MDU Resources (MDU) 10-Q 8 May 15 2015 Q1 Quarterly report Financial data

Document_And_Entity_Informatio

Document And Entity Information	3 Months Ended
	Mar. 31, 2015	1-May-15
Entity Information [Line Items]
Entity registrant name	MDU RESOURCES GROUP INC
Entity central index key	67716
Current fiscal year end date	-19
Entity filer category	Large Accelerated Filer
Entity common stock, shares outstanding		194,770,955
Document fiscal year focus	2015
Document fiscal period focus	Q1
Document type	10-Q
Amendment flag	FALSE
Document period end date	31-Mar-15

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating revenues:
Electric, natural gas distribution and pipeline and energy services	$427,444	$491,541
Exploration and production, construction materials and contracting, construction services and other	491,066	551,312
Total operating revenues	918,510	1,042,853
Operating expenses:
Fuel and purchased power	23,819	26,544
Purchased natural gas sold	202,960	244,892
Cost of crude oil	2,270	0
Operation and maintenance:
Electric, natural gas distribution and pipeline and energy services	79,425	67,284
Exploration and production, construction materials and contracting, construction services and other	440,993	445,951
Depreciation, depletion and amortization	95,507	99,557
Taxes, other than income	47,483	55,721
Write-down of oil and natural gas properties	500,400	0
Total operating expenses	1,392,857	939,949
Operating income (loss)	-474,347	102,904
Other income	2,324	2,183
Interest expense	23,149	20,971
Income (loss) before income taxes	-495,172	84,116
Income taxes	-185,727	27,932
Income (loss) from continuing operations	-309,445	56,184
Loss from discontinued operations, net of tax (Note 10)	0	-45
Net income (loss)	-309,445	56,139
Net loss attributable to noncontrolling interest	-3,528	-523
Dividends declared on preferred stocks	171	171
Earnings (loss) on common stock	($306,088)	$56,491
Earnings (loss) per common share - basic:
Earnings (loss) before discontinued operations	($1.57)	$0.30
Discontinued operations, net of tax	$0	$0
Earnings (loss) per common share - basic	($1.57)	$0.30
Earnings (loss) per common share - diluted:
Earnings (loss) before discontinued operations	($1.57)	$0.30
Discontinued operations, net of tax	$0	$0
Earnings (loss) per common share - diluted	($1.57)	$0.30
Dividends declared per common share	$0.18	$0.18
Weighted average common shares outstanding - basic	194,479	189,820
Weighted average common shares outstanding - diluted	194,479	190,432

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Consolidated Statements of Comprehensive Income [Abstract]
Net income (loss)	($309,445)	$56,139
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $60 and $204 for the three months ended in 2015 and 2014, respectively	99	344
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $230 and $168 for the three months ended in 2015 and 2014, respectively	375	275
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(68) and $28 for the three months ended in 2015 and 2014, respectively	-112	46
Reclassification adjustment for loss on foreign currency translation adjustment included in net income, net of tax of $490 and $0 for the three months ended in 2015 and 2014, respectively	802	0
Foreign currency translation adjustment	690	46
Net unrealized gain on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(11) and $(19) for the three months ended in 2015 and 2014, respectively	-21	-36
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $19 and $20 for the three months ended in 2015 and 2014, respectively	36	38
Net unrealized gain on available-for-sale investments	15	2
Other comprehensive income	1,179	667
Comprehensive income (loss)	-308,266	56,806
Comprehensive loss attributable to noncontrolling interest	-3,528	-523
Comprehensive income (loss) attributable to common stockholders	($304,738)	$57,329

Consolidated_Statements_of_Com1

Consolidated Statements of Comprehensive Income - Parenthetical (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax	$60	$204
Amortization of postretirement liability losses included in net periodic benefit cost, tax	230	168
Foreign currency translation adjustments arising during the period, tax	-68	28
Reclassification adjustment for foreign currency translation (gain) loss included in net income, tax	490	0
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	-11	-19
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income, tax	$19	$20

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$122,241		$81,855		$83,700
Receivables, net	564,090		693,318		690,761
Inventories	336,598		300,811		301,332
Deferred income taxes	32,987		23,806		29,427
Commodity derivative instruments	7,127		18,335		81
Prepayments and other current assets	98,503		76,848		99,229
Total current assets	1,161,546		1,194,973		1,204,530
Investments	118,407		117,920		113,763
Property, plant and equipment	9,299,713		9,693,171		9,150,269
Less accumulated depreciation, depletion and amortization	4,233,193		4,166,407		3,954,442
Net property, plant and equipment	5,066,520		5,526,764		5,195,827
Deferred charges and other assets:
Goodwill	635,204	[1]	635,204	[1]	636,039	[1]
Other intangible assets, net	9,166		9,840		12,296
Other	326,542		325,277		246,394
Total deferred charges and other assets	970,912		970,321		894,729
Total assets	7,317,385		7,809,978		7,408,849
Current liabilities:
Short-term borrowings	16,100		0		0
Long-term debt due within one year	409,292		269,449		12,227
Accounts payable	259,881		382,671		399,935
Taxes payable	48,436		45,631		61,847
Dividends payable	35,687		35,607		33,980
Accrued compensation	37,797		62,775		40,016
Commodity derivative instruments	0		0		12,186
Other accrued liabilities	179,672		172,561		185,287
Total current liabilities	986,865		968,694		745,478
Long-term debt	1,780,694		1,825,278		2,093,605
Deferred credits and other liabilities:
Deferred income taxes	804,757		952,413		899,420
Other liabilities	810,626		813,809		720,542
Total deferred credits and other liabilities	1,615,383		1,766,222		1,619,962
Equity:
Preferred stocks	15,000		15,000		15,000
Common stockholders' equity:
Authorized - 500,000,000 shares, $1.00 par value. Shares issued - 195,191,129 at March 31, 2015,191,838,720 at March 31, 2014 and 194,754,812 at December 31, 2014	195,191		194,755		191,839
Other paid-in-capital	1,214,867		1,207,188		1,110,221
Retained earnings	1,421,220		1,762,827		1,625,692
Accumulated other comprehensive loss	-40,924		-42,103		-37,538
Treasury stock at cost - 538,921 shares	-3,626		-3,626		-3,626
Total common stockholders' equity	2,786,728		3,119,041		2,886,588
Total stockholders' equity	2,801,728		3,134,041		2,901,588
Noncontrolling interest	132,715		115,743		48,216
Total equity	2,934,443		3,249,784		2,949,804
Total liabilities and equity	$7,317,385		$7,809,978		$7,408,849
[1]	Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Stockholders' equity:
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, par value	$1	$1	$1
Common stock, shares issued	195,191,129	194,754,812	191,838,720
Treasury shares	538,921	538,921	538,921

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating activities:
Net income (loss)	($309,445)	$56,139
Loss from discontinued operations, net of tax	0	-45
Income (loss) from continuing operations	-309,445	56,184
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	95,507	99,557
Deferred income taxes	-160,633	35,965
Unrealized loss on commodity derivatives	11,208	6,712
Write-down of oil and natural gas properties	500,400	0
Changes in current assets and liabilities, net of acquisitions:
Receivables	101,371	25,611
Inventories	-44,408	-19,809
Other current assets	-20,349	-22,324
Accounts payable	-53,334	-11,525
Other current liabilities	-16,119	-28,355
Other noncurrent changes	-9,537	-4,987
Net cash provided by continuing operations	94,661	137,029
Net cash provided by discontinued operations	0	8
Net cash provided by operating activities	94,661	137,037
Investing activities:
Capital expenditures	-189,463	-179,646
Acquisitions, net of cash acquired	0	-206,304
Net proceeds from sale or disposition of property and other	26,801	5,179
Investments	2,449	458
Net cash used in continuing operations	-160,213	-380,313
Net cash provided by discontinued operations	0	0
Net cash used in investing activities	-160,213	-380,313
Financing activities:
Issuance of short-term borrowings	16,100	0
Repayment of short-term borrowings	0	-11,500
Issuance of long-term debt	149,332	309,501
Repayment of long-term debt	-54,162	-58,232
Proceeds from issuance of common stock	9,864	54,843
Dividends paid	-35,607	-33,737
Excess tax benefit on stock-based compensation	0	4,833
Contribution from noncontrolling interest	20,500	16,000
Net cash provided by continuing operations	106,027	281,708
Net cash provided by discontinued operations	0	0
Net cash provided by financing activities	106,027	281,708
Effect of exchange rate changes on cash and cash equivalents	-89	43
Increase in cash and cash equivalents	40,386	38,475
Cash and cash equivalents - beginning of year	81,855	45,225
Cash and cash equivalents - end of period	$122,241	$83,700

Basis_of_presentation

Basis of presentation	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of presentation
	The accompanying consolidated interim financial statements were prepared in conformity with the basis of presentation reflected in the consolidated financial statements included in the Company's 2014 Annual Report, and the standards of accounting measurement set forth in the interim reporting guidance in the ASC and any amendments thereto adopted by the FASB. Interim financial statements do not include all disclosures provided in annual financial statements and, accordingly, these financial statements should be read in conjunction with those appearing in the 2014 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses. Management has also evaluated the impact of events occurring after March 31, 2015, up to the date of issuance of these consolidated interim financial statements.

Seasonality_of_operations

Seasonality of operations	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Seasonality of operations	Seasonality of operations
	Some of the Company's operations are highly seasonal and revenues from, and certain expenses for, such operations may fluctuate significantly among quarterly periods. Accordingly, the interim results for particular businesses, and for the Company as a whole, may not be indicative of results for the full fiscal year.

Accounts_receivable_and_allowa

Accounts receivable and allowance for doubtful accounts	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Accounts receivable and allowance for doubtful accounts	Accounts receivable and allowance for doubtful accounts
	Accounts receivable consist primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts. The total balance of receivables past due 90 days or more was $29.3 million, $22.6 million and $30.9 million at March 31, 2015 and 2014, and December 31, 2014, respectively.
	The allowance for doubtful accounts is determined through a review of past due balances and other specific account data. Account balances are written off when management determines the amounts to be uncollectible. The Company's allowance for doubtful accounts at March 31, 2015 and 2014, and December 31, 2014, was $9.4 million, $10.9 million and $9.5 million, respectively.

Inventories_and_natural_gas_in

Inventories and natural gas in storage	3 Months Ended
	Mar. 31, 2015
Inventory Disclosure [Abstract]
Inventories and natural gas in storage	Inventories and natural gas in storage
	Inventories, other than natural gas in storage for the Company's regulated operations, are stated at the lower of average cost or market value. 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. The portion of the cost of natural gas in storage expected to be used within one year is included in inventories. Inventories consisted of:
		March 31, 2015		March 31, 2014		December 31, 2014
		(In thousands)
	Aggregates held for resale	$	112,029	$	104,106	$	108,161
	Asphalt oil	89,578		66,292		42,135
	Materials and supplies	65,599		68,809		65,683
	Merchandise for resale	15,688		22,463		24,420
	Natural gas in storage (current)	9,303		6,129		19,302
	Other	44,401		33,533		41,110
	Total	$	336,598	$	301,332	$	300,811
	The remainder of natural gas in storage, which largely represents the cost of gas required to maintain pressure levels for normal operating purposes, is included in other assets and was $49.3 million, $47.4 million and $49.3 million at March 31, 2015 and 2014, and December 31, 2014, respectively.

Oil_and_natural_gas_properties

Oil and natural gas properties	3 Months Ended
	Mar. 31, 2015
Property, Plant and Equipment [Abstract]
Oil and natural gas properties	Oil and natural gas properties
	The company uses 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. Any conveyances of properties, including gains or losses on abandonments of properties, are generally treated as adjustments to the cost of the properties with no gain or loss recognized.
	Capitalized costs are 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, are used to calculate the ceiling test. Accordingly, the Company was required to write down its oil and natural gas producing properties. The noncash write-down amounted to $500.4 million ($315.3 million after tax) for the three months ended March 31, 2015.
	At March 31, 2014 and December 31, 2014, the Company's full-cost ceiling exceeded the Company's capitalized cost. Various factors, including lower SEC Defined Prices, market differentials, changes in estimates of proved reserve quantities, unsuccessful results of exploration and development efforts or changes in operating and development costs could result in future noncash write-downs of the Company's oil and natural gas properties.
	SEC Defined Prices for each quarter for the last 12 months were as follows:
	SEC Defined Prices for the 12 months ended	NYMEX		Henry Hub		Ventura
		Oil Price		Gas Price		Gas Price
		(per Bbl)		(per MMBtu)		(per MMBtu)
	March 31, 2015	$	82.72	$	3.87	$	3.96
	December 31, 2014	94.99		4.34		7.71
	September 30, 2014	99.08		4.24		7.6
	June 30, 2014	100.27		4.1		7.47
	For purposes of comparison, first-of-the-month prices were as follows:
		NYMEX		Henry Hub		Ventura
		Oil Price		Gas Price		Gas Price
		(per Bbl)		(per MMBtu)		(per MMBtu)
	Apr-15	$	50.09	$	2.63	$	2.45
	May-15	59.15		2.57		2.51
	Given the current oil and natural gas pricing environment, the Company believes it is likely it will have noncash write-downs of its oil and natural gas properties in future quarters until such time as commodity prices begin to recover.

Earnings_per_common_share

Earnings per common share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Earnings per common share	Earnings (loss) per common share
	Basic earnings (loss) per common share were computed by dividing earnings (loss) on common stock by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings per common share were computed by dividing earnings on common stock by the total of the weighted average number of shares of common stock outstanding during the applicable period, plus the effect of outstanding performance share awards. Diluted loss per common share for the period ended March 31, 2015, was computed by dividing the loss on common stock by the weighted average number of shares of common stock outstanding during the period. Due to the loss on common stock for the period ended March 31, 2015, the effect of outstanding performance share awards was excluded from the computation of diluted loss per common share as their effect was antidilutive. In 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 calculations was as follows:
		Three Months Ended
		March 31,
		2015	2014
		(In thousands)
	Weighted average common shares outstanding - basic	194,479	189,820
	Effect of dilutive performance share awards	—	612
	Weighted average common shares outstanding - diluted	194,479	190,432
	Shares excluded from the calculation of diluted earnings per share	87	—

Cash_flow_information

Cash flow information	3 Months Ended
	Mar. 31, 2015
Supplemental Cash Flow Information [Abstract]
Cash flow information	Cash flow information
	Cash expenditures for interest and income taxes were as follows:
		Three Months Ended
		March 31,
		2015			2014
		(In thousands)
	Interest, net of amounts capitalized and AFUDC - borrowed of $2.6 million and $2.4 million in 2015 and 2014, respectively	$	23,874		$	20,850
	Income taxes paid (refunded), net	$	(1,339	)	$	9,435
	Noncash investing transactions were as follows:
		March 31,
		2015			2014
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	54,134		$	65,736

New_accounting_standards

New accounting standards	3 Months Ended
	Mar. 31, 2015
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
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 will be effective for the Company on January 1, 2017. In April 2015, the FASB tentatively decided to defer the effective date one year and allow entities to early adopt. If these proposals are adopted, the guidance would 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 evaluating the effects the adoption of the new revenue guidance will have on its results of operations, financial position, cash flows and disclosures, as well as its method of adoption.
	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 will be effective for the Company on January 1, 2016, and is to be applied retrospectively. Early adoption of this guidance is permitted. The guidance will require a reclassification of the debt issuance costs on the Consolidated Balance Sheets, but will not impact the Company's results of operations or cash flows.

Comprehensive_income_loss

Comprehensive income (loss)	3 Months Ended
	Mar. 31, 2015
Comprehensive income (loss) [Abstract]
Comprehensive income (loss)	Comprehensive income (loss)
	The after-tax changes in the components of accumulated other comprehensive loss were as follows:
	Three Months Ended March 31, 2015	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,071	)	$	(38,218	)	$	(829	)	$	15		$	(42,103	)
	Other comprehensive loss before reclassifications	—			—			(112		)	(21		)	(133		)
	Amounts reclassified from accumulated other comprehensive loss	99			375			802			36			1,312
	Net current-period other comprehensive income	99			375			690			15			1,179
	Balance at end of period	$	(2,972	)	$	(37,843	)	$	(139	)	$	30		$	(40,924	)
	Three Months Ended March 31, 2014	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,765	)	$	(33,807	)	$	(667	)	$	34		$	(38,205	)
	Other comprehensive income (loss) before reclassifications	—			—			46			(36		)	10
	Amounts reclassified from accumulated other comprehensive loss	344			275			—			38			657
	Net current-period other comprehensive income	344			275			46			2			667
	Balance at end of period	$	(3,421	)	$	(33,532	)	$	(621	)	$	36		$	(37,538	)
	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended						Location on Consolidated Statements of Income
		31-Mar-15			31-Mar-14
		(In thousands)
	Reclassification adjustment for loss on derivative instruments included in net income (loss):
	Commodity derivative instruments	$	—		$	(388	)	Operating revenues
	Interest rate derivative instruments	(159		)	(160		)	Interest expense
		(159		)	(548		)
		60			204			Income taxes
		(99		)	(344		)
	Amortization of postretirement liability losses included in net periodic benefit cost	(605		)	(443		)	(a)
		230			168			Income taxes
		(375		)	(275		)
	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)	(55		)	(58		)	Other income
		19			20			Income taxes
		(36		)	(38		)
	Total reclassifications	$	(1,312	)	$	(657	)
	(a) Included in net periodic benefit cost (credit). For more information, see Note 16.

Discontinued_operations

Discontinued operations	3 Months Ended
	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Discontinued operations	Discontinued operations
	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 in the first quarter of 2014, which are reflected in discontinued operations in the consolidated financial statements and accompanying notes at March 31, 2014. Discontinued operations are included in the Other category.

Goodwill_and_other_intangible_

Goodwill and other intangible assets	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and other intangible assets	Goodwill and other intangible assets
	The changes in the carrying amount of goodwill were as follows:
	Three Months Ended	Balance			Goodwill			Balance
	31-Mar-15	as of			Acquired			as of
		January 1,			During			March 31, 2015*
		2015*			the Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	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 energy services segment, which occurred in prior periods.
	Three Months Ended	Balance			Goodwill			Balance
	31-Mar-14	as of			Acquired			as of
		January 1,			During the			March 31, 2014*
		2014*			Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Year Ended	Balance			Goodwill			Balance
	31-Dec-14	as of			Acquired			as of
		January 1,			During the			December 31,
		2014*			Year/Other			2014*
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			(835		)	103,441
	Total	$	636,039		$	(835	)	$	635,204
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Other amortizable intangible assets were as follows:
		March 31, 2015			March 31, 2014			December 31, 2014
		(In thousands)
	Customer relationships	$	20,975		$	21,310		$	21,310
	Accumulated amortization	(15,649		)	(14,230		)	(15,556		)
		5,326			7,080			5,754
	Noncompete agreements	4,409			5,580			5,080
	Accumulated amortization	(3,504		)	(4,335		)	(4,098		)
		905			1,245			982
	Other	8,300			10,920			10,921
	Accumulated amortization	(5,365		)	(6,949		)	(7,817		)
		2,935			3,971			3,104
	Total	$	9,166		$	12,296		$	9,840
	Amortization expense for amortizable intangible assets for the three months ended March 31, 2015 and 2014, was $700,000 and $800,000, respectively. Estimated amortization expense for amortizable intangible assets is $2.5 million in 2015, $2.2 million in 2016, $1.9 million in 2017, $1.0 million in 2018, $900,000 in 2019 and $1.4 million thereafter.

Derivative_instruments

Derivative instruments	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative instruments
	The Company's policy allows the use of derivative instruments as part of an overall energy price, foreign currency and interest rate risk management program to efficiently manage and minimize commodity price, foreign currency and interest rate risk. As of March 31, 2015, the Company had no outstanding foreign currency or interest rate hedges.
	The fair value of derivative instruments must be estimated as of the end of each reporting period and is recorded on the Consolidated Balance Sheets as an asset or a liability.
	Fidelity
	At March 31, 2015 and 2014, and December 31, 2014, Fidelity held oil swap and collar agreements with total forward notional volumes of 958,000, 2.7 million and 270,000 Bbl, respectively, and natural gas swap agreements with total forward notional volumes of 2.8 million, 14.7 million and 5.0 million MMBtu, respectively. Fidelity utilizes these derivative instruments to manage a portion of the market risk associated with fluctuations in the price of oil and natural gas on its forecasted sales of oil and natural gas production.
	Effective April 1, 2013, Fidelity elected to de-designate all commodity derivative contracts previously designated as cash flow hedges and elected to discontinue hedge accounting prospectively for all of its commodity derivative instruments. When the criteria for hedge accounting is not met or when hedge accounting is not elected, realized gains and losses and unrealized gains and losses are both recorded in operating revenues on the Consolidated Statements of Income. As a result of discontinuing hedge accounting on commodity derivative instruments, gains and losses on the oil and natural gas derivative instruments remain in accumulated other comprehensive income (loss) as of the de-designation date and are reclassified into earnings in future periods as the underlying hedged transactions affect earnings. At April 1, 2013, accumulated other comprehensive income (loss) included $1.8 million of unrealized gains, representing the mark-to-market value of the Company's commodity derivative instruments that qualified as cash flow hedges as of the balance sheet date, which the Company has subsequently reclassified into earnings.
	Prior to April 1, 2013, changes in the fair value attributable to the effective portion of the hedging instruments, net of tax, were recorded in stockholders' equity as a component of accumulated other comprehensive income (loss). To the extent that the hedges were not effective or did not qualify for hedge accounting, the ineffective portion of the changes in fair market value was recorded directly in earnings. Gains and losses on the oil and natural gas derivative instruments were reclassified from accumulated other comprehensive income (loss) into operating revenues on the Consolidated Statements of Income at the date the oil and natural gas quantities were settled.
	Certain of Fidelity's derivative instruments contain cross-default provisions that state if Fidelity or any of its affiliates fails to make payment with respect to certain indebtedness, in excess of specified amounts, the counterparties could require early settlement or termination of the derivative instruments in liability positions. Fidelity had no derivative instruments that were in a liability position with credit-risk-related contingent features at March 31, 2015 and December 31, 2014. The aggregate fair value of Fidelity's derivative instruments with credit-risk related contingent features that were in a liability position at March 31, 2014, were $12.2 million. The aggregate fair value of assets that would have been needed to settle the instruments immediately if the credit-risk-related contingent features were triggered on March 31, 2014, were $12.2 million.
	Centennial
	Centennial has historically entered into interest rate derivative instruments to manage a portion of its interest rate exposure on the forecasted issuance of long-term debt. As of March 31, 2015 and 2014, and December 31, 2014, Centennial had no outstanding interest rate swap agreements.
	Fidelity and Centennial
	The gains and losses on derivative instruments were as follows:
		Three Months Ended
		March 31,
		2015			2014
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of loss reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	$	—		$	244
	Interest rate derivatives designated as cash flow hedges:
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	99			100
	Commodity derivatives not designated as hedging instruments:
	Amount of loss recognized in operating revenues, before tax	(11,208		)	(6,712		)
	Over the next 12 months net losses of approximately $400,000 (after tax) are estimated to be reclassified from accumulated other comprehensive income (loss) into earnings, as the hedged transactions affect earnings.
	The location and fair value of the gross amount of the Company's derivative instruments on the Consolidated Balance Sheets were as follows:
	Asset	Location on	Fair Value at March 31, 2015			Fair Value at March 31, 2014			Fair Value at December 31, 2014
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	7,127		$	81		$	18,335
		Other assets - noncurrent	—			249			—
	Total asset derivatives		$	7,127		$	330		$	18,335
	Liability	Location on	Fair Value at March 31, 2015			Fair Value at March 31, 2014			Fair Value at December 31, 2014
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	—		$	12,186		$	—
	Total liability derivatives		$	—		$	12,186		$	—
	All of the Company's commodity derivative instruments at March 31, 2015 and 2014, and December 31, 2014, were subject to legally enforceable master netting agreements. However, the Company's policy is to not offset fair value amounts for derivative instruments and, as a result, the Company's derivative assets and liabilities are presented gross on the Consolidated Balance Sheets. The gross derivative assets and liabilities (excluding settlement receivables and payables that may be subject to the same master netting agreements) presented on the Consolidated Balance Sheets and the amount eligible for offset under the master netting agreements is presented in the following table:
	March 31, 2015	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	7,127		$	—		$	7,127
	Total assets	$	7,127		$	—		$	7,127
	March 31, 2014	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	330		$	(330	)	$	—
	Total assets	$	330		$	(330	)	$	—
	Liabilities:
	Commodity derivatives	$	12,186		$	(330	)	$	11,856
	Total liabilities	$	12,186		$	(330	)	$	11,856
	December 31, 2014	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	18,335		$	—		$	18,335
	Total assets	$	18,335		$	—		$	18,335

Fair_value_measurements

Fair value measurements	3 Months Ended
	Mar. 31, 2015
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 $67.8 million, $63.3 million and $65.8 million, at March 31, 2015 and 2014, and December 31, 2014, respectively, are classified as Investments on the Consolidated Balance Sheets. The net unrealized gains on these investments were $2.0 million and $900,000 for the three months ended March 31, 2015 and 2014, 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:
	March 31, 2015	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,792	$	58	$	(18	)	$	7,832
	U.S. Treasury securities	2,337		9		(4		)	2,342
	Total	$	10,129	$	67	$	(22	)	$	10,174
	March 31, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,943	$	63	$	(17	)	$	7,989
	U.S. Treasury securities	2,069		9		—			2,078
	Total	$	10,012	$	72	$	(17	)	$	10,067
	December 31, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	6,594	$	60	$	(18	)	$	6,636
	U.S. Treasury securities	3,574		—		(19		)	3,555
	Total	$	10,168	$	60	$	(37	)	$	10,191
	The fair value of the Company's money market funds approximates cost.
	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 consist of investments in short-term unsecured promissory notes and the value is based on comparable market transactions taking into consideration the credit quality of the issuer. 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.
	The estimated fair value of the Company's Level 2 commodity derivative instruments is based upon futures prices, volatility and time to maturity, among other things. Counterparty statements are utilized to determine the value of the commodity derivative instruments and are reviewed and corroborated using various methodologies and significant observable inputs. The Company's and the counterparties' nonperformance risk is also evaluated.
	Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value. For the three months ended March 31, 2015 and 2014, there were no transfers between Levels 1 and 2.
	The Company's assets and liabilities measured at fair value on a recurring basis were as follows:
		Fair Value Measurements at March 31, 2015, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at March 31, 2015
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	16,945	$	—		$	16,945
	Insurance contract*	—		67,797		—			67,797
	Available-for-sale securities:
	Mortgage-backed securities	—		7,832		—			7,832
	U.S. Treasury securities	—		2,342		—			2,342
	Commodity derivative instruments	—		7,127		—			7,127
	Total assets measured at fair value	$	—	$	102,043	$	—		$	102,043
	* The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies, 32 percent in fixed-income investments, 1 percent in target date investments and 1 percent in cash equivalents.
		Fair Value Measurements at March 31, 2014, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at March 31, 2014
		(Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	20,267	$	—		$	20,267
	Insurance contract*	—		63,269		—			63,269
	Available-for-sale securities:
	Mortgage-backed securities	—		7,989		—			7,989
	U.S. Treasury securities	—		2,078		—			2,078
	Commodity derivative instruments	—		330		—			330
	Total assets measured at fair value	$	—	$	93,933	$	—		$	93,933
	Liabilities:
	Commodity derivative instruments	$	—	$	12,186	$	—		$	12,186
	Total liabilities measured at fair value	$	—	$	12,186	$	—		$	12,186
	* The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 27 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 16 percent in fixed-income investments.
		Fair Value Measurements at December 31, 2014, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2014
		(Level 1)				(Level 3)
		(In thousands)
	Assets:
	Money market funds	$	—	$	18,473	$	—		$	18,473
	Insurance contract*	—		65,831		—			65,831
	Available-for-sale securities:
Mortgage-backed securities	—		6,636		—			6,636
U.S. Treasury securities	—		3,555		—			3,555
Commodity derivative instruments	—		18,335		—			18,335
Total assets measured at fair value	$	—	$	112,830	$	—		$	112,830
* The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
The Company's long-term debt is not measured at fair value on the Consolidated Balance Sheets and the fair value is being provided for disclosure purposes only. The fair value was based on discounted future cash flows using current market interest rates. The estimated fair value of the Company's Level 2 long-term debt was as follows:
	Carrying		Fair
	Amount		Value
	(In thousands)
Long-term debt at March 31, 2015	$	2,189,986	$	2,340,681
Long-term debt at March 31, 2014	$	2,105,832	$	2,186,839
Long-term debt at December 31, 2014	$	2,094,727	$	2,239,445
The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values.

Equity

Equity	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Equity	Equity
	A summary of the changes in equity was as follows:
	Three Months Ended March 31, 2015	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2014	$	3,134,041		$	115,743		$	3,249,784
	Net loss	(305,917		)	(3,528		)	(309,445		)
	Other comprehensive income	1,179			—			1,179
	Dividends declared on preferred stocks	(171		)	—			(171		)
	Dividends declared on common stock	(35,515		)	—			(35,515		)
	Stock-based compensation	(121		)	—			(121		)
	Net tax deficit on stock-based compensation	(1,632		)	—			(1,632		)
	Issuance of common stock	9,864			—			9,864
	Contribution from noncontrolling interest	—			20,500			20,500
	Balance at March 31, 2015	$	2,801,728		$	132,715		$	2,934,443
	Three Months Ended March 31, 2014	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2013	$	2,823,164		$	32,738		$	2,855,902
	Net income (loss)	56,662			(523		)	56,139
	Other comprehensive income	667			—			667
	Dividends declared on preferred stocks	(171		)	—			(171		)
	Dividends declared on common stock	(33,809		)	—			(33,809		)
	Stock-based compensation	1,336			—			1,336
	Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	(5,564		)	—			(5,564		)
	Excess tax benefit on stock-based compensation	4,729			—			4,729
	Issuance of common stock	54,574			—			54,574
	Contribution from noncontrolling interest	—			16,001			16,001
	Balance at March 31, 2014	$	2,901,588		$	48,216		$	2,949,804

Business_segment_data

Business segment data	3 Months Ended
	Mar. 31, 2015
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 energy services segment provides natural gas transportation, underground storage, processing and gathering 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 recently commenced operations of Dakota Prairie Refinery in conjunction with Calumet to refine crude oil. The facility has begun producing diesel fuel and is expected to begin sales of diesel during May 2015. This segment also provides cathodic protection and other energy-related services.
	The exploration and production segment is engaged in oil and natural gas development and production activities in the Rocky Mountain and Mid-Continent/Gulf States regions of the United States. The Company intends to market its exploration and production business in the future. The plan to market this business has been delayed due to low oil prices.
	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 communications 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 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 included Centennial Resources' investment in the Brazilian Transmission Lines.
	The information below follows the same accounting policies as described in Note 1 of the Company's Notes to Consolidated Financial Statements in the 2014 Annual Report. Information on the Company's businesses was as follows:
	Three Months Ended March 31, 2015	External		Inter-			Earnings (Loss)
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	71,776	$	—		$	8,328
	Natural gas distribution	330,573		—			21,450
	Pipeline and energy services	25,095		21,341			4,018
		427,444		21,341			33,796
	Exploration and production	49,710		5,226			(328,904		)
	Construction materials and contracting	205,658		948			(14,635		)
	Construction services	235,403		11,695			4,760
	Other	295		1,772			(255		)
		491,066		19,641			(339,034		)
	Intersegment eliminations	—		(40,982		)	(850		)
	Total	$	918,510	$	—		$	(306,088	)
	Three Months Ended March 31, 2014	External		Inter-			Earnings (Loss)
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	73,647	$	—		$	11,033
	Natural gas distribution	374,233		—			27,263
	Pipeline and energy services	43,661		18,276			4,349
		491,541		18,276			42,645
	Exploration and production	116,669		20,867			20,939
	Construction materials and contracting	164,423		4,017			(23,574		)
	Construction services	269,892		3,738			16,568
	Other	328		1,724			264
		551,312		30,346			14,197
	Intersegment eliminations	—		(48,622		)	(351		)
	Total	$	1,042,853	$	—		$	56,491
	Earnings from electric, natural gas distribution and pipeline and energy services are substantially all from regulated operations. Earnings from exploration and production, construction materials and contracting, construction services and other are all from nonregulated operations.

Employee_benefit_plans

Employee benefit plans	3 Months Ended
	Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Pension and other postretirement benefit plans	Employee benefit plans
	Pension and other postretirement plans
	The Company has noncontributory defined benefit pension plans and other postretirement benefit plans for certain eligible employees. Components of net periodic benefit cost for the Company's pension and other postretirement benefit plans were as follows:
								Other
								Postretirement
		Pension Benefits						Benefits
	Three Months Ended March 31,	2015			2014			2015			2014
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	40		$	33		$	483		$	379
	Interest cost	4,364			4,440			914			858
	Expected return on assets	(5,373		)	(5,125		)	(1,175		)	(1,067		)
	Amortization of prior service cost (credit)	18			18			(342		)	(348		)
	Amortization of net actuarial loss	1,735			1,313			461			318
	Net periodic benefit cost, including amount capitalized	784			679			341			140
	Less amount capitalized	76			95			29			29
	Net periodic benefit cost	$	708		$	584		$	312		$	111
	Nonqualified benefit plans
	In addition to the qualified plan defined pension benefits reflected in the table, the Company 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 to their beneficiaries upon death for a 15-year period. The Company's net periodic benefit cost for this plan for the three months ended March 31, 2015 and 2014, was $1.7 million and $1.7 million, respectively.
	Multiemployer plans
	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. The plan administrator will determine Knife River's withdrawal liability. For the three months ended March 31, 2015, the Company accrued an additional withdrawal liability of approximately $2.4 million (approximately $1.5 million after tax). The total withdrawal liability is currently estimated at $16.4 million (approximately $9.8 million after tax). The assessed withdrawal liability for this plan may be significantly different from the current estimate.

Regulatory_matters_and_revenue

Regulatory matters and revenues subject to refund	3 Months Ended
	Mar. 31, 2015
Regulated Operations [Abstract]
Regulatory matters and revenues subject to refund	Regulatory matters and revenues subject to refund
	On August 11, 2014, Montana-Dakota filed an application with the MTPSC for a natural gas rate increase. Montana-Dakota requested a total increase of approximately $3.0 million annually or approximately 3.6 percent above current rates. The requested increase includes the costs associated with the increased investment in facilities, including ongoing investment in new and replacement distribution facilities, depreciation and taxes associated with the increased investment as well as an increase in Montana-Dakota's operation and maintenance expenses. On February 3, 2015, the MTPSC approved an interim increase of $2.0 million or approximately 2.3 percent, subject to refund, to be effective with service rendered on and after February 6, 2015. On March 18, 2015, Montana-Dakota and the Montana Consumer Counsel filed a settlement agreement that resolved all issues of the application and reflected a natural gas rate increase of $2.5 million annually or approximately 3.0 percent. An amended stipulation reflecting minor changes in rate design was submitted on March 24, 2015. On April 28, 2015, the MTPSC approved the settlement rates, which are expected to be effective with service rendered on and after May 11, 2015.
	On October 3, 2014, Montana-Dakota filed an application with the WYPSC for a natural gas rate increase. Montana-Dakota requested a total increase of approximately $788,000 annually or approximately 4.1 percent above current rates. The requested increase includes the costs associated with the increased investment in facilities, including ongoing investment in new and replacement distribution facilities and the associated operation and maintenance expenses, depreciation and taxes associated with the increase in investment. On April 16, 2015, Montana-Dakota and the Wyoming Office of Consumer Advocate filed a stipulation and agreement that resolved all issues between the parties and reflected a natural gas rate increase of $501,000 annually or approximately 2.6 percent. The WYPSC has scheduled a hearing for this matter on May 19, 2015.
	On November 14, 2014, Montana-Dakota filed an application with the NDPSC for approval to implement the rate adjustment associated with the electric generation resource recovery rider approved by the NDPSC on August 20, 2014. On January 7, 2015, the NDPSC approved the rate adjustments of $5.3 million annually to be effective with service rendered on and after January 9, 2015.
	On December 22, 2014, Montana-Dakota filed an application for advance determination of prudence and a certificate of public convenience and necessity with the NDPSC for the Thunder Spirit Wind project. This project will provide energy, capacity and renewable energy credits to Montana-Dakota's electric customers in North Dakota, Montana and South Dakota. The NDPSC has scheduled a hearing for this matter on May 14, 2015.
	On February 6, 2015, Montana-Dakota filed an application with the NDPSC for a natural gas rate increase. Montana-Dakota requested a total increase of approximately $4.3 million annually or approximately 3.4 percent above current rates. The requested increase includes the costs associated with the increased investment in facilities, including ongoing investment in new and replacement distribution facilities, depreciation and taxes associated with the increased investment as well as an increase in Montana-Dakota's operation and maintenance expenses. Montana-Dakota requested an interim increase of $4.3 million or 3.4 percent, subject to refund, which was approved by the NDPSC on March 11, 2015, effective with service rendered on or after April 7, 2015. A technical hearing has been scheduled for July 20-21, 2015. This matter is pending before the NDPSC.
	On March 31, 2015, Cascade filed an application with the OPUC for a natural gas rate increase. Cascade requested a total increase of approximately $3.6 million annually or approximately 5.1 percent above current rates. The requested increase includes the costs associated with the increased investment in facilities, including ongoing investment in new and replacement distribution facilities and the associated operation and maintenance expenses, depreciation and taxes associated with the increase in investment, as well as environmental remediation expenses.
	On April 10, 2015, Montana-Dakota submitted a request to the NDPSC to update the environmental cost recovery rider to reflect actual costs incurred through February 2015 and projected costs through June 2016 related to the recovery of Montana-Dakota's share of the costs resulting from the environmental retrofit required to be installed at the Big Stone Station. The request also includes costs associated with the environmental upgrade required at the Lewis & Clark Station to comply with the EPA's MATS rule. The filing also requests a revision to the environmental cost recovery rider that will allow future recovery of ongoing reagent costs required to meet environmental standards as a monthly adjustment. A total of $8.1 million is requested to be recovered under the adjustment. If approved, the rates would be effective July 1, 2015, through June 30, 2016.

Contingencies

Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies
	The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries. 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 $25.8 million, $31.4 million and $27.6 million for contingencies, including litigation, production taxes, royalty claims and environmental matters at March 31, 2015 and 2014, and December 31, 2014, respectively, which include amounts that may have been accrued for matters discussed in Litigation and Environmental matters within this note.
	Litigation
	Natural Gas Gathering Operations In January 2010, SourceGas filed an application with the Colorado State District Court to compel WBI Energy Midstream to arbitrate a dispute regarding operating pressures under a natural gas gathering contract on one of WBI Energy Midstream's pipeline gathering systems in Montana. WBI Energy Midstream resisted the application and sought a declaratory order interpreting the gathering contract. In May 2010, the Colorado State District Court granted the application and ordered WBI Energy Midstream into arbitration. In October 2010, the arbitration panel issued an award in favor of SourceGas for approximately $26.6 million. The Colorado Court of Appeals issued a decision on May 24, 2012, reversing the Colorado State District Court order compelling arbitration, vacating the final award and remanding the case to the Colorado State District Court to determine SourceGas's claims and WBI Energy Midstream's counterclaims. On remand of the matter to the Colorado State District Court, SourceGas may assert claims similar to those asserted in the arbitration proceeding.
	In a related matter, Omimex filed a complaint against WBI Energy Midstream in Montana Seventeenth Judicial District Court in July 2010 alleging WBI Energy Midstream breached a separate gathering contract with Omimex as a result of the increased operating pressures demanded by SourceGas on the same natural gas gathering system. 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.
	Exploration and Production During the ordinary course of its business, Fidelity is subject to audit for various production related taxes by certain state and federal tax authorities for varying periods as well as claims for royalty obligations under lease agreements for oil and gas production. Disputes may exist regarding facts and questions of law relating to the tax and royalty obligations.
	On May 15, 2013, Austin Holdings, LLC filed an action against Fidelity in Wyoming State District Court alleging Fidelity violated the Wyoming Royalty Payment Act and implied lease covenants by deducting production costs from and by failing to properly report and pay royalties for coalbed methane gas production in Wyoming. The plaintiff, in addition to declaratory and injunctive relief, seeks class certification for similarly situated persons and an unspecified amount of monetary damages on behalf of the class for unpaid royalties, interest, reporting violations and attorney fees. The Company reached a settlement of the matter, subject to final court approval, for an amount that is not material.
	Construction Materials Until the fall of 2011 when it discontinued active mining operations at the pit, JTL operated the Target Range Gravel Pit in Missoula County, Montana under a 1975 reclamation contract pursuant to the Montana Opencut Mining Act. In September 2009, the Montana DEQ sent a letter asserting JTL was in violation of the Montana Opencut Mining Act by conducting mining operations outside a permitted area. JTL filed a complaint in Montana First Judicial District Court in June 2010, seeking a declaratory order that the reclamation contract is a valid permit under the Montana Opencut Mining Act. The Montana DEQ filed an answer and counterclaim to the complaint in August 2011, alleging JTL was in violation of the Montana Opencut Mining Act and requesting imposition of penalties of not more than $3.7 million plus not more than $5,000 per day from the date of the counterclaim. The Company believes the operation of the Target Range Gravel Pit was conducted under a valid permit; however, the imposition of civil penalties is reasonably possible. The Company filed an application for amendment of its opencut mining permit and intends to resolve this matter through settlement or continuation of the Montana First Judicial District Court litigation.
	Former Employee Litigation On August 6, 2012, a former employee and his spouse filed actions against Connolly-Pacific and others in California Superior Court alleging the former employee contracted acute myelogenous leukemia from exposure to substances while employed as a seaman by the defendants. The plaintiffs request compensatory damages of approximately $23.8 million plus punitive damages, costs and interest. Connolly-Pacific is contesting the claims and believes it has meritorious defenses to them. Connolly-Pacific will seek insurance coverage for defense costs and any liability incurred in the litigation.
	Construction Services Bombard Mechanical is a third-party defendant in litigation pending in Nevada State District Court in which the plaintiff claims damages attributable to defects in the construction of a 48 story residential tower built in 2008 for which Bombard Mechanical performed plumbing and mechanical work as a subcontractor. On March 12, 2015, the plaintiff submitted cost of repair estimates totaling approximately $26 million for alleged defects related to plumbing and mechanical system defects. Bombard Mechanical is being defended in the action under a policy of insurance subject to a reservation of rights.
	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 $70 million. It is not possible to estimate the cost of a corrective action plan until the remedial investigation and feasibility study have been completed, the EPA has decided on a strategy and a ROD has been published. Corrective action will be taken after the development of a proposed plan and ROD on the harbor site is issued. 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 administrative action.
	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 $1.7 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 and December 1, 2014.
	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. 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 Department of Ecology 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 $12.4 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 until the next general rate case. 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 Department of Ecology 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.
	Guarantees
	In connection with the sale of the Brazilian Transmission Lines, 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.
	WBI Holdings has guaranteed certain of Fidelity's oil and natural gas swap agreement obligations. There is no fixed maximum amount guaranteed in relation to the oil and natural gas swap agreements as the amount of the obligation is dependent upon oil and natural gas commodity prices. The amount of derivative activity entered into by the subsidiary is limited by corporate policy. The guarantees of the oil and natural gas swap agreements at March 31, 2015, expire in 2015; however, Fidelity may continue to enter into additional derivative instruments and, as a result, WBI Holdings from time to time may issue additional guarantees on these derivative instruments. There were no amounts outstanding by Fidelity at March 31, 2015. In the event Fidelity defaults under its obligations, WBI Holdings would be required to make payments under its guarantees.
	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, natural gas transportation and sales agreements, gathering contracts and certain other guarantees. At March 31, 2015, the fixed maximum amounts guaranteed under these agreements aggregated $76.8 million. The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate $42.6 million in 2015; $14.5 million in 2016; $1.2 million in 2017; $500,000 in 2018; $500,000 in 2019; $13.5 million, which is subject to expiration on a specified number of days after the receipt of written notice; and $4.0 million, which has no scheduled maturity date. The amount outstanding by subsidiaries of the Company under the above guarantees was $200,000 and was reflected on the Consolidated Balance Sheet at March 31, 2015. In the event of default under these guarantee obligations, the subsidiary issuing the guarantee for that particular obligation would be required to make payments under its guarantee.
	Certain subsidiaries have outstanding letters of credit to third parties related to insurance policies and other agreements, some of which are guaranteed by other subsidiaries of the Company. At March 31, 2015, the fixed maximum amounts guaranteed under these letters of credit, aggregated $42.5 million. In 2015 and 2016, $14.3 million and $28.2 million, respectively, of letters of credit are scheduled to expire. The amount outstanding by subsidiaries of the Company under the above letters of credit was $300,000 and was reflected on the Consolidated Balance Sheet at March 31, 2015. 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.
	Centennial and WBI Holdings have guaranteed certain debt obligations of Dakota Prairie Refining. For more information, see Variable interest entities in this note.
	WBI Holdings has an outstanding guarantee to WBI Energy Transmission. This guarantee is related to a natural gas transportation and storage agreement that guarantees the performance of Prairielands. At March 31, 2015, the fixed maximum amount guaranteed under this agreement was $4.0 million and is scheduled to expire in 2016. In the event of Prairielands' default in its payment obligations, WBI Holdings would be required to make payment under its guarantee. The amount outstanding by Prairielands under the above guarantee was $1.1 million. The amount outstanding under this guarantee was not reflected on the Consolidated Balance Sheet at March 31, 2015, because this intercompany transaction was eliminated in consolidation.
	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 March 31, 2015.
	In the normal course of business, Centennial has surety bonds related to construction contracts and reclamation obligations of its subsidiaries. In the event a subsidiary of Centennial does not fulfill a bonded obligation, Centennial would be responsible to the surety bond company for completion of the bonded contract or obligation. A large portion of the surety bonds is expected to expire within the next 12 months; however, Centennial will likely continue to enter into surety bonds for its subsidiaries in the future. At March 31, 2015, approximately $536.7 million of surety bonds were outstanding, which were not reflected on the Consolidated Balance Sheet.
	Variable interest entities
	The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary.
	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 have 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 are $150 million and $75 million, respectively. Capital commitments in excess of $300 million are being shared equally between WBI Energy and Calumet. The total project cost is currently estimated at approximately $425 million to $435 million. Dakota Prairie Refining entered into a term loan for project debt financing of $75 million on April 22, 2013. The operating agreement provides for allocation of profits and losses consistent with ownership interests; however, deductions attributable to project financing debt will be allocated to Calumet. Calumet's future cash distributions from Dakota Prairie Refining will be decreased by the principal and interest to be paid on the project debt, while the cash distributions to WBI Energy will not be decreased. Pursuant to the operating agreement, Centennial agreed to guarantee Dakota Prairie Refining's obligation under the term loan.
	On December 1, 2014, Dakota Prairie Refining entered into a $50 million revolving credit agreement with an expiration date of December 1, 2015. Pursuant to the revolving credit agreement, WBI Holdings has guaranteed 50 percent of the credit agreement and Calumet has issued a letter of credit supporting 50 percent of the credit agreement. The credit agreement is used to meet the operational needs of the facility.
	Dakota Prairie Refining has been determined to be a VIE, and the Company has determined that it is the primary beneficiary as it has an obligation to absorb losses that could be potentially significant to the VIE through WBI Energy's equity investment and Centennial's guarantee of the third-party term loan. Accordingly, the Company consolidates Dakota Prairie Refining in its financial statements and records a noncontrolling interest for Calumet's ownership interest.
	Dakota Prairie Refinery has commenced operations. The facility has begun producing diesel fuel and is expected to begin sales of diesel during May 2015. The assets of Dakota Prairie Refining shall be used solely for the benefit of Dakota Prairie Refining. The total assets and liabilities of Dakota Prairie Refining reflected on the Company's Consolidated Balance Sheets were as follows:
		March 31, 2015		March 31, 2014		December 31, 2014
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	10,784	$	22,996	$	21,376
	Accounts receivable	2,335		—		2,759
	Inventories	7,902		—		5,311
	Other current assets	2,926		1,135		4,019
	Total current assets	23,947		24,131		33,465
	Net property, plant and equipment	425,944		207,260		398,984
	Deferred charges and other assets:
	Other	4,562		—		3,400
	Total deferred charges and other assets	4,562		—		3,400
	Total assets	$	454,453	$	231,391	$	435,849
	LIABILITIES
	Current liabilities:
	Short-term borrowings	$	16,100	$	—	$	—
	Long-term debt due within one year	3,000		3,000		3,000
	Accounts payable	23,654		16,103		55,089
	Taxes payable	569		113		648
	Accrued compensation	683		164		727
	Other accrued liabilities	1,016		580		899
	Total current liabilities	45,022		19,960		60,363
	Long-term debt	69,000		72,000		69,000
	Total liabilities	$	114,022	$	91,960	$	129,363
	Fuel Contract On October 10, 2012, the Coyote Station entered into a new coal supply agreement with Coyote Creek that will replace a coal supply agreement expiring in May 2016. The new agreement provides for the purchase of coal necessary to supply the coal requirements of the Coyote Station for the period May 2016 through December 2040.
	The new coal supply agreement creates a variable interest in Coyote Creek due to the transfer of all operating and economic risk to the Coyote Station owners, as the agreement is structured so the price of the coal will cover all costs of operations as well as future reclamation costs. The Coyote Station owners are also providing a guarantee of the value of the assets of Coyote Creek as they would be required to buy the assets at book value should they terminate the contract prior to the end of the contract term and are providing a guarantee of the value of the equity of Coyote Creek in that they are required to buy the entity at the end of the contract term at equity value. Although the Company has determined that Coyote Creek is a VIE, the Company has concluded that it is not the primary beneficiary of Coyote Creek because the authority to direct the activities of the entity is shared by the four unrelated owners of the Coyote Station, with no primary beneficiary existing. As a result, Coyote Creek is not required to be consolidated in the Company's financial statements.
	At March 31, 2015, Coyote Creek was not yet operational. The assets and liabilities of Coyote Creek and exposure to loss as a result of the Company's involvement with the VIE, based on the Company's ownership percentage, at March 31, 2015, was $20.8 million.

Inventories_and_natural_gas_in1

Inventories and natural gas in storage (Tables)	3 Months Ended
	Mar. 31, 2015
Inventory Disclosure [Abstract]
Inventories	Inventories consisted of:
		March 31, 2015		March 31, 2014		December 31, 2014
		(In thousands)
	Aggregates held for resale	$	112,029	$	104,106	$	108,161
	Asphalt oil	89,578		66,292		42,135
	Materials and supplies	65,599		68,809		65,683
	Merchandise for resale	15,688		22,463		24,420
	Natural gas in storage (current)	9,303		6,129		19,302
	Other	44,401		33,533		41,110
	Total	$	336,598	$	301,332	$	300,811

Oil_and_natural_gas_SEC_define

Oil and natural gas SEC defined prices (Tables)	3 Months Ended
	Mar. 31, 2015
Property, Plant and Equipment [Abstract]
SEC defined pricing	SEC Defined Prices for each quarter for the last 12 months were as follows:
	SEC Defined Prices for the 12 months ended	NYMEX		Henry Hub		Ventura
		Oil Price		Gas Price		Gas Price
		(per Bbl)		(per MMBtu)		(per MMBtu)
	March 31, 2015	$	82.72	$	3.87	$	3.96
	December 31, 2014	94.99		4.34		7.71
	September 30, 2014	99.08		4.24		7.6
	June 30, 2014	100.27		4.1		7.47
	For purposes of comparison, first-of-the-month prices were as follows:
		NYMEX		Henry Hub		Ventura
		Oil Price		Gas Price		Gas Price
		(per Bbl)		(per MMBtu)		(per MMBtu)
	Apr-15	$	50.09	$	2.63	$	2.45
	May-15	59.15		2.57		2.51

Earnings_per_common_share_Tabl

Earnings per common share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Weighted average common shares outstanding	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings (loss) per share calculations was as follows:
		Three Months Ended
		March 31,
		2015	2014
		(In thousands)
	Weighted average common shares outstanding - basic	194,479	189,820
	Effect of dilutive performance share awards	—	612
	Weighted average common shares outstanding - diluted	194,479	190,432
	Shares excluded from the calculation of diluted earnings per share	87	—

Cash_flow_information_Tables

Cash flow information (Tables)	3 Months Ended
	Mar. 31, 2015
Supplemental Cash Flow Information [Abstract]
Supplemental cash flow information	Cash expenditures for interest and income taxes were as follows:
		Three Months Ended
		March 31,
		2015			2014
		(In thousands)
	Interest, net of amounts capitalized and AFUDC - borrowed of $2.6 million and $2.4 million in 2015 and 2014, respectively	$	23,874		$	20,850
	Income taxes paid (refunded), net	$	(1,339	)	$	9,435
	Noncash investing transactions were as follows:
		March 31,
		2015			2014
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	54,134		$	65,736

Comprehensive_income_loss_Tabl

Comprehensive income (loss) (Tables)	3 Months Ended
	Mar. 31, 2015
Comprehensive income (loss) [Abstract]
Comprehensive income (loss)	The after-tax changes in the components of accumulated other comprehensive loss were as follows:
	Three Months Ended March 31, 2015	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,071	)	$	(38,218	)	$	(829	)	$	15		$	(42,103	)
	Other comprehensive loss before reclassifications	—			—			(112		)	(21		)	(133		)
	Amounts reclassified from accumulated other comprehensive loss	99			375			802			36			1,312
	Net current-period other comprehensive income	99			375			690			15			1,179
	Balance at end of period	$	(2,972	)	$	(37,843	)	$	(139	)	$	30		$	(40,924	)
	Three Months Ended March 31, 2014	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,765	)	$	(33,807	)	$	(667	)	$	34		$	(38,205	)
	Other comprehensive income (loss) before reclassifications	—			—			46			(36		)	10
	Amounts reclassified from accumulated other comprehensive loss	344			275			—			38			657
	Net current-period other comprehensive income	344			275			46			2			667
	Balance at end of period	$	(3,421	)	$	(33,532	)	$	(621	)	$	36		$	(37,538	)
Reclassification out of accumulated other comprehensive income	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended						Location on Consolidated Statements of Income
		31-Mar-15			31-Mar-14
		(In thousands)
	Reclassification adjustment for loss on derivative instruments included in net income (loss):
	Commodity derivative instruments	$	—		$	(388	)	Operating revenues
	Interest rate derivative instruments	(159		)	(160		)	Interest expense
		(159		)	(548		)
		60			204			Income taxes
		(99		)	(344		)
	Amortization of postretirement liability losses included in net periodic benefit cost	(605		)	(443		)	(a)
		230			168			Income taxes
		(375		)	(275		)
	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)	(55		)	(58		)	Other income
		19			20			Income taxes
		(36		)	(38		)
	Total reclassifications	$	(1,312	)	$	(657	)
	(a) Included in net periodic benefit cost (credit). For more information, see Note 16.

Goodwill_and_other_intangible_1

Goodwill and other intangible assets (Tables)	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying amount of goodwill	The changes in the carrying amount of goodwill were as follows:
	Three Months Ended	Balance			Goodwill			Balance
	31-Mar-15	as of			Acquired			as of
		January 1,			During			March 31, 2015*
		2015*			the Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	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 energy services segment, which occurred in prior periods.
	Three Months Ended	Balance			Goodwill			Balance
	31-Mar-14	as of			Acquired			as of
		January 1,			During the			March 31, 2014*
		2014*			Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Year Ended	Balance			Goodwill			Balance
	31-Dec-14	as of			Acquired			as of
		January 1,			During the			December 31,
		2014*			Year/Other			2014*
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			(835		)	103,441
	Total	$	636,039		$	(835	)	$	635,204
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
Other amortizable intangible assets	Other amortizable intangible assets were as follows:
		March 31, 2015			March 31, 2014			December 31, 2014
		(In thousands)
	Customer relationships	$	20,975		$	21,310		$	21,310
	Accumulated amortization	(15,649		)	(14,230		)	(15,556		)
		5,326			7,080			5,754
	Noncompete agreements	4,409			5,580			5,080
	Accumulated amortization	(3,504		)	(4,335		)	(4,098		)
		905			1,245			982
	Other	8,300			10,920			10,921
	Accumulated amortization	(5,365		)	(6,949		)	(7,817		)
		2,935			3,971			3,104
	Total	$	9,166		$	12,296		$	9,840

Derivative_instruments_Tables

Derivative instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of derivative instruments, gain (losses)	The gains and losses on derivative instruments were as follows:
		Three Months Ended
		March 31,
		2015			2014
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of loss reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	$	—		$	244
	Interest rate derivatives designated as cash flow hedges:
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	99			100
	Commodity derivatives not designated as hedging instruments:
	Amount of loss recognized in operating revenues, before tax	(11,208		)	(6,712		)
Derivative instruments	The location and fair value of the gross amount of the Company's derivative instruments on the Consolidated Balance Sheets were as follows:
	Asset	Location on	Fair Value at March 31, 2015			Fair Value at March 31, 2014			Fair Value at December 31, 2014
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	7,127		$	81		$	18,335
		Other assets - noncurrent	—			249			—
	Total asset derivatives		$	7,127		$	330		$	18,335
	Liability	Location on	Fair Value at March 31, 2015			Fair Value at March 31, 2014			Fair Value at December 31, 2014
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	—		$	12,186		$	—
	Total liability derivatives		$	—		$	12,186		$	—
Offsetting assets and liabilities master netting	The gross derivative assets and liabilities (excluding settlement receivables and payables that may be subject to the same master netting agreements) presented on the Consolidated Balance Sheets and the amount eligible for offset under the master netting agreements is presented in the following table:
	March 31, 2015	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	7,127		$	—		$	7,127
	Total assets	$	7,127		$	—		$	7,127
	March 31, 2014	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	330		$	(330	)	$	—
	Total assets	$	330		$	(330	)	$	—
	Liabilities:
	Commodity derivatives	$	12,186		$	(330	)	$	11,856
	Total liabilities	$	12,186		$	(330	)	$	11,856
	December 31, 2014	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	18,335		$	—		$	18,335
	Total assets	$	18,335		$	—		$	18,335

Fair_value_measurements_Tables

Fair value measurements (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Available-for-sale securities	Details of available-for-sale securities were as follows:
	March 31, 2015	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,792	$	58	$	(18	)	$	7,832
	U.S. Treasury securities	2,337		9		(4		)	2,342
	Total	$	10,129	$	67	$	(22	)	$	10,174
	March 31, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,943	$	63	$	(17	)	$	7,989
	U.S. Treasury securities	2,069		9		—			2,078
	Total	$	10,012	$	72	$	(17	)	$	10,067
	December 31, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	6,594	$	60	$	(18	)	$	6,636
	U.S. Treasury securities	3,574		—		(19		)	3,555
	Total	$	10,168	$	60	$	(37	)	$	10,191
Assets and liabilities measured at fair value on a recurring basis	The Company's assets and liabilities measured at fair value on a recurring basis were as follows:
		Fair Value Measurements at March 31, 2015, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at March 31, 2015
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	16,945	$	—		$	16,945
	Insurance contract*	—		67,797		—			67,797
	Available-for-sale securities:
	Mortgage-backed securities	—		7,832		—			7,832
	U.S. Treasury securities	—		2,342		—			2,342
	Commodity derivative instruments	—		7,127		—			7,127
	Total assets measured at fair value	$	—	$	102,043	$	—		$	102,043
	* The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies, 32 percent in fixed-income investments, 1 percent in target date investments and 1 percent in cash equivalents.
		Fair Value Measurements at March 31, 2014, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at March 31, 2014
		(Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	20,267	$	—		$	20,267
	Insurance contract*	—		63,269		—			63,269
	Available-for-sale securities:
	Mortgage-backed securities	—		7,989		—			7,989
	U.S. Treasury securities	—		2,078		—			2,078
	Commodity derivative instruments	—		330		—			330
	Total assets measured at fair value	$	—	$	93,933	$	—		$	93,933
	Liabilities:
	Commodity derivative instruments	$	—	$	12,186	$	—		$	12,186
	Total liabilities measured at fair value	$	—	$	12,186	$	—		$	12,186
	* The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 27 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 16 percent in fixed-income investments.
		Fair Value Measurements at December 31, 2014, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2014
		(Level 1)				(Level 3)
		(In thousands)
	Assets:
	Money market funds	$	—	$	18,473	$	—		$	18,473
	Insurance contract*	—		65,831		—			65,831
	Available-for-sale securities:
	Mortgage-backed securities	—		6,636		—			6,636
	U.S. Treasury securities	—		3,555		—			3,555
	Commodity derivative instruments	—		18,335		—			18,335
	Total assets measured at fair value	$	—	$	112,830	$	—		$	112,830
	* The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
Fair value of long term debt outstanding	The estimated fair value of the Company's Level 2 long-term debt was as follows:
		Carrying		Fair
		Amount		Value
		(In thousands)
	Long-term debt at March 31, 2015	$	2,189,986	$	2,340,681
	Long-term debt at March 31, 2014	$	2,105,832	$	2,186,839
	Long-term debt at December 31, 2014	$	2,094,727	$	2,239,445

Equity_Tables

Equity (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Summary of changes in equity	A summary of the changes in equity was as follows:
	Three Months Ended March 31, 2015	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2014	$	3,134,041		$	115,743		$	3,249,784
	Net loss	(305,917		)	(3,528		)	(309,445		)
	Other comprehensive income	1,179			—			1,179
	Dividends declared on preferred stocks	(171		)	—			(171		)
	Dividends declared on common stock	(35,515		)	—			(35,515		)
	Stock-based compensation	(121		)	—			(121		)
	Net tax deficit on stock-based compensation	(1,632		)	—			(1,632		)
	Issuance of common stock	9,864			—			9,864
	Contribution from noncontrolling interest	—			20,500			20,500
	Balance at March 31, 2015	$	2,801,728		$	132,715		$	2,934,443
	Three Months Ended March 31, 2014	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2013	$	2,823,164		$	32,738		$	2,855,902
	Net income (loss)	56,662			(523		)	56,139
	Other comprehensive income	667			—			667
	Dividends declared on preferred stocks	(171		)	—			(171		)
	Dividends declared on common stock	(33,809		)	—			(33,809		)
	Stock-based compensation	1,336			—			1,336
	Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	(5,564		)	—			(5,564		)
	Excess tax benefit on stock-based compensation	4,729			—			4,729
	Issuance of common stock	54,574			—			54,574
	Contribution from noncontrolling interest	—			16,001			16,001
	Balance at March 31, 2014	$	2,901,588		$	48,216		$	2,949,804

Business_segment_data_Tables

Business segment data (Tables)	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Information on the Company's businesses	Information on the Company's businesses was as follows:
	Three Months Ended March 31, 2015	External		Inter-			Earnings (Loss)
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	71,776	$	—		$	8,328
	Natural gas distribution	330,573		—			21,450
	Pipeline and energy services	25,095		21,341			4,018
		427,444		21,341			33,796
	Exploration and production	49,710		5,226			(328,904		)
	Construction materials and contracting	205,658		948			(14,635		)
	Construction services	235,403		11,695			4,760
	Other	295		1,772			(255		)
		491,066		19,641			(339,034		)
	Intersegment eliminations	—		(40,982		)	(850		)
	Total	$	918,510	$	—		$	(306,088	)
	Three Months Ended March 31, 2014	External		Inter-			Earnings (Loss)
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	73,647	$	—		$	11,033
	Natural gas distribution	374,233		—			27,263
	Pipeline and energy services	43,661		18,276			4,349
		491,541		18,276			42,645
	Exploration and production	116,669		20,867			20,939
	Construction materials and contracting	164,423		4,017			(23,574		)
	Construction services	269,892		3,738			16,568
	Other	328		1,724			264
		551,312		30,346			14,197
	Intersegment eliminations	—		(48,622		)	(351		)
	Total	$	1,042,853	$	—		$	56,491

Employee_benefit_plans_Tables

Employee benefit plans (Tables)	3 Months Ended
	Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Schedule of net benefit costs	Components of net periodic benefit cost for the Company's pension and other postretirement benefit plans were as follows:
								Other
								Postretirement
		Pension Benefits						Benefits
	Three Months Ended March 31,	2015			2014			2015			2014
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	40		$	33		$	483		$	379
	Interest cost	4,364			4,440			914			858
	Expected return on assets	(5,373		)	(5,125		)	(1,175		)	(1,067		)
	Amortization of prior service cost (credit)	18			18			(342		)	(348		)
	Amortization of net actuarial loss	1,735			1,313			461			318
	Net periodic benefit cost, including amount capitalized	784			679			341			140
	Less amount capitalized	76			95			29			29
	Net periodic benefit cost	$	708		$	584		$	312		$	111

Contingencies_Commitment_and_C

Contingencies Commitment and Contingencies Disclosure (Tables)	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Limited liability project reflected on consolidated balance sheet	The total assets and liabilities of Dakota Prairie Refining reflected on the Company's Consolidated Balance Sheets were as follows:
		March 31, 2015		March 31, 2014		December 31, 2014
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	10,784	$	22,996	$	21,376
	Accounts receivable	2,335		—		2,759
	Inventories	7,902		—		5,311
	Other current assets	2,926		1,135		4,019
	Total current assets	23,947		24,131		33,465
	Net property, plant and equipment	425,944		207,260		398,984
	Deferred charges and other assets:
	Other	4,562		—		3,400
	Total deferred charges and other assets	4,562		—		3,400
	Total assets	$	454,453	$	231,391	$	435,849
	LIABILITIES
	Current liabilities:
	Short-term borrowings	$	16,100	$	—	$	—
	Long-term debt due within one year	3,000		3,000		3,000
	Accounts payable	23,654		16,103		55,089
	Taxes payable	569		113		648
	Accrued compensation	683		164		727
	Other accrued liabilities	1,016		580		899
	Total current liabilities	45,022		19,960		60,363
	Long-term debt	69,000		72,000		69,000
	Total liabilities	$	114,022	$	91,960	$	129,363

Accounts_receivable_and_allowa1

Accounts receivable and allowance for doubtful accounts (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Millions, unless otherwise specified
Receivables [Abstract]
Receivables past due 90 days or more	$29.30	$30.90	$22.60
Allowance for doubtful accounts receivable	$9.40	$9.50	$10.90

Inventories_and_natural_gas_in2

Inventories and natural gas in storage (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Inventory Disclosure [Abstract]
Aggregates held for resale	$112,029,000	$108,161,000	$104,106,000
Asphalt oil	89,578,000	42,135,000	66,292,000
Materials and supplies	65,599,000	65,683,000	68,809,000
Merchandise for resale	15,688,000	24,420,000	22,463,000
Natural gas in storage (current)	9,303,000	19,302,000	6,129,000
Other	44,401,000	41,110,000	33,533,000
Total	336,598,000	300,811,000	301,332,000
Natural gas in storage noncurrent	$49,300,000	$49,300,000	$47,400,000

Oil_and_natural_gas_impairment

Oil and natural gas impairment (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Property, Plant and Equipment [Abstract]
Write-down of oil and natural gas properties	$500,400,000	$0
Impairment of oil and gas properties, after tax	$315,300,000

SEC_defined_prices_Details_2

SEC defined prices (Details 2)	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	1-May-15	Apr. 01, 2015
NYMEX Oil Price [Member]
SEC Defined Pricing [Line Items]
SEC Defined Prices	82.72	94.99	99.08	100.27
Henry Hub Gas Price [Member]
SEC Defined Pricing [Line Items]
SEC Defined Prices	3.87	4.34	4.24	4.1
Ventura Gas Price [Member]
SEC Defined Pricing [Line Items]
SEC Defined Prices	3.96	7.71	7.6	7.47
Subsequent event [Member] | NYMEX Oil Price [Member]
SEC Defined Pricing [Line Items]
SEC Defined Prices					59.15	50.09
Subsequent event [Member] | Henry Hub Gas Price [Member]
SEC Defined Pricing [Line Items]
SEC Defined Prices					2.57	2.63
Subsequent event [Member] | Ventura Gas Price [Member]
SEC Defined Pricing [Line Items]
SEC Defined Prices					2.51	2.45

Earnings_per_common_share_Deta

Earnings per common share (Details)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Weighted average common shares outstanding - basic	194,479,000	189,820,000
Effect of dilutive performance share awards	0	612,000
Weighted average common shares outstanding - diluted	194,479,000	190,432,000
Shares excluded from the calculation of diluted earnings per share	87,000	0

Cash_flow_information_Details_

Cash flow information (Details 1) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Supplemental Cash Flow Information [Abstract]
Interest, net of amounts capitalized and AFUDC - borrowed of $2.6 million and $2.4 million in 2015 and 2014, respectively	$23,874	$20,850
Income taxes paid (refunded), net	-1,339	9,435
Property, plant and equipment additions in accounts payable	$54,134	$65,736

Cash_flow_information_Cash_flo

Cash flow information Cash flow information (Details 2) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Supplemental Cash Flow Information [Abstract]
Capitalized interest and AFUDC borrowed	$2.60	$2.40

Comprehensive_income_loss_Deta

Comprehensive income (loss) (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	($42,103)	($38,205)
Other comprehensive income (loss) before reclassifications	-133	10
Amounts reclassified from accumulated other comprehensive loss	1,312	657
Net current-period other comprehensive income (loss)	1,179	667
Balance	-40,924	-37,538
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	-3,071	-3,765
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	99	344
Net current-period other comprehensive income (loss)	99	344
Balance	-2,972	-3,421
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	-38,218	-33,807
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	375	275
Net current-period other comprehensive income (loss)	375	275
Balance	-37,843	-33,532
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	-829	-667
Other comprehensive income (loss) before reclassifications	-112	46
Amounts reclassified from accumulated other comprehensive loss	802	0
Net current-period other comprehensive income (loss)	690	46
Balance	-139	-621
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	15	34
Other comprehensive income (loss) before reclassifications	-21	-36
Amounts reclassified from accumulated other comprehensive loss	36	38
Net current-period other comprehensive income (loss)	15	2
Balance	$30	$36

Reclassification_out_of_accumu

Reclassification out of accumulated other comprehensive income (loss) (Details 2) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Operating revenues	$918,510	$1,042,853
Interest expense	-23,149	-20,971
Income before income taxes	-495,172	84,116
Other income	2,324	2,183
Income taxes	185,727	-27,932
Net income (loss)	-309,445	56,139
Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income before income taxes	-159	-548
Income taxes	60	204
Net income (loss)	-99	-344
Amortization of postretirement liability losses included in net periodic benefit cost
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net periodic benefit cost	-605	-443
Income taxes	230	168
Net income (loss)	-375	-275
Reclassification adjustment for loss on foreign currency translation adjustment included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	-1,292	0
Income taxes	490	0
Net income (loss)	-802	0
Reclassification adjustment for loss on available-for-sale investments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	-55	-58
Income taxes	19	20
Net income (loss)	-36	-38
Total reclassifications
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net income (loss)	-1,312	-657
Commodity derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Operating revenues	0	-388
Interest rate derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	($159)	($160)

Goodwill_rollforward_Details

Goodwill rollforward (Details) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Goodwill [Roll Forward]
Balance as of beginning of period	$635,204,000	[1]	$636,039,000	[1]	$636,039,000	[1]
Goodwill acquired during the year	0		0
Goodwill acquired during the year/other					-835,000
Balance as of end of period	635,204,000	[1]	636,039,000	[1]	635,204,000	[1]
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	345,736,000		345,736,000		345,736,000
Goodwill acquired during the year	0		0
Goodwill acquired during the year/other					0
Balance as of end of period	345,736,000		345,736,000		345,736,000
Pipeline and energy services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
Goodwill acquired during the year	0		0
Goodwill acquired during the year/other					0
Balance as of end of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
Goodwill, impaired, accumulated impairment loss	12,300,000		12,300,000		12,300,000
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	176,290,000		176,290,000		176,290,000
Goodwill acquired during the year	0		0
Goodwill acquired during the year/other					0
Balance as of end of period	176,290,000		176,290,000		176,290,000
Construction services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	103,441,000		104,276,000		104,276,000
Goodwill acquired during the year	0		0
Goodwill acquired during the year/other					-835,000
Balance as of end of period	$103,441,000		$104,276,000		$103,441,000
[1]	Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.

Other_intangible_assets_Detail

Other intangible assets (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$9,166,000	$12,296,000	$9,840,000
Amortization of intangible assets	700,000	800,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2015	2,500,000
2016	2,200,000
2017	1,900,000
2018	1,000,000
2019	900,000
Thereafter	1,400,000
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	20,975,000	21,310,000	21,310,000
Intangible assets, accumulated amortization	-15,649,000	-14,230,000	-15,556,000
Intangible assets, net (excluding goodwill)	5,326,000	7,080,000	5,754,000
Noncompete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	4,409,000	5,580,000	5,080,000
Intangible assets, accumulated amortization	-3,504,000	-4,335,000	-4,098,000
Intangible assets, net (excluding goodwill)	905,000	1,245,000	982,000
Other intangible assets [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	8,300,000	10,920,000	10,921,000
Intangible assets, accumulated amortization	-5,365,000	-6,949,000	-7,817,000
Intangible assets, net (excluding goodwill)	$2,935,000	$3,971,000	$3,104,000

Derivative_instruments_Details

Derivative instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Mar. 31, 2013
Derivative instruments [Line Items]
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI				$1,800,000
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	0	0	12,200,000
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments			12,200,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	($400,000)
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	958,000	270,000	2,700,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	2,800,000	5,000,000	14,700,000

Gains_and_losses_on_derivative

Gains and losses on derivative instruments (Details 2) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	$99	$344
Designated as Hedging Instrument [Member] | Commodity derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	0	244
Designated as Hedging Instrument [Member] | Interest rate derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	99	100
Not designated as hedging instrument [Member] | Commodity derivatives [Member]
Derivative instruments [Line Items]
Amount of gain (loss) recognized in operating revenues, before tax	($11,208)	($6,712)

Derivative_instruments_Details1

Derivative instruments (Details 3) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$7,127	$18,335	$330
Derivative liability fair value gross liability	0	0	12,186
Commodity contract [Member] | Not designated as hedging instrument [Member] | Commodity derivative instruments - current assets [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	7,127	18,335	81
Commodity contract [Member] | Not designated as hedging instrument [Member] | Other assets noncurrent [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	0	0	249
Commodity contract [Member] | Not designated as hedging instrument [Member] | Commodity derivative instruments - current liabilities [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	$0	$0	$12,186

Subject_to_master_netting_agre

Subject to master netting agreements (Details 4) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	$7,127	$18,335	$330
Derivative liability fair value gross liability	0	0	12,186
Commodity derivative instruments assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	7,127	18,335	330
Gross amount not offset	0	0	-330
Derivative asset, fair value, net	7,127	18,335	0
Total assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	7,127	18,335	330
Gross amount not offset	0	0	-330
Derivative asset, fair value, net	7,127	18,335	0
Commodity derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability			12,186
Gross amount not offset			-330
Derivative liability, fair value, net			11,856
Total liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability			12,186
Gross amount not offset			-330
Derivative liability, fair value, net			$11,856

Availableforsale_securities_De

Available-for-sale securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$67,800,000	$63,300,000	$65,800,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	2,000,000	900,000
Available-for-sale securities [Abstract]
Cost	10,129,000	10,012,000	10,168,000
Gross unrealized gains	67,000	72,000	60,000
Gross unrealized losses	-22,000	-17,000	-37,000
Fair value	10,174,000	10,067,000	10,191,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	7,792,000	7,943,000	6,594,000
Gross unrealized gains	58,000	63,000	60,000
Gross unrealized losses	-18,000	-17,000	-18,000
Fair value	7,832,000	7,989,000	6,636,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	2,337,000	2,069,000	3,574,000
Gross unrealized gains	9,000	9,000	0
Gross unrealized losses	-4,000	0	-19,000
Fair value	$2,342,000	$2,078,000	$3,555,000

Fair_value_measurements_Detail

Fair value measurements (Details 2) (USD $)	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
In Thousands, unless otherwise specified
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	20.00%		20.00%		29.00%
Percentage investment in common stock of small-cap companies	18.00%		18.00%		27.00%
Percentage investment in common stock of large-cap companies	28.00%		29.00%		28.00%
Percentage in fixed-income and other investments	32.00%		32.00%		16.00%
Percentage Investment in Target Date Investments	1.00%
Percentage investment in cash and cash equivalents	1.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	102,043		112,830		93,933
Liabilities, fair value disclosure					12,186
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	16,945		18,473		20,267
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	67,797	[1]	65,831	[2]	63,269	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,832		6,636		7,989
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,342		3,555		2,078
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,127		18,335		330
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Liabilities, fair value disclosure					0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	102,043		112,830		93,933
Liabilities, fair value disclosure					12,186
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	16,945		18,473		20,267
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	67,797	[1]	65,831	[2]	63,269	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,832		6,636		7,989
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,342		3,555		2,078
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,127		18,335		330
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Liabilities, fair value disclosure					0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					12,186
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					0
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					12,186
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					0
[1]	The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies, 32 percent in fixed-income investments, 1 percent in target date investments and 1 percent in cash equivalents.
[2]	The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
[3]	The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 27 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 16 percent in fixed-income investments.

Fair_value_measurements_Detail1

Fair value measurements (Details 3) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified
Carrying amount [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt	$2,189,986	$2,094,727	$2,105,832
Fair value [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$2,340,681	$2,239,445	$2,186,839

Equity_Details

Equity (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Balance
Total stockholders' equity	$3,134,041
Noncontrolling interest	115,743
Total equity	3,249,784	2,855,902
Net loss attributable to noncontrolling interest	-3,528	-523
Net income (loss)	-309,445	56,139
Other comprehensive income	1,179	667
Dividends declared on preferred stocks	-171	-171
Dividends declared on common stock	-35,515	-33,809
Stock-based compensation	-121	1,336
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings		-5,564
Excess tax benefit (net tax deficit) on stock-based compensation	-1,632	4,729
Issuance of common stock	9,864	54,574
Contribution from noncontrolling interest	20,500	16,001
Total stockholders' equity	2,801,728	2,901,588
Noncontrolling interest	132,715	48,216
Total equity	2,934,443	2,949,804
Total stockholders’ equity [Member]
Balance
Total stockholders' equity	3,134,041	2,823,164
Net income (loss) attributable to parent	-305,917	56,662
Other comprehensive income	1,179	667
Dividends declared on preferred stocks	-171	-171
Dividends declared on common stock	-35,515	-33,809
Stock-based compensation	-121	1,336
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings		-5,564
Excess tax benefit (net tax deficit) on stock-based compensation	-1,632	4,729
Issuance of common stock	9,864	54,574
Contribution from noncontrolling interest	0	0
Total stockholders' equity	2,801,728	2,901,588
Noncontrolling interest [Member]
Balance
Noncontrolling interest	115,743	32,738
Net loss attributable to noncontrolling interest	-3,528	-523
Other comprehensive income	0	0
Dividends declared on preferred stocks	0	0
Dividends declared on common stock	0	0
Stock-based compensation	0	0
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings		0
Excess tax benefit (net tax deficit) on stock-based compensation	0	0
Issuance of common stock	0	0
Contribution from noncontrolling interest	20,500	16,001
Noncontrolling interest	$132,715	$48,216

Business_segment_data_Details

Business segment data (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$918,510	$1,042,853
Intersegment operating revenues	0	0
Earnings on common stock	-306,088	56,491
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	427,444	491,541
Intersegment operating revenues	21,341	18,276
Earnings on common stock	33,796	42,645
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	491,066	551,312
Intersegment operating revenues	19,641	30,346
Earnings on common stock	-339,034	14,197
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	71,776	73,647
Intersegment operating revenues	0	0
Earnings on common stock	8,328	11,033
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	330,573	374,233
Intersegment operating revenues	0	0
Earnings on common stock	21,450	27,263
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	25,095	43,661
Intersegment operating revenues	21,341	18,276
Earnings on common stock	4,018	4,349
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	49,710	116,669
Intersegment operating revenues	5,226	20,867
Earnings on common stock	-328,904	20,939
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	205,658	164,423
Intersegment operating revenues	948	4,017
Earnings on common stock	-14,635	-23,574
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	235,403	269,892
Intersegment operating revenues	11,695	3,738
Earnings on common stock	4,760	16,568
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	295	328
Intersegment operating revenues	1,772	1,724
Earnings on common stock	-255	264
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment operating revenues	-40,982	-48,622
Earnings on common stock	($850)	($351)

Employee_benefit_plans_Details

Employee benefit plans (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$40	$33
Interest cost	4,364	4,440
Expected return on assets	-5,373	-5,125
Amortization of prior service cost (credit)	18	18
Amortization of net actuarial loss	1,735	1,313
Net periodic benefit cost, including amount capitalized	784	679
Less amount capitalized	76	95
Net periodic benefit cost	708	584
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	483	379
Interest cost	914	858
Expected return on assets	-1,175	-1,067
Amortization of prior service cost (credit)	-342	-348
Amortization of net actuarial loss	461	318
Net periodic benefit cost, including amount capitalized	341	140
Less amount capitalized	29	29
Net periodic benefit cost	312	111
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$1,700	$1,700

Employee_benefit_plans_Multiem

Employee benefit plans Multiemployer Plan (Details 2) (Multiemployer plans, pension [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Additional Multiemployer Plan Withdrawal Obligation	$2.40
Additional Estimated Multiemployer Plan Withdrawal Obligation After Tax	1.5
Multiemployer plans, withdrawal obligation	16.4
Estimated multiemployer plan, withdrawal obligation, after tax	$9.80

Regulatory_matters_and_revenue1

Regulatory matters and revenues subject to refund (Details) (USD $)	0 Months Ended
	Mar. 18, 2015	Feb. 03, 2015	Aug. 11, 2014	Oct. 03, 2014	Jan. 07, 2015	Mar. 11, 2015	Feb. 06, 2015	Mar. 31, 2015	Apr. 16, 2015	Apr. 10, 2015
MTPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested			$3,000,000
Percent above current rates requested			3.60%
Interim rate increase (decrease), amount		2,000,000
Interim rate increase (decrease), percent		2.30%
Settlement stipulation	2,500,000
Settlement stipulation percent	3.00%
WYPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested				788,000
Percent above current rates requested				4.10%
NDPSC-Generation [Member]
Regulatory matters and revenues subject to refund [Line Items]
Annual amount recovered					5,300,000
NDPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested							4,300,000
Percent above current rates requested							3.40%
Interim rate increase (decrease), amount						4,300,000
Interim rate increase (decrease), percent						3.40%
OPUC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested								3,600,000
Percent above current rates requested								5.10%
Subsequent event [Member] | WYPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Settlement stipulation									501,000
Settlement stipulation percent									2.60%
Subsequent event [Member] | NDPSC-Environmental [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested										$8,100,000

Litigation_Details

Litigation (Details) (USD $)	0 Months Ended
	Aug. 06, 2012	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Oct. 31, 2010	Aug. 31, 2011	Mar. 12, 2015
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters		$25,800,000	$27,600,000	$31,400,000
Natural gas gathering operations [Member]
Loss Contingencies [Line Items]
Arbitration award					26,600,000
Litigation related to construction materials [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss						3,700,000
Additional amount per day of potential penalties						5,000
Former employee litigation [Member]
Loss Contingencies [Line Items]
Plaintiffs request compensatory damages	23,800,000
Litigation related to construction services [Member]
Loss Contingencies [Line Items]
Damage Repair Estimate							$26,000,000

Enviromental_matters_Details_2

Enviromental matters (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2015
Portland Harbor Site [Member]
Site Contingency [Line Items]
Environmental matters investigative costs	$70,000,000
Eugene, OR Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, low estimate, loss exposure	500,000
Site contingency, high estimate, loss exposure	11,000,000
Estimated proportional share of cleanup liability	50.00%
Accrual for environmental loss contingencies	1,700,000
Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, low estimate, loss exposure	340,000
Site contingency, high estimate, loss exposure	6,400,000
Accrual for environmental loss contingencies	12,400,000
Bellingham, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$8,000,000

Guarantees_Details_3

Guarantees (Details 3) (USD $)	Mar. 31, 2015
Guarantor Obligations [Line Items]
Amount of hedging obligations guaranteed reflected on balance sheet	$0
Guarantor obligations, maximum exposure, undiscounted	76,800,000
Fixed maximum amounts guaranteed by year 2015	42,600,000
Fixed maximum amounts guaranteed by year 2016	14,500,000
Fixed maximum amounts guaranteed by year 2017	1,200,000
Fixed maximum amounts guaranteed by year 2018	500,000
Fixed maximum amounts guaranteed by year 2019	500,000
Fixed maximum amounts guaranteed by year expires after certain number of days after written notice	13,500,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	200,000
Letters of credit	42,500,000
Letters of credit set to expire - 2015	14,300,000
Letters of credit set to expire - 2016	28,200,000
Outstanding letters of credit	300,000
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	4,000,000
Amount outstanding by a related party under the guarantee	1,100,000
Amount of surety bonds outstanding	$536,700,000

Commitment_and_contingencies_v

Commitment and contingencies variable interest entities (Details 4) (USD $)	Mar. 31, 2015	Dec. 01, 2014	Dec. 31, 2014	Mar. 31, 2014
Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of ownership	50.00%
Total project costs	$300,000,000
Portion of capital commitment	150,000,000
Partner portion of capital commitment	75,000,000
Excess capital commitments	300,000,000
Project cost low range estimate	425,000,000
Project cost high range estimate	435,000,000
Term loan for project debt financing	75,000,000
Line of credit facility, maximum borrowing capacity		50,000,000
Dakota Prairie Refining, LLC [Member] | Cash and cash equivalents [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	10,784,000		21,376,000	22,996,000
Dakota Prairie Refining, LLC [Member] | Accounts receivable [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	2,335,000		2,759,000	0
Dakota Prairie Refining, LLC [Member] | Inventories [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	7,902,000		5,311,000	0
Dakota Prairie Refining, LLC [Member] | Other current assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	2,926,000		4,019,000	1,135,000
Dakota Prairie Refining, LLC [Member] | Total current assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	23,947,000		33,465,000	24,131,000
Dakota Prairie Refining, LLC [Member] | Net property, plant and equipment [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	425,944,000		398,984,000	207,260,000
Dakota Prairie Refining, LLC [Member] | Other [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	4,562,000		3,400,000	0
Dakota Prairie Refining, LLC [Member] | Total deferred charges and other assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	4,562,000		3,400,000	0
Dakota Prairie Refining, LLC [Member] | Total assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	454,453,000		435,849,000	231,391,000
Dakota Prairie Refining, LLC [Member] | Short-term borrowings [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	16,100,000		0	0
Dakota Prairie Refining, LLC [Member] | Long-term debt due within one year [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	3,000,000		3,000,000	3,000,000
Dakota Prairie Refining, LLC [Member] | Accounts payable [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	23,654,000		55,089,000	16,103,000
Dakota Prairie Refining, LLC [Member] | Taxes payable [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	569,000		648,000	113,000
Dakota Prairie Refining, LLC [Member] | Accrued compensation [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	683,000		727,000	164,000
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	1,016,000		899,000	580,000
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	45,022,000		60,363,000	19,960,000
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	69,000,000		69,000,000	72,000,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	114,022,000		129,363,000	91,960,000
Fuel contract [Member]
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$20,800,000
WBI Holdings [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of credit agreement guaranteed		50.00%
Noncontrolling interest [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of credit agreement guaranteed		50.00%