MDU Resources (MDU) 10-Q 8 Aug 14 2014 Q2 Quarterly report Financial data

Document_And_Entity_Informatio

Document And Entity Information	6 Months Ended
	Jun. 30, 2014	Aug. 01, 2014
Entity Information [Line Items]	'	'
Entity registrant name	'MDU RESOURCES GROUP INC	'
Entity central index key	'0000067716	'
Current fiscal year end date	'--12-31	'
Entity filer category	'Large Accelerated Filer	'
Entity common stock, shares outstanding	'	193,946,489
Document fiscal year focus	'2014	'
Document fiscal period focus	'Q2	'
Document type	'10-Q	'
Amendment flag	'false	'
Document period end date	30-Jun-14	'

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating revenues:	'	'	'	'
Electric, natural gas distribution and pipeline and energy services	$254,689	$227,442	$746,231	$651,565
Exploration and production, construction materials and contracting, construction services and other	839,357	833,153	1,390,668	1,340,633
Total operating revenues	1,094,046	1,060,595	2,136,899	1,992,198
Operating expenses:	'	'	'	'
Fuel and purchased power	21,046	18,169	47,590	39,777
Purchased natural gas sold	84,415	70,255	329,307	269,442
Operation and maintenance:	'	'	'	'
Electric, natural gas distribution and pipeline and energy services	61,764	76,627	129,047	142,730
Exploration and production, construction materials and contracting, construction services and other	675,330	661,495	1,121,282	1,055,511
Depreciation, depletion and amortization	103,126	95,289	202,683	188,850
Taxes, other than income	49,431	47,382	105,152	99,979
Total operating expenses	995,112	969,217	1,935,061	1,796,289
Operating income	98,934	91,378	201,838	195,909
Loss from equity method investments	-297	-7	-245	-319
Other income	2,777	1,436	4,908	2,677
Interest expense	21,516	21,427	42,487	42,300
Income before income taxes	79,898	71,380	164,014	155,967
Income taxes	27,118	24,988	55,050	52,983
Income from continuing operations	52,780	46,392	108,964	102,984
Income (loss) from discontinued operations, net of tax (Note 11)	547	-59	502	-136
Net income	53,327	46,333	109,466	102,848
Net loss attributable to noncontrolling interest	-779	-179	-1,302	-179
Dividends declared on preferred stocks	171	171	342	342
Earnings on common stock	$53,935	$46,341	$110,426	$102,685
Earnings per common share - basic:	'	'	'	'
Earnings before discontinued operations	$0.28	$0.25	$0.58	$0.54
Discontinued operations, net of tax	$0	$0	$0	$0
Earnings per common share - basic	$0.28	$0.25	$0.58	$0.54
Earnings per common share - diluted:	'	'	'	'
Earnings before discontinued operations	$0.28	$0.24	$0.58	$0.54
Discontinued operations, net of tax	$0	$0	$0	$0
Earnings per common share - diluted	$0.28	$0.24	$0.58	$0.54
Dividends declared per common share	$0.18	$0.17	$0.36	$0.35
Weighted average common shares outstanding - basic	192,060	188,831	190,946	188,831
Weighted average common shares outstanding - diluted	192,659	189,463	191,543	189,460

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Consolidated Statements of Comprehensive Income [Abstract]	'	'	'	'
Net income	$53,327	$46,333	$109,466	$102,848
Net unrealized gain (loss) on derivative instruments qualifying as hedges:	'	'	'	'
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax of $0 and $52 for the three months ended and $0 and $(3,116) for the six months ended in 2014 and 2013, respectively	0	254	0	-5,594
Reclassification adjustment for (gain) loss on derivative instruments included in net income, net of tax of $10 and $(322) for the three months ended and $213 and $(1,948) for the six months ended in 2014 and 2013, respectively	13	-395	357	-3,168
Net unrealized gain (loss) on derivative instruments qualifying as hedges	13	-141	357	-8,762
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $150 and $543 for the three months ended and $318 and $862 for the six months ended in 2014 and 2013, respectively	245	424	520	1,072
Foreign currency translation adjustment recognized during the period, net of tax of $26 and $(234) for the three months ended and $54 and $(197) for the six months ended in 2014 and 2013, respectively	42	-390	88	-302
Net unrealized gain (loss) on available-for-sale investments:	'	'	'	'
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $4 and $(77) for the three months ended and $5 and $(100) for the six months ended in 2014 and 2013, respectively	8	-142	10	-187
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $17 and $23 for the three months ended and $17 and $42 for the six months ended in 2014 and 2013, respectively	32	43	32	79
Net unrealized gain (loss) on available-for-sale investments	40	-99	42	-108
Other comprehensive income (loss)	340	-206	1,007	-8,100
Comprehensive income	53,667	46,127	110,473	94,748
Comprehensive loss attributable to noncontrolling interest	-779	-179	-1,302	-179
Comprehensive income attributable to common stockholders	$54,446	$46,306	$111,775	$94,927

Consolidated_Statements_of_Com1

Consolidated Statements of Comprehensive Income - Parenthetical (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net unrealized gain (loss) on derivative instruments arising during the period, tax	$0	$52	$0	($3,116)
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax	10	-322	213	-1,948
Amortization of postretirement liability losses included in net periodic benefit cost, tax	150	543	318	862
Foreign currency translation adjustments arising during the period, tax	26	-234	54	-197
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	4	-77	5	-100
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income, tax	$17	$23	$17	$42

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
In Thousands, unless otherwise specified
Current assets:	'		'		'
Cash and cash equivalents	$90,318		$45,225		$114,971
Receivables, net	731,247		713,067		734,765
Inventories	331,422		282,391		345,885
Deferred income taxes	29,110		25,048		27,959
Commodity derivative instruments	129		1,447		9,797
Prepayments and other current assets	93,980		49,510		58,870
Total current assets	1,276,206		1,116,688		1,292,247
Investments	116,594		112,939		106,508
Property, plant and equipment	9,390,538		8,803,866		8,454,204
Less accumulated depreciation, depletion and amortization	4,011,858		3,872,487		3,709,679
Net property, plant and equipment	5,378,680		4,931,379		4,744,525
Deferred charges and other assets:	'		'		'
Goodwill	636,039	[1]	636,039	[1]	636,039	[1]
Other intangible assets, net	11,266		13,099		15,312
Other	249,532		251,188		297,040
Total deferred charges and other assets	896,837		900,326		948,391
Total assets	7,668,317		7,061,332		7,091,671
Current liabilities:	'		'		'
Short-term borrowings	0		11,500		31,600
Long-term debt due within one year	42,215		12,277		69,091
Accounts payable	424,201		404,961		411,621
Taxes payable	48,985		74,175		89,896
Dividends payable	34,388		33,737		32,745
Accrued compensation	50,024		69,661		44,159
Commodity derivative instruments	17,449		7,483		1,388
Other accrued liabilities	179,402		171,106		185,389
Total current liabilities	796,664		784,900		865,889
Long-term debt	2,144,879		1,842,286		1,937,663
Deferred credits and other liabilities:	'		'		'
Deferred income taxes	925,813		859,306		782,838
Other liabilities	736,519		718,938		810,639
Total deferred credits and other liabilities	1,662,332		1,578,244		1,593,477
Equity:	'		'		'
Preferred stocks	15,000		15,000		15,000
Common stockholders' equity:	'		'		'
Authorized - 500,000,000 shares, $1.00 par value. Shares issued - 194,138,654 at June 30, 2014, 189,369,450 at June 30, 2013 and 189,868,780 at December 31, 2013	194,139		189,869		189,369
Other paid-in-capital	1,186,900		1,056,996		1,040,379
Retained earnings	1,645,291		1,603,130		1,494,419
Accumulated other comprehensive loss	-37,198		-38,205		-56,821
Treasury stock at cost - 538,921 shares	-3,626		-3,626		-3,626
Total common stockholders' equity	2,985,506		2,808,164		2,663,720
Total stockholders' equity	3,000,506		2,823,164		2,678,720
Noncontrolling interest	63,936		32,738		15,922
Total equity	3,064,442		2,855,902		2,694,642
Total liabilities and equity	$7,668,317		$7,061,332		$7,091,671
[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 $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
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	194,138,654	189,868,780	189,369,450
Treasury shares	538,921	538,921	538,921

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013
Operating activities:	'	'
Net income	$109,466	$102,848
Income (loss) from discontinued operations, net of tax	502	-136
Income from continuing operations	108,964	102,984
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation, depletion and amortization	202,683	188,850
Loss, net of distributions, from equity method investments	245	1,491
Deferred income taxes	60,141	19,790
Unrealized (gain) loss on commodity derivatives	11,908	-7,215
Excess tax benefit on stock-based compensation	-4,729	0
Changes in current assets and liabilities, net of acquisitions:	'	'
Receivables	-8,501	-65,637
Inventories	-48,857	-29,923
Other current assets	-37,460	-18,044
Accounts payable	-23,277	18,940
Other current liabilities	-36,447	23,071
Other noncurrent changes	-13,638	-741
Net cash provided by continuing operations	211,032	233,566
Net cash provided by (used in) discontinued operations	-491	360
Net cash provided by operating activities	210,541	233,926
Investing activities:	'	'
Capital expenditures	-390,126	-431,439
Acquisitions, net of cash acquired	-206,304	0
Net proceeds from sale or disposition of property and other	11,917	20,884
Investments	-1,208	16
Net cash used in continuing operations	-585,721	-410,539
Net cash provided by discontinued operations	0	0
Net cash used in investing activities	-585,721	-410,539
Financing activities:	'	'
Issuance of short-term borrowings	0	29,600
Repayment of short-term borrowings	-11,500	0
Issuance of long-term debt	441,447	450,461
Repayment of long-term debt	-111,539	-214,473
Proceeds from issuance of common stock	132,268	0
Dividends paid	-67,717	-32,915
Excess tax benefit on stock-based compensation	4,729	0
Contribution from noncontrolling interest	32,500	10,000
Net cash provided by continuing operations	420,188	242,673
Net cash provided by discontinued operations	0	0
Net cash provided by financing activities	420,188	242,673
Effect of exchange rate changes on cash and cash equivalents	85	-131
Increase in cash and cash equivalents	45,093	65,929
Cash and cash equivalents - beginning of year	45,225	49,042
Cash and cash equivalents - end of period	$90,318	$114,971

Basis_of_presentation

Basis of presentation	6 Months Ended
	Jun. 30, 2014
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 2013 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 2013 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 June 30, 2014, up to the date of issuance of these consolidated interim financial statements.

Seasonality_of_operations

Seasonality of operations	6 Months Ended
	Jun. 30, 2014
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	6 Months Ended
	Jun. 30, 2014
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 $29.6 million, $35.1 million and $36.4 million at June 30, 2014 and 2013, and December 31, 2013, 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 June 30, 2014 and 2013, and December 31, 2013, was $9.6 million, $10.6 million and $10.1 million, respectively.

Inventories_and_natural_gas_in

Inventories and natural gas in storage	6 Months Ended
	Jun. 30, 2014
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:
		June 30,		June 30,		December 31,
		2014		2013		2013
		(In thousands)
	Aggregates held for resale	$	112,129	$	103,503	$	101,568
	Asphalt oil	76,525		91,837		38,099
	Materials and supplies	70,938		74,648		69,808
	Merchandise for resale	25,507		27,330		21,720
	Natural gas in storage (current)	10,903		14,287		16,417
	Other	35,420		34,280		34,779
	Total	$	331,422	$	345,885	$	282,391
	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 $47.4 million, $48.6 million and $48.3 million at June 30, 2014 and 2013, and December 31, 2013, respectively.

Impairment_of_longlived_assets

Impairment of long-lived assets	6 Months Ended
	Jun. 30, 2014
Asset Impairment Charges [Abstract]	'
Impairment of long-lived assets	'
	Impairment of long-lived assets
	During the second quarter of 2013, the Company recognized an impairment of coalbed natural gas gathering assets at the pipeline and energy services segment of $14.5 million ($9.0 million after tax), which is recorded in operation and maintenance expense on the Consolidated Statements of Income. The impairment is related to coalbed natural gas gathering assets located in Wyoming and Montana where there has been a significant decline in natural gas development and production activity largely due to low natural gas prices. The coalbed natural gas gathering assets were written down to fair value that was determined using the income approach. For more information on this nonrecurring fair value measurement, see Note 15.

Earnings_per_common_share

Earnings per common share	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Earnings per common share	'
	Earnings per common share
	Basic earnings per common share were computed by dividing earnings on common stock by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings per common share were computed by dividing earnings on common stock by the total of the weighted average number of shares of common stock outstanding during the applicable period, plus the effect of outstanding performance share awards. Common stock outstanding includes issued shares less shares held in treasury. Net income was the same for both the basic and diluted earnings per share calculations. A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows:
		Three Months Ended		Six Months Ended
		June 30,		June 30,
		2014	2013	2014	2013
		(In thousands)
	Weighted average common shares outstanding - basic	192,060	188,831	190,946	188,831
	Effect of dilutive performance share awards	599	632	597	629
	Weighted average common shares outstanding - diluted	192,659	189,463	191,543	189,460
	Shares excluded from the calculation of diluted earnings per share	—	—	—	—

Cash_flow_information

Cash flow information	6 Months Ended
	Jun. 30, 2014
Supplemental Cash Flow Information [Abstract]	'
Cash flow information	'
	Cash flow information
	Cash expenditures for interest and income taxes were as follows:
		Six Months Ended
		June 30,
		2014		2013
		(In thousands)
	Interest, net of amount capitalized	$	39,441	$	41,440
	Income taxes paid (refunded), net	$	39,984	$	(2,649	)
	Noncash investing transactions were as follows:
		June 30,
		2014		2013
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	95,833	$	77,073

New_accounting_standards

New accounting standards	6 Months Ended
	Jun. 30, 2014
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]	'
New accounting standard	'
	New Accounting Standard
	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. 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.

Comprehensive_income_loss

Comprehensive income (loss)	6 Months Ended
	Jun. 30, 2014
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 June 30, 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	$	(3,421	)	$	(33,532	)	$	(621	)	$	36		$	(37,538	)
	Other comprehensive income (loss) before reclassifications	—			—			42			8			50
	Amounts reclassified from accumulated other comprehensive loss	13			245			—			32			290
	Net current-period other comprehensive income	13			245			42			40			340
	Balance at end of period	$	(3,408	)	$	(33,287	)	$	(579	)	$	76		$	(37,198	)
	Three Months Ended June 30, 2013	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	$	(2,603	)	$	(53,699	)	$	(423	)	$	110		$	(56,615	)
	Other comprehensive income (loss) before reclassifications	254			—			(390		)	(142		)	(278		)
	Amounts reclassified from accumulated other comprehensive loss	(395		)	424			—			43			72
	Net current-period other comprehensive income (loss)	(141		)	424			(390		)	(99		)	(206		)
	Balance at end of period	$	(2,744	)	$	(53,275	)	$	(813	)	$	11		$	(56,821	)
	Six Months Ended June 30, 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	—			—			88			10			98
	Amounts reclassified from accumulated other comprehensive loss	357			520			—			32			909
	Net current-period other comprehensive income	357			520			88			42			1,007
	Balance at end of period	$	(3,408	)	$	(33,287	)	$	(579	)	$	76		$	(37,198	)
	Six Months Ended June 30, 2013	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	$	6,018		$	(54,347	)	$	(511	)	$	119		$	(48,721	)
	Other comprehensive income (loss) before reclassifications	(5,594		)	—			(302		)	(187		)	(6,083		)
	Amounts reclassified from accumulated other comprehensive loss	(3,168		)	1,072			—			79			(2,017		)
	Net current-period other comprehensive income (loss)	(8,762		)	1,072			(302		)	(108		)	(8,100		)
	Balance at end of period	$	(2,744	)	$	(53,275	)	$	(813	)	$	11		$	(56,821	)
	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended						Six Months Ended						Location on Consolidated Statements of Income
		June 30,						June 30,
		2014			2013			2014			2013
		(In thousands)
	Reclassification adjustment for gain (loss) on derivative instruments included in net income:
	Commodity derivative instruments	$	137		$	1,381		$	(250	)	$	5,896		Operating revenues
	Interest rate derivative instruments	(160		)	(664		)	(320		)	(780		)	Interest expense
		(23		)	717			(570		)	5,116
		10			(322		)	213			(1,948		)	Income taxes
		(13		)	395			(357		)	3,168
	Amortization of postretirement liability losses included in net periodic benefit cost	(395		)	(967		)	(838		)	(1,934		)	(a)
		150			543			318			862			Income taxes
		(245		)	(424		)	(520		)	(1,072		)
	Reclassification adjustment for loss on available-for-sale investments included in net income	(49		)	(66		)	(49		)	(121		)	Other income
		17			23			17			42			Income taxes
		(32		)	(43		)	(32		)	(79		)
	Total reclassifications	$	(290	)	$	(72	)	$	(909	)	$	2,017
	(a) Included in net periodic benefit cost (credit). For more information, see Note 19.

Acquisition

Acquisition	6 Months Ended
	Jun. 30, 2014
Business Combinations [Abstract]	'
Acquisition	'
	Acquisition
	On February 10, 2014, the Company entered into agreements to purchase working interests and leasehold positions in oil and natural gas production assets in the southern Powder River Basin of Wyoming. The effective date of the acquisition was October 1, 2013, and the closing occurred on March 6, 2014. The purchase price was $206.3 million, including purchase price adjustments.
	The acquisition was accounted for under the acquisition method of accounting and, accordingly, the acquired assets and liabilities assumed have been recorded at their respective fair values as of the date of acquisition. The results of operations of the acquired properties are included in the financial statements since the date of the acquisition. Pro forma financial amounts reflecting the effects of the acquisition are not presented, as such acquisition was not material to the Company's financial position or results of operations.

Discontinued_operations

Discontinued operations	6 Months Ended
	Jun. 30, 2014
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 incurs legal expenses and had a benefit related to the resolution of this matter in the second quarter of 2014, which are reflected in discontinued operations in the consolidated financial statements and accompanying notes. Discontinued operations are included in the Other category. For more information, see Note 21.

Equity_method_investments

Equity method investments	6 Months Ended
	Jun. 30, 2014
Equity Method Investments and Joint Ventures [Abstract]	'
Equity method investments	'
	Equity method investments
	Investments in companies in which the Company has the ability to exercise significant influence over operating and financial policies are accounted for using the equity method. At June 30, 2014, the Company had no significant equity method investments.
	In August 2006, MDU Brasil acquired ownership interests in the Brazilian Transmission Lines. The electric transmission lines are primarily in northeastern and southern Brazil. The transmission contracts provide for revenues denominated in the Brazilian Real, annual inflation adjustments and change in tax law adjustments. The functional currency for the Brazilian Transmission Lines is the Brazilian Real.
	In 2009, multiple sales agreements were signed for the Company to sell its ownership interests in the Brazilian Transmission Lines. In November 2010, the Company completed the sale of its entire ownership interest in ENTE and ERTE and 59.96 percent of the Company's ownership interest in ECTE. The Company's remaining interest in ECTE is being sold over a four-year period. In August 2013 and 2012, and November 2011, the Company completed the sale of one-fourth of the remaining interest in each year. The Company recognized an immaterial gain in 2013. The Company's remaining ownership interest in ECTE is being accounted for under the cost method.
	At June 30, 2013, the equity method investments had total assets of $114.8 million and long-term debt of $56.2 million. The Company's investment in its equity method investments was approximately $5.5 million, including undistributed earnings of $2.0 million, at June 30, 2013.

Goodwill_and_other_intangible_

Goodwill and other intangible assets	6 Months Ended
	Jun. 30, 2014
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:
	Six Months Ended	Balance			Goodwill			Balance
	30-Jun-14	as of			Acquired			as of
		January 1,			During			June 30, 2014*
		2014*			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	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.
	Six Months Ended	Balance			Goodwill			Balance
	30-Jun-13	as of			Acquired			as of
		January 1,			During the			June 30, 2013*
		2013*			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-13	as of			Acquired			as of
		January 1,			During the			December 31,
		2013*			Year			2013*
		(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.
	Other amortizable intangible assets were as follows:
		June 30,			June 30,			December 31,
		2014			2013			2013
		(In thousands)
	Customer relationships	$	21,310		$	21,310		$	21,310
	Accumulated amortization	(14,734		)	(12,715		)	(13,726		)
		6,576			8,595			7,584
	Noncompete agreements	5,080			6,186			6,186
	Accumulated amortization	(3,936		)	(4,557		)	(4,840		)
		1,144			1,629			1,346
	Other	10,921			10,979			10,995
	Accumulated amortization	(7,375		)	(5,891		)	(6,826		)
		3,546			5,088			4,169
	Total	$	11,266		$	15,312		$	13,099
	Amortization expense for amortizable intangible assets for the three and six months ended June 30, 2014, was $1.0 million and $1.8 million, respectively. Amortization expense for amortizable intangible assets for the three and six months ended June 30, 2013, was $1.0 million and $1.8 million, respectively. Estimated amortization expense for amortizable intangible assets is $3.3 million in 2014, $2.5 million in 2015, $2.2 million in 2016, $1.9 million in 2017, $1.0 million in 2018 and $2.2 million thereafter.

Derivative_instruments

Derivative instruments	6 Months Ended
	Jun. 30, 2014
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 June 30, 2014, 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 June 30, 2014 and 2013, and December 31, 2013, Fidelity held oil swap and collar agreements with total forward notional volumes of 2.5 million, 3.1 million and 2.9 million Bbl, respectively, and natural gas swap agreements with total forward notional volumes of 11.0 million, 21.4 million and 18.3 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. The Company expects to reclassify into earnings from accumulated other comprehensive income (loss) the remaining value related to de-designating commodity derivative instruments over the next 6 months.
	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.
	There were no components of the derivative instruments' gain or loss excluded from the assessment of hedge effectiveness. Gains and losses must be reclassified into earnings as a result of the discontinuance of cash flow hedges if it is probable that the original forecasted transactions will not occur, and there were no such reclassifications.
	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. The aggregate fair value of Fidelity's derivative instruments with credit-risk-related contingent features that are in a liability position at June 30, 2014 and 2013, and December 31, 2013, were $17.4 million, $1.4 million and $7.5 million, respectively. 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 June 30, 2014 and 2013, and December 31, 2013, were $17.4 million, $1.4 million and $7.5 million, respectively.
	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 June 30, 2014 and 2013, and December 31, 2013, Centennial had no outstanding interest rate swap agreements.
	Fidelity and Centennial
	The gains and losses on derivative instruments were as follows:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of loss recognized in accumulated other comprehensive loss (effective portion), net of tax	$	—		$	—		$	—		$	(6,154	)
	Amount of (gain) loss reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(86		)	(871		)	158			(3,714		)
	Amount of loss recognized in operating revenues (ineffective portion), before tax	—			—			—			(1,422		)
	Interest rate derivatives designated as cash flow hedges:
	Amount of gain recognized in accumulated other comprehensive loss (effective portion), net of tax	—			254			—			560
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	99			475			199			546
	Amount of loss recognized in interest expense (ineffective portion), before tax	—			(610		)	—			(769		)
	Commodity derivatives not designated as hedging instruments:
	Amount of gain (loss) recognized in operating revenues, before tax	(5,196		)	13,047			(11,908		)	8,637
	Over the next 12 months net losses of approximately $535,000 (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 June 30, 2014			Fair Value at June 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	129		$	9,797		$	1,447
		Other assets - noncurrent	131			1,447			503
	Total asset derivatives		$	260		$	11,244		$	1,950
	Liability	Location on	Fair Value at June 30, 2014			Fair Value at June 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	17,449		$	1,388		$	7,483
	Total liability derivatives		$	17,449		$	1,388		$	7,483
	All of the Company's commodity derivative instruments at June 30, 2014 and 2013, and December 31, 2013, 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:
	30-Jun-14	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	260		$	(260	)	$	—
	Total assets	$	260		$	(260	)	$	—
	Liabilities:
	Commodity derivatives	$	17,449		$	(260	)	$	17,189
	Total liabilities	$	17,449		$	(260	)	$	17,189
	30-Jun-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	11,244		$	(1,388	)	$	9,856
	Total assets	$	11,244		$	(1,388	)	$	9,856
	Liabilities:
	Commodity derivatives	$	1,388		$	(1,388	)	$	—
	Total liabilities	$	1,388		$	(1,388	)	$	—
	31-Dec-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
Assets:
Commodity derivatives	$	1,950		$	(1,950	)	$	—
Total assets	$	1,950		$	(1,950	)	$	—
Liabilities:
Commodity derivatives	$	7,483		$	(1,950	)	$	5,533
Total liabilities	$	7,483		$	(1,950	)	$	5,533

Fair_value_measurements

Fair value measurements	6 Months Ended
	Jun. 30, 2014
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 $64.4 million, $54.0 million and $62.4 million, at June 30, 2014 and 2013, and December 31, 2013, respectively, are classified as Investments on the Consolidated Balance Sheets. The net unrealized gains on these investments were $1.1 million and $2.0 million for the three and six months ended June 30, 2014, respectively. The net unrealized gains on these investments were $700,000 and $5.1 million for the three and six months ended June 30, 2013, 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:
	June 30, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,989	$	91	$	(5	)	$	8,075
	U.S. Treasury securities	2,066		30		—			2,096
	Total	$	10,055	$	121	$	(5	)	$	10,171
	June 30, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,035	$	58	$	(41	)	$	8,052
	U.S. Treasury securities	1,920		15		(15		)	1,920
	Total	$	9,955	$	73	$	(56	)	$	9,972
	December 31, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,151	$	69	$	(27	)	$	8,193
	U.S. Treasury securities	1,906		15		(4		)	1,917
	Total	$	10,057	$	84	$	(31	)	$	10,110
	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.
	The estimated fair value of the Company's Level 2 interest rate derivative instruments is measured using quoted market prices or pricing models using prevailing market interest rates as of the measurement date. Counterparty statements are utilized to determine the value of the interest rate 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 six months ended June 30, 2014 and 2013, 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 June 30, 2014, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at June 30, 2014
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	19,990	$	—		$	19,990
	Insurance contract*	—		64,449		—			64,449
	Available-for-sale securities:
	Mortgage-backed securities	—		8,075		—			8,075
	U.S. Treasury securities	—		2,096		—			2,096
	Commodity derivative instruments	—		260		—			260
	Total assets measured at fair value	$	—	$	94,870	$	—		$	94,870
	Liabilities:
	Commodity derivative instruments	$	—	$	17,449	$	—		$	17,449
	Total liabilities measured at fair value	$	—	$	17,449	$	—		$	17,449
	*  The insurance contract invests approximately 21 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, 31 percent in fixed-income investments and 1 percent in cash equivalents.
		Fair Value Measurements at June 30, 2013, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at June 30, 2013
		(Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	29,902	$	—		$	29,902
	Insurance contract*	—		54,039		—			54,039
	Available-for-sale securities:
	Mortgage-backed securities	—		8,052		—			8,052
	U.S. Treasury securities	—		1,920		—			1,920
	Commodity derivative instruments	—		11,244		—			11,244
	Total assets measured at fair value	$	—	$	105,157	$	—		$	105,157
	Liabilities:
	Commodity derivative instruments	$	—	$	1,388	$	—		$	1,388
	Total liabilities measured at fair value	$	—	$	1,388	$	—		$	1,388
	*  The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 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, 2013, Using
		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2013
						(Level 3)
	(In thousands)
Assets:
Money market funds	$	—	$	19,227	$	—		$	19,227
Insurance contract*	—		62,370		—			62,370
Available-for-sale securities:
Mortgage-backed securities	—		8,193		—			8,193
U.S. Treasury securities	—		1,917		—			1,917
Commodity derivative instruments	—		1,950		—			1,950
Total assets measured at fair value	$	—	$	93,657	$	—		$	93,657
Liabilities:
Commodity derivative instruments	$	—	$	7,483	$	—		$	7,483
Total liabilities measured at fair value	$	—	$	7,483	$	—		$	7,483
* The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.
The Company applies the provisions of the fair value measurement standard to its nonrecurring, non-financial measurements, including long-lived asset impairments. These assets are not measured at fair value on an ongoing basis but are subject to fair value adjustments only in certain circumstances. The Company reviews the carrying value of its long-lived assets, excluding goodwill and oil and natural gas properties, whenever events or changes in circumstances indicate that such carrying amounts may not be recoverable. During the second quarter of 2013, coalbed natural gas gathering assets were reviewed for impairment and found to be impaired and were written down to their estimated fair value using the income approach. Under this approach, fair value is determined by using the present value of future estimated cash flows. The factors used to determine the estimated future cash flows include, but are not limited to, internal estimates of gathering revenue, future commodity prices and operating costs and equipment salvage values. The estimated cash flows are discounted using a rate that approximates the weighted average cost of capital of a market participant. These fair value inputs are not typically observable. At June 30, 2013, coalbed natural gas gathering assets were written down to the nonrecurring fair value measurement of $9.7 million. The fair value of these coalbed natural gas gathering assets have been categorized as Level 3 (Significant Unobservable Inputs) in the fair value hierarchy.
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 June 30, 2014	$	2,187,094	$	2,283,351
Long-term debt at June 30, 2013	$	2,006,754	$	2,090,208
Long-term debt at December 31, 2013	$	1,854,563	$	1,912,590
The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values.

Longterm_debt

Long-term debt	6 Months Ended
	Jun. 30, 2014
Debt Disclosure [Abstract]	'
Long-term debt	'
	Long-term debt
	On May 8, 2014, the Company amended its revolving credit agreement to increase the borrowing limit to $175.0 million and extend the termination date to May 8, 2019.
	The Company entered into a $150.0 million note purchase agreement on January 28, 2014, and issued $50.0 million of Senior Notes on April 15, 2014, with a due date of April 15, 2044, at an interest rate of 5.2 percent. The remaining $100.0 million of Senior Notes was issued on July 15, 2014, with due dates ranging from July 15, 2024 to July 15, 2026, at a weighted average interest rate of 4.3 percent.
	On May 8, 2014, Centennial entered into an amended and restated revolving credit agreement which increased the borrowing limit to $650.0 million and extended the termination date to May 8, 2019. The credit agreement contains customary covenants and provisions, including a covenant of Centennial not to permit, as of the end of any fiscal quarter, the ratio of total consolidated debt to total consolidated capitalization to be greater than 65 percent. Other covenants include restrictions on the sale of certain assets, limitations on subsidiary indebtedness and the making of certain loans and investments.
	Centennial's revolving credit agreement contains cross-default provisions. These provisions state that if Centennial or any subsidiary of Centennial fails to make any payment with respect to any indebtedness or contingent obligation, in excess of a specified amount, under any agreement that causes such indebtedness to be due prior to its stated maturity or the contingent obligation to become payable, then Centennial will be in default under the revolving credit agreement.
	Centennial entered into two separate two year $125.0 million term loan agreements with variable interest rates on March 31, 2014 and April 2, 2014, respectively. These agreements contain customary covenants and default provisions, including covenants not to permit, as of the end of any fiscal quarter, the ratio of Centennial's total debt to total capitalization to be greater than 65 percent. The covenants also include certain limitations on subsidiary indebtedness and restrictions on the sale of certain assets and on the making of certain loans and investments.
	In addition, borrowings outstanding that were classified as long-term debt under the Company’s and Centennial’s commercial paper programs totaled $268.0 million at June 30, 2014, compared to $153.9 million at December 31, 2013, respectively.

Equity

Equity	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Equity	'
	Equity
	A summary of the changes in equity was as follows:
	Six Months Ended June 30, 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)	110,768			(1,302		)	109,466
	Other comprehensive income	1,007			—			1,007
	Dividends declared on preferred stocks	(342		)	—			(342		)
	Dividends declared on common stock	(68,025		)	—			(68,025		)
	Stock-based compensation	2,796			—			2,796
	Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	(5,564		)	—			(5,564		)
	Net tax benefit on stock-based compensation	4,729			—			4,729
	Issuance of common stock	131,973			—			131,973
	Contribution from noncontrolling interest	—			32,500			32,500
	Balance at June 30, 2014	$	3,000,506		$	63,936		$	3,064,442
	Six Months Ended June 30, 2013	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2012	$	2,648,248		$	—		$	2,648,248
	Net income (loss)	103,027			(179		)	102,848
	Other comprehensive loss	(8,100		)	—			(8,100		)
	Dividends declared on preferred stocks	(342		)	—			(342		)
	Dividends declared on common stock	(65,147		)	—			(65,147		)
	Stock-based compensation	2,453			—			2,453
	Net tax deficit on stock-based compensation	(1,419		)	—			(1,419		)
	Contribution from noncontrolling interest	—			16,101			16,101
	Balance at June 30, 2013	$	2,678,720		$	15,922		$	2,694,642

Business_segment_data

Business segment data	6 Months Ended
	Jun. 30, 2014
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 Company also has an investment in a foreign country, which consists of Centennial Resources' investment in ECTE.
	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 is constructing the Dakota Prairie Refinery in conjunction with Calumet to refine crude oil and also provides cathodic protection and other energy-related services.
	The exploration and production segment is engaged in oil and natural gas acquisition, exploration, development and production activities in the Rocky Mountain and Mid-Continent/Gulf States regions of the United States.
	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 specializes in constructing and maintaining electric and communication lines, gas pipelines, fire suppression systems, and external lighting and traffic signalization equipment. This segment also provides utility excavation services and inside electrical wiring, cabling and mechanical services, sells and distributes electrical materials, and manufactures and distributes specialty equipment.
	The Other category includes the activities of Centennial Capital, which insures various types of risks as a captive insurer for certain of the Company's subsidiaries. The function of the captive insurer is to fund the deductible layers of the insured companies' general liability, automobile liability and pollution liability coverages. Centennial Capital also owns certain real and personal property. The Other category also includes Centennial Resources' investment in ECTE.
	The information below follows the same accounting policies as described in Note 1 of the Company's Notes to Consolidated Financial Statements in the 2013 Annual Report. Information on the Company's businesses was as follows:
	Three Months Ended June 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	65,149	$	—		$	7,823
	Natural gas distribution	146,077		—			(4,494		)
	Pipeline and energy services	43,463		7,888			5,789
		254,689		7,888			9,118
	Exploration and production	129,309		10,271			19,180
	Construction materials and contracting	434,452		8,106			10,554
	Construction services	275,109		7,273			14,307
	Other	487		1,744			1,608
		839,357		27,394			45,649
	Intersegment eliminations	—		(35,282		)	(832		)
	Total	$	1,094,046	$	—		$	53,935
	Three Months Ended June 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	56,981	$	—		$	4,410
	Natural gas distribution	127,584		—			(5,893		)
	Pipeline and energy services	42,877		7,999			(6,395		)
		227,442		7,999			(7,878		)
	Exploration and production	137,053		12,556			32,995
	Construction materials and contracting	418,345		12,958			10,025
	Construction services	277,259		2,340			12,915
	Other	496		1,839			340
		833,153		29,693			56,275
	Intersegment eliminations	—		(37,692		)	(2,056		)
	Total	$	1,060,595	$	—		$	46,341
	Six Months Ended June 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	138,796	$	—		$	18,856
	Natural gas distribution	520,311		—			22,768
	Pipeline and energy services	87,124		26,164			10,138
		746,231		26,164			51,762
	Exploration and production	245,976		31,139			40,120
	Construction materials and contracting	598,875		12,123			(13,019		)
	Construction services	545,002		11,010			30,875
	Other	815		3,468			1,872
		1,390,668		57,740			59,848
	Intersegment eliminations	—		(83,904		)	(1,184		)
	Total	$	2,136,899	$	—		$	110,426
	Six Months Ended June 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	121,635	$	—		$	14,235
Natural gas distribution	459,337		—			26,624
Pipeline and energy services	70,593		26,717			(4,064		)
	651,565		26,717			36,795
Exploration and production	252,415		22,369			53,279
Construction materials and contracting	580,323		17,251			(10,557		)
Construction services	507,065		3,914			24,579
Other	830		3,657			645
	1,340,633		47,191			67,946
Intersegment eliminations	—		(73,908		)	(2,056		)
Total	$	1,992,198	$	—		$	102,685
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	6 Months Ended
	Jun. 30, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]	'
Pension and other postretirement benefit plans	'
	Employee benefit 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 June 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	31		$	37		$	380		$	334
	Interest cost	4,405			4,106			924			667
	Expected return on assets	(5,484		)	(4,875		)	(1,242		)	(1,065		)
	Amortization of prior service cost (credit)	18			18			(348		)	(364		)
	Amortization of net actuarial loss	1,121			1,716			6			407
	Net periodic benefit cost, including amount capitalized	91			1,002			(280		)	(21		)
	Less amount capitalized	73			158			(19		)	61
	Net periodic benefit cost (credit)	$	18		$	844		$	(261	)	$	(82	)
								Other
								Postretirement
		Pension Benefits						Benefits
	Six Months Ended June 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	64		$	77		$	759		$	838
	Interest cost	8,845			8,124			1,782			1,607
	Expected return on assets	(10,609		)	(9,958		)	(2,309		)	(2,172		)
	Amortization of prior service cost (credit)	36			36			(696		)	(728		)
	Amortization of net actuarial loss	2,434			3,580			324			1,078
	Net periodic benefit cost, including amount capitalized	770			1,859			(140		)	623
	Less amount capitalized	168			268			10			90
	Net periodic benefit cost (credit)	$	602		$	1,591		$	(150	)	$	533
	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 and six months ended June 30, 2014, was $1.6 million and $3.3 million, respectively. The Company's net periodic benefit cost for this plan for the three and six months ended June 30, 2013, was $1.8 million and $3.6 million, respectively.

Regulatory_matters_and_revenue

Regulatory matters and revenues subject to refund	6 Months Ended
	Jun. 30, 2014
Regulated Operations [Abstract]	'
Regulatory matters and revenues subject to refund	'
	Regulatory matters and revenues subject to refund
	On April 8, 2014, Montana-Dakota submitted a request to the NDPSC to update the environmental cost recovery rider to reflect actual costs incurred through February 2014 and projected costs through June 2015 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 NDPSC approved the proposed rider on July 10, 2014, reflecting an annual amount of $8.6 million to be recovered under the rider. The rider was effective with service rendered on and after July 15, 2014.
	On September 18, 2013, Montana-Dakota filed an application with the NDPSC for a natural gas rate increase. Montana-Dakota requested a total increase of $6.8 million annually or approximately 6.4 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, an operations building, automated meter reading and a new customer billing system. An interim increase of $4.3 million annually or approximately 4.0 percent went into effect for service rendered on or after November 17, 2013. On December 30, 2013, the NDPSC approved a settlement agreement for an increase in the same amount as the interim increase. A hearing on the rate design portion of the case was held February 5, 2014. The NDPSC voted to approve an order approving the allocation of the revenue increase to each rate class and the rate design on April 9, 2014. Final rates were implemented May 1, 2014.
	On February 27, 2014, Montana-Dakota filed an application with the NDPSC for approval of an electric generation resource recovery rider for recovery of Montana-Dakota's investment in the 88-MW simple-cycle natural gas turbine and associated facilities currently under construction near Mandan, ND. Montana-Dakota requested recovery of $7.4 million annually or approximately 4.6 percent above current rates. Advance determination of prudence and a certificate of public convenience and necessity were received from the NDPSC on April 11, 2012. On March 12, 2014, the NDPSC suspended the filing pending further review. The NDPSC held a hearing regarding this matter on May 28, 2014. The matter is pending before the NDPSC.
	On October 31, 2013, WBI Energy Transmission filed a general natural gas rate change application with the FERC based on an increase in investments of $312 million, increased operating costs, and the effect of lower storage and off system volumes. On April 30, 2014, WBI Energy Transmission reached a settlement in principle with FERC Trial Staff and all active parties to resolve the rate case. WBI Energy Transmission filed settlement rates to take effect on an interim basis, effective May 1, 2014, pending final approval of the settlement. On June 4, 2014, WBI Energy Transmission submitted to the FERC an Uncontested Offer of Settlement. On June 11, 2014, the Presiding Administrative Law Judge issued a Certification of Uncontested Settlement recommending FERC approval of the settlement without modification. The matter is pending before the FERC. Based on the adjusted base period volumes filed in the case, the annual increase in revenues is approximately $11.5 million.

Contingencies

Contingencies	6 Months Ended
	Jun. 30, 2014
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 $32.1 million, $32.9 million and $29.5 million for contingencies, including litigation, production taxes, royalty claims and environmental matters, at June 30, 2014 and 2013, and December 31, 2013, respectively, which include amounts that may have been accrued for matters discussed in Litigation and Environmental matters within this note.
	Litigation
	Guarantee Obligation Under a Construction Contract Centennial guaranteed CEM's obligations under a construction contract with LPP for a 550-MW combined-cycle electric generating facility near Hobbs, New Mexico. Centennial Resources sold CEM in July 2007 to Bicent. In February 2009, Centennial received a Notice and Demand from LPP under the guarantee agreement alleging that CEM did not meet certain of its obligations under the construction contract and demanding that Centennial indemnify LPP against all losses, damages, claims, costs, charges and expenses arising from CEM's alleged failures. In December 2009, LPP submitted a demand for arbitration of its dispute with CEM to the American Arbitration Association seeking compensatory damages of $149.7 million. An arbitration award was issued January 13, 2012, awarding LPP $22.0 million. Centennial subsequently received a demand from LPP for payment of the arbitration award plus interest and attorneys' fees. An accrual related to the guarantee as a result of the arbitration award was recorded in discontinued operations on the Consolidated Statement of Income in the fourth quarter of 2011. CEM filed a petition with the New York Supreme Court to vacate the arbitration award in favor of LPP. The New York Supreme Court granted CEM's petition to vacate the arbitration award on November 20, 2012, and entered an order and judgment to that effect on June 5, 2013. LPP appealed the order and judgment and on February 20, 2014, the New York Supreme Court Appellate Division ruled the arbitration award was properly vacated. LPP filed a motion with the New York Court of Appeals for leave to appeal the decision of the New York Supreme Court Appellate Division. The motion was denied on June 10, 2014. Due to the vacation of the arbitration award, the accrual that was previously recorded in 2011 was reversed in the fourth quarter of 2012. The effect of this was recorded in discontinued operations on the Consolidated Statement of Income. For more information regarding discontinued operations, see Note 11.
	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.
	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, but a decision has not been issued.
	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. Fidelity believes it has meritorious defenses against class certification and the claims.
	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 is preparing a staff report which will recommend a cleanup alternative for the site. 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 an order reauthorizing the deferred accounting for the 12 months starting November 30, 2013.
	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 $12.6 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
	Centennial guaranteed CEM's obligations under a construction contract. For more information, see Litigation in this note.
	In connection with the sale of the Brazilian Transmission Lines, as discussed in Note 12, 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 June 30, 2014, expire in the years ranging from 2014 to 2015; however, Fidelity continues to enter into additional derivative instruments and, as a result, WBI Holdings from time to time may issue additional guarantees on these derivative instruments. The amount outstanding by Fidelity was $14.5 million and was reflected on the Consolidated Balance Sheet at June 30, 2014. 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 June 30, 2014, the fixed maximum amounts guaranteed under these agreements aggregated $76.7 million. The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate $21.2 million in 2014; $35.7 million in 2015; $700,000 in 2016; $600,000 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 June 30, 2014. 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 June 30, 2014, the fixed maximum amounts guaranteed under these letters of credit, aggregated $36.3 million. In 2014 and 2015, $7.0 million and $29.3 million, respectively, of letters of credit are scheduled to expire. There were no amounts outstanding under the above letters of credit at June 30, 2014.
	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 June 30, 2014, 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 June 30, 2014, 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 June 30, 2014.
	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 June 30, 2014, approximately $666.9 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 $300.0 million, under the agreement are $150.0 million and $75.0 million, respectively. Capital commitments in excess of $300.0 million are expected to be shared equally between WBI Energy and Calumet. The total project cost is currently estimated at $350 million. Dakota Prairie Refining entered into a term loan for project debt financing of $75.0 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.
	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.
	Construction of Dakota Prairie Refinery began in early 2013 and the plant is not yet operational. Therefore, the results of operations of Dakota Prairie Refining did not have a material effect on the Company's Consolidated Statements of Income. 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:
		30-Jun-14		30-Jun-13		31-Dec-13
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	32,283	$	63,089	$	4,774
	Accounts receivable	—		5		—
	Other current assets	2,136		—		26
	Total current assets	34,419		63,094		4,800
	Net property, plant and equipment	254,079		75,216		172,073
	Total assets	$	288,498	$	138,310	$	176,873
	LIABILITIES
	Current liabilities:
	Long-term debt due within one year	$	3,000	$	3,000	$	3,000
	Accounts payable	28,150		21,057		8,904
	Taxes payable	225		—		5
	Accrued compensation	256		—		26
	Other accrued liabilities	494		300		461
	Total current liabilities	32,125		24,357		12,396
	Long-term debt	69,000		72,000		72,000
	Total liabilities	$	101,125	$	96,357	$	84,396
	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 June 30, 2014, 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 June 30, 2014, was $9.7 million.

Subsequent_event

Subsequent event	6 Months Ended
	Jun. 30, 2014
Subsequent Events [Abstract]	'
Subsequent events [Text Block]	'
	Subsequent Events
	The Company entered into a $150.0 million note purchase agreement on January 28, 2014. On April 15, 2014, the Company issued $50.0 million of Senior Notes with a due date of April 15, 2044, at an interest rate of 5.2 percent. The remaining $100.0 million of Senior Notes was issued on July 15, 2014, with due dates ranging from July 15, 2024 to July 15, 2026, at a weighted average interest rate of 4.3 percent.
	Fidelity entered into a purchase and sale agreement on July 17, 2014, to sell certain oil and natural gas properties in Mountrail County, North Dakota for a sale price of approximately $200.0 million, subject to accounting and purchase price adjustments customary with dispositions of this type. The effective date of the disposition is May 1, 2014, with the expected closing date to occur on or before September 30, 2014, conditioned upon the buyer completing a due diligence process, including environmental reviews, and both parties satisfying other standard closing conditions.

Inventories_and_natural_gas_in1

Inventories and natural gas in storage (Tables)	6 Months Ended
	Jun. 30, 2014
Inventory Disclosure [Abstract]	'
Inventories	'
	Inventories consisted of:
		June 30,		June 30,		December 31,
		2014		2013		2013
		(In thousands)
	Aggregates held for resale	$	112,129	$	103,503	$	101,568
	Asphalt oil	76,525		91,837		38,099
	Materials and supplies	70,938		74,648		69,808
	Merchandise for resale	25,507		27,330		21,720
	Natural gas in storage (current)	10,903		14,287		16,417
	Other	35,420		34,280		34,779
	Total	$	331,422	$	345,885	$	282,391

Earnings_per_common_share_Tabl

Earnings per common share (Tables)	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Weighted average common shares outstanding	'
	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows:
		Three Months Ended		Six Months Ended
		June 30,		June 30,
		2014	2013	2014	2013
		(In thousands)
	Weighted average common shares outstanding - basic	192,060	188,831	190,946	188,831
	Effect of dilutive performance share awards	599	632	597	629
	Weighted average common shares outstanding - diluted	192,659	189,463	191,543	189,460
	Shares excluded from the calculation of diluted earnings per share	—	—	—	—

Cash_flow_information_Tables

Cash flow information (Tables)	6 Months Ended
	Jun. 30, 2014
Supplemental Cash Flow Information [Abstract]	'
Supplemental cash flow information	'
	Cash expenditures for interest and income taxes were as follows:
		Six Months Ended
		June 30,
		2014		2013
		(In thousands)
	Interest, net of amount capitalized	$	39,441	$	41,440
	Income taxes paid (refunded), net	$	39,984	$	(2,649	)
	Noncash investing transactions were as follows:
		June 30,
		2014		2013
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	95,833	$	77,073

Comprehensive_income_loss_Tabl

Comprehensive income (loss) (Tables)	6 Months Ended
	Jun. 30, 2014
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 June 30, 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	$	(3,421	)	$	(33,532	)	$	(621	)	$	36		$	(37,538	)
	Other comprehensive income (loss) before reclassifications	—			—			42			8			50
	Amounts reclassified from accumulated other comprehensive loss	13			245			—			32			290
	Net current-period other comprehensive income	13			245			42			40			340
	Balance at end of period	$	(3,408	)	$	(33,287	)	$	(579	)	$	76		$	(37,198	)
	Three Months Ended June 30, 2013	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	$	(2,603	)	$	(53,699	)	$	(423	)	$	110		$	(56,615	)
	Other comprehensive income (loss) before reclassifications	254			—			(390		)	(142		)	(278		)
	Amounts reclassified from accumulated other comprehensive loss	(395		)	424			—			43			72
	Net current-period other comprehensive income (loss)	(141		)	424			(390		)	(99		)	(206		)
	Balance at end of period	$	(2,744	)	$	(53,275	)	$	(813	)	$	11		$	(56,821	)
	Six Months Ended June 30, 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	—			—			88			10			98
	Amounts reclassified from accumulated other comprehensive loss	357			520			—			32			909
	Net current-period other comprehensive income	357			520			88			42			1,007
	Balance at end of period	$	(3,408	)	$	(33,287	)	$	(579	)	$	76		$	(37,198	)
	Six Months Ended June 30, 2013	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	$	6,018		$	(54,347	)	$	(511	)	$	119		$	(48,721	)
	Other comprehensive income (loss) before reclassifications	(5,594		)	—			(302		)	(187		)	(6,083		)
	Amounts reclassified from accumulated other comprehensive loss	(3,168		)	1,072			—			79			(2,017		)
	Net current-period other comprehensive income (loss)	(8,762		)	1,072			(302		)	(108		)	(8,100		)
	Balance at end of period	$	(2,744	)	$	(53,275	)	$	(813	)	$	11		$	(56,821	)
Reclassification out of accumulated other comprehensive income	'
	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended						Six Months Ended						Location on Consolidated Statements of Income
		June 30,						June 30,
		2014			2013			2014			2013
		(In thousands)
	Reclassification adjustment for gain (loss) on derivative instruments included in net income:
	Commodity derivative instruments	$	137		$	1,381		$	(250	)	$	5,896		Operating revenues
	Interest rate derivative instruments	(160		)	(664		)	(320		)	(780		)	Interest expense
		(23		)	717			(570		)	5,116
		10			(322		)	213			(1,948		)	Income taxes
		(13		)	395			(357		)	3,168
	Amortization of postretirement liability losses included in net periodic benefit cost	(395		)	(967		)	(838		)	(1,934		)	(a)
		150			543			318			862			Income taxes
		(245		)	(424		)	(520		)	(1,072		)
	Reclassification adjustment for loss on available-for-sale investments included in net income	(49		)	(66		)	(49		)	(121		)	Other income
		17			23			17			42			Income taxes
		(32		)	(43		)	(32		)	(79		)
	Total reclassifications	$	(290	)	$	(72	)	$	(909	)	$	2,017
	(a) Included in net periodic benefit cost (credit). For more information, see Note 19.

Goodwill_and_other_intangible_1

Goodwill and other intangible assets (Tables)	6 Months Ended
	Jun. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Changes in the carrying amount of goodwill	'
	The changes in the carrying amount of goodwill were as follows:
	Six Months Ended	Balance			Goodwill			Balance
	30-Jun-14	as of			Acquired			as of
		January 1,			During			June 30, 2014*
		2014*			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	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.
	Six Months Ended	Balance			Goodwill			Balance
	30-Jun-13	as of			Acquired			as of
		January 1,			During the			June 30, 2013*
		2013*			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-13	as of			Acquired			as of
		January 1,			During the			December 31,
		2013*			Year			2013*
		(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.
Other amortizable intangible assets	'
	Other amortizable intangible assets were as follows:
		June 30,			June 30,			December 31,
		2014			2013			2013
		(In thousands)
	Customer relationships	$	21,310		$	21,310		$	21,310
	Accumulated amortization	(14,734		)	(12,715		)	(13,726		)
		6,576			8,595			7,584
	Noncompete agreements	5,080			6,186			6,186
	Accumulated amortization	(3,936		)	(4,557		)	(4,840		)
		1,144			1,629			1,346
	Other	10,921			10,979			10,995
	Accumulated amortization	(7,375		)	(5,891		)	(6,826		)
		3,546			5,088			4,169
	Total	$	11,266		$	15,312		$	13,099

Derivative_instruments_Tables

Derivative instruments (Tables)	6 Months Ended
	Jun. 30, 2014
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						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of loss recognized in accumulated other comprehensive loss (effective portion), net of tax	$	—		$	—		$	—		$	(6,154	)
	Amount of (gain) loss reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(86		)	(871		)	158			(3,714		)
	Amount of loss recognized in operating revenues (ineffective portion), before tax	—			—			—			(1,422		)
	Interest rate derivatives designated as cash flow hedges:
	Amount of gain recognized in accumulated other comprehensive loss (effective portion), net of tax	—			254			—			560
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	99			475			199			546
	Amount of loss recognized in interest expense (ineffective portion), before tax	—			(610		)	—			(769		)
	Commodity derivatives not designated as hedging instruments:
	Amount of gain (loss) recognized in operating revenues, before tax	(5,196		)	13,047			(11,908		)	8,637
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 June 30, 2014			Fair Value at June 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	129		$	9,797		$	1,447
		Other assets - noncurrent	131			1,447			503
	Total asset derivatives		$	260		$	11,244		$	1,950
	Liability	Location on	Fair Value at June 30, 2014			Fair Value at June 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	17,449		$	1,388		$	7,483
	Total liability derivatives		$	17,449		$	1,388		$	7,483
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:
	30-Jun-14	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	260		$	(260	)	$	—
	Total assets	$	260		$	(260	)	$	—
	Liabilities:
	Commodity derivatives	$	17,449		$	(260	)	$	17,189
	Total liabilities	$	17,449		$	(260	)	$	17,189
	30-Jun-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	11,244		$	(1,388	)	$	9,856
	Total assets	$	11,244		$	(1,388	)	$	9,856
	Liabilities:
	Commodity derivatives	$	1,388		$	(1,388	)	$	—
	Total liabilities	$	1,388		$	(1,388	)	$	—
	31-Dec-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	1,950		$	(1,950	)	$	—
	Total assets	$	1,950		$	(1,950	)	$	—
	Liabilities:
	Commodity derivatives	$	7,483		$	(1,950	)	$	5,533
	Total liabilities	$	7,483		$	(1,950	)	$	5,533

Fair_value_measurements_Tables

Fair value measurements (Tables)	6 Months Ended
	Jun. 30, 2014
Fair Value Disclosures [Abstract]	'
Available-for-sale securities	'
	Details of available-for-sale securities were as follows:
	June 30, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,989	$	91	$	(5	)	$	8,075
	U.S. Treasury securities	2,066		30		—			2,096
	Total	$	10,055	$	121	$	(5	)	$	10,171
	June 30, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,035	$	58	$	(41	)	$	8,052
	U.S. Treasury securities	1,920		15		(15		)	1,920
	Total	$	9,955	$	73	$	(56	)	$	9,972
	December 31, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,151	$	69	$	(27	)	$	8,193
	U.S. Treasury securities	1,906		15		(4		)	1,917
	Total	$	10,057	$	84	$	(31	)	$	10,110
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 June 30, 2014, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at June 30, 2014
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	19,990	$	—		$	19,990
	Insurance contract*	—		64,449		—			64,449
	Available-for-sale securities:
	Mortgage-backed securities	—		8,075		—			8,075
	U.S. Treasury securities	—		2,096		—			2,096
	Commodity derivative instruments	—		260		—			260
	Total assets measured at fair value	$	—	$	94,870	$	—		$	94,870
	Liabilities:
	Commodity derivative instruments	$	—	$	17,449	$	—		$	17,449
	Total liabilities measured at fair value	$	—	$	17,449	$	—		$	17,449
	*  The insurance contract invests approximately 21 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, 31 percent in fixed-income investments and 1 percent in cash equivalents.
		Fair Value Measurements at June 30, 2013, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at June 30, 2013
		(Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	29,902	$	—		$	29,902
	Insurance contract*	—		54,039		—			54,039
	Available-for-sale securities:
	Mortgage-backed securities	—		8,052		—			8,052
	U.S. Treasury securities	—		1,920		—			1,920
	Commodity derivative instruments	—		11,244		—			11,244
	Total assets measured at fair value	$	—	$	105,157	$	—		$	105,157
	Liabilities:
	Commodity derivative instruments	$	—	$	1,388	$	—		$	1,388
	Total liabilities measured at fair value	$	—	$	1,388	$	—		$	1,388
	*  The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 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, 2013, Using
		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2013
						(Level 3)
		(In thousands)
	Assets:
	Money market funds	$	—	$	19,227	$	—		$	19,227
	Insurance contract*	—		62,370		—			62,370
	Available-for-sale securities:
	Mortgage-backed securities	—		8,193		—			8,193
	U.S. Treasury securities	—		1,917		—			1,917
	Commodity derivative instruments	—		1,950		—			1,950
	Total assets measured at fair value	$	—	$	93,657	$	—		$	93,657
	Liabilities:
	Commodity derivative instruments	$	—	$	7,483	$	—		$	7,483
	Total liabilities measured at fair value	$	—	$	7,483	$	—		$	7,483
	* The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.
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 June 30, 2014	$	2,187,094	$	2,283,351
	Long-term debt at June 30, 2013	$	2,006,754	$	2,090,208
	Long-term debt at December 31, 2013	$	1,854,563	$	1,912,590

Equity_Tables

Equity (Tables)	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Summary of changes in equity	'
	A summary of the changes in equity was as follows:
	Six Months Ended June 30, 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)	110,768			(1,302		)	109,466
	Other comprehensive income	1,007			—			1,007
	Dividends declared on preferred stocks	(342		)	—			(342		)
	Dividends declared on common stock	(68,025		)	—			(68,025		)
	Stock-based compensation	2,796			—			2,796
	Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	(5,564		)	—			(5,564		)
	Net tax benefit on stock-based compensation	4,729			—			4,729
	Issuance of common stock	131,973			—			131,973
	Contribution from noncontrolling interest	—			32,500			32,500
	Balance at June 30, 2014	$	3,000,506		$	63,936		$	3,064,442
	Six Months Ended June 30, 2013	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2012	$	2,648,248		$	—		$	2,648,248
	Net income (loss)	103,027			(179		)	102,848
	Other comprehensive loss	(8,100		)	—			(8,100		)
	Dividends declared on preferred stocks	(342		)	—			(342		)
	Dividends declared on common stock	(65,147		)	—			(65,147		)
	Stock-based compensation	2,453			—			2,453
	Net tax deficit on stock-based compensation	(1,419		)	—			(1,419		)
	Contribution from noncontrolling interest	—			16,101			16,101
	Balance at June 30, 2013	$	2,678,720		$	15,922		$	2,694,642

Business_segment_data_Tables

Business segment data (Tables)	6 Months Ended
	Jun. 30, 2014
Segment Reporting [Abstract]	'
Information on the Company's businesses	'
	Information on the Company's businesses was as follows:
	Three Months Ended June 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	65,149	$	—		$	7,823
	Natural gas distribution	146,077		—			(4,494		)
	Pipeline and energy services	43,463		7,888			5,789
		254,689		7,888			9,118
	Exploration and production	129,309		10,271			19,180
	Construction materials and contracting	434,452		8,106			10,554
	Construction services	275,109		7,273			14,307
	Other	487		1,744			1,608
		839,357		27,394			45,649
	Intersegment eliminations	—		(35,282		)	(832		)
	Total	$	1,094,046	$	—		$	53,935
	Three Months Ended June 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	56,981	$	—		$	4,410
	Natural gas distribution	127,584		—			(5,893		)
	Pipeline and energy services	42,877		7,999			(6,395		)
		227,442		7,999			(7,878		)
	Exploration and production	137,053		12,556			32,995
	Construction materials and contracting	418,345		12,958			10,025
	Construction services	277,259		2,340			12,915
	Other	496		1,839			340
		833,153		29,693			56,275
	Intersegment eliminations	—		(37,692		)	(2,056		)
	Total	$	1,060,595	$	—		$	46,341
	Six Months Ended June 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	138,796	$	—		$	18,856
	Natural gas distribution	520,311		—			22,768
	Pipeline and energy services	87,124		26,164			10,138
		746,231		26,164			51,762
	Exploration and production	245,976		31,139			40,120
	Construction materials and contracting	598,875		12,123			(13,019		)
	Construction services	545,002		11,010			30,875
	Other	815		3,468			1,872
		1,390,668		57,740			59,848
	Intersegment eliminations	—		(83,904		)	(1,184		)
	Total	$	2,136,899	$	—		$	110,426
	Six Months Ended June 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	121,635	$	—		$	14,235
	Natural gas distribution	459,337		—			26,624
	Pipeline and energy services	70,593		26,717			(4,064		)
		651,565		26,717			36,795
	Exploration and production	252,415		22,369			53,279
	Construction materials and contracting	580,323		17,251			(10,557		)
	Construction services	507,065		3,914			24,579
	Other	830		3,657			645
		1,340,633		47,191			67,946
Intersegment eliminations	—		(73,908		)	(2,056		)
Total	$	1,992,198	$	—		$	102,685

Employee_benefit_plans_Tables

Employee benefit plans (Tables)	6 Months Ended
	Jun. 30, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [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 June 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	31		$	37		$	380		$	334
	Interest cost	4,405			4,106			924			667
	Expected return on assets	(5,484		)	(4,875		)	(1,242		)	(1,065		)
	Amortization of prior service cost (credit)	18			18			(348		)	(364		)
	Amortization of net actuarial loss	1,121			1,716			6			407
	Net periodic benefit cost, including amount capitalized	91			1,002			(280		)	(21		)
	Less amount capitalized	73			158			(19		)	61
	Net periodic benefit cost (credit)	$	18		$	844		$	(261	)	$	(82	)
								Other
								Postretirement
		Pension Benefits						Benefits
	Six Months Ended June 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	64		$	77		$	759		$	838
	Interest cost	8,845			8,124			1,782			1,607
	Expected return on assets	(10,609		)	(9,958		)	(2,309		)	(2,172		)
	Amortization of prior service cost (credit)	36			36			(696		)	(728		)
	Amortization of net actuarial loss	2,434			3,580			324			1,078
	Net periodic benefit cost, including amount capitalized	770			1,859			(140		)	623
	Less amount capitalized	168			268			10			90
	Net periodic benefit cost (credit)	$	602		$	1,591		$	(150	)	$	533

Contingencies_Commitment_and_C

Contingencies Commitment and Contingencies Disclosure (Tables)	6 Months Ended
	Jun. 30, 2014
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:
		30-Jun-14		30-Jun-13		31-Dec-13
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	32,283	$	63,089	$	4,774
	Accounts receivable	—		5		—
	Other current assets	2,136		—		26
	Total current assets	34,419		63,094		4,800
	Net property, plant and equipment	254,079		75,216		172,073
	Total assets	$	288,498	$	138,310	$	176,873
	LIABILITIES
	Current liabilities:
	Long-term debt due within one year	$	3,000	$	3,000	$	3,000
	Accounts payable	28,150		21,057		8,904
	Taxes payable	225		—		5
	Accrued compensation	256		—		26
	Other accrued liabilities	494		300		461
	Total current liabilities	32,125		24,357		12,396
	Long-term debt	69,000		72,000		72,000
	Total liabilities	$	101,125	$	96,357	$	84,396

Accounts_receivable_and_allowa1

Accounts receivable and allowance for doubtful accounts (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
In Millions, unless otherwise specified
Receivables [Abstract]	'	'	'
Receivables past due 90 days or more	$29.60	$36.40	$35.10
Allowance for doubtful accounts receivable	$9.60	$10.10	$10.60

Inventories_and_natural_gas_in2

Inventories and natural gas in storage (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Inventory Disclosure [Abstract]	'	'	'
Aggregates held for resale	$112,129,000	$101,568,000	$103,503,000
Asphalt oil	76,525,000	38,099,000	91,837,000
Materials and supplies	70,938,000	69,808,000	74,648,000
Merchandise for resale	25,507,000	21,720,000	27,330,000
Natural gas in storage (current)	10,903,000	16,417,000	14,287,000
Other	35,420,000	34,779,000	34,280,000
Total	331,422,000	282,391,000	345,885,000
Natural gas in storage noncurrent	$47,400,000	$48,300,000	$48,600,000

Impairment_of_longlived_assets1

Impairment of long-lived assets (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2013
Asset Impairment Charges [Abstract]	'
Asset impairment charges	$14.50
Asset impairment charges (after tax)	$9

Earnings_per_common_share_Deta

Earnings per common share (Details)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]	'	'	'	'
Weighted average common shares outstanding - basic	192,060	188,831	190,946	188,831
Effect of dilutive performance share awards	599	632	597	629
Weighted average common shares outstanding - diluted	192,659	189,463	191,543	189,460
Shares excluded from the calculation of diluted earnings per share	0	0	0	0

Cash_flow_information_Details

Cash flow information (Details) (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013
Supplemental Cash Flow Information [Abstract]	'	'
Interest, net of amount capitalized	$39,441	$41,440
Income taxes paid (refunded), net	39,984	-2,649
Property, plant and equipment additions in accounts payable	$95,833	$77,073

Comprehensive_income_loss_Deta

Comprehensive income (loss) (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	($37,538)	($56,615)	($38,205)	($48,721)
Other comprehensive income (loss) before reclassifications	50	-278	98	-6,083
Amounts reclassified from accumulated other comprehensive loss	290	72	909	-2,017
Net current-period other comprehensive income (loss)	340	-206	1,007	-8,100
Balance	-37,198	-56,821	-37,198	-56,821
Net unrealized gain (loss) on derivative instruments qualifying as hedges	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	-3,421	-2,603	-3,765	6,018
Other comprehensive income (loss) before reclassifications	0	254	0	-5,594
Amounts reclassified from accumulated other comprehensive loss	13	-395	357	-3,168
Net current-period other comprehensive income (loss)	13	-141	357	-8,762
Balance	-3,408	-2,744	-3,408	-2,744
Postretirement liability adjustment	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	-33,532	-53,699	-33,807	-54,347
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	245	424	520	1,072
Net current-period other comprehensive income (loss)	245	424	520	1,072
Balance	-33,287	-53,275	-33,287	-53,275
Foreign currency translation adjustment	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	-621	-423	-667	-511
Other comprehensive income (loss) before reclassifications	42	-390	88	-302
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	42	-390	88	-302
Balance	-579	-813	-579	-813
Net unrealized gain (loss) on available-for-sale investments	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	36	110	34	119
Other comprehensive income (loss) before reclassifications	8	-142	10	-187
Amounts reclassified from accumulated other comprehensive loss	32	43	32	79
Net current-period other comprehensive income (loss)	40	-99	42	-108
Balance	$76	$11	$76	$11

Reclassification_out_of_accumu

Reclassification out of accumulated other comprehensive income (loss) (Details 2) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Revenues	$1,094,046	$1,060,595	$2,136,899	$1,992,198
Interest expense	-21,516	-21,427	-42,487	-42,300
Other income	2,777	1,436	4,908	2,677
Income before income taxes	79,898	71,380	164,014	155,967
Income taxes	27,118	24,988	55,050	52,983
Net income (loss)	53,327	46,333	109,466	102,848
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	-23	717	-570	5,116
Income taxes	10	-322	213	-1,948
Net income (loss)	-13	395	-357	3,168
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	-395	-967	-838	-1,934
Income taxes	150	543	318	862
Net income (loss)	-245	-424	-520	-1,072
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	-49	-66	-49	-121
Income taxes	17	23	17	42
Net income (loss)	-32	-43	-32	-79
Total reclassifications	'	'	'	'
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Net income (loss)	-290	-72	-909	2,017
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]	'	'	'	'
Revenues	137	1,381	-250	5,896
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	($160)	($664)	($320)	($780)

Acquisition_Details

Acquisition (Details) (USD $)	0 Months Ended
In Millions, unless otherwise specified	Mar. 06, 2014
Business Combinations [Abstract]	'
Business acquisition, cost of acquired entity, purchase price	$206.30

Equity_method_investments_Deta

Equity method investments (Details) (USD $)	Jun. 30, 2013	Nov. 30, 2010	Aug. 31, 2013	Aug. 31, 2012	Nov. 30, 2011	Nov. 30, 2010	Nov. 30, 2010	Nov. 30, 2010
In Millions, unless otherwise specified		ECTE [Member]	ECTE [Member]	ECTE [Member]	ECTE [Member]	ECTE [Member]	ENTE [Member]	ERTE [Member]
Schedule of Equity Method Investments [Line Items]	'	'	'	'	'	'	'	'
Percentage ownership interest in equity method investment sold	'	'	'	'	'	59.96%	100.00%	100.00%
Number of years over which remaining ownership interest in equity method investment will be sold	'	'4 years	'	'	'	'	'	'
Portion of remaining interest sold	'	'	25.00%	25.00%	25.00%	'	'	'
Total assets of equity method investments	$114.80	'	'	'	'	'	'	'
Total long-term debt of equity method investments	56.2	'	'	'	'	'	'	'
Investment in equity method investments	5.5	'	'	'	'	'	'	'
Undistributed earnings of equity method investments	$2	'	'	'	'	'	'	'

Goodwill_rollforward_Details

Goodwill rollforward (Details) (USD $)	6 Months Ended				12 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	$636,039,000	[1]	$636,039,000	[1]	$636,039,000	[1]
Goodwill acquired during the year	0		0		0
Balance as of end of period	636,039,000	[1]	636,039,000	[1]	636,039,000	[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		0
Balance as of end of period	345,736,000		345,736,000		345,736,000
Pipeline and energy services [Member]	'		'		'
Goodwill [Line Items]	'		'		'
Goodwill, impaired, accumulated impairment loss	12,300,000		12,300,000		12,300,000
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		0
Balance as of end of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
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		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	104,276,000		104,276,000		104,276,000
Goodwill acquired during the year	0		0		0
Balance as of end of period	$104,276,000		$104,276,000		$104,276,000
[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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, net (excluding goodwill)	$11,266,000	$15,312,000	$11,266,000	$15,312,000	$13,099,000
Amortization of intangible assets	1,000,000	1,000,000	1,800,000	1,800,000	'
Estimated amortization expense for amortizable intangible assets [Abstract]	'	'	'	'	'
2014	3,300,000	'	3,300,000	'	'
2015	2,500,000	'	2,500,000	'	'
2016	2,200,000	'	2,200,000	'	'
2017	1,900,000	'	1,900,000	'	'
2018	1,000,000	'	1,000,000	'	'
Thereafter	2,200,000	'	2,200,000	'	'
Customer relationships [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	21,310,000	21,310,000	21,310,000	21,310,000	21,310,000
Intangible assets, accumulated amortization	-14,734,000	-12,715,000	-14,734,000	-12,715,000	-13,726,000
Intangible assets, net (excluding goodwill)	6,576,000	8,595,000	6,576,000	8,595,000	7,584,000
Noncompete agreements [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	5,080,000	6,186,000	5,080,000	6,186,000	6,186,000
Intangible assets, accumulated amortization	-3,936,000	-4,557,000	-3,936,000	-4,557,000	-4,840,000
Intangible assets, net (excluding goodwill)	1,144,000	1,629,000	1,144,000	1,629,000	1,346,000
Other intangible assets [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	10,921,000	10,979,000	10,921,000	10,979,000	10,995,000
Intangible assets, accumulated amortization	-7,375,000	-5,891,000	-7,375,000	-5,891,000	-6,826,000
Intangible assets, net (excluding goodwill)	$3,546,000	$5,088,000	$3,546,000	$5,088,000	$4,169,000

Derivative_instruments_Details

Derivative instruments (Details) (USD $)	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	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
Amount of time remaining to transfer accumulated other comprehensive income to earnings	'6 months	'	'	'
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	-535,000	'	'	'
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	17,400,000	7,500,000	1,400,000	'
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments	$17,400,000	$7,500,000	$1,400,000	'
Oil swap and\or collar [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Derivative, nonmonetary notional amount	2,500,000	2,900,000	3,100,000	'
Natural gas swap and\or collar [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Derivative, nonmonetary notional amount	11,000,000	18,300,000	21,400,000	'

Gains_and_losses_on_derivative

Gains and losses on derivative instruments (Details 2) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$0	$254	$0	($5,594)
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	13	-395	357	-3,168
Commodity derivatives designated as cash flow hedges [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	0	0	0	-6,154
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	-86	-871	158	-3,714
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	0	0	-1,422
Interest rate derivatives designated as cash flow hedges [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	0	254	0	560
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	99	475	199	546
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	-610	0	-769
Commodity derivatives not designated as hedging instruments:	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in operating revenues, before tax	($5,196)	$13,047	($11,908)	$8,637

Derivative_instruments_Details1

Derivative instruments (Details 3) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'	'
Derivative asset, fair value, gross asset	$260	$1,950	$11,244
Derivative liability fair value gross liability	17,449	7,483	1,388
Not designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current assets [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments assets at fair value	129	1,447	9,797
Not designated as hedging instrument [Member] | Commodity contract [Member] | Other assets noncurrent [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments assets at fair value	131	503	1,447
Not designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current liabilities [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments liabilities at fair value	$17,449	$7,483	$1,388

Subject_to_master_netting_agre

Subject to master netting agreements (Details 4) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
In Thousands, unless otherwise specified
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	$260	$1,950	$11,244
Derivative liability fair value gross liability	17,449	7,483	1,388
Commodity derivatives [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	260	1,950	11,244
Gross amount not offset	-260	-1,950	-1,388
Derivative asset, fair value, net	0	0	9,856
Total assets [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	260	1,950	11,244
Gross amount not offset	-260	-1,950	-1,388
Derivative asset, fair value, net	0	0	9,856
Commodity derivatives - liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	17,449	7,483	1,388
Gross amount not offset	-260	-1,950	-1,388
Derivative liability, fair value, net	17,189	5,533	0
Total liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	17,449	7,483	1,388
Gross amount not offset	-260	-1,950	-1,388
Derivative liability, fair value, net	$17,189	$5,533	$0

Availableforsale_securities_De

Available-for-sale securities (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Available-for-sale securities [Line Items]	'	'	'	'	'
Investments used to satisfy obligations under nonqualified benefit plans	$64,400,000	$54,000,000	$64,400,000	$54,000,000	$62,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	1,100,000	700,000	2,000,000	5,100,000	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	10,055,000	9,955,000	10,055,000	9,955,000	10,057,000
Gross unrealized gains	'	'	121,000	73,000	84,000
Gross unrealized losses	'	'	-5,000	-56,000	-31,000
Fair value	10,171,000	9,972,000	10,171,000	9,972,000	10,110,000
Mortgage backed securities [Member]	'	'	'	'	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	7,989,000	8,035,000	7,989,000	8,035,000	8,151,000
Gross unrealized gains	'	'	91,000	58,000	69,000
Gross unrealized losses	'	'	-5,000	-41,000	-27,000
Fair value	8,075,000	8,052,000	8,075,000	8,052,000	8,193,000
US Treasury securities [Member]	'	'	'	'	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	2,066,000	1,920,000	2,066,000	1,920,000	1,906,000
Gross unrealized gains	'	'	30,000	15,000	15,000
Gross unrealized losses	'	'	0	-15,000	-4,000
Fair value	$2,096,000	$1,920,000	$2,096,000	$1,920,000	$1,917,000

Fair_value_measurements_Detail

Fair value measurements (Details 2) (USD $)	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
Concentration risks, percentage [Abstract]	'		'		'
Percentage investment in common stock of mid-cap companies	21.00%		29.00%		28.00%
Percentage investment in common stock of small-cap companies	18.00%		28.00%		28.00%
Percentage investment in common stock of large-cap companies	29.00%		28.00%		28.00%
Percentage in fixed-income and other investments	31.00%		15.00%		16.00%
Percentage investment in cash and cash equivalents	1.00%		'		'
Assets, Fair Value Disclosure, Nonrecurring	'		'		$9,700,000
Fair value, measurements, recurring [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	94,870,000		93,657,000		105,157,000
Liabilities, fair value disclosure	17,449,000		7,483,000		1,388,000
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Liabilities, fair value disclosure	17,449,000		7,483,000		1,388,000
Fair value, measurements, recurring [Member] | Money market funds [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	19,990,000		19,227,000		29,902,000
Fair value, measurements, recurring [Member] | Insurance contract [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	64,449,000	[1]	62,370,000	[2]	54,039,000	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	8,075,000		8,193,000		8,052,000
Fair value, measurements, recurring [Member] | US Treasury securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	2,096,000		1,917,000		1,920,000
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	260,000		1,950,000		11,244,000
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		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Liabilities, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Mortgage backed securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0		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	94,870,000		93,657,000		105,157,000
Liabilities, fair value disclosure	17,449,000		7,483,000		1,388,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Liabilities, fair value disclosure	17,449,000		7,483,000		1,388,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	19,990,000		19,227,000		29,902,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	64,449,000	[1]	62,370,000	[2]	54,039,000	[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	8,075,000		8,193,000		8,052,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	2,096,000		1,917,000		1,920,000
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	260,000		1,950,000		11,244,000
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		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Liabilities, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Mortgage backed securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member]	'		'		'
Fair value measurements [Line Items]	'		'		'
Assets, fair value disclosure	0		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
[1]	The insurance contract invests approximately 21 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, 31 percent in fixed-income investments and 1 percent in cash equivalents.
[2]	The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.
[3]	The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 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 $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
In Thousands, unless otherwise specified
Carrying amount [Member]	'	'	'
Fair value, balance sheet grouping [Line Items]	'	'	'
Long-term debt	$2,187,094	$1,854,563	$2,006,754
Fair value [Member]	'	'	'
Fair value, balance sheet grouping [Line Items]	'	'	'
Long-term debt, fair value	$2,283,351	$1,912,590	$2,090,208

Longterm_debt_Details

Long-term debt (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013	8-May-14	Apr. 15, 2014	Jan. 28, 2014	Apr. 02, 2014	Mar. 31, 2014	8-May-14	Jul. 15, 2014
In Millions, unless otherwise specified			The Company [Member]	The Company [Member]	The Company [Member]	Centennial Energy Holdings, Inc [Member]	Centennial Energy Holdings, Inc [Member]	Centennial Energy Holdings, Inc [Member]	Subsequent event [Member]
									The Company [Member]
Line of Credit Facility [Line Items]	'	'	'	'	'	'	'	'	'
Note purchase agreement	'	'	'	'	$150	'	'	'	'
Senior notes issued	'	'	'	50	'	'	'	'	100
Senior notes weighted average interest rate	'	'	'	5.20%	'	'	'	'	4.30%
Facility, maximum borrowing capacity	'	'	175	'	'	'	'	650	'
Ratio of total debt to total capitalization as specified in debt convenants	'	'	'	'	'	65.00%	65.00%	65.00%	'
Debt instrument, term	'	'	'	'	'	'2 years	'2 years	'	'
Term loan agreement	'	'	'	'	'	125	125	'	'
Long-term commercial paper	$268	$153.90	'	'	'	'	'	'	'

Equity_Details

Equity (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Balance	'	'	'	'
Total stockholders' equity	'	'	$2,823,164	'
Noncontrolling interest	'	'	32,738	'
Total equity	'	'	2,855,902	2,648,248
Net loss attributable to noncontrolling interest	-779	-179	-1,302	-179
Net income (loss)	53,327	46,333	109,466	102,848
Other comprehensive income	340	-206	1,007	-8,100
Dividends declared on preferred stocks	'	'	-342	-342
Dividends declared on common stock	'	'	-68,025	-65,147
Stock-based compensation	'	'	2,796	2,453
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	'	'	-5,564	'
Net tax benefit on stock-based compensation	'	'	4,729	-1,419
Issuance of common stock	'	'	131,973	'
Contribution from noncontrolling interest	'	'	32,500	16,101
Total stockholders' equity	3,000,506	2,678,720	3,000,506	2,678,720
Noncontrolling interest	63,936	15,922	63,936	15,922
Total equity	3,064,442	2,694,642	3,064,442	2,694,642
Total stockholders’ equity [Member]	'	'	'	'
Balance	'	'	'	'
Total stockholders' equity	'	'	2,823,164	2,648,248
Net income (loss) attributable to parent	'	'	110,768	103,027
Other comprehensive income	'	'	1,007	-8,100
Dividends declared on preferred stocks	'	'	-342	-342
Dividends declared on common stock	'	'	-68,025	-65,147
Stock-based compensation	'	'	2,796	2,453
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	'	'	-5,564	'
Net tax benefit on stock-based compensation	'	'	4,729	-1,419
Issuance of common stock	'	'	131,973	'
Contribution from noncontrolling interest	'	'	0	0
Total stockholders' equity	3,000,506	2,678,720	3,000,506	2,678,720
Noncontrolling interest [Member]	'	'	'	'
Balance	'	'	'	'
Noncontrolling interest	'	'	32,738	0
Net loss attributable to noncontrolling interest	'	'	-1,302	-179
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	'
Net tax benefit on stock-based compensation	'	'	0	0
Issuance of common stock	'	'	0	'
Contribution from noncontrolling interest	'	'	32,500	16,101
Noncontrolling interest	$63,936	$15,922	$63,936	$15,922

Business_segment_data_Details

Business segment data (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	$1,094,046	$1,060,595	$2,136,899	$1,992,198
Intersegment operating revenues	0	0	0	0
Earnings on common stock	53,935	46,341	110,426	102,685
Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	254,689	227,442	746,231	651,565
Intersegment operating revenues	7,888	7,999	26,164	26,717
Earnings on common stock	9,118	-7,878	51,762	36,795
Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	839,357	833,153	1,390,668	1,340,633
Intersegment operating revenues	27,394	29,693	57,740	47,191
Earnings on common stock	45,649	56,275	59,848	67,946
Electric [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	65,149	56,981	138,796	121,635
Intersegment operating revenues	0	0	0	0
Earnings on common stock	7,823	4,410	18,856	14,235
Natural gas distribution [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	146,077	127,584	520,311	459,337
Intersegment operating revenues	0	0	0	0
Earnings on common stock	-4,494	-5,893	22,768	26,624
Pipeline and energy services [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	43,463	42,877	87,124	70,593
Intersegment operating revenues	7,888	7,999	26,164	26,717
Earnings on common stock	5,789	-6,395	10,138	-4,064
Exploration and production [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	129,309	137,053	245,976	252,415
Intersegment operating revenues	10,271	12,556	31,139	22,369
Earnings on common stock	19,180	32,995	40,120	53,279
Construction materials and contracting [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	434,452	418,345	598,875	580,323
Intersegment operating revenues	8,106	12,958	12,123	17,251
Earnings on common stock	10,554	10,025	-13,019	-10,557
Construction services [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	275,109	277,259	545,002	507,065
Intersegment operating revenues	7,273	2,340	11,010	3,914
Earnings on common stock	14,307	12,915	30,875	24,579
Other [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	487	496	815	830
Intersegment operating revenues	1,744	1,839	3,468	3,657
Earnings on common stock	1,608	340	1,872	645
Intersegment Eliminations [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	0	0	0	0
Intersegment operating revenues	-35,282	-37,692	-83,904	-73,908
Earnings on common stock	($832)	($2,056)	($1,184)	($2,056)

Employee_benefit_plans_Details

Employee benefit plans (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension benefits [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Service cost	$31	$37	$64	$77
Interest cost	4,405	4,106	8,845	8,124
Expected return on assets	-5,484	-4,875	-10,609	-9,958
Amortization of prior service cost (credit)	18	18	36	36
Amortization of net actuarial loss	1,121	1,716	2,434	3,580
Net periodic benefit cost, including amount capitalized	91	1,002	770	1,859
Less amount capitalized	73	158	168	268
Net periodic benefit cost (credit)	18	844	602	1,591
Other postretirement benefits [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Service cost	380	334	759	838
Interest cost	924	667	1,782	1,607
Expected return on assets	-1,242	-1,065	-2,309	-2,172
Amortization of prior service cost (credit)	-348	-364	-696	-728
Amortization of net actuarial loss	6	407	324	1,078
Net periodic benefit cost, including amount capitalized	-280	-21	-140	623
Less amount capitalized	-19	61	10	90
Net periodic benefit cost (credit)	-261	-82	-150	533
Supplemental employee retirement plans [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Net periodic benefit cost (credit)	$1,600	$1,800	$3,300	$3,600

Regulatory_matters_and_revenue1

Regulatory matters and revenues subject to refund (Details) (USD $)	0 Months Ended
In Millions, unless otherwise specified	Feb. 27, 2014	Nov. 17, 2013	Sep. 18, 2013	Oct. 31, 2013	Jul. 10, 2014
	NDPSC [Member]	NDPSC [Member]	NDPSC [Member]	FERC [Member]	Subsequent event [Member]
					NDPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]	'	'	'	'	'
Annual amount recovered	'	'	'	'	$8.60
Rate increase requested	7.4	'	6.8	'	'
Percent above current rates requested	4.60%	'	6.40%	'	'
Interim rate increase (decrease), amount	'	4.3	'	'	'
Interim rate increase, percent	'	4.00%	'	'	'
Increased expenditures covered by change application	'	'	'	312	'
Approximate annual increase in revenues	'	'	'	$11.50	'

Litigation_Details

Litigation (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Aug. 31, 2011	Aug. 06, 2012	Oct. 31, 2010	Jan. 13, 2012	Dec. 31, 2009
				Litigation related to construction materials [Member]	Former employee litigation [Member]	Natural gas gathering operations [Member]	Guarantee obligation under construction contract [Member]	Guarantee obligation under construction contract [Member]
Loss Contingencies [Line Items]	'	'	'	'	'	'	'	'
Potential liabilities related to litigation and environmental matters	$32,100,000	$29,500,000	$32,900,000	'	'	'	'	'
Amount of compensatory damages in LPP's notice of demand	'	'	'	'	'	'	'	149,700,000
Arbitration award	'	'	'	'	'	26,600,000	22,000,000	'
Loss contingency, estimate of possible loss	'	'	'	3,700,000	'	'	'	'
Additional amount per day of potential penalties	'	'	'	5,000	'	'	'	'
Plaintiffs request compensatory damages	'	'	'	'	$23,800,000	'	'	'

Enviromental_matters_Details_2

Enviromental matters (Details 2) (USD $)	6 Months Ended
	Jun. 30, 2014
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	12,600,000
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 $)	Jun. 30, 2014
Guarantor Obligations [Line Items]	'
Amount of hedging obligations guaranteed reflected on balance sheet	$14,500,000
Guarantor obligations, maximum exposure, undiscounted	76,700,000
Fixed maximum amounts guaranteed by year 2014	21,200,000
Fixed maximum amounts guaranteed by year 2015	35,700,000
Fixed maximum amounts guaranteed by year 2016	700,000
Fixed maximum amounts guaranteed by year 2017	600,000
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	36,300,000
Letters of credit set to expire - 2014	7,000,000
Letters of credit set to expire - 2015	29,300,000
Outstanding letters of credit	0
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	$666,900,000

Commitment_and_contingencies_v

Commitment and contingencies variable interest entities (Details 4) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
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 estimate	350,000,000	'	'
Term loan for project debt financing	75,000,000	'	'
Dakota Prairie Refining, LLC [Member] | Cash and Cash Equivalents [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	32,283,000	4,774,000	63,089,000
Dakota Prairie Refining, LLC [Member] | Accounts Receivable [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	0	0	5,000
Dakota Prairie Refining, LLC [Member] | Other current assets [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	2,136,000	26,000	0
Dakota Prairie Refining, LLC [Member] | Total current assets [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	34,419,000	4,800,000	63,094,000
Dakota Prairie Refining, LLC [Member] | Net property, plant and equipment [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	254,079,000	172,073,000	75,216,000
Dakota Prairie Refining, LLC [Member] | Total assets [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	288,498,000	176,873,000	138,310,000
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	28,150,000	8,904,000	21,057,000
Dakota Prairie Refining, LLC [Member] | Taxes Payable [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	225,000	5,000	0
Dakota Prairie Refining, LLC [Member] | Accrued Compensation [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	256,000	26,000	0
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	494,000	461,000	300,000
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	32,125,000	12,396,000	24,357,000
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	69,000,000	72,000,000	72,000,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	101,125,000	84,396,000	96,357,000
Fuel contract [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$9,700,000	'	'

Subsequent_event_Details

Subsequent event (Details) (USD $)	Apr. 15, 2014	Jan. 28, 2014	Jul. 17, 2014	Jul. 15, 2014
In Millions, unless otherwise specified	The Company [Member]	The Company [Member]	Subsequent event [Member]	Subsequent event [Member]
				The Company [Member]
Subsequent Event [Line Items]	'	'	'	'
Note purchase agreement	'	$150	'	'
Senior notes issued	50	'	'	100
Senior notes weighted average interest rate	5.20%	'	'	4.30%
Significant disposal, or sale proceeds	'	'	$200	'