived intangible assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 5,925,000	$ 7,342,000
Amortization of intangible assets	2,500,000	2,500,000	$ 3,200,000
Estimated amortization expense for amortizable intangible assets
2,017	2,200,000
2,018	1,200,000
2,019	1,000,000
2,020	500,000
2,021	200,000
Thereafter	800,000
Customer relationships [Member]
Finite-lived intangible assets [Line Items]
Intangible assets, gross	17,145,000	20,975,000
Intangible assets, less accumulated amortization	13,917,000	16,845,000
Intangible assets, net (excluding goodwill)	3,228,000	4,130,000
Noncompete agreements [Member]
Finite-lived intangible assets [Line Items]
Intangible assets, gross	2,430,000	4,409,000
Intangible assets, less accumulated amortization	1,658,000	3,655,000
Intangible assets, net (excluding goodwill)	772,000	754,000
Other [Member}
Finite-lived intangible assets [Line Items]
Intangible assets, gross	7,768,000	8,304,000
Intangible assets, less accumulated amortization	5,843,000	5,846,000
Intangible assets, net (excluding goodwill)	$ 1,925,000	$ 2,458,000

Regulatory assets and liabili71

Regulatory assets and liabilities (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Regulatory assets and liabilities
Regulatory assets:		$ 307,320	$ 263,258
Regulatory liabilities:		241,852	243,118
Net regulatory position		65,468	20,140
Regulatory assets not earning a rate of return		255,400	224,700
Plant removal and decommissioning costs [Member}
Regulatory assets and liabilities
Regulatory liabilities:	[1]	176,972	182,981
Taxes refundable to customers [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	11,010	17,060
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	9,099	4,764
Natural gas costs refundable through rate adjustments [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[2]	25,580	20,884
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1],[2]	$ 19,191	17,429
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Estimated recovery period	[3]	(e)
Regulatory assets:	[4]	$ 176,025	185,832
Taxes recoverable from customers [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Over plant lives
Regulatory assets:	[4]	$ 28,278	27,682
Manufactured gas plant sites remediation [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	0
Regulatory assets:	[4]	$ 18,259	18,617
Asset Retirement Obligation Costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	0
Regulatory assets:	[4]	$ 42,580	8,000
Natural gas costs recoverable through rate adjustments [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 1 year
Regulatory assets:	[6]	$ 2,242	547
Long-term debt refinancing costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 21 years
Regulatory assets:	[4]	$ 6,248	7,031
Costs related to identifying generation development [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 10 years
Regulatory assets:	[4]	$ 3,407	3,808
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Largely within 1- 4 years
Regulatory assets:	[4],[6]	$ 30,281	$ 11,741
[1]	Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets.
[2]	Included in other accrued liabilities on the Consolidated Balance Sheets.
[3]	Recovered as expense is incurred or cash contributions are made.
[4]	Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
[5]	Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
[6]	Included in prepayments and other current assets on the Consolidated Balance Sheets.

Fair value measurements Fair va

Fair value measurements Fair value measurements insurance contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 70.9	$ 67.5
Net unrealized gain (loss) of investments used to satisfy obligations under nonqualified benefit plans	$ 3.4	$ 1.7	$ 3.4

Available-for-sale securities (

Available-for-sale securities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities [Line Items]
Available-for-sale securities, amortized cost basis	$ 10,546	$ 10,443
Available-for-sale securities, gross unrealized gains	8	19
Available-for-sale securities, gross unrealized losses	(105)	(55)
Available-for-sale securities	10,449	10,407
Mortgage-backed securities
Available-for-sale securities [Line Items]
Available-for-sale securities, amortized cost basis	10,546	9,128
Available-for-sale securities, gross unrealized gains	8	19
Available-for-sale securities, gross unrealized losses	(105)	(49)
Available-for-sale securities	$ 10,449	9,098
U.S. Treasury securities
Available-for-sale securities [Line Items]
Available-for-sale securities, amortized cost basis		1,315
Available-for-sale securities, gross unrealized gains		0
Available-for-sale securities, gross unrealized losses		(6)
Available-for-sale securities		$ 1,309

Fair value measurements (Detail

Fair value measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015
Fair value measurements [Line Items]
Nonrecurring fair value measurement					$ 10,800	$ 1,100
Concentration risks, percentage [Abstract]
Percentage in fixed-income investments	52.00%		63.00%
Percentage investment in common stock of large-cap companies	22.00%		19.00%
Percentage investment in common stock of mid-cap companies	13.00%		9.00%
Percentage investment in common stock of small-cap companies	10.00%		7.00%
Percentage investment in target date investments	1.00%		1.00%
Percentage investment in cash and cash equivalents	2.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets:	$ 82,972		$ 79,286
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets:	1,602		1,420
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets:	70,921	[1]	67,459	[2]
Fair value, measurements, recurring [Member] | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	10,449		9,098
Fair value, measurements, recurring [Member] | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			1,309
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contract
Fair value measurements [Line Items]
Assets:	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			0
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2)
Fair value measurements [Line Items]
Assets:	82,972		79,286
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Money market funds
Fair value measurements [Line Items]
Assets:	1,602		1,420
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Insurance contract
Fair value measurements [Line Items]
Assets:	70,921	[1]	67,459	[2]
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	10,449		9,098
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			1,309
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Money market funds
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Insurance contract
Fair value measurements [Line Items]
Assets:	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	$ 0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			$ 0
[1]	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.
[2]	The insurance contract invests approximately 63 percent in fixed-income investments, 19 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.

Fair value measurements (Deta75

Fair value measurements (Details 4) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 1,790,159	$ 1,796,163
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	1,790,159	1,796,163
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 1,841,885	$ 1,819,828

Credit facilities (Details)

Credit facilities (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Line of credit facility and other debt [Abstract]
Letters of credit at end of period		$ 0
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | MDU Resources Group, Inc [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity		175,000,000
Amount outstanding, end of period	[1]	$ 111,000,000	$ 44,500,000
Expiration date		May 8, 2019
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | Centennial Energy Holdings, Inc [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity		$ 500,000,000
Amount outstanding, end of period	[1]	$ 151,000,000	18,000,000
Expiration date		Sep. 23, 2021
Long-term debt [Member] | Revolving credit facility [Member] | MDU Resources Group, Inc [Member]
Line of credit facility and other debt [Abstract]
Letters of credit at end of period		$ 0
Option to increase borrowings, maximum amount		225,000,000
Long-term debt [Member] | Revolving credit facility [Member] | Cascade Natural Gas Corporation [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	[2]	50,000,000
Amount outstanding, end of period		0	0
Letters of credit at end of period	[3]	$ 2,200,000
Expiration date		Jul. 9, 2018
Option to increase borrowings, maximum amount		$ 75,000,000
Long-term debt [Member] | Revolving credit facility [Member] | Intermountain Gas Company [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	[4]	65,000,000
Amount outstanding, end of period		20,900,000	$ 47,900,000
Letters of credit at end of period		$ 0
Expiration date		Jul. 13, 2018
Option to increase borrowings, maximum amount		$ 90,000,000
Long-term debt [Member] | Revolving credit facility [Member] | Centennial Energy Holdings, Inc [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity		500,000,000
Letters of credit at end of period		0
Option to increase borrowings, maximum amount		$ 600,000,000
[1]	Amount outstanding under commercial paper program.
[2]	Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.
[3]	Outstanding letter(s) of credit reduce the amount available under the credit agreement.
[4]	Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.

Long-term borrowings (Details 2

Long-term borrowings (Details 2) - USD ($) $ in Millions		Mar. 21, 2017	Dec. 31, 2016	Nov. 21, 2016	Nov. 09, 2016
MDU Resources Group, Inc [Member]
Debt instrument [Line Items]
Note purchase agreement				$ 100
Senior notes				$ 40
Interest rate				4.15%
MDU Resources Group, Inc [Member] | Revolving credit facility [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Ratio of funded debt to total capitalization as specified in debt convenants			65.00%
Ratio of funded debt to capitalization - Company alone, as specified in debt convenants			65.00%
MDU Energy Capital, LLC [Member] | Master shelf agreement [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Ratio of total debt to adjusted total capitalization as specified in debt convenants			70.00%
Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants			65.00%
Ratio of total debt to total capitalization as specified in debt convenants.			65.00%
Ratio - EBIT to interest expense			150.00%
Cascade Natural Gas Corporation [Member] | Revolving credit facility [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Facility limit, maximum borrowing capacity	[1]		$ 50
Ratio of total debt to total capitalization as specified in debt convenants.			65.00%
Intermountain Gas Company [Member]
Debt instrument [Line Items]
Senior notes					$ 30
Interest rate					4.00%
Intermountain Gas Company [Member] | Revolving credit facility [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Facility limit, maximum borrowing capacity	[2]		$ 65
Ratio of total debt to total capitalization as specified in debt convenants.			65.00%
Centennial Energy Holdings, Inc [Member] | Master shelf agreement [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt convenants.			60.00%
Ratio of EBITDA to interest expense			175.00%
Centennial Energy Holdings, Inc [Member] | Revolving credit facility [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Facility limit, maximum borrowing capacity			$ 500
Ratio of total debt to total capitalization as specified in debt convenants.			65.00%
Facility, decrease borrowing capacity			$ 150
WBI Energy Transmission, Inc. [Member] | Uncommitted long term private shelf agreement [Member] | Long-term debt [Member]
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt convenants.			55.00%
Long-term private shelf agreement			$ 175
Long-term private shelf agreement issuance capacity			200
Notes outstanding			100
Remaining capacity under uncommitted private shelf agreement			$ 100
Subsequent Event [Member] | MDU Resources Group, Inc [Member]
Debt instrument [Line Items]
Senior notes		$ 60
Weighted average interest rate		3.61%
[1]	Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.
[2]	Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.

Long-term debt outstanding (Det

Long-term debt outstanding (Details 3) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Long term debt outstanding [Abstract]
Long-term debt	$ 1,790,159	$ 1,796,163
Long-term debt due within one year	43,598	238,539
Long-term debt	$ 1,746,561	1,557,624
Senior notes [Member]
Debt instrument [Line Items]
Weighted average interest rate	4.87%
Long term debt outstanding [Abstract]
Long-term debt	$ 1,437,831	1,616,246
Commercial paper [Member]
Debt instrument [Line Items]
Weighted average interest rate	1.27%
Long term debt outstanding [Abstract]
Long-term debt	$ 262,000	62,500
Medium-term notes [Member]
Debt instrument [Line Items]
Weighted average interest rate	6.68%
Long term debt outstanding [Abstract]
Long-term debt	$ 50,000	50,000
Other notes [Member]
Debt instrument [Line Items]
Weighted average interest rate	5.25%
Long term debt outstanding [Abstract]
Long-term debt	$ 24,471	24,589
Credit agreements [Member]
Debt instrument [Line Items]
Weighted average interest rate	3.14%
Long term debt outstanding [Abstract]
Long-term debt	$ 21,793	48,906
Long-term debt [Member]
Long term debt outstanding [Abstract]
Unamortized debt issuance costs	(5,832)	(6,069)
Discount	$ (104)	$ (9)

Schedule of debt maturities (De

Schedule of debt maturities (Details 4) $ in Thousands	Dec. 31, 2016USD ($)
Long-term debt maturities [Line Items]
2,017	$ 43,598
2,018	169,449
2,019	162,154
2,020	15,021
2,021	151,013
Thereafter	$ 1,254,860

Asset Retirement Obligations (D

Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset retirement obligation [Roll Forward]
Balance, beginning of period	$ 242,224	$ 27,211
Liabilities incurred	15,114	2,751
Liabilities settled	(4,338)	(1,708)
Accretion expense	13,918	2,134
Revisions in estimates	48,052	211,836
Balance, end of period	$ 314,970	$ 242,224

Preferred Stocks (Details)

Preferred Stocks (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Preferred stocks	$ 15,000	$ 15,000
Preferred stock redemption price upon liquidation (in dollars per share)	$ 100
Cumulative Preferred Stock, Par Value $100 Issuable in Series [Member]
Class of Stock [Line Items]
Authorized preferred cumulative shares (in shares)	500,000	500,000
Cumulative preferred shares, par value (in dollars per share)	$ 100	$ 100
Preferred stock, series 4.50% [Member]
Class of Stock [Line Items]
Preferred stocks	$ 10,000	$ 10,000
Preferred shares outstanding (in shares)	100,000	100,000
Preferred stock, $100 par value	4.50%	4.50%	4.50%
Preferred stock, dividends per share, declared	$ 4.50	$ 4.50	$ 4.50
Preferred stock, redemption per share amount	$ 105
Preferred stock, series 4.70% [Member]
Class of Stock [Line Items]
Preferred stocks	$ 5,000	$ 5,000
Preferred shares outstanding (in shares)	50,000	50,000
Preferred stock, $100 par value	4.70%	4.70%	4.70%
Preferred stock, dividends per share, declared	$ 4.70	$ 4.70	$ 4.70
Preferred stock, redemption per share amount	$ 102
Cumulative preferred stock A, without par value, issuable in series [Member]
Class of Stock [Line Items]
Authorized preferred cumulative shares (in shares)	1,000,000	1,000,000
Cumulative preference stock , without par value, issuable in series [Member]
Class of Stock [Line Items]
Authorized preferred cumulative shares (in shares)	500,000	500,000

Common Stock (Details)

Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Dividends declared per common share	$ 0.7550	$ 0.7350	$ 0.7150
Company's subsidiaries net assets restricted from being used to transfer funds to the Company	$ 1,300
Credit agreement limitation on company ratio of funded debt to capitalization (excluding subsidiaries)	65.00%
Company's (excluding its subsidiaries) net assets restricted from use for dividend payments	$ 351
MDU Resources Group, Inc. [Member]
Shares of common stock reserved for original issuance under the K-Plan (in shares)	7.8
Centennial [Member]
Maximum distributions to the company as a ratio of average consolidated indebtedness to consolidated EBITDA	300.00%

Stock based compensation plans

Stock based compensation plans (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation [Abstract]
Share-based compensation arrangement, number of shares available for grant	5.5
Share-based compensation expense, net of tax	$ 3.3	$ 2.9	$ 4.4
Share-based compensation nonvested awards total compensation cost not yet recognized	$ 4.9
Share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition	1 year 6 months

Stock awards (Details 2)

Stock awards (Details 2) - Nonemployee director stock compensation plan [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award [Line Items]
Nonemployee director stock compensation shares issued	37,218	58,181	43,088
Fair value of nonemployee director stock compensation shares issued	$ 1.1	$ 1.1	$ 1.1

Performance share awards (Detai

Performance share awards (Details 3) - Performance shares [Member] - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range, minimum	0.00%
Participant target grant of shares, percentage rate range, maximum	200.00%
Historical volatility rate	50.00%
Implied volatility rate	50.00%
Weighted average grant-date fair value	$ 14.60	$ 18.98	$ 41.13
Expected volatility rate, minimum	29.25%	22.86%	18.94%
Expected volatility rate, maximum	32.51%	24.61%	20.43%
Risk-free interest rate, minimum	0.47%	0.05%	0.03%
Risk-free interest rate, maximum	0.92%	1.07%	0.74%
Weighted average discounted dividends per share	$ 1.56	$ 1.57	$ 2.15
Fair value of vested shares	$ 953,000	$ 0	$ 16,600,000
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Nonvested at beginning of period	565,896
Nonvested at beginning of period	$ 27.90
Granted	324,205
Weighted average grant-date fair value	$ 14.60	$ 18.98	$ 41.13
Vested	58,401
Vested	$ 29.01
Forfeited	167,512
Forfeited	$ 27.30
Nonvested at end of period	664,188	565,896
Nonvested at end of period	$ 21.47	$ 27.90
Grant date February 2014 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	136,901
Grant date February 2015 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	200,112
Grant date June 2015 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	14,441
Grant date February 2016 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	310,583
Grant Date March 2016 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	2,151

Components of income before inc

Components of income before income taxes from continuing operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
United States	$ 326,252	$ 248,379	$ 249,501
Foreign	(24)	(1,326)	(52)
Income before income taxes	$ 326,228	$ 247,053	$ 249,449

Income tax expense (benefit) (D

Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 81,989	$ 85,897	$ 8,837
State	13,190	10,093	622
Foreign	2	30	0
Current income taxes	95,181	96,020	9,459
Deferred:
Federal	(2,102)	(19,632)	52,041
State	1,184	(5,304)	1,913
Investment tax credit - net	(1,131)	(420)	1,009
Deferred income taxes	(2,049)	(25,356)	54,963
Total income tax expense	$ 93,132	$ 70,664	$ 64,422

Components of deferred tax asse

Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Postretirement	$ 87,872	$ 97,666
Compensation-related	44,995	33,714
Alternative minimum tax credit carryforward	29,338	28,169
Federal renewable energy credit	16,944	3,400
Customer advances	13,524	12,623
Legal and environmental contingencies	9,895	6,377
Asset retirement obligations	8,867	8,694
Other	46,957	43,306
Total deferred tax assets	258,392	233,949
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	774,838	756,444
Postretirement	70,670	71,835
Intangible asset amortization	26,413	23,950
Other	45,580	36,359
Total deferred tax liabilities	917,501	888,588
Valuation allowance	9,117	8,990
Deferred Tax Liabilities, Net	$ 668,226	$ 663,629

Carryforwards (Details 4)

Carryforwards (Details 4) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
State and local jurisdiction [Member]
Carryforwards [Line Items]
Operating loss carryforwards	$ 114.7	$ 116.2
Investment tax credit carryforward, amount	20.7	13.9
Federal and State Taxing Authority [Member]
Carryforwards [Line Items]
Tax credit carryforward, amount	$ 43.3	$ 21.3

Deferred tax reconciliation (De

Deferred tax reconciliation (Details 5) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Change in net deferred income tax liability from the preceding table	$ 4,597
Deferred taxes associated with other comprehensive income	(825)
Other	(5,821)
Deferred income taxes	$ (2,049)	$ (25,356)	$ 54,963

Income tax expense (benefit) st

Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ 114,179,000	$ 86,468,000	$ 87,308,000
Federal statutory rate	35.00%	35.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax	$ 9,027,000	$ 8,208,000	$ 7,019,000
State income tax rate	2.80%	3.30%	2.80%
Federal renewable energy credit	$ (13,544,000)	$ (3,400,000)	$ (3,655,000)
Federal renewable energy credit rate	(4.20%)	(1.40%)	(1.50%)
Tax compliance and uncertain tax positions	$ (3,028,000)	$ (2,607,000)	$ (8,568,000)
Tax compliance and uncertain tax positions	(0.90%)	(1.00%)	(3.40%)
Domestic production activities	$ (6,251,000)	$ (6,842,000)	$ (3,993,000)
Domestic production activities	(1.90%)	(2.80%)	(1.60%)
Other	$ (7,251,000)	$ (11,163,000)	$ (13,689,000)
Other rate	(2.30%)	(4.50%)	(5.50%)
Total income tax expense	$ 93,132,000	$ 70,664,000	$ 64,422,000
Total income tax expense rate	28.50%	28.60%	25.80%
Temporary differences [Abstract]
Undistributed earnings from foreign operations	$ 2,400,000
Deferred tax liabilities, undistributed foreign earnings	$ 889,000

Unrecognized tax benefits (Deta

Unrecognized tax benefits (Details 7) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 0	$ 105,000	$ 7,845,000
Settlements	0	0	(7,740,000)
Lapse of statute of limitations	0	(105,000)	0
Balance at end of year	0	0	105,000
Interest (income) expense recognized in income tax expense	(92,000)	122,000	$ 387,000
Amount of interest income accrued for an overpayment of income taxes.	$ 54,000
Accrued liability for payment of interest		$ 94,000

Cash flow information (Details)

Cash flow information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest, net of amount capitalized and AFUDC - borrowed of $914, $9,288 and $10,069 in 2016, 2015 and 2014, respectively	$ 87,920	$ 88,775	$ 81,195
Income taxes paid, net	105,908	61,405	80,090
Property, plant and equipment additions in accounts payable	22,712	39,754	12,791
Capitalized interest and AFUDC borrowed	914	9,288	10,069
Continuing and discontinued operations [Member]
Income taxes paid, net	$ 1,300	$ 2,400	$ 69,800

Business segment data (Details)

Business segment data (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business segment data [Line Items]
External operating revenues		$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 1,016,826	$ 1,198,342	$ 938,039	$ 860,845	$ 4,128,828	$ 4,014,052	$ 4,115,073
Intersegment operating revenues										0	0	0
Depreciation, depletion and amortization										216,318	211,747	203,084
Interest expense										87,848	91,179	86,871
Income taxes										93,132	70,664	64,422
Earnings (loss) on common stock before income (loss) from discontinued operations										232,411	175,704	184,342
Income (loss) from discontinued operations, net of tax	[1]									(300,354)	(834,080)	109,311
Loss from discontinued operations attributable to noncontrolling interest										(131,691)	(35,256)	(3,895)
Total earnings (loss) on common stock										63,748	(623,120)	297,548
Capital expenditures	[2]									372,420	572,134	434,457
Total assets		6,284,467				6,565,154				6,284,467	6,565,154	7,805,405
Property, plant and equipment		6,510,229				6,387,702				6,510,229	6,387,702
Less accumulated depreciation, depletion and amortization		2,578,902				2,489,322				2,578,902	2,489,322	2,385,202
Net property, plant and equipment		3,931,327				3,898,380				3,931,327	3,898,380	3,520,320
Additional information [Abstract]
Net noncash capital expenditure-related accounts payable and AFUDC										(15,800)	35,300	5,100
Electric:
Business segment data [Line Items]
Depreciation, depletion and amortization										50,220	37,583	35,008
Interest expense										24,982	17,421	15,595
Income taxes										1,449	11,523	12,442
Capital expenditures										111,134	332,876	185,121
Total assets	[3]	1,406,694				1,325,858				1,406,694	1,325,858	1,028,001
Property, plant and equipment	[3]	1,888,613				1,786,148				1,888,613	1,786,148	1,457,101
Natural gas distribution:
Business segment data [Line Items]
Depreciation, depletion and amortization										65,426	64,756	54,700
Interest expense										30,405	29,471	27,217
Income taxes										9,181	11,377	11,350
Capital expenditures										126,272	130,793	120,613
Total assets	[3]	2,099,296				2,038,433				2,099,296	2,038,433	1,935,271
Property, plant and equipment	[3]	2,179,413				2,076,581				2,179,413	2,076,581	1,904,759
Pipeline and midstream:
Business segment data [Line Items]
Depreciation, depletion and amortization										24,885	27,981	29,749
Interest expense										7,903	9,895	9,946
Income taxes										12,408	7,505	12,232
Capital expenditures										34,467	18,315	61,754
Total assets		550,615				591,651				550,615	591,651	651,925
Property, plant and equipment		672,199				758,729				672,199	758,729	818,388
Construction materials and contracting:
Business segment data [Line Items]
Depreciation, depletion and amortization										58,413	65,937	68,557
Interest expense										15,265	15,183	16,368
Income taxes										60,625	41,619	18,586
Capital expenditures										37,845	48,126	37,896
Total assets		1,220,459				1,261,963				1,220,459	1,261,963	1,260,534
Property, plant and equipment		1,549,375				1,553,428				1,549,375	1,553,428	1,529,942
Construction services:
Business segment data [Line Items]
Depreciation, depletion and amortization										15,307	13,420	12,874
Interest expense										4,059	3,959	4,176
Income taxes										17,748	16,432	24,753
Capital expenditures										60,344	38,269	26,942
Total assets		513,093				442,845				513,093	442,845	437,322
Property, plant and equipment		171,361				163,279				171,361	163,279	144,395
Other
Business segment data [Line Items]
Depreciation, depletion and amortization										2,067	2,070	2,196
Interest expense										5,854	15,853	13,823
Income taxes										(2,028)	(9,834)	(11,136)
Capital expenditures										2,358	3,755	2,131
Total assets	[4]	283,255				287,940				283,255	287,940	315,495
Property, plant and equipment		49,268				49,537				49,268	49,537	50,937
Discontinued operations, held-for-sale
Business segment data [Line Items]
Total assets		$ 211,055				$ 616,464				211,055	616,464	2,176,857
Eliminations
Business segment data [Line Items]
Intersegment operating revenues										(57,430)	(78,786)	(136,302)
Interest expense										(620)	(603)	(254)
Income taxes										(6,251)	(7,958)	(3,805)
Earnings (loss) on common stock before income (loss) from discontinued operations	[1]									6,251	5,016	(6,095)
Regulated:
Business segment data [Line Items]
External operating revenues										1,141,454	1,149,038	1,246,903
Intersegment operating revenues										48,794	49,065	45,013
Earnings (loss) on common stock before income (loss) from discontinued operations										91,384	80,201	82,655
Regulated: | Electric:
Business segment data [Line Items]
External operating revenues										322,356	280,615	277,874
Intersegment operating revenues										0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations										42,222	35,914	36,731
Regulated: | Natural gas distribution:
Business segment data [Line Items]
External operating revenues										766,115	817,419	921,986
Intersegment operating revenues										0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations										27,102	23,607	30,484
Regulated: | Pipeline and midstream:
Business segment data [Line Items]
External operating revenues										52,983	51,004	47,043
Intersegment operating revenues										48,794	49,065	45,013
Earnings (loss) on common stock before income (loss) from discontinued operations										22,060	20,680	15,440
Nonregulated:
Business segment data [Line Items]
External operating revenues										2,987,374	2,865,014	2,868,170
Intersegment operating revenues										8,636	29,721	91,289
Earnings (loss) on common stock before income (loss) from discontinued operations										134,776	90,487	107,782
Nonregulated: | Pipeline and midstream:
Business segment data [Line Items]
External operating revenues										39,602	54,281	64,494
Intersegment operating revenues										223	554	742
Earnings (loss) on common stock before income (loss) from discontinued operations										1,375	(7,430)	9,226
Nonregulated: | Construction materials and contracting:
Business segment data [Line Items]
External operating revenues										1,873,696	1,901,530	1,740,089
Intersegment operating revenues										574	2,752	25,241
Earnings (loss) on common stock before income (loss) from discontinued operations										102,687	89,096	51,510
Nonregulated: | Construction services:
Business segment data [Line Items]
External operating revenues										1,072,663	907,767	1,062,055
Intersegment operating revenues										609	18,660	57,474
Earnings (loss) on common stock before income (loss) from discontinued operations										33,945	23,762	54,432
Nonregulated: | Other
Business segment data [Line Items]
External operating revenues										1,413	1,436	1,532
Intersegment operating revenues										7,230	7,755	7,832
Earnings (loss) on common stock before income (loss) from discontinued operations										$ (3,231)	$ (14,941)	$ (7,386)
[1]	Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.
[2]	Capital expenditures for 2016, 2015 and 2014 include noncash capital expenditure-related accounts payable and AFUDC, totaling $(15.8) million, $35.3 million and $5.1 million, respectively.
[3]	Includes allocations of common utility property
[4]	Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets)

Change in benefit obligations a

Change in benefit obligations and plan assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 442,960	$ 475,337
Service cost	0	86	$ 129
Interest cost	17,218	17,141	17,682
Plan participants' contributions	0	0
Actuarial (gain) loss	1,882	(24,875)
Benefits paid	(25,753)	(24,729)
Benefit obligation at end of year	436,307	442,960	475,337
Change in net plan assets:
Fair value of plan assets at beginning of year	332,667	354,363
Actual gain (loss) on plan assets	26,595	(10,879)
Employer contribution	0	13,912
Plan participants' contributions	0	0
Benefits paid	(25,753)	(24,729)
Fair value of net plan assets at end of year	333,509	332,667	354,363
Funded status - under	(102,798)	(110,293)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	0	0
Other accrued liabilities (current)	0	0
Other liabilities (noncurrent)	(102,798)	(110,293)
Net amount recognized	(102,798)	(110,293)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	198,668	208,671
Prior service cost (credit)	0	0
Total	198,668	208,671
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	92,734	99,012
Service cost	1,647	1,816	1,518
Interest cost	3,688	3,607	3,521
Plan participants' contributions	1,405	1,408
Actuarial (gain) loss	(3,872)	(5,873)
Benefits paid	(6,298)	(7,236)
Benefit obligation at end of year	89,304	92,734	99,012
Change in net plan assets:
Fair value of plan assets at beginning of year	82,593	87,586
Actual gain (loss) on plan assets	4,184	258
Employer contribution	962	577
Plan participants' contributions	1,405	1,408
Benefits paid	(6,298)	(7,236)
Fair value of net plan assets at end of year	82,846	82,593	$ 87,586
Funded status - under	(6,458)	(10,141)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	13,131	5,095
Other accrued liabilities (current)	(538)	(421)
Other liabilities (noncurrent)	(19,051)	(14,815)
Net amount recognized	(6,458)	(10,141)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	17,470	22,484
Prior service cost (credit)	(13,003)	(14,374)
Total	$ 4,467	$ 8,110

Benefit obligations in excess o

Benefit obligations in excess of plan assets (Details 2) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 436,307	$ 442,960
Accumulated benefit obligation	436,307	442,960
Fair value of plan assets	$ 333,509	$ 332,667

Components of net periodic bene

Components of net periodic benefit cost (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 86	$ 129
Interest cost	17,218	17,141	17,682
Expected return on assets	(20,924)	(22,254)	(21,218)
Amortization of prior service cost (credit)	0	36	71
Recognized net actuarial loss	6,215	7,016	4,869
Curtailment loss	0	258	0
Net periodic benefit cost (credit), including amount capitalized	2,509	2,283	1,533
Less amount capitalized	381	316	388
Net periodic benefit cost (credit)	2,128	1,967	1,145
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	(3,789)	8,257	77,238
Amortization of actuarial loss	(6,215)	(7,016)	(4,869)
Amortization of prior service (cost) credit	0	(294)	(71)
Total recognized in accumulated other comprehensive (income) loss	(10,004)	947	72,298
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	(7,876)	2,914	73,443
Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,647	1,816	1,518
Interest cost	3,688	3,607	3,521
Expected return on assets	(4,533)	(4,795)	(4,617)
Amortization of prior service cost (credit)	(1,371)	(1,371)	(1,393)
Recognized net actuarial loss	1,491	1,960	649
Curtailment loss	0	0	0
Net periodic benefit cost (credit), including amount capitalized	922	1,217	(322)
Less amount capitalized	(52)	120	(21)
Net periodic benefit cost (credit)	974	1,097	(301)
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	(3,523)	(1,336)	15,114
Amortization of actuarial loss	(1,491)	(1,960)	(649)
Amortization of prior service (cost) credit	1,371	1,371	1,393
Total recognized in accumulated other comprehensive (income) loss	(3,643)	(1,925)	15,858
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	$ (2,669)	$ (828)	$ 15,557

Estimated net loss and prior se

Estimated net loss and prior service credit (Details 4)	12 Months Ended
	Dec. 31, 2016USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amortization of net gains (losses)	$ (6,400,000)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amortization of net gains (losses)	(900,000)
Defined benefit plan, amortization of net prior service cost (credit)	$ (1,400,000)

Weighted average assumptions (D

Weighted average assumptions (Details 5)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Used to determine benefit obligation
Discount rate	3.83%	4.00%
Expected return on plan assets	6.75%	6.75%
Used to determine benefit cost
Discount rate	4.00%	3.70%
Expected return on plan assets	6.75%	7.00%
Other Postretirement Benefits
Used to determine benefit obligation
Discount rate	3.86%	4.06%
Expected return on plan assets	5.75%	5.75%
Rate of compensation increase	3.00%	3.00%
Used to determine benefit cost
Discount rate	4.06%	3.74%
Expected return on plan assets	5.75%	6.00%
Rate of compensation increase	3.00%	3.00%

Expected rate of return (Detail

Expected rate of return (Details 6)	12 Months Ended
	Dec. 31, 2016
Equity securities [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	50.00%
Defined benefit plan, target plan asset allocations range minimum	40.00%
Equity securities [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	40.00%
Defined benefit plan, target plan asset allocations range minimum	30.00%
Fixed Income Securities [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	60.00%
Defined benefit plan, target plan asset allocations range minimum	50.00%
Fixed Income Securities [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	70.00%
Defined benefit plan, target plan asset allocations range minimum	60.00%

Health care rate assumptions an

Health care rate assumptions and cost trend rate (Details 7) - Other Postretirement Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate - ultimate	4.50%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate	2,024	2,021
Percentage excess over the health care cost trend rate that retiree contributions will generally increase each year	6.00%
Effect on total of service and interest cost components, one percentage point increase	$ 255
Effect on total of service and interest cost components, one percentage point decrease	(210)
Effect on postretirement benefit obligations, one percentage point increase	5,741
Effect on postretirement benefit obligations, one percentage point decrease	$ (4,834)
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year	8.60%	4.00%
Health care cost trend rate - ultimate		5.00%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate assumed for next year	10.70%	8.00%
Health care cost trend rate - ultimate		6.00%

Fair value - pension and postre

Fair value - pension and postretirement (Details 8) - USD ($)	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 333,509,000		$ 332,667,000		$ 354,363,000
Percentage investment in common stock of large-cap U.S. companies	20.00%		19.00%
Percentage investment in US Government securities	7.00%
Percentage investment in corporate bonds	21.00%		16.00%
Percentage investment in common stock of international companies	29.00%		29.00%
Percentage investment in cash equivalents	8.00%		16.00%
Percentage investment in common stock of mid-cap U.S. companies			6.00%
Percentage investment in other investments	15.00%		14.00%
Estimated future employer contributions in next fiscal year	$ 2,000,000
Pension Benefits | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	6,347,000		$ 8,379,000
Pension Benefits | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,348,000		15,135,000
Pension Benefits | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1,584,000		2,332,000
Pension Benefits | Collective and Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	226,107,000	[1]	217,968,000	[2]
Pension Benefits | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	68,677,000		62,145,000
Pension Benefits | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,002,000		11,680,000
Pension Benefits | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	6,396,000		12,111,000
Pension Benefits | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	331,461,000		329,750,000
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,348,000		15,135,000
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1,584,000		2,332,000
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	162,055,000	[1]	154,400,000	[2]
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	4,352,000		5,288,000
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	179,339,000		177,155,000
Pension Benefits | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	6,347,000		8,379,000
Pension Benefits | Significant Other Observable Inputs (Level 2) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Other Observable Inputs (Level 2) | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Other Observable Inputs (Level 2) | Collective and Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	64,052,000	[1]	63,568,000	[2]
Pension Benefits | Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	68,677,000		62,145,000
Pension Benefits | Significant Other Observable Inputs (Level 2) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,002,000		11,680,000
Pension Benefits | Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,044,000		6,823,000
Pension Benefits | Significant Other Observable Inputs (Level 2) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	152,122,000		152,595,000
Pension Benefits | Significant Unobservable Inputs (Level 3) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | Collective and Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0	[1]	0	[2]
Pension Benefits | Significant Unobservable Inputs (Level 3) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 82,846,000		$ 82,593,000		$ 87,586,000
Percentage investment in common stock of large-cap U.S. companies	25.00%		19.00%
Percentage investment in US Government securities	20.00%		22.00%
Percentage investment in corporate bonds	38.00%		36.00%
Percentage investment in other investments	8.00%		13.00%
Percentage investment in mortgage backed securities	9.00%		10.00%
Estimated future employer contributions in next fiscal year	$ 900,000
Other Postretirement Benefits | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	250,000		$ 3,261,000
Other Postretirement Benefits | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,328,000		2,274,000
Other Postretirement Benefits | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	5,000		9,000
Other Postretirement Benefits | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,263,000	[3]	77,044,000	[4]
Other Postretirement Benefits | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	82,846,000		82,588,000
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,333,000		2,283,000
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,328,000		2,274,000
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	5,000		9,000
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0	[3]	0	[4]
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,513,000		80,305,000
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	250,000		3,261,000
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,263,000	[3]	77,044,000	[4]
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 0	[3]	$ 0	[4]
[1]	Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in corporate bonds, 20 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 7 percent in U.S. Government securities and 15 percent in other investments.
[2]	Collective and mutual funds invest approximately 29 percent in common stock of international companies, 19 percent in common stock of large-cap U.S. companies, 16 percent in corporate bonds, 16 percent in cash equivalents, 6 percent in common stock of mid-cap U.S. companies and 14 percent in other investments.
[3]	The insurance contract invests approximately 38 percent in corporate bonds, 25 percent in common stock of large-cap U.S. companies, 20 percent in U.S. Government securities, 9 percent in mortgage-backed securities and 8 percent in other investments.
[4]	The insurance contract invests approximately 36 percent in corporate bonds, 22 percent in U.S. Government securities, 19 percent in common stock of large-cap U.S. companies, 10 percent in mortgage-backed securities and 13 percent in other investments.

Estimated future benefit paymen

Estimated future benefit payments and subsidies (Details 9) $ in Thousands	Dec. 31, 2016USD ($)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	$ 5,410
Prescription drug subsidy receipts, next twelve months	168
Expected future benefit payments, year two	5,573
Prescription drug subsidy receipts, year two	165
Expected future benefit payments, year three	5,603
Prescription drug subsidy receipts, year three	160
Expected future benefit payments, year four	5,500
Prescription drug subsidy receipts, Year Four	154
Expected future benefit payments, year five	5,511
Prescription drug subsidy receipts, year five	146
Expected future benefit payments, five fiscal years thereafter	27,956
Prescription drug subsidy receipts, after year five	568
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	24,798
Expected future benefit payments, year two	25,054
Expected future benefit payments, year three	25,271
Expected future benefit payments, year four	25,616
Expected future benefit payments, year five	25,987
Expected future benefit payments, five fiscal years thereafter	$ 132,224

Nonqualified defined benefit pl

Nonqualified defined benefit plan (Details 10) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans		$ 111	$ 105.2
Supplemental employee retirement plan, defined benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		1.8	7.1	$ 6.6
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	$ 3.3
Projected benefit obligation		101.8	110.8
Accumulated benefit obligation		$ 101.8	$ 104.6
Discount rate to determine benefit obligation		3.56%	3.77%
Rate of compensation increase to determine benefit obligation		0.00%	4.00%
Discount rate to determine net periodic benefit cost		3.77%	3.51%
Rate of compensation increase to determine net periodic benefit cost		4.00%	4.00%
Expected future benefit payments, next twelve months		$ 6.7
Expected future benefit payments, year two		7.1
Expected future benefit payments, year three		7.3
Expected future benefit payments, year four		7.7
Expected future benefit payments, year five		7.7
Expected future benefit payments, five fiscal years thereafter		$ 36.4

Defined contribution plans (Det

Defined contribution plans (Details 11) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Nonqualified Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 395,000	$ 207,000	$ 104,000
Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 40,900,000	$ 36,800,000	$ 34,400,000

Employee Benefit Plans Investme

Employee Benefit Plans Investments in nonqualified plans (Details 12) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items]
Investments in nonqualified benefit plans	$ 111	$ 105.2
Equity securities [Member]
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items]
Investments in nonqualified benefit plans	62.5	54.2
Life insurance carried on plan participants [Member]
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items]
Investments in nonqualified benefit plans	35.5	34.3
Other investments [Member]
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items]
Investments in nonqualified benefit plans	$ 13	$ 16.7

Multiemployer plans (Details 13

Multiemployer plans (Details 13) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Multiemployer Plans [Line Items]
Multiemployer plan red zone status funded percentage, maximum		65.00%
Multiemployer plan yellow zone status funded percentage, minimum		65.00%
Mulitiemployer plan yellow status funded percentage, maximum		80.00%
Multiemployer plan green zone status funded percentage, minimum		80.00%
Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions		$ 44,527,000	$ 39,112,000	$ 42,817,000
Alaska Laborers-Employers Retirement Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		916,028,298
Multiemployer plan number		1
Multiemployer plans, certified zone status		Yellow	Yellow
Multiemployer plans, certified zone status, date		Jun. 30, 2016	Jun. 30, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 766,000	$ 917,000	666,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Dec. 31, 2016
Edison Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		936,061,681
Multiemployer plan number		1
Multiemployer plans, certified zone status		Green	Green
Multiemployer plans, certified zone status, date		Dec. 31, 2016	Dec. 31, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		No
Multiemployer plan, period contributions		$ 6,242,000	$ 5,517,000	9,061,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Dec. 31, 2017
IBEW Local No. 82 Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		316,127,268
Multiemployer plan number		1
Multiemployer plans, certified zone status		Green	Red
Multiemployer plans, certified zone status, date		Jun. 30, 2016	Jun. 30, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 2,560,000	$ 2,252,000	1,392,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Dec. 1, 2019
IBEW Local No. 357 Pension Plan A [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		886,023,284
Multiemployer plan number		1
Multiemployer plans, certified zone status		Green	Green
Multiemployer plans, funding improvement plan and rehabilitation plan		No
Multiemployer plan, period contributions		$ 3,016,000	$ 1,896,000	3,575,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		May 31, 2018
IBEW Local 648 Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		316,134,845
Multiemployer plan number		1
Multiemployer plans, certified zone status		Red	Red
Multiemployer plans, certified zone status, date		Feb. 29, 2016	Feb. 28, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 773,000	$ 745,000	1,110,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Sep. 2, 2018
Idaho Plumbers and Pipefitters Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		826,010,346
Multiemployer plan number		1
Multiemployer plans, certified zone status		Green	Green
Multiemployer plans, certified zone status, date		May 31, 2016	May 31, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		No
Multiemployer plan, period contributions		$ 1,221,000	$ 1,169,000	1,125,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Sep. 30, 2019
Local Union 212 IBEW Pension Trust Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		316,127,280
Multiemployer plan number		1
Multiemployer plans, certified zone status		Yellow	Yellow
Multiemployer plans, certified zone status, date		Apr. 30, 2016	Apr. 30, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 1,146,000	$ 937,000	568,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Jun. 2, 2019
National Automatic Sprinkler Industry Pension Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		526,054,620
Multiemployer plan number		1
Multiemployer plans, certified zone status		Red	Red
Multiemployer plans, certified zone status, date		Dec. 31, 2016	Dec. 31, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 775,000	$ 677,000	608,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date, first		Jul. 31, 2018
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, Last		Mar. 31, 2021
National Electrical Benefit Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		530,181,657
Multiemployer plan number		1
Multiemployer plans, certified zone status		Green	Green
Multiemployer plans, funding improvement plan and rehabilitation plan		No
Multiemployer plan, period contributions		$ 6,366,000	$ 5,271,000	6,476,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date, first		Jan. 1, 2017
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, Last		May 31, 2020
Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	[1]	836,011,320
Multiemployer plan number		1
Multiemployer plans, certified zone status		Red	Red
Multiemployer plans, certified zone status, date		Dec. 31, 2016	Dec. 31, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 0	$ 0	68,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date	[2]	Oct. 31, 2005
Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] | Withdrawal from Multiemployer Defined Benefit Plan [Member]
Multiemployer Plans [Line Items]
Multiemployer discounted future quarterly payments		$ 42,000
Multiemployer withdrawal liability prior accrual		16,400,000
Multiemployer plans, withdrawal obligation		$ 5,200,000
Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		956,052,257
Multiemployer plan number		1
Multiemployer plans, certified zone status		Red	Red
Multiemployer plans, certified zone status, date		Dec. 31, 2016	Dec. 31, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 1,087,000	$ 714,000	676,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Jun. 30, 2017
Southwest Marine Pension Trust [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number		956,123,404
Multiemployer plan number		1
Multiemployer plans, certified zone status		Red	Red
Multiemployer plans, funding improvement plan and rehabilitation plan		Implemented
Multiemployer plan, period contributions		$ 50,000	$ 26,000	31,000
Multiemployer plans, surcharge		No
Multiemployer plans, collective-bargaining arrangement, expiration date		Jan. 31, 2019
Multiemployer Plan, Individually Insignificant Multiemployer Plans [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions		$ 20,525,000	$ 18,991,000	$ 17,461,000
[1]	The Company withdrew from the plan as of October 26, 2014, as discussed later.
[2]	Plan includes collective bargaining agreements which have expired. The agreements contain provisions that automatically renew the existing contracts in lieu of a new negotiated collective bargaining agreement.

Multiemployer other postretirem

Multiemployer other postretirement and defined contribution plans (Details 14) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Multiemployer Plans, Defined Contribution [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 23.8	$ 19.5	$ 22
Multiemployer Plans, Postretirement Benefit [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 36.1	$ 31.4	$ 34.6

Jointly owned facilities (Detai

Jointly owned facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Big Stone Station:
Jointly owned facilities [Line Items]
Utility plant in service	$ 157,144	$ 157,761
Less accumulated depreciation	49,568	48,242
Utility plant in services net	107,576	109,519
Coyote Station:
Jointly owned facilities [Line Items]
Utility plant in service	156,334	140,895
Less accumulated depreciation	105,928	94,755
Utility plant in services net	50,406	46,140
Wygen III:
Jointly owned facilities [Line Items]
Utility plant in service	66,251	65,023
Less accumulated depreciation	7,550	6,788
Utility plant in services net	$ 58,701	$ 58,235

Regulatory matters (Details)

Regulatory matters (Details) - USD ($)

Jan. 17, 2017

Jan. 10, 2017

Dec. 28, 2016

Dec. 21, 2016

Dec. 12, 2016

Dec. 02, 2016

Nov. 23, 2016

Nov. 21, 2016

Oct. 27, 2016

Oct. 14, 2016

Sep. 06, 2016

Aug. 12, 2016

Jun. 10, 2016

Jun. 01, 2016

Apr. 29, 2016

Mar. 09, 2016

Feb. 10, 2016

Jan. 05, 2016

Jan. 01, 2016

Sep. 30, 2015

MNPUC [Member]

Regulatory matters [Line Items]

Rate increase requested

$ 1,600,000

Percent above current rates requested

6.40%

Interim rate increase (decrease), amount

$ 1,500,000

Interim rate increase (decrease), percentage

6.40%

Public utilities, approved rate increase (decrease), amount

$ 1,100,000

Public utilities, approved rate increase (decrease), percentage

5.20%

NDPSC-Renewable Rider [Member]

Regulatory matters [Line Items]

Interim rate increase (decrease), amount

$ 15,100,000

Public utilities, requested return on equity, percentage

10.50%

NDPSC-Electric Rider [Member]

Regulatory matters [Line Items]

Interim rate increase (decrease), amount

$ 9,700,000

Public utilities, requested return on equity, percentage

10.50%

NDPSC-Transmission Adjustment [Member]

Regulatory matters [Line Items]

Interim rate increase (decrease), amount

$ 6,800,000

Public utilities, requested return on equity, percentage

10.50%

OPUC [Member]

Regulatory matters [Line Items]

Rate increase requested

$ 1,900,000

Percent above current rates requested

2.80%

Public utilities, approved rate increase (decrease), amount

$ 754,000

WUTC-Pipeline Replacement [Member]

Regulatory matters [Line Items]

Public utilities, replacement cost recovery mechanism

$ 4,600,000

Public utilities, requested additional revenue, percentage

2.00%

Incremental pipeline replacement investment

$ 2,400,000

Incremental operation and maintenance costs

$ 2,200,000

Incremental pipeline recovery investment, allowed amount

$ 1,900,000

WYPSC [Member]

Regulatory matters [Line Items]

Rate increase requested

$ 3,200,000

Percent above current rates requested

13.10%

Settlement stipulation

$ 2,700,000

Settlement stipulation percent

11.10%

IPUC [Member]

Regulatory matters [Line Items]

Rate increase requested

$ 10,200,000

Percent above current rates requested

4.10%

Public utilities, requested rate increase (decrease), amended, amount

$ 9,600,000

NDPSC [Member]

Regulatory matters [Line Items]

Rate increase requested

$ 13,400,000

Percent above current rates requested

6.60%

Interim rate increase (decrease), amount

$ 11,700,000

$ 13,000,000

Interim rate increase (decrease), percentage

5.80%

6.50%

MTPSC [Member]

Regulatory matters [Line Items]

Public utilities gas and electric tax tracking adjustment

$ 814,000

MNPUC-Gas Cost Tariff [Member]

Regulatory matters [Line Items]

Public utilities, gas utility infrastructure cost tariff, requested

$ 456,000

Subsequent Event [Member] | MISO [Member]

Regulatory matters [Line Items]

Revenue requirement

$ 11,100,000

Subsequent Event [Member] | MTPSC [Member]

Regulatory matters [Line Items]

Public utilities gas tax tracking adjustment, approved

$ 474,000

Public utilities electric tax tracking adjustment, approved

$ 251,000

Commitments and Contingencies C

Commitments and Contingencies Commitment and Contingencies Accrued Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Loss Contingency Accrual	$ 31.8	$ 19.5

Enviromental matters (Details 2

Enviromental matters (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2016	Jan. 06, 2017
Portland Harbor site [Member]
Site Contingency [Line Items]
Environmental matters investigative costs	$ 100,000,000
Eugene, OR manufactured gas plant site [Member]
Site Contingency [Line Items]
Estimated proportional share of cleanup liability	50.00%
Accrual for environmental loss contingencies	$ 1,600,000
Bremerton, WA manufactured gas plant site [Member]
Site Contingency [Line Items]
Accrual for environmental loss contingencies	12,500,000
Bellingham, WA manufactured gas plant site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	8,000,000
Subsequent Event [Member] | Portland Harbor site [Member]
Site Contingency [Line Items]
Environmental Matters, Estimated Costs		$ 1,000,000,000
Minimum [Member] | Eugene, OR manufactured gas plant site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	500,000
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] | Eugene, OR manufactured gas plant site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	11,000,000
Maximum [Member] | Bremerton, WA manufactured gas plant site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 6,400,000

Operating leases (Details 3)

Operating leases (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Leases, Annual minimum lease payments due [Abstract]
2,017	$ 51.7
2,018	43.3
2,019	33.9
2,020	23.2
2,021	9.4
Thereafter	42
Rent expense
Rent expense	$ 65	$ 53.9	$ 46.9

Purchase commitments (Details 4

Purchase commitments (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Purchase commitments due
2,017	$ 367.7
2,018	215.7
2,019	189.4
2,020	138
2,021	130.6
Thereafter	859.5
Amounts purchased under various commitments	$ 539.3	$ 842.1	$ 759
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Term of unrecorded unconditional purchase obligation	1 year
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Term of unrecorded unconditional purchase obligation	44 years

Guarantees (Details 5)

Guarantees (Details 5)	Dec. 31, 2016USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 98,700,000
Fixed maximum amounts guaranteed by year 2017	33,600,000
Fixed maximum amounts guaranteed by year 2018	4,500,000
Fixed maximum amounts guaranteed by year 2019	56,600,000
Fixed maximum amounts guaranteed by year no maturity date	4,000,000
Amount outstanding under guarantees	0
Letters of credit	30,800,000
Outstanding letters of credit	0
Amount of surety bonds outstanding	516,100,000
Financial Guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 63,800,000

Variable interest entities (Det

Variable interest entities (Details 6) - USD ($) $ in Thousands	Dec. 31, 2016	Jun. 27, 2016	Dec. 31, 2015	Feb. 07, 2013
Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Total project costs	$ 300,000
Portion of capital commitment	150,000
Partner portion of capital commitment	75,000
Excess capital commitments	300,000
Term loan for project debt financing	75,000
Fuel contract [Member]
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 43,300
Cash and cash equivalents [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			$ 851
Accounts receivable [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			7,693
Inventories [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			13,176
Other current assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			6,215
Total current assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			27,935
Net property, plant and equipment [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			425,123
Other [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			9,626
Total deferred charges and other assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			9,626
Total assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets			462,684
Short-term borrowings [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			45,500
Long-term debt due within one year [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			5,250
Accounts payable [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			24,766
Taxes payable [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			1,391
Accrued compensation [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			938
Other accrued liabilities [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			4,953
Total current liabilities [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			82,798
Long-term debt [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			63,750
Total liabilities [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities			$ 146,548
Calumet [Member]
Variable Interest Entities [Line Items]
Previous percentage of ownership				50.00%
Calumet [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Previous percentage of ownership				50.00%
WBI Energy [Member]
Variable Interest Entities [Line Items]
Previous percentage of ownership				50.00%
Percentage of ownership acquired		50.00%
WBI Energy [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Previous percentage of ownership				50.00%
Percentage of ownership acquired		50.00%

Quarterly Financial Informat117

Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues	$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 1,016,826	$ 1,198,342	$ 938,039	$ 860,845	$ 4,128,828	$ 4,014,052	$ 4,115,073
Operating expenses	904,613	1,061,883	954,983	798,229	934,896	1,070,514	878,330	810,537	3,719,708	3,694,277	3,787,891
Operating income	111,486	146,684	88,965	61,985	81,930	127,828	59,709	50,308	409,120	319,775	327,182
Income from continuing operations	66,547	88,386	46,298	31,865	55,902	73,886	26,061	20,540	$ 233,096	$ 176,389	$ 185,027
Loss from discontinued operations attributable to the Company, net of tax	(816)	(5,400)	(155,451)	(6,996)	(3,368)	(213,334)	(255,665)	(326,457)
Net income (loss) attributable to parent	$ 65,731	$ 82,986	$ (109,153)	$ 24,869	$ 52,534	$ (139,448)	$ (229,604)	$ (305,917)
Earnings (loss) per common share - basic [Abstract]
Earnings before discontinued operations	$ 0.34	$ 0.45	$ 0.24	$ 0.16	$ 0.29	$ 0.38	$ 0.13	$ 0.10	$ 1.19	$ 0.90	$ 0.96
Discontinued operations attributable to the Company, net of tax	0	(0.03)	(0.80)	(0.03)	(0.02)	(1.10)	(1.31)	(1.67)	(0.86)	(4.10)	0.59
Earnings (loss) per common share - basic	$ 0.34	$ 0.42	$ (0.56)	$ 0.13	$ 0.27	$ (0.72)	$ (1.18)	$ (1.57)	$ 0.33	$ (3.20)	$ 1.55
Weighted average common shares outstanding - basic	195,304	195,304	195,304	195,284	195,266	195,151	194,805	194,479	195,299	194,928	192,507
Earnings per common share - diluted [Abstract]
Earnings before discontinued operations	$ 0.33	$ 0.45	$ 0.24	$ 0.16	$ 0.29	$ 0.38	$ 0.13	$ 0.10	$ 1.19	$ 0.90	$ 0.96
Discontinued operations attributable to the Company, net of tax	0	(0.03)	(0.80)	(0.03)	(0.02)	(1.10)	(1.31)	(1.67)	(0.86)	(4.10)	0.59
Earnings (loss) per common share - diluted	$ 0.33	$ 0.42	$ (0.56)	$ 0.13	$ 0.27	$ (0.72)	$ (1.18)	$ (1.57)	$ 0.33	$ (3.20)	$ 1.55
Weighted average common shares outstanding - diluted	195,889	195,811	195,699	195,284	195,324	195,169	194,838	194,566	195,618	194,986	192,587
Additional Information [Abstract]
Asset impairment charges						$ 14,100	$ 3,000
Withdrawal from multiemployer defined benefit plan [Member] | Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member]
Additional Information [Abstract]
Multiemployer plan withdrawal increase (decrease)	$ (11,100)							$ 2,400

Capitalized costs and AD&D rela

Capitalized costs and AD&D related to oil and natural gas producing activities (Details) $ in Thousands	Dec. 31, 2014USD ($)
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Subject to amortization	$ 3,205,036
Not subject to amortization	132,141
Total capitalized costs	3,337,177
Less accumulated depreciation, depletion and amortization	1,752,566
Net capitalized costs	$ 1,584,611

Capital expenditures related to

Capital expenditures related to oil and natural gas producing activities (Details 2) $ in Thousands	12 Months Ended
	Dec. 31, 2014USD ($)
Acquisitions:
Proved properties	$ 87,919	[1]
Unproved properties	138,683	[1]
Exploration	16,879	[1]
Development	331,400	[1]
Total capital expenditures	574,881	[1]
Net change in property, plant and equipment related to future liabilities for asset retirement obligations that are excluded	$ (9,000)
[1]	Excludes net reductions to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of oil and natural gas wells of $9.0 million for the year ended December 31, 2014.

Estimated quantities of proved

Estimated quantities of proved reserves (Details 3)	12 Months Ended
	Dec. 31, 2016MBoeMMcfMBbls	Dec. 31, 2015MBoeMMcfMBbls	Dec. 31, 2014MBoeMMcfMBbls
Proved developed and undeveloped reserves:
Balance at beginning of year, BOE | MBoe	13,321	91,940	80,695
Production, BOE | MBoe	0	(6,471)	(8,998)
Extensions and discoveries, BOE | MBoe	0	888	24,025
Improved recovery, BOE | MBoe	0	0	0
Purchases of proved reserves, BOE | MBoe	0	0	6,748
Sales revisions, BOE | MBoe	(13,321)	(57,097)	(14,796)
Revisions of previous estimate, BOE | MBoe	0	(15,939)	4,266
Balance at end of year, BOE | MBoe	0	13,321	91,940
Oil (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	12,687	43,918	41,019
Production	0	(3,286)	(4,919)
Extensions and discoveries	0	744	9,654
Improved recovery	0	0	0
Purchases of proved reserves	0	0	5,463
Sales of proved reserves	(12,687)	(16,474)	(4,945)
Revisions of previous estimates	0	(12,215)	(2,354)
Balance at end of year	0	12,687	43,918
NGL (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	211	7,187	6,602
Production	0	(393)	(609)
Extensions and discoveries	0	29	3,634
Improved recovery	0	0	0
Purchases of proved reserves	0	0	0
Sales of proved reserves	(211)	(6,864)	(3,109)
Revisions of previous estimates	0	252	669
Balance at end of year	0	211	7,187
Natural Gas (MMcf)
Proved developed and undeveloped reserves:
Balance at beginning of year | MMcf	2,531	245,011	198,445
Production | MMcf	0	(16,747)	(20,822)
Extensions and discoveries | MMcf	0	681	64,420
Improved recovery | MMcf	0	0	0
Purchases of proved reserves | MMcf	0	0	7,711
Sales of proved reserves | MMcf	(2,531)	(202,560)	(40,451)
Revisions of previous estimates | MMcf	0	(23,854)	35,708
Balance at end of year | MMcf	0	2,531	245,011

Proved reserves (Details 4)

Proved reserves (Details 4)	Dec. 31, 2016MBoeMMcfMBbls	Dec. 31, 2015MBoeMMcfMBbls	Dec. 31, 2014MBoeMMcfMBbls	Dec. 31, 2013MBoeMMcfMBbls
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, energy | MBoe	0	11,865	65,086
Proved undeveloped reserves, energy | MBoe	0	1,456	26,854
Balance at end of year, energy | MBoe	0	13,321	91,940	80,695
Changes in proved and undeveloped reserves [Abstract]
Change in PUD reserve | MBoe	(1,500)
Oil (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	0	11,380	30,130
Proved undeveloped reserves, volume	0	1,307	13,788
Balance at end of year, volume	0	12,687	43,918	41,019
NGL (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	0	144	4,217
Proved undeveloped reserves, volume	0	67	2,970
Balance at end of year, volume	0	211	7,187	6,602
Natural Gas (MMcf)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume | MMcf	0	2,033	184,437
Proved undeveloped reserves, volume | MMcf	0	498	60,574
Balance at end of year, volume | MMcf	0	2,531	245,011	198,445

Schedule I-Condensed Financial

Schedule I-Condensed Financial Information of Registrant (Details)	12 Months Ended
	Dec. 31, 2016
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule I-Condensed Financial Information of Registrant	Summary of Significant Accounting Policies Basis of presentation The condensed financial information reported in Schedule I is being presented to comply with Rule 12-04 of Regulation S-X. The information is unconsolidated and is presented for the parent company only, which is comprised of MDU Resources Group, Inc. (the Company) and Montana-Dakota and Great Plains, public utility divisions of the Company. In Schedule I, investments in subsidiaries are presented under the equity method of accounting where the assets and liabilities of the subsidiaries are not consolidated. The investments in net assets of the subsidiaries are recorded on the Condensed Balance Sheets. The income (loss) from subsidiaries is reported as equity in earnings (loss) of subsidiaries on the Condensed Statements of Income. The consolidated financial statements of MDU Resources Group, Inc. reflect certain businesses as discontinued operations. These statements should be read in conjunction with the consolidated financial statements and notes thereto of MDU Resources Group, Inc. Earnings (loss) per common share Please refer to the Consolidated Statements of Income of the registrant for earnings (loss) per common share. In addition, see Item 8 - Note 1 for information on the computation of earnings (loss) per common share. Note 2 - Debt The Company has long-term debt obligations outstanding of $681.8 million at December 31, 2016 , with annual maturities of $100,000 in 2017 , $100.1 million in 2018 , $111.1 million in 2019 , $100,000 in 2020 and $470.4 million scheduled to mature in years after 2021 . For more information on debt, see Item 8 - Note 6 . Note 3 - Dividends The Company depends on earnings from its divisions and dividends from its subsidiaries to pay dividends on common stock. Cash dividends paid to the Company by subsidiaries were $115.8 million , $110.6 million and $105.6 million for the years ended December 31, 2016 , 2015 and 2014 , respectively.

Condensed Statements of Income

Condensed Statements of Income and Comprehensive Income (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 1,016,826	$ 1,198,342	$ 938,039	$ 860,845	$ 4,128,828	$ 4,014,052	$ 4,115,073
Operating expenses	904,613	1,061,883	954,983	798,229	934,896	1,070,514	878,330	810,537	3,719,708	3,694,277	3,787,891
Operating income	$ 111,486	$ 146,684	$ 88,965	$ 61,985	$ 81,930	$ 127,828	$ 59,709	$ 50,308	409,120	319,775	327,182
Other income									4,956	18,457	9,138
Interest expense									87,848	91,179	86,871
Income before income taxes									326,228	247,053	249,449
Income taxes									93,132	70,664	64,422
Earnings (loss) on common stock									63,748	(623,120)	297,548
Comprehensive income (loss)									65,848	(617,480)	294,335
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues									561,266	556,112	628,578
Operating expenses									469,062	478,198	547,820
Operating income									92,204	77,914	80,758
Other income									1,491	8,318	5,271
Interest expense									31,519	23,562	21,055
Income before income taxes									62,176	62,670	64,974
Income taxes									6,355	15,882	16,819
Equity in earnings of subsidiaries from continuing operations									177,275	129,601	136,872
Net income from continuing operations									233,096	176,389	185,027
Equity in earnings (loss) of subsidiaries from discontinued operations attributable to the company									(168,663)	(798,824)	113,206
Dividends declared on preferred stocks									685	685	685
Earnings (loss) on common stock									63,748	(623,120)	297,548
Comprehensive income (loss)									$ 65,848	$ (617,480)	$ 294,335

Condensed Balance Sheets (Detai

Condensed Balance Sheets (Details 3) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 46,107	$ 83,903		$ 60,479		$ 52,566
Receivables, net	630,243	582,475
Inventories	238,273	240,551
Prepayments and other current assets	48,461	29,528
Total current assets	977,475	991,304
Investments	125,866	119,704
Property, plant and equipment	6,510,229	6,387,702
Less accumulated depreciation, depletion and amortization	2,578,902	2,489,322		2,385,202
Net property, plant and equipment	3,931,327	3,898,380		3,520,320
Deferred charges and other assets:
Goodwill	631,791	635,204	[1]	635,204	[1]
Other	415,419	351,603
Total deferred charges and other assets	1,249,799	1,555,766
Total assets	6,284,467	6,565,154		7,805,405
Current liabilities:
Long-term debt due within one year	43,598	238,539
Accounts payable	279,962	286,061
Taxes payable	48,164	46,880
Dividends payable	37,767	36,784
Accrued compensation	65,867	45,192
Other accrued liabilities	184,377	167,322
Total current liabilities	669,659	947,261
Long-term debt	1,746,561	1,557,624
Deferred credits and other liabilities:
Deferred income taxes	668,226	663,629
Other	883,777	812,342
Total deferred credits and other liabilities	1,552,003	1,539,721
Commitments and contingencies (Notes 14, 16 and 17)
Equity:
Preferred stocks	15,000	15,000
Authorized - 500,000,000 shares, $1.00 par value
Issued - 195,843,297 shares in 2016 and 195,804,665 shares in 2015	195,843	195,805
Other paid-in capital	1,232,478	1,230,119
Retained earnings	912,282	996,355
Accumulated other comprehensive loss	(35,733)	(37,148)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total common stockholders' equity	2,301,244	2,381,505
Total stockholders' equity	2,316,244	2,396,505
Total liabilities and equity	$ 6,284,467	$ 6,565,154
Condensed Balance Sheet (Parenthetical)
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1
Common stock, shares issued	195,843,297	195,804,665
Treasury stock, shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 4,159	$ 2,921		$ 6,120		$ 5,051
Receivables, net	80,467	70,511
Accounts receivable from subsidiaries	34,424	33,129
Inventories	17,352	16,883
Prepayments and other current assets	24,531	7,876
Total current assets	160,933	131,320
Investments	70,370	66,784
Investment in subsidiaries	1,603,874	1,722,351
Property, plant and equipment	2,502,264	2,378,994
Less accumulated depreciation, depletion and amortization	756,191	711,209
Net property, plant and equipment	1,746,073	1,667,785
Deferred charges and other assets:
Goodwill	4,812	4,812
Other	183,654	184,080
Total deferred charges and other assets	188,466	188,892
Total assets	3,769,716	3,777,132
Current liabilities:
Long-term debt due within one year	110	109
Accounts payable	37,697	54,275
Accounts payable to subsidiaries	5,592	6,622
Taxes payable	14,992	10,995
Dividends payable	37,767	36,784
Accrued compensation	16,086	7,539
Other accrued liabilities	34,929	40,931
Total current liabilities	147,173	157,255
Long-term debt	679,667	623,048
Deferred credits and other liabilities:
Deferred income taxes	270,126	255,069
Other	356,506	345,255
Total deferred credits and other liabilities	626,632	600,324
Equity:
Preferred stocks	15,000	15,000
Authorized - 500,000,000 shares, $1.00 par value
Issued - 195,843,297 shares in 2016 and 195,804,665 shares in 2015	195,843	195,805
Other paid-in capital	1,232,478	1,230,119
Retained earnings	912,282	996,355
Accumulated other comprehensive loss	(35,733)	(37,148)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total common stockholders' equity	2,301,244	2,381,505
Total stockholders' equity	2,316,244	2,396,505
Total liabilities and equity	$ 3,769,716	$ 3,777,132
Condensed Balance Sheet (Parenthetical)
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1	$ 1
Common stock, shares issued	195,843,297	195,804,655
Treasury stock, shares	538,921	538,921
[1]	Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods.

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 462,209	$ 661,832	$ 587,062
Capital expenditures	(388,183)	(536,832)	(429,336)
Net proceeds from sale or disposition of property and other	44,826	54,569	28,899
Investments	(1,396)	1,515	(1,041)
Net cash used in investing activities	(305,095)	(382,453)	(904,190)
Issuance of long-term debt	309,064	345,920	606,168
Repayment of long-term debt	(315,647)	(566,498)	(365,247)
Proceeds from issuance of common stock	0	21,898	150,060
Dividends paid	(147,156)	(142,835)	(136,712)
Excess tax benefit on stock-based compensation	0	0	4,729
Tax withholding on stock-based compensation	(323)	0	(5,564)
Net cash provided by financing activities	(194,914)	(255,730)	325,196
Increase (decrease) in cash and cash equivalents	(37,796)	23,424	7,913
Cash and cash equivalents - beginning of year	83,903	60,479	52,566
Cash and cash equivalents - end of year	46,107	83,903	60,479
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	238,125	255,273	208,208
Capital expenditures	(159,570)	(349,985)	(223,251)
Net proceeds from sale or disposition of property and other	3,784	3,268	1,552
Investments in and advances to subsidiaries	(5,000)	(7,000)	(134,451)
Advances from subsidiaries	15,000	100,000	64,500
Investments	(129)	5	(794)
Net cash used in investing activities	(145,915)	(253,712)	(292,444)
Issuance of long-term debt	106,420	224,185	148,959
Repayment of long-term debt	(50,010)	(108,008)	(76,432)
Proceeds from issuance of common stock	0	21,898	150,060
Dividends paid	(147,156)	(142,835)	(136,712)
Excess tax benefit on stock-based compensation	0	0	3,326
Tax withholding on stock-based compensation	(226)	0	(3,896)
Net cash provided by financing activities	(90,972)	(4,760)	85,305
Increase (decrease) in cash and cash equivalents	1,238	(3,199)	1,069
Cash and cash equivalents - beginning of year	2,921	6,120	5,051
Cash and cash equivalents - end of year	$ 4,159	$ 2,921	$ 6,120

Notes to Condensed Financial St

Notes to Condensed Financial Statements (Details 5) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Long-term debt	$ 1,790,159,000	$ 1,796,163,000
2,017	43,598,000
2,018	169,449,000
2,019	162,154,000
2,020	15,021,000
Mature in years after 2021	1,254,860,000
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Long-term debt	681,800,000
2,017	100,000
2,018	100,100,000
2,019	111,100,000
2,020	100,000
Mature in years after 2021	470,400,000
Cash dividends paid to parent company by consolidated subsidiaries	$ 115,800,000	$ 110,600,000	$ 105,600,000

Schedule II - Consolidated Valu

Schedule II - Consolidated Valuation and Qualifying Accounts (Details)	12 Months Ended
	Dec. 31, 2016
Valuation and Qualifying Accounts [Abstract]
Schedule II - Valuation and Qualifying Accounts	Consolidated Valuation and Qualifying Accounts For the years ended December 31, 2016 , 2015 and 2014 Additions Description Balance at Beginning of Year Charged to Costs and Expenses Other * Deductions ** Balance at End of Year (In thousands) Allowance for doubtful accounts: 2016 $ 9,835 $ 8,302 $ 851 $ 8,509 $ 10,479 2015 9,511 11,343 1,012 12,031 9,835 2014 10,085 8,548 1,335 10,457 9,511 * Recoveries. ** Uncollectible accounts written off. All other schedules are omitted because of the absence of the conditions under which they are required, or because the information required is included in the Company's Consolidated Financial Statements and Notes thereto.

Schedule II - Consolidated V128

Schedule II - Consolidated Valuation and Qualifying Accounts (Details 2) - Allowance for Doubtful Accounts [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts:
Balance, beginning of period		$ 9,835	$ 9,511	$ 10,085
Charged to costs and expenses		8,302	11,343	8,548
Other	[1]	851	1,012	1,335
Deductions	[2]	8,509	12,031	10,457
Balance, end of period		$ 10,479	$ 9,835	$ 9,511
[1]	Recoveries.
[2]	Uncollectible accounts written off.