MDU Resources (MDU) 10-Q 7 Nov 13 2013 Q3 Quarterly report Financial data

Document_And_Entity_Informatio

Document And Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 31, 2013
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	'	188,830,529
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating revenues:	'	'	'	'
Electric, natural gas distribution and pipeline and energy services	$192,103	$184,863	$843,670	$784,399
Exploration and production, construction materials and contracting, construction services and other	1,093,679	988,655	2,434,310	2,209,889
Total operating revenues	1,285,782	1,173,518	3,277,980	2,994,288
Operating expenses:	'	'	'	'
Fuel and purchased power	19,983	17,634	59,760	51,247
Purchased natural gas sold	35,826	35,199	305,268	279,038
Operation and maintenance:	'	'	'	'
Electric, natural gas distribution and pipeline and energy services	64,078	67,830	206,808	188,945
Exploration and production, construction materials and contracting, construction services and other	870,252	793,850	1,925,762	1,793,347
Depreciation, depletion and amortization	99,966	91,850	288,816	260,858
Taxes, other than income	45,804	41,090	145,784	132,017
Write-down of oil and natural gas properties (Note 5)	0	160,100	0	160,100
Total operating expenses	1,135,909	1,207,553	2,932,198	2,865,552
Operating income (loss)	149,873	-34,035	345,782	128,736
Earnings (loss) from equity method investments	-61	2,388	-380	4,025
Other income	2,326	1,702	5,003	4,050
Interest expense	21,012	19,840	63,312	56,929
Income (loss) before income taxes	131,126	-49,785	287,093	79,882
Income taxes	46,576	-20,253	99,559	24,516
Income (loss) from continuing operations	84,550	-29,532	187,534	55,366
Income (loss) from discontinued operations, net of tax (Note 11)	-118	-139	-254	4,867
Net income (loss)	84,432	-29,671	187,280	60,233
Net loss attributable to noncontrolling interest	-24	0	-204	0
Dividends declared on preferred stocks	171	171	514	514
Earnings (loss) on common stock	$84,285	($29,842)	$186,970	$59,719
Earnings (loss) per common share -- basic:	'	'	'	'
Earnings (loss) before discontinued operations	$0.45	($0.16)	$0.99	$0.29
Discontinued operations, net of tax	$0	$0	$0	$0.03
Earnings (loss) per common share -- basic	$0.45	($0.16)	$0.99	$0.32
Earnings (loss) per common share -- diluted:	'	'	'	'
Earnings (loss) before discontinued operations	$0.44	($0.16)	$0.99	$0.29
Discontinued operations, net of tax	$0	$0	$0	$0.03
Earnings (loss) per common share -- diluted	$0.44	($0.16)	$0.99	$0.32
Dividends declared per common share	$0.17	$0.17	$0.52	$0.50
Weighted average common shares outstanding - basic	188,831	188,831	188,831	188,824
Weighted average common shares outstanding - diluted	189,638	188,831	189,634	189,029

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Consolidated Statements of Comprehensive Income [Abstract]	'	'	'	'
Net income (loss)	$84,432	($29,671)	$187,280	$60,233
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 $(5,377) for the three months ended and $(3,116) and $4,570 for the nine months ended in 2013 and 2012, respectively	0	-9,125	-5,594	7,962
Reclassification adjustment for gain on derivative instruments included in net income, net of tax of $(297) and $(4,570) for the three months ended and $(2,246) and $(4,126) for the nine months ended in 2013 and 2012, respectively	-510	-7,782	-3,678	-7,029
Net unrealized gain (loss) on derivative instruments qualifying as hedges	-510	-16,907	-9,272	933
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 $(5) and $4 for the three months ended and $(106) and $(22) for the nine months ended in 2013 and 2012, respectively	-10	7	-197	-41
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $20 and $17 for the three months ended and $63 and $54 for the nine months ended in 2013 and 2012, respectively	38	32	117	101
Net unrealized gain (loss) on available-for-sale investments	28	39	-80	60
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $166 and $1,027 for the three and nine months ended in 2013	271	0	1,344	0
Foreign currency translation adjustment recognized during the period, net of tax of $(12) and $(10) for the three months ended and $(209) and $(275) for the nine months ended in 2013 and 2012, respectively	-20	-8	-351	-443
Reclassification adjustment for loss on foreign currency translation adjustment included in net income, net of tax of $70 and $2 for the three months ended and $70 and $2 for the nine months ended in 2013 and 2012, respectively	115	3	143	3
Foreign currency translation adjustment	95	-5	-208	-440
Other comprehensive income (loss)	-116	-16,873	-8,216	553
Comprehensive income (loss)	84,316	-46,544	179,064	60,786
Comprehensive loss attributable to noncontrolling interest	-24	0	-204	0
Comprehensive income (loss) attributable to common stockholders	$84,340	($46,544)	$179,268	$60,786

Consolidated_Statements_of_Com1

Consolidated Statements of Comprehensive Income - Parenthetical (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net unrealized gain (loss) on derivative instruments arising during the period, tax	$0	($5,377)	($3,116)	$4,570
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax	-297	-4,570	-2,246	-4,126
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	-5	4	-106	-22
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, tax	20	17	63	54
Amortization of postretirement liability losses included in net periodic benefit cost, tax	166	0	1,027	0
Foreign currency translation adjustments arising during the period, tax	-12	-10	-209	-275
Reclassification adjustment for foreign currency translation (gain) loss included in net income, tax	$70	$2	$70	$2

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
In Thousands, unless otherwise specified
Current assets:	'		'		'
Cash and cash equivalents	$66,174		$49,042		$74,242
Receivables, net	787,311		678,123		743,274
Inventories	314,571		317,415		315,767
Deferred income taxes	26,284		22,846		25,345
Commodity derivative instruments	4,373		18,304		19,193
Prepayments and other current assets	56,257		42,351		71,579
Total current assets	1,254,970		1,128,081		1,249,400
Investments	108,664		103,243		102,139
Property, plant and equipment	8,651,334		8,107,751		8,129,872
Less accumulated depreciation, depletion and amortization	3,796,052		3,608,912		3,546,927
Net property, plant and equipment	4,855,282		4,498,839		4,582,945
Deferred charges and other assets:	'		'		'
Goodwill	636,039	[1]	636,039	[1]	636,039	[1]
Other intangible assets, net	14,092		17,129		18,015
Other	298,061		299,160		314,133
Total deferred charges and other assets	948,192		952,328		968,187
Total assets	7,167,108		6,682,491		6,902,671
Current liabilities:	'		'		'
Short-term borrowings	7,000		28,200		11,000
Long-term debt due within one year	44,024		134,108		240,564
Accounts payable	437,740		388,015		402,241
Taxes payable	80,392		46,475		54,903
Dividends payable	32,745		171		31,800
Accrued compensation	62,746		48,448		48,792
Commodity derivative instruments	9,740		0		2,072
Other accrued liabilities	171,420		204,698		233,773
Total current liabilities	845,807		850,115		1,025,145
Long-term debt	1,967,872		1,610,867		1,502,413
Deferred credits and other liabilities:	'		'		'
Deferred income taxes	808,011		755,102		797,249
Other liabilities	794,928		818,159		834,934
Total deferred credits and other liabilities	1,602,939		1,573,261		1,632,183
Equity:	'		'		'
Preferred stocks	15,000		15,000		15,000
Common stockholders' equity:	'		'		'
Authorized - 500,000,000 shares, $1.00 par value. Shares issued - 189,369,450 at September 30, 2013 and 2012 and December 31, 2012	189,369		189,369		189,369
Other paid-in capital	1,041,787		1,039,080		1,038,066
Retained earnings	1,546,000		1,457,146		1,550,569
Accumulated other comprehensive loss	-56,937		-48,721		-46,448
Treasury stock at cost - 538,921 shares	-3,626		-3,626		-3,626
Total common stockholders' equity	2,716,593		2,633,248		2,727,930
Total stockholders' equity	2,731,593		2,648,248		2,742,930
Noncontrolling interest	18,897		0		0
Total equity	2,750,490		2,648,248		2,742,930
Total liabilities and equity	$7,167,108		$6,682,491		$6,902,671
[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 $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
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	189,369,450	189,369,450	189,369,450
Treasury shares	538,921	538,921	538,921

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Operating activities:	'	'
Net income (loss)	$187,280	$60,233
Income (loss) from discontinued operations, net of tax	-254	4,867
Income from continuing operations	187,534	55,366
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation, depletion and amortization	288,816	260,858
Earnings (loss), net of distributions, from equity method investments	1,736	-1,086
Deferred income taxes	46,212	40,310
Unrealized loss on commodity derivatives	5,379	523
Write-down of oil and natural gas properties	0	160,100
Changes in current assets and liabilities, net of acquisitions:	'	'
Receivables	-107,482	-89,596
Inventories	1,562	-40,386
Other current assets	-15,397	-18,512
Accounts payable	25,817	21,811
Other current liabilities	18,680	-32,994
Other noncurrent changes	-24,149	-20,206
Net cash provided by continuing operations	428,708	336,188
Net cash provided by (used in) discontinued operations	254	-6,826
Net cash provided by operating activities	428,962	329,362
Investing activities:	'	'
Capital expenditures	-648,465	-629,776
Acquisitions, net of cash acquired	0	-67,253
Net proceeds from sale or disposition of property and other	40,985	31,090
Investments	218	11,188
Proceeds from sale of equity method investment	1,896	2,394
Net cash used in continuing operations	-605,366	-652,357
Net cash provided by discontinued operations	0	0
Net cash used in investing activities	-605,366	-652,357
Financing activities:	'	'
Issuance of short-term borrowings	5,000	2,900
Issuance of long-term debt	497,318	400,443
Repayment of long-term debt	-255,980	-73,459
Proceeds from issuance of common stock	0	88
Dividends paid	-65,660	-95,394
Excess tax benefit on stock-based compensation	0	26
Contribution from noncontrolling interest	13,000	0
Net cash provided by continuing operations	193,678	234,604
Net cash provided by discontinued operations	0	0
Net cash provided by financing activities	193,678	234,604
Effect of exchange rate changes on cash and cash equivalents	-142	-139
Increase (decrease) in cash and cash equivalents	17,132	-88,530
Cash and cash equivalents - beginning of year	49,042	162,772
Cash and cash equivalents - end of period	$66,174	$74,242

Basis_of_presentation

Basis of presentation	9 Months Ended
	Sep. 30, 2013
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 2012 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 2012 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses. Management has also evaluated the impact of events occurring after September 30, 2013, up to the date of issuance of these consolidated interim financial statements.

Seasonality_of_operations

Seasonality of operations	9 Months Ended
	Sep. 30, 2013
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	9 Months Ended
	Sep. 30, 2013
Receivables [Abstract]	'
Accounts receivable and allowance for doubtful accounts	'
	Accounts receivable and allowance for doubtful accounts
	Accounts receivable consists primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts. The total balance of receivables past due 90 days or more was $31.1 million, $35.1 million and $34.3 million as of September 30, 2013 and 2012, and December 31, 2012, 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 as of September 30, 2013 and 2012, and December 31, 2012, was $9.6 million, $10.5 million and $10.8 million, respectively.

Inventories_and_natural_gas_in

Inventories and natural gas in storage	9 Months Ended
	Sep. 30, 2013
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:
		September 30,		September 30,		December 31,
		2013		2012		2012
		(In thousands)
	Aggregates held for resale	$	104,784	$	88,632	$	87,715
	Asphalt oil	43,078		47,084		67,480
	Materials and supplies	71,370		75,551		69,390
	Merchandise for resale	23,713		30,827		31,172
	Natural gas in storage (current)	37,689		41,091		29,030
	Other	33,937		32,582		32,628
	Total	$	314,571	$	315,767	$	317,415
	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 $48.6 million, $50.3 million, and $49.7 million at September 30, 2013 and 2012, and December 31, 2012, respectively.

Oil_and_natural_gas_properties

Oil and natural gas properties	9 Months Ended
	Sep. 30, 2013
Oil and Gas Property [Abstract]	'
Oil and natural gas properties	'
	Oil and natural gas properties
	The Company uses the full-cost method of accounting for its oil and natural gas production activities. Under this method, all costs incurred in the acquisition, exploration and development of oil and natural gas properties are capitalized and amortized on the units-of-production method based on total proved reserves. Any conveyances of properties, including gains or losses on abandonments of properties, are generally treated as adjustments to the cost of the properties with no gain or loss recognized.
	Capitalized costs are subject to a "ceiling test" that limits such costs to the aggregate of the present value of future net cash flows from proved reserves discounted at 10 percent, as mandated under the rules of the SEC, plus the cost of unproved properties not subject to amortization, less applicable income taxes. Proved reserves and associated future cash flows are determined based on SEC Defined Prices. If capitalized costs, less accumulated amortization and related deferred income taxes, exceed the full-cost ceiling at the end of any quarter, a permanent noncash write-down is required to be charged to earnings in that quarter regardless of subsequent price changes.
	The Company's capitalized costs under the full-cost method of accounting exceeded the full-cost ceiling at September 30, 2012, largely the result of lower SEC Defined Prices, primarily lower natural gas prices. Accordingly, the Company was required to write down its oil and natural gas producing properties. The noncash write-down amounted to $160.1 million ($100.9 million after tax) for the three and nine months ended September 30, 2012.
	The Company hedged a portion of its oil and natural gas production and the effects of the cash flow hedges were used in determining the full-cost ceiling at September 30, 2012. The Company would have recognized an additional write-down of its oil and natural gas properties of $19.5 million ($12.3 million after tax) at September 30, 2012, if the effects of cash flow hedges had not been considered in calculating the full-cost ceiling. For more information on the Company's cash flow hedges, see Note 14.
	At September 30, 2013, the Company's full-cost ceiling exceeded the Company's capitalized costs. However, there is risk that lower SEC Defined Prices, market differentials, changes in estimates of proved reserve quantities, unsuccessful results of exploration and development efforts or changes in operating and development costs could result in additional future noncash write-downs of the Company's oil and natural gas properties.

Impairment_of_longlived_assets

Impairment of long-lived assets	9 Months Ended
	Sep. 30, 2013
Asset Impairment Charges [Abstract]	'
Impairment of long-lived assets	'
	Impairment of long-lived assets
	During the second quarters of 2013 and 2012, the Company recognized impairments of coalbed natural gas gathering assets at the pipeline and energy services segment of $14.5 million ($9.0 million after tax) and $2.7 million ($1.7 million after tax), respectively, which are recorded in operation and maintenance expense on the Consolidated Statements of Income. The impairments are 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	9 Months Ended
	Sep. 30, 2013
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		Nine Months Ended
		September 30,		September 30,
		2013	2012	2013	2012
		(In thousands)
	Weighted average common shares outstanding - basic	188,831	188,831	188,831	188,824
	Effect of dilutive stock options and performance share awards	807	—	803	205
	Weighted average common shares outstanding - diluted	189,638	188,831	189,634	189,029
	Shares excluded from the calculation of diluted earnings per share	—	434	—	—

Cash_flow_information

Cash flow information	9 Months Ended
	Sep. 30, 2013
Supplemental Cash Flow Elements [Abstract]	'
Cash flow information	'
	Cash flow information
	Cash expenditures for interest and income taxes were as follows:
		Nine Months Ended
		September 30,
		2013		2012
		(In thousands)
	Interest, net of amount capitalized	$	60,281	$	57,956
	Income taxes paid	$	30,262	$	3,210
	Noncash investing transactions were as follows:
		September 30,
		2013		2012
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	85,646	$	68,636

New_accounting_standards

New accounting standards	9 Months Ended
	Sep. 30, 2013
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]	'
New accounting standards	'
	New accounting standards
	Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income In February 2013, the FASB issued guidance on the reporting of amounts reclassified out of accumulated other comprehensive income. This guidance requires an entity to report the effect of significant reclassifications out of accumulated other comprehensive income on the respective line items in net income if the amount being reclassified is required to be reclassified in its entirety to net income. Entities may present this information either on the face of the statement where net income is presented or in the notes. This guidance was effective for the Company on January 1, 2013, and is to be applied prospectively. The guidance required additional disclosures, however it did not impact the Company's results of operations, financial position or cash flows.
	Disclosures about Offsetting Assets and Liabilities In December 2011, the FASB issued guidance on the disclosure requirements related to balance sheet offsetting. The new disclosure requirements relate to the nature of an entity's rights of offset and related arrangements associated with its financial instruments and derivative instruments. In January 2013, the FASB issued guidance clarifying the scope of the disclosures related to balance sheet offsetting. The amendments clarify that this guidance only applies to derivative instruments, repurchase agreements and securities lending transactions that are either offset or subject to an enforceable master netting arrangement. The guidance was effective for the Company on January 1, 2013, and must be applied retrospectively. The guidance required additional disclosures, however it did not impact the Company's results of operations, financial position or cash flows.

Comprehensive_income_loss

Comprehensive income (loss)	9 Months Ended
	Sep. 30, 2013
Comprehensive income (loss) [Abstract]	'
Comprehensive income (loss)	'
	Comprehensive income (loss)
	The after-tax changes in the components of accumulated other comprehensive loss as of September 30, 2013, were as follows:
		Net Unrealized Gain (Loss) on Derivative			Net Unrealized Gain (Loss) on Available-for-sale Investments			Postretirement			Foreign Currency Translation Adjustment			Total Accumulated
		Instruments						Liability Adjustment						Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at December 31, 2012	$	6,018		$	119		$	(54,347	)	$	(511	)	$	(48,721	)
	Other comprehensive income (loss) before reclassifications	(5,594		)	(197		)	—			(351		)	(6,142		)
	Amounts reclassified from accumulated other comprehensive loss	(3,678		)	117			1,344			143			(2,074		)
	Net current-period other comprehensive income (loss)	(9,272		)	(80		)	1,344			(208		)	(8,216		)
	Balance at September 30, 2013	$	(3,254	)	$	39		$	(53,003	)	$	(719	)	$	(56,937	)
	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended			Nine Months Ended			Location on Consolidated Statements of Income
		30-Sep-13			30-Sep-13
		(In thousands)
	Reclassification adjustment for gain (loss) on derivative instruments included in net income
	Commodity derivative instruments	$	1,007		$	6,903		Operating revenues
	Interest rate derivative instruments	(200		)	(979		)	Interest expense
		807			5,924
		(297		)	(2,246		)	Income taxes
		510			3,678
	Amortization of postretirement liability losses included in net periodic benefit cost	(437		)	(2,371		)	(a)
		166			1,027			Income taxes
		(271		)	(1,344		)
	Reclassification adjustment for loss on available-for-sale investments included in net income	(58		)	(180		)	Other income
		20			63			Income taxes
		(38		)	(117		)
	Reclassification adjustment for loss on foreign currency translation adjustment included in net income	(185		)	(213		)	Earnings (loss) from equity method investments
		70			70			Earnings (loss) from equity method investments
		(115		)	(143		)
	Total reclassifications	$	86		$	2,074
	(a) Included in net periodic benefit cost (credit). For more information, see Note 18.

Discontinued_operations

Discontinued operations	9 Months Ended
	Sep. 30, 2013
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 had 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 has accrued liabilities related to this matter. In the second quarter of 2012, discontinued operations reflected a net benefit largely related to estimated insurance recoveries related to this matter. These items are reflected as discontinued operations in the consolidated financial statements and accompanying notes. Discontinued operations are included in the Other category. For more information, see Note 20.

Equity_method_investments

Equity method investments	9 Months Ended
	Sep. 30, 2013
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 September 30, 2013, 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 interest 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 remaining interest in ECTE is being purchased 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 and 2012. Alusa, CEMIG and CELESC hold the remaining ownership interests in ECTE. The Company's remaining ownership interest in ECTE is being accounted for under the cost method.
	At September 30, 2012 and December 31, 2012, the equity method investments had total assets of $110.6 million and $129.0 million, respectively, and long-term debt of $28.2 million and $65.5 million, respectively. The Company's investment in its equity method investments was approximately $7.4 million and $6.9 million, including undistributed earnings of $4.1 million and $3.4 million, at September 30, 2012 and December 31, 2012, respectively.

Goodwill_and_other_intangible_

Goodwill and other intangible assets	9 Months Ended
	Sep. 30, 2013
Goodwill and Intangible Assets Disclosure [Abstract]	'
Goodwill and other intangible assets	'
	Goodwill and other intangible assets
	The changes in the carrying amount of goodwill were as follows:
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-13	as of			Acquired			as of
		January 1,			During			September 30, 2013*
		2013*			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.
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-12	as of			Acquired			as of September 30, 2012*
		January 1,			During the
		2012*			Year**
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	103,168			1,108			104,276
	Total	$	634,931		$	1,108		$	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.
	** Includes contingent consideration that was not material related to an acquisition in a prior period.
	Year Ended	Balance			Goodwill			Balance
	31-Dec-12	as of			Acquired			as of
		January 1,			During the			December 31,
		2012*			Year**			2012*
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	103,168			1,108			104,276
	Total	$	634,931		$	1,108		$	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.
	** Includes contingent consideration that was not material related to an acquisition in a prior period.
	Other amortizable intangible assets were as follows:
		September 30,			September 30,			December 31,
		2013			2012			2012
		(In thousands)
	Customer relationships	$	21,310		$	21,310		$	21,310
	Accumulated amortization	(13,221		)	(11,192		)	(11,701		)
		8,089			10,118			9,609
	Noncompete agreements	6,186			7,236			7,236
	Accumulated amortization	(4,706		)	(5,198		)	(5,326		)
		1,480			2,038			1,910
	Other	10,995			10,979			10,979
	Accumulated amortization	(6,472		)	(5,120		)	(5,369		)
		4,523			5,859			5,610
	Total	$	14,092		$	18,015		$	17,129
	Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2013, was $1.2 million and $3.0 million, respectively. Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2012, was $1.0 million and $2.9 million, respectively. Estimated amortization expense for amortizable intangible assets is $3.7 million in 2013, $3.4 million in 2014, $2.6 million in 2015, $2.2 million in 2016, $1.9 million in 2017 and $3.3 million thereafter.

Derivative_instruments

Derivative instruments	9 Months Ended
	Sep. 30, 2013
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 September 30, 2013, the Company had no outstanding foreign currency or interest rate hedges. The following information should be read in conjunction with Notes 1 and 7 in the Company's Notes to Consolidated Financial Statements in the 2012 Annual Report.
	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.
	Cascade
	Cascade has historically utilized natural gas swap agreements to manage a portion of its regulated natural gas supply portfolio in order to manage fluctuations in the price of natural gas related to core customers in accordance with authority granted by the WUTC and OPUC. Core customers consist of residential, commercial and smaller industrial customers. As of September 30, 2013 and December 31, 2012, Cascade has no outstanding swap agreements. As of September 30, 2012, Cascade held a natural gas swap agreement with total forward notional volumes of 31,000 MMBtu. Periodic changes in the fair market value of the derivative instruments are recorded on the Consolidated Balance Sheets as a regulatory asset or a regulatory liability, and settlements of these arrangements are expected to be recovered through the purchased gas cost adjustment mechanism. Gains and losses on the settlements of these derivative instruments are recorded as a component of purchased natural gas sold on the Consolidated Statements of Income as they are recovered through the purchased gas cost adjustment mechanism. Under the terms of these arrangements, Cascade either pays or receives settlement payments based on the difference between the fixed strike price and the monthly index price applicable to each contract. For the three and nine months ended September 30, 2012, the change in the fair market value of the derivative instrument of $175,000 and $384,000, respectively, was recorded as a decrease to regulatory assets.
	Cascade's derivative instrument contains a cross-default provision that states that if Cascade fails to pay certain of its indebtedness, in excess of specified amounts, the counterparty may require early settlement or termination of the derivative instrument in a liability position. The fair value of Cascade's derivative instrument with the credit-risk-related contingent feature that was in a liability position at September 30, 2012, was $53,000. The aggregate fair value of assets that would have been needed to settle the instrument immediately if the credit-risk-related contingent feature were triggered on September 30, 2012, was $53,000.
	Fidelity
	At September 30, 2013 and 2012, and December 31, 2012, Fidelity held oil swap and collar agreements with total forward notional volumes of 3.9 million, 3.3 million and 2.6 million Bbl, respectively, and natural gas swap agreements with total forward notional volumes of 16.5 million, 8.2 million and 11.0 million MMBtu, respectively. In addition, at September 30, 2012, Fidelity held natural gas basis swap agreements with total forward notional volumes of 874,000 MMBtu. Fidelity utilizes these derivative instruments to manage a portion of the market risk associated with fluctuations in the price of oil and natural gas and basis differentials 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 15 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.
	Centennial
	As of September 30, 2013, Centennial had no outstanding interest rate swap agreements. At September 30, 2012 and December 31, 2012, Centennial held interest rate swap agreements with total notional amounts of $60.0 million and $50.0 million, respectively, which were designated as cash flow hedging instruments. Centennial entered into these interest rate derivative instruments to manage a portion of its interest rate exposure on the forecasted issuance of long-term debt.
	Changes in the fair value attributable to the effective portion of hedging instruments, net of tax, are recorded in stockholders' equity as a component of accumulated other comprehensive income (loss). To the extent that the hedges are not effective, the ineffective portion of the changes in fair market value is recorded directly in earnings. Gains and losses on the interest rate derivatives are reclassified from accumulated other comprehensive income (loss) into interest expense on the Consolidated Statements of Income in the same period the hedged item affects earnings.
	Fidelity and Centennial
	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.
	The gains and losses on derivative instruments were as follows:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$	—		$	(8,622	)	$	(6,153	)	$	9,822
	Amount of gain reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(634		)	(7,788		)	(4,349		)	(7,056		)
	Amount of loss recognized in operating revenues (ineffective portion), before tax	—			(529		)	(1,422		)	(917		)
	Interest rate derivatives designated as cash flow hedges:
	Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	—			(503		)	559			(1,860		)
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	124			6			671			27
	Amount of loss recognized in interest expense (ineffective portion), before tax	—			—			(769		)	—
	Commodity derivatives not designated as hedging instruments:
	Amount of gain (loss) recognized in operating revenues, before tax	(12,594		)	(654		)	(3,957		)	394
	Based on September 30, 2013, fair values, over the next 12 months net gains of approximately $28,000 (after tax) are estimated to be reclassified from accumulated other comprehensive income (loss) into earnings, as the hedged transactions affect earnings.
	Certain of Fidelity's derivative instruments contain cross-default provisions that state if Fidelity or any of its affiliates fail to pay certain of their indebtedness, in excess of specified amounts, the counterparties may 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 September 30, 2013 and 2012, and December 31, 2012, were $9.9 million, $9.9 million and $6.3 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 September 30, 2013 and 2012, and December 31, 2012, were $9.9 million, $9.9 million and $6.3 million, respectively.
	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 September 30, 2013			Fair Value at September 30, 2012			Fair Value at December 31, 2012
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	—		$	18,619		$	18,084
		Other assets - noncurrent	—			3,463			—
			—			22,082			18,084
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	4,373			574			220
		Other assets - noncurrent	1,771			63			—
			6,144			637			220
	Total asset derivatives		$	6,144		$	22,719		$	18,304
	Liability	Location on	Fair Value at September 30, 2013			Fair Value at September 30, 2012			Fair Value at December 31, 2012
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	—		$	1,958		$	—
		Other liabilities - noncurrent	—			83			—
	Interest rate derivatives	Other accrued liabilities	—			7,779			6,255
			—			9,820			6,255
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	9,740			114			—
		Other liabilities - noncurrent	149			—			—
			9,889			114			—
	Total liability derivatives		$	9,889		$	9,934		$	6,255
	All of the Company's commodity and interest rate derivative instruments at September 30, 2013 and 2012, and December 31, 2012, 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-Sep-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
Assets:
Commodity derivatives	$	6,144		$	(4,939	)	$	1,205
Total assets	$	6,144		$	(4,939	)	$	1,205
Liabilities:
Commodity derivatives	$	9,889		$	(4,939	)	$	4,950
Total liabilities	$	9,889		$	(4,939	)	$	4,950
30-Sep-12	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
	(In thousands)
Assets:
Commodity derivatives	$	22,719		$	(2,102	)	$	20,617
Total assets	$	22,719		$	(2,102	)	$	20,617
Liabilities:
Commodity derivatives	$	2,155		$	(2,102	)	$	53
Interest rate derivatives	7,779			—			7,779
Total liabilities	$	9,934		$	(2,102	)	$	7,832
31-Dec-12	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
	(In thousands)
Assets:
Commodity derivatives	$	18,304		$	—		$	18,304
Total assets	$	18,304		$	—		$	18,304
Liabilities:
Interest rate derivatives	$	6,255		$	—		$	6,255
Total liabilities	$	6,255		$	—		$	6,255

Fair_value_measurements

Fair value measurements	9 Months Ended
	Sep. 30, 2013
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 $58.1 million, $48.4 million and $48.9 million, as of September 30, 2013 and 2012, and December 31, 2012, respectively, are classified as Investments on the Consolidated Balance Sheets. The net unrealized gains on these investments were $4.1 million and $9.2 million for the three and nine months ended September 30, 2013, respectively. The net unrealized gains on these investments was $2.4 million and $4.7 million for the three and nine months ended September 30, 2012. The change in fair value, which is considered part of the cost of the plan, is classified in operation and maintenance expense on the Consolidated Statements of Income.
	The Company did not elect the fair value option, which records gains and losses in income, for its available-for-sale securities, which include mortgage-backed securities and U.S. Treasury securities. These available-for-sale securities are recorded at fair value and are classified as Investments on the Consolidated Balance Sheets. Unrealized gains or losses are recorded in accumulated other comprehensive income (loss). Details of available-for-sale securities were as follows:
	September 30, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,051	$	70	$	(20	)	$	8,101
	U.S. Treasury securities	1,912		15		(4		)	1,923
	Total	$	9,963	$	85	$	(24	)	$	10,024
	September 30, 2012	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,391	$	175	$	(2	)	$	8,564
	U.S. Treasury securities	1,758		47		—			1,805
	Total	$	10,149	$	222	$	(2	)	$	10,369
	December 31, 2012	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,054	$	144	$	(3	)	$	8,195
	U.S. Treasury securities	1,763		43		—			1,806
	Total	$	9,817	$	187	$	(3	)	$	10,001
	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 three and nine months ended September 30, 2013 and 2012, there were no transfers between Levels 1 and 2.
	The Company's assets and liabilities measured at fair value on a recurring basis are as follows:
		Fair Value Measurements at September 30, 2013, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2013
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	21,019	$	—		$	21,019
	Insurance contract*	—		58,142		—			58,142
	Available-for-sale securities:
	Mortgage-backed securities	—		8,101		—			8,101
	U.S. Treasury securities	—		1,923		—			1,923
	Commodity derivative instruments	—		6,144		—			6,144
	Total assets measured at fair value	$	—	$	95,329	$	—		$	95,329
	Liabilities:
	Commodity derivative instruments	$	—	$	9,889	$	—		$	9,889
	Total liabilities measured at fair value	$	—	$	9,889	$	—		$	9,889
	*  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 and other investments.
		Fair Value Measurements at September 30, 2012, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2012
		(Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	21,816	$	—		$	21,816
	Insurance contract*	—		48,384		—			48,384
	Available-for-sale securities:
	Mortgage-backed securities	—		8,564		—			8,564
	U.S. Treasury securities	—		1,805		—			1,805
	Commodity derivative instruments	—		22,719		—			22,719
	Total assets measured at fair value	$	—	$	103,288	$	—		$	103,288
	Liabilities:
	Commodity derivative instruments	$	—	$	2,155	$	—		$	2,155
	Interest rate derivative instruments	—		7,779		—			7,779
	Total liabilities measured at fair value	$	—	$	9,934	$	—		$	9,934
	*  The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.
		Fair Value Measurements at December 31, 2012, Using
		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2012
(Level 3)
	(In thousands)
Assets:
Money market funds	$	—	$	24,240	$	—		$	24,240
Insurance contract*	—		48,898		—			48,898
Available-for-sale securities:
Mortgage-backed securities	—		8,195		—			8,195
U.S. Treasury securities	—		1,806		—			1,806
Commodity derivative instruments	—		18,304		—			18,304
Total assets measured at fair value	$	—	$	101,443	$	—		$	101,443
Liabilities:
Interest rate derivative instruments	$	—	$	6,255	$	—		$	6,255
Total liabilities measured at fair value	$	—	$	6,255	$	—		$	6,255
* The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other 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 quarters of 2013 and 2012, 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, 2012, certain coalbed natural gas gathering assets were written down to the nonrecurring fair value measurement of $2.5 million. At June 30, 2013, additional 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 September 30, 2013	$	2,011,896	$	2,106,887
Long-term debt at September 30, 2012	$	1,742,977	$	1,906,673
Long-term debt at December 31, 2012	$	1,744,975	$	1,888,135
The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values.

Income_taxes

Income taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Income taxes	'
	Income taxes
	In connection with the income tax examination for the 2007 through 2009 tax years, the Company recorded income tax expense of $2.2 million for unrecognized tax positions in the first quarter of 2012.
	In addition, the Company had a reduction of deferred income tax expense of $2.5 million in the first quarter of 2012, due to a deferred income tax rate reduction related to state income tax apportionment.
	It is likely that substantially all of the unrecognized tax benefits of $14.9 million, as well as interest, at September 30, 2013, will be settled in the next 12 months due to the anticipated settlement of federal and state audits.
	In September 2013, the Internal Revenue Service released final regulations relating to the capitalization of tangible personal property which are effective for tax years beginning on or after January 1, 2014. The Company does not expect these new regulations to have a material effect on its results of operations, financial position or cash flows.

Business_segment_data

Business segment data	9 Months Ended
	Sep. 30, 2013
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 Dakota Prairie Refinery 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 regions of the United States and in and around the Gulf of Mexico.
	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 2012 Annual Report. Information on the Company's businesses was as follows:
	Three Months Ended September 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	68,314	$	—		$	11,417
	Natural gas distribution	77,417		—			(11,204		)
	Pipeline and energy services	46,372		4,906			5,310
		192,103		4,906			5,523
	Exploration and production	119,234		10,714			17,434
	Construction materials and contracting	706,982		7,422			49,159
	Construction services	267,038		3,097			12,154
	Other	425		1,859			1,217
		1,093,679		23,092			79,964
	Intersegment eliminations	—		(27,998		)	(1,202		)
	Total	$	1,285,782	$	—		$	84,285
	Three Months Ended September 30, 2012	External		Inter-			Loss
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	63,492	$	—		$	11,000
	Natural gas distribution	80,069		—			(8,782		)
	Pipeline and energy services	41,302		7,046			3,273
		184,863		7,046			5,491
	Exploration and production	100,380		8,076			(87,748		)
	Construction materials and contracting	641,500		8,508			41,889
	Construction services	246,358		834			9,863
	Other	417		1,948			663
		988,655		19,366			(35,333		)
	Intersegment eliminations	—		(26,412		)	—
	Total	$	1,173,518	$	—		$	(29,842	)
	Nine Months Ended September 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	189,949	$	—		$	25,652
	Natural gas distribution	536,756		—			15,420
	Pipeline and energy services	116,965		31,623			1,247
		843,670		31,623			42,319
	Exploration and production	371,648		33,083			70,713
	Construction materials and contracting	1,287,305		24,673			38,602
	Construction services	774,103		7,011			36,733
	Other	1,254		5,516			1,862
		2,434,310		70,283			147,910
	Intersegment eliminations	—		(101,906		)	(3,259		)
	Total	$	3,277,980	$	—		$	186,970
	Nine Months Ended September 30, 2012	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	174,410	$	—		$	22,977
Natural gas distribution	504,805		—			10,314
Pipeline and energy services	105,184		36,393			21,884
	784,399		36,393			55,175
Exploration and production	289,106		25,114			(56,860		)
Construction materials and contracting	1,229,731		11,756			24,748
Construction services	688,368		1,078			29,951
Other	2,684		4,303			6,705
	2,209,889		42,251			4,544
Intersegment eliminations	—		(78,644		)	—
Total	$	2,994,288	$	—		$	59,719
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	9 Months Ended
	Sep. 30, 2013
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 September 30,	2013			2012			2013			2012
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	39		$	349		$	419		$	437
	Interest cost	4,062			4,407			804			943
	Expected return on assets	(4,979		)	(5,865		)	(1,086		)	(1,222		)
	Amortization of prior service cost (credit)	18			(22		)	(364		)	(534		)
	Amortization of net actuarial loss	1,793			1,887			327			356
	Amortization of net transition obligation	—			—			—			531
	Curtailment gain	—			(1,023		)	—			—
	Net periodic benefit cost, including amount capitalized	933			(267		)	100			511
	Less amount capitalized	157			185			47			314
	Net periodic benefit cost (credit)	$	776		$	(452	)	$	53		$	197
								Other
								Postretirement
		Pension Benefits						Benefits
	Nine Months Ended September 30,	2013			2012			2013			2012
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	116		$	1,044		$	1,257		$	1,310
	Interest cost	12,186			13,223			2,411			3,124
	Expected return on assets	(14,937		)	(17,596		)	(3,258		)	(3,667		)
	Amortization of prior service cost (credit)	54			(64		)	(1,092		)	(1,078		)
	Amortization of net actuarial loss	5,373			5,670			1,405			1,769
	Amortization of net transition obligation	—			—			—			1,594
	Curtailment gain	—			(1,023		)	—			—
	Net periodic benefit cost, including amount capitalized	2,792			1,254			723			3,052
	Less amount capitalized	425			615			137			635
	Net periodic benefit cost	$	2,367		$	639		$	586		$	2,417
	In 2010, all benefit and service accruals for nonunion and certain union plans were frozen. In 2011 and effective September 30, 2012, all benefit and service accruals for certain additional union employees were frozen. These employees are eligible to receive additional defined contribution plan benefits.
	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 nine months ended September 30, 2013, was $1.8 million and $5.5 million, respectively. The Company's net periodic benefit cost for this plan for the three and nine months ended September 30, 2012, was $2.0 million and $6.1 million, respectively.
	In 2012, the Company modified health care coverage for certain retirees. Effective January 1, 2013, post-65 coverage is replaced by a fixed-dollar subsidy for certain retirees and spouses to be used to purchase individual insurance through an exchange.

Regulatory_matters_and_revenue

Regulatory matters and revenues subject to refund	9 Months Ended
	Sep. 30, 2013
Regulated Operations [Abstract]	'
Regulatory matters and revenues subject to refund	'
	Regulatory matters and revenues subject to refund
	On September 26, 2012, Montana-Dakota filed an application with the MTPSC for a natural gas rate increase. Montana-Dakota requested a total increase of $3.5 million annually or approximately 5.9 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. Montana-Dakota requested an interim increase, subject to refund, of $1.7 million or approximately 2.9 percent. On April 12, 2013, the MTPSC issued an interim order authorizing an interim increase of $850,000 annually to be effective with service rendered on or after April 15, 2013, subject to refund. A hearing was held August 5-6, 2013.
	On December 21, 2012, Montana-Dakota filed an application with the SDPUC for a natural gas rate increase. Montana-Dakota requested a total increase of $1.5 million annually or approximately 3.3 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. On June 19, 2013, Montana-Dakota filed a notice of intent to implement an interim rate increase, subject to refund, of $1.5 million effective with service rendered on or after July 22, 2013. On October 24, 2013, Montana-Dakota and the SDPUC Staff filed a settlement stipulation, which reflects a natural gas rate increase of $900,000 annually or approximately 2.0 percent. On November 5, 2013, the SDPUC approved the settlement stipulation, with rates to be effective with service rendered on and after December 1, 2013.
	On February 11, 2013, Montana-Dakota filed an application with the NDPSC for approval of an environmental cost recovery rider for recovery of Montana-Dakota's share of the costs resulting from the environmental retrofit required to be installed at the Big Stone Station. The costs proposed to be recovered are associated with the ongoing construction costs for the installation of the BART air-quality control system. On February 27, 2013, the NDPSC suspended the filing pending further review. On May 31, 2013, Montana-Dakota filed revisions to its filing to reflect revised budget amounts. A hearing was held on September 16, 2013.
	On June 14, 2013, Montana-Dakota filed for an advance determination of prudence with the NDPSC to add filterable particulate matter pollution control equipment at Montana-Dakota's Lewis & Clark generating station to comply with the Mercury and Air Toxics Standards rule, projected to be completed in 2016. Project cost is estimated to be $27.7 million. An informal hearing was held on September 25, 2013. On October 9, 2013, the NDPSC issued an order approving the advance determination of prudence.
	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. Montana-Dakota requested an interim increase, subject to refund, of $4.5 million or approximately 4.2 percent. On October 9, 2013, the NDPSC approved the interim increase to be effective with service rendered on or after November 17, 2013. On October 23, 2013, Montana-Dakota and the NDPSC Advocacy Staff filed a settlement agreement that resolved the revenue requirement portion of the application and reflected a natural gas rate increase of $4.3 million annually or approximately 4.0 percent, and agreed that Montana-Dakota will only implement $4.3 million of interim rate relief. The NDPSC has scheduled an informal hearing on the settlement on November 13, 2013.
	On October 31, 2013, WBI Energy Transmission filed a general natural gas rate change application with the FERC for an increase of $28.9 million annually to cover increased investments of $312 million, increased operating costs, and the effect of lower storage and off system volumes. WBI Energy Transmission requested the rates to be effective December 1, 2013.

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2013
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 $30.8 million, $41.6 million and $22.5 million for contingencies, including litigation, production taxes, royalty claims and environmental matters, as of September 30, 2013 and 2012, and December 31, 2012, 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. On October 19, 2012, Centennial moved to intervene in the New York Supreme Court action to vacate the arbitration award and also filed a complaint with the New York Supreme Court seeking a declaration that LPP is not entitled to indemnification from Centennial under the guaranty for the arbitration award. 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. Due to the vacation of the arbitration award, the Company no longer believes the loss related to this matter to be probable and thus the liability that was previously recorded in 2011 was reversed in the fourth quarter of 2012. We believe that it is reasonably possible that a loss related to this matter could result if LPP is successful in its appeal, the arbitration award is affirmed and LPP continues to assert its demand against Centennial under the guarantee for payment of the arbitration award, attorneys' fees and interest. 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.
	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. An arbitration hearing was held in August 2010. In October 2010, the arbitration panel issued an award in favor of SourceGas for approximately $26.6 million. As a result, WBI Energy Midstream, which is included in the pipeline and energy services segment, recorded a $26.6 million charge ($16.5 million after tax) in the third quarter of 2010. On April 20, 2011, the Colorado State District Court confirmed the arbitration award as a court judgment. WBI Energy Midstream filed an appeal from the Colorado State District Court's order and judgment to the Colorado Court of Appeals. 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. As a result of the Colorado Court of Appeals decision, in the second quarter of 2012, WBI Energy Midstream changed its estimated loss related to this matter. This resulted in a reduction of expense of $24.1 million ($15.0 million after tax), which is largely reflected in operation and maintenance expense on the Consolidated Statements of Income. On August 2, 2012, SourceGas filed a petition for writ of certiorari with the Colorado Supreme Court for review of the Colorado Court of Appeals decision which was denied on July 22, 2013. WBI Energy Midstream anticipates that 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. Expert reports submitted by Omimex contended its damages as a result of the increased operating pressures were $16.1 million to $22.6 million, however, the experts revised their calculation of Omimex's damages to $1.0 million. The parties subsequently settled the breach of contract claim and, subject to a 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.
	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 $1.3 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 plans to apply for a renewal to defer the environmental remediation costs.
	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. Cascade has entered into an administrative settlement agreement and consent order with the EPA regarding the scope and schedule for a remedial investigation and feasibility study for the site. Cascade has accrued $12.5 million for the remedial investigation, feasibility study and remediation of this site. In April 2010, Cascade filed a petition with the WUTC for authority to defer the costs, which are included in other noncurrent assets, incurred in relation to the environmental remediation of this site 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 and collar agreement obligations. There is no fixed maximum amount guaranteed in relation to the oil and natural gas swap and collar 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 and collar agreements at September 30, 2013, expire in the years ranging from 2013 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 $6.3 million and was reflected on the Consolidated Balance Sheet at September 30, 2013. 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 September 30, 2013, the fixed maximum amounts guaranteed under these agreements aggregated $63.6 million. The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate $13.0 million in 2013; $30.9 million in 2014; $1.6 million in 2015; $300,000 in 2016; $100,000 in 2017; $200,000 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 $300,000 and was reflected on the Consolidated Balance Sheet at September 30, 2013. In the event of default under these guarantee obligations, the subsidiary issuing the guarantee for that particular obligation would be required to make payments under its guarantee.
	Certain subsidiaries have outstanding letters of credit to third parties related to insurance policies and other agreements, some of which are guaranteed by other subsidiaries of the Company. At September 30, 2013, the fixed maximum amounts guaranteed under these letters of credit, aggregated $38.2 million. In 2013 and 2014, $7.7 million and $30.5 million, respectively, of letters of credit are scheduled to expire. There were no amounts outstanding under the above letters of credit at September 30, 2013.
	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 September 30, 2013, the fixed maximum amount guaranteed under this agreement was $5.0 million and is scheduled to expire in 2014. 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 $900,000. The amount outstanding under this guarantee was not reflected on the Consolidated Balance Sheet at September 30, 2013, 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 September 30, 2013.
	In the normal course of business, Centennial has surety bonds related to construction contracts and reclamation obligations of its subsidiaries. In the event a subsidiary of Centennial does not fulfill a bonded obligation, Centennial would be responsible to the surety bond company for completion of the bonded contract or obligation. A large portion of the surety bonds is expected to expire within the next 12 months; however, Centennial will likely continue to enter into surety bonds for its subsidiaries in the future. As of September 30, 2013, approximately $552 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. GAAP provides a framework for identifying VIEs and determining when a company should include the assets, liabilities, noncontrolling interest, and results of activities of a VIE in its consolidated financial statements.
	A VIE should be consolidated if a party with an ownership, contractual, or other financial interest in the VIE (a variable interest holder) has the power to direct the VIE's most significant activities and the obligation to absorb losses or right to receive benefits of the VIE that could be significant to the VIE. A variable interest holder that consolidates the VIE is called the primary beneficiary. Upon consolidation, the primary beneficiary generally must initially record all of the VIE's assets, liabilities, and noncontrolling interests at fair value and subsequently account for the VIE as if it were consolidated.
	The Company's evaluation of whether it qualifies as the primary beneficiary of a VIE is highly complex and involves significant judgments, estimates and assumptions and includes a qualitative analysis of the activities that most significantly impact the VIE's economic performance and whether the Company has the power to direct those activities, the design of the entity, the rights of the parties, and the purpose of the arrangement.
	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 fifty percent ownership interest in Dakota Prairie Refining. WBI Energy's and Calumet's capital commitments, based on a total project cost of $300 million, under the agreement are $150 million and $75 million, respectively. Dakota Prairie Refining entered into a term loan for project debt financing of $75 million on April 22, 2013. The 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 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-Sep-13
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	23,146
	Accounts receivable	1
	Other current assets	25
	Total current assets	23,172
	Net property, plant and equipment	123,297
	Total assets	$	146,469
	LIABILITIES
	Current liabilities:
	Accounts payable	$	20,313
	Long-term debt due within one year	3,000
	Other accrued liabilities	363
	Total current liabilities	23,676
	Long-term debt	72,000
	Total liabilities	$	95,676
	Fuel Contract On October 10, 2012, the Coyote Station entered into a new coal supply agreement with Coyote Creek which 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, of which the Company is a 25.0 percent owner, 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 September 30, 2013, 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 September 30, 2013, was $7.4 million.

Subsquent_events

Subsquent events	9 Months Ended
	Sep. 30, 2013
Subsequent Events [Abstract]	'
Subsequent events [Text Block]	'
	Subsequent events
	On October 1, 2013, MDU Energy Capital entered into a $30.0 million credit agreement with a maturity date of March 30, 2014.
	Cascade entered into a note purchase agreement on October 30, 2013, and issued $50.0 million of Senior Notes with due dates ranging from August 2025 to August 2028 at a weighted average interest rate of 4.24 percent.
	Intermountain entered into a note purchase agreement on October 30, 2013, and issued $50.0 million of Senior Notes with due dates ranging from October 2025 to October 2028 at a weighted average interest rate of 4.21 percent.
	On November 1, 2013, Centennial issued $25.0 million of Senior Notes under a note purchase agreement dated June 27, 2013, with due dates ranging from November 2023 to November 2028 at a weighted average interest rate of 4.84 percent.

Inventories_and_natural_gas_in1

Inventories and natural gas in storage (Tables)	9 Months Ended
	Sep. 30, 2013
Inventory Disclosure [Abstract]	'
Inventories	'
	Inventories consisted of:
		September 30,		September 30,		December 31,
		2013		2012		2012
		(In thousands)
	Aggregates held for resale	$	104,784	$	88,632	$	87,715
	Asphalt oil	43,078		47,084		67,480
	Materials and supplies	71,370		75,551		69,390
	Merchandise for resale	23,713		30,827		31,172
	Natural gas in storage (current)	37,689		41,091		29,030
	Other	33,937		32,582		32,628
	Total	$	314,571	$	315,767	$	317,415

Earnings_per_common_share_Tabl

Earnings per common share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Weighted average common shares outstanding	'
	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows:
		Three Months Ended		Nine Months Ended
		September 30,		September 30,
		2013	2012	2013	2012
		(In thousands)
	Weighted average common shares outstanding - basic	188,831	188,831	188,831	188,824
	Effect of dilutive stock options and performance share awards	807	—	803	205
	Weighted average common shares outstanding - diluted	189,638	188,831	189,634	189,029
	Shares excluded from the calculation of diluted earnings per share	—	434	—	—

Cash_flow_information_Tables

Cash flow information (Tables)	9 Months Ended
	Sep. 30, 2013
Supplemental Cash Flow Information [Abstract]	'
Supplemental cash flow information	'
	Cash expenditures for interest and income taxes were as follows:
		Nine Months Ended
		September 30,
		2013		2012
		(In thousands)
	Interest, net of amount capitalized	$	60,281	$	57,956
	Income taxes paid	$	30,262	$	3,210
	Noncash investing transactions were as follows:
		September 30,
		2013		2012
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	85,646	$	68,636

Comprehensive_income_loss_Tabl

Comprehensive income (loss) (Tables)	9 Months Ended
	Sep. 30, 2013
Comprehensive income (loss) [Abstract]	'
Comprehensive income (loss)	'
	The after-tax changes in the components of accumulated other comprehensive loss as of September 30, 2013, were as follows:
		Net Unrealized Gain (Loss) on Derivative			Net Unrealized Gain (Loss) on Available-for-sale Investments			Postretirement			Foreign Currency Translation Adjustment			Total Accumulated
		Instruments						Liability Adjustment						Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at December 31, 2012	$	6,018		$	119		$	(54,347	)	$	(511	)	$	(48,721	)
	Other comprehensive income (loss) before reclassifications	(5,594		)	(197		)	—			(351		)	(6,142		)
	Amounts reclassified from accumulated other comprehensive loss	(3,678		)	117			1,344			143			(2,074		)
	Net current-period other comprehensive income (loss)	(9,272		)	(80		)	1,344			(208		)	(8,216		)
	Balance at September 30, 2013	$	(3,254	)	$	39		$	(53,003	)	$	(719	)	$	(56,937	)
Reclassification out of accumulated other comprehensive income	'
	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended			Nine Months Ended			Location on Consolidated Statements of Income
		30-Sep-13			30-Sep-13
		(In thousands)
	Reclassification adjustment for gain (loss) on derivative instruments included in net income
	Commodity derivative instruments	$	1,007		$	6,903		Operating revenues
	Interest rate derivative instruments	(200		)	(979		)	Interest expense
		807			5,924
		(297		)	(2,246		)	Income taxes
		510			3,678
	Amortization of postretirement liability losses included in net periodic benefit cost	(437		)	(2,371		)	(a)
		166			1,027			Income taxes
		(271		)	(1,344		)
	Reclassification adjustment for loss on available-for-sale investments included in net income	(58		)	(180		)	Other income
		20			63			Income taxes
		(38		)	(117		)
	Reclassification adjustment for loss on foreign currency translation adjustment included in net income	(185		)	(213		)	Earnings (loss) from equity method investments
		70			70			Earnings (loss) from equity method investments
		(115		)	(143		)
	Total reclassifications	$	86		$	2,074
	(a) Included in net periodic benefit cost (credit). For more information, see Note 18.

Goodwill_and_other_intangible_1

Goodwill and other intangible assets (Tables)	9 Months Ended
	Sep. 30, 2013
Goodwill and Intangible Assets Disclosure [Abstract]	'
Changes in the carrying amount of goodwill	'
	The changes in the carrying amount of goodwill were as follows:
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-13	as of			Acquired			as of
		January 1,			During			September 30, 2013*
		2013*			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.
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-12	as of			Acquired			as of September 30, 2012*
		January 1,			During the
		2012*			Year**
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	103,168			1,108			104,276
	Total	$	634,931		$	1,108		$	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.
	** Includes contingent consideration that was not material related to an acquisition in a prior period.
	Year Ended	Balance			Goodwill			Balance
	31-Dec-12	as of			Acquired			as of
		January 1,			During the			December 31,
		2012*			Year**			2012*
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	103,168			1,108			104,276
	Total	$	634,931		$	1,108		$	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.
	** Includes contingent consideration that was not material related to an acquisition in a prior period.
Other amortizable intangible assets	'
	Other amortizable intangible assets were as follows:
		September 30,			September 30,			December 31,
		2013			2012			2012
		(In thousands)
	Customer relationships	$	21,310		$	21,310		$	21,310
	Accumulated amortization	(13,221		)	(11,192		)	(11,701		)
		8,089			10,118			9,609
	Noncompete agreements	6,186			7,236			7,236
	Accumulated amortization	(4,706		)	(5,198		)	(5,326		)
		1,480			2,038			1,910
	Other	10,995			10,979			10,979
	Accumulated amortization	(6,472		)	(5,120		)	(5,369		)
		4,523			5,859			5,610
	Total	$	14,092		$	18,015		$	17,129

Derivative_instruments_Tables

Derivative instruments (Tables)	9 Months Ended
	Sep. 30, 2013
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						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$	—		$	(8,622	)	$	(6,153	)	$	9,822
	Amount of gain reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(634		)	(7,788		)	(4,349		)	(7,056		)
	Amount of loss recognized in operating revenues (ineffective portion), before tax	—			(529		)	(1,422		)	(917		)
	Interest rate derivatives designated as cash flow hedges:
	Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	—			(503		)	559			(1,860		)
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	124			6			671			27
	Amount of loss recognized in interest expense (ineffective portion), before tax	—			—			(769		)	—
	Commodity derivatives not designated as hedging instruments:
	Amount of gain (loss) recognized in operating revenues, before tax	(12,594		)	(654		)	(3,957		)	394
Derivative instruments (Tables)	'
	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 September 30, 2013			Fair Value at September 30, 2012			Fair Value at December 31, 2012
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	—		$	18,619		$	18,084
		Other assets - noncurrent	—			3,463			—
			—			22,082			18,084
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	4,373			574			220
		Other assets - noncurrent	1,771			63			—
			6,144			637			220
	Total asset derivatives		$	6,144		$	22,719		$	18,304
	Liability	Location on	Fair Value at September 30, 2013			Fair Value at September 30, 2012			Fair Value at December 31, 2012
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	—		$	1,958		$	—
		Other liabilities - noncurrent	—			83			—
	Interest rate derivatives	Other accrued liabilities	—			7,779			6,255
			—			9,820			6,255
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	9,740			114			—
		Other liabilities - noncurrent	149			—			—
			9,889			114			—
	Total liability derivatives		$	9,889		$	9,934		$	6,255
	All of the Company's commodity and interest rate derivative instruments at September 30, 2013 and 2012, and December 31, 2012, 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-Sep-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	6,144		$	(4,939	)	$	1,205
	Total assets	$	6,144		$	(4,939	)	$	1,205
	Liabilities:
	Commodity derivatives	$	9,889		$	(4,939	)	$	4,950
	Total liabilities	$	9,889		$	(4,939	)	$	4,950
	30-Sep-12	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	22,719		$	(2,102	)	$	20,617
	Total assets	$	22,719		$	(2,102	)	$	20,617
	Liabilities:
	Commodity derivatives	$	2,155		$	(2,102	)	$	53
	Interest rate derivatives	7,779			—			7,779
	Total liabilities	$	9,934		$	(2,102	)	$	7,832
	31-Dec-12	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	18,304		$	—		$	18,304
	Total assets	$	18,304		$	—		$	18,304
	Liabilities:
	Interest rate derivatives	$	6,255		$	—		$	6,255
	Total liabilities	$	6,255		$	—		$	6,255

Fair_value_measurements_Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]	'
Available-for-sale securities	'
	Details of available-for-sale securities were as follows:
	September 30, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,051	$	70	$	(20	)	$	8,101
	U.S. Treasury securities	1,912		15		(4		)	1,923
	Total	$	9,963	$	85	$	(24	)	$	10,024
	September 30, 2012	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,391	$	175	$	(2	)	$	8,564
	U.S. Treasury securities	1,758		47		—			1,805
	Total	$	10,149	$	222	$	(2	)	$	10,369
	December 31, 2012	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,054	$	144	$	(3	)	$	8,195
	U.S. Treasury securities	1,763		43		—			1,806
	Total	$	9,817	$	187	$	(3	)	$	10,001
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 are as follows:
		Fair Value Measurements at September 30, 2013, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2013
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	21,019	$	—		$	21,019
	Insurance contract*	—		58,142		—			58,142
	Available-for-sale securities:
	Mortgage-backed securities	—		8,101		—			8,101
	U.S. Treasury securities	—		1,923		—			1,923
	Commodity derivative instruments	—		6,144		—			6,144
	Total assets measured at fair value	$	—	$	95,329	$	—		$	95,329
	Liabilities:
	Commodity derivative instruments	$	—	$	9,889	$	—		$	9,889
	Total liabilities measured at fair value	$	—	$	9,889	$	—		$	9,889
	*  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 and other investments.
		Fair Value Measurements at September 30, 2012, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2012
		(Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	21,816	$	—		$	21,816
	Insurance contract*	—		48,384		—			48,384
	Available-for-sale securities:
	Mortgage-backed securities	—		8,564		—			8,564
	U.S. Treasury securities	—		1,805		—			1,805
	Commodity derivative instruments	—		22,719		—			22,719
	Total assets measured at fair value	$	—	$	103,288	$	—		$	103,288
	Liabilities:
	Commodity derivative instruments	$	—	$	2,155	$	—		$	2,155
	Interest rate derivative instruments	—		7,779		—			7,779
	Total liabilities measured at fair value	$	—	$	9,934	$	—		$	9,934
	*  The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.
		Fair Value Measurements at December 31, 2012, Using
		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2012
						(Level 3)
		(In thousands)
	Assets:
	Money market funds	$	—	$	24,240	$	—		$	24,240
	Insurance contract*	—		48,898		—			48,898
	Available-for-sale securities:
	Mortgage-backed securities	—		8,195		—			8,195
	U.S. Treasury securities	—		1,806		—			1,806
	Commodity derivative instruments	—		18,304		—			18,304
	Total assets measured at fair value	$	—	$	101,443	$	—		$	101,443
	Liabilities:
	Interest rate derivative instruments	$	—	$	6,255	$	—		$	6,255
	Total liabilities measured at fair value	$	—	$	6,255	$	—		$	6,255
	* The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other 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 September 30, 2013	$	2,011,896	$	2,106,887
	Long-term debt at September 30, 2012	$	1,742,977	$	1,906,673
	Long-term debt at December 31, 2012	$	1,744,975	$	1,888,135

Business_segment_data_Tables

Business segment data (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Information on the Company's businesses	'
	Information on the Company's businesses was as follows:
	Three Months Ended September 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	68,314	$	—		$	11,417
	Natural gas distribution	77,417		—			(11,204		)
	Pipeline and energy services	46,372		4,906			5,310
		192,103		4,906			5,523
	Exploration and production	119,234		10,714			17,434
	Construction materials and contracting	706,982		7,422			49,159
	Construction services	267,038		3,097			12,154
	Other	425		1,859			1,217
		1,093,679		23,092			79,964
	Intersegment eliminations	—		(27,998		)	(1,202		)
	Total	$	1,285,782	$	—		$	84,285
	Three Months Ended September 30, 2012	External		Inter-			Loss
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	63,492	$	—		$	11,000
	Natural gas distribution	80,069		—			(8,782		)
	Pipeline and energy services	41,302		7,046			3,273
		184,863		7,046			5,491
	Exploration and production	100,380		8,076			(87,748		)
	Construction materials and contracting	641,500		8,508			41,889
	Construction services	246,358		834			9,863
	Other	417		1,948			663
		988,655		19,366			(35,333		)
	Intersegment eliminations	—		(26,412		)	—
	Total	$	1,173,518	$	—		$	(29,842	)
	Nine Months Ended September 30, 2013	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	189,949	$	—		$	25,652
	Natural gas distribution	536,756		—			15,420
	Pipeline and energy services	116,965		31,623			1,247
		843,670		31,623			42,319
	Exploration and production	371,648		33,083			70,713
	Construction materials and contracting	1,287,305		24,673			38,602
	Construction services	774,103		7,011			36,733
	Other	1,254		5,516			1,862
		2,434,310		70,283			147,910
	Intersegment eliminations	—		(101,906		)	(3,259		)
	Total	$	3,277,980	$	—		$	186,970
	Nine Months Ended September 30, 2012	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	174,410	$	—		$	22,977
	Natural gas distribution	504,805		—			10,314
	Pipeline and energy services	105,184		36,393			21,884
		784,399		36,393			55,175
	Exploration and production	289,106		25,114			(56,860		)
	Construction materials and contracting	1,229,731		11,756			24,748
	Construction services	688,368		1,078			29,951
	Other	2,684		4,303			6,705
		2,209,889		42,251			4,544
Intersegment eliminations	—		(78,644		)	—
Total	$	2,994,288	$	—		$	59,719

Employee_benefit_plans_Tables

Employee benefit plans (Tables)	9 Months Ended
	Sep. 30, 2013
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 September 30,	2013			2012			2013			2012
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	39		$	349		$	419		$	437
	Interest cost	4,062			4,407			804			943
	Expected return on assets	(4,979		)	(5,865		)	(1,086		)	(1,222		)
	Amortization of prior service cost (credit)	18			(22		)	(364		)	(534		)
	Amortization of net actuarial loss	1,793			1,887			327			356
	Amortization of net transition obligation	—			—			—			531
	Curtailment gain	—			(1,023		)	—			—
	Net periodic benefit cost, including amount capitalized	933			(267		)	100			511
	Less amount capitalized	157			185			47			314
	Net periodic benefit cost (credit)	$	776		$	(452	)	$	53		$	197
								Other
								Postretirement
		Pension Benefits						Benefits
	Nine Months Ended September 30,	2013			2012			2013			2012
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	116		$	1,044		$	1,257		$	1,310
	Interest cost	12,186			13,223			2,411			3,124
	Expected return on assets	(14,937		)	(17,596		)	(3,258		)	(3,667		)
	Amortization of prior service cost (credit)	54			(64		)	(1,092		)	(1,078		)
	Amortization of net actuarial loss	5,373			5,670			1,405			1,769
	Amortization of net transition obligation	—			—			—			1,594
	Curtailment gain	—			(1,023		)	—			—
	Net periodic benefit cost, including amount capitalized	2,792			1,254			723			3,052
	Less amount capitalized	425			615			137			635
	Net periodic benefit cost	$	2,367		$	639		$	586		$	2,417

Contingencies_Commitment_and_C

Contingencies Commitment and Contingencies Disclosure (Tables)	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Limited liability project reflected on consolidated balance sheet [Table Text Block]	'
	The total assets and liabilities of Dakota Prairie Refining reflected on the Company's Consolidated Balance Sheets were as follows:
		30-Sep-13
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	23,146
	Accounts receivable	1
	Other current assets	25
	Total current assets	23,172
	Net property, plant and equipment	123,297
	Total assets	$	146,469
	LIABILITIES
	Current liabilities:
	Accounts payable	$	20,313
	Long-term debt due within one year	3,000
	Other accrued liabilities	363
	Total current liabilities	23,676
	Long-term debt	72,000
	Total liabilities	$	95,676

Accounts_receivable_and_allowa1

Accounts receivable and allowance for doubtful accounts (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
In Millions, unless otherwise specified
Receivables [Abstract]	'	'	'
Receivables past due 90 days or more	$31.10	$34.30	$35.10
Allowance for doubtful accounts receivable	$9.60	$10.80	$10.50

Inventories_and_natural_gas_in2

Inventories and natural gas in storage (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Inventory Disclosure [Abstract]	'	'	'
Aggregates held for resale	$104,784,000	$87,715,000	$88,632,000
Asphalt oil	43,078,000	67,480,000	47,084,000
Materials and supplies	71,370,000	69,390,000	75,551,000
Merchandise for resale	23,713,000	31,172,000	30,827,000
Natural gas in storage (current)	37,689,000	29,030,000	41,091,000
Other	33,937,000	32,628,000	32,582,000
Total	314,571,000	317,415,000	315,767,000
Natural gas in storage noncurrent	$48,600,000	$49,700,000	$50,300,000

Oil_and_natural_gas_properties1

Oil and natural gas properties (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Oil and Gas Property [Abstract]	'	'	'	'
Discount rate used in calculating present value of future net cash flows from proved reserves	'	10.00%	'	10.00%
Write-down of oil and natural gas properties	$0	$160,100,000	$0	$160,100,000
Write-down of oil and natural gas properties, after tax	'	100,900,000	'	100,900,000
Effects of cash flow hedges not considered in calculating ceiling limitation, amount	'	19,500,000	'	19,500,000
Effects of cash flow hedges not considered in calculating ceiling limitation amount, after tax	'	$12,300,000	'	$12,300,000

Impairment_of_longlived_assets1

Impairment of long-lived assets (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2013	Jun. 30, 2012
Asset Impairment Charges [Abstract]	'	'
Asset impairment charges	$14.50	$2.70
Asset impairment charges (after tax)	$9	$1.70

Earnings_per_common_share_Deta

Earnings per common share (Details)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]	'	'	'	'
Weighted average common shares outstanding - basic	188,831	188,831	188,831	188,824
Effect of dilutive stock options and performance share awards	807	0	803	205
Weighted average common shares outstanding - diluted	189,638	188,831	189,634	189,029
Shares excluded from the calculation of diluted earnings per share	0	434	0	0

Cash_flow_information_Details

Cash flow information (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Supplemental Cash Flow Information [Abstract]	'	'
Interest, net of amount capitalized	$60,281	$57,956
Income taxes paid	30,262	3,210
Property, plant and equipment additions in accounts payable	$85,646	$68,636

Comprehensive_income_loss_Deta

Comprehensive income (loss) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Balance at December 31, 2012	'	'	($48,721)	'
Other comprehensive income (loss) before reclassifications	'	'	-6,142	'
Amounts reclassified from accumulated other comprehensive loss	'	'	-2,074	'
Net current-period other comprehensive income (loss)	-116	-16,873	-8,216	553
Balance at September 30, 2013	-56,937	-46,448	-56,937	-46,448
Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges	'	'	'	'
Balance at December 31, 2012	'	'	6,018	'
Other comprehensive income (loss) before reclassifications	'	'	-5,594	'
Amounts reclassified from accumulated other comprehensive loss	'	'	-3,678	'
Net current-period other comprehensive income (loss)	'	'	-9,272	'
Balance at September 30, 2013	-3,254	'	-3,254	'
Net Unrealized Gain (Loss) on Available-for-sale Investments	'	'	'	'
Balance at December 31, 2012	'	'	119	'
Other comprehensive income (loss) before reclassifications	'	'	-197	'
Amounts reclassified from accumulated other comprehensive loss	'	'	117	'
Net current-period other comprehensive income (loss)	'	'	-80	'
Balance at September 30, 2013	39	'	39	'
Postretirement liability adjustment	'	'	'	'
Balance at December 31, 2012	'	'	-54,347	'
Other comprehensive income (loss) before reclassifications	'	'	0	'
Amounts reclassified from accumulated other comprehensive loss	'	'	1,344	'
Net current-period other comprehensive income (loss)	'	'	1,344	'
Balance at September 30, 2013	-53,003	'	-53,003	'
Foreign Currency Translation Adjustment	'	'	'	'
Balance at December 31, 2012	'	'	-511	'
Other comprehensive income (loss) before reclassifications	'	'	-351	'
Amounts reclassified from accumulated other comprehensive loss	'	'	143	'
Net current-period other comprehensive income (loss)	'	'	-208	'
Balance at September 30, 2013	($719)	'	($719)	'

Reclassification_out_of_accumu

Reclassification out of accumulated other comprehensive income (Details 2) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Revenues	$1,285,782	$1,173,518	$3,277,980	$2,994,288
Interest expense	21,012	19,840	63,312	56,929
Income before income taxes	131,126	-49,785	287,093	79,882
Income taxes	46,576	-20,253	99,559	24,516
Earnings (loss) from equity method investments	-61	2,388	-380	4,025
Net income (loss)	84,432	-29,671	187,280	60,233
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	807	'	5,924	'
Income taxes	-297	'	-2,246	'
Net income (loss)	510	'	3,678	'
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	-437	'	-2,371	'
Income taxes	166	'	1,027	'
Net income (loss)	-271	'	-1,344	'
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	-58	'	-180	'
Income taxes	20	'	63	'
Net income (loss)	-38	'	-117	'
Reclassification adjustment for loss on foreign currency translation adjustment included in net income [Member]	'	'	'	'
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Earnings (loss) from equity method investments	-185	'	-213	'
Earnings (loss) from equity method investments, tax	70	'	70	'
Net income (loss)	-115	'	-143	'
Total reclassifications	'	'	'	'
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Net income (loss)	86	'	2,074	'
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	1,007	'	6,903	'
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	($200)	'	($979)	'

Equity_method_investments_Deta

Equity method investments (Details) (USD $)	Dec. 31, 2012	Sep. 30, 2012	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]	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	$129	$110.60	'	'	'	'	'	'
Total long-term debt of equity method investments	65.5	28.2	'	'	'	'	'	'
Investment in equity method investments	6.9	7.4	'	'	'	'	'	'
Undistributed earnings of equity method investments	$3.40	$4.10	'	'	'	'	'	'

Goodwill_rollforward_Details

Goodwill rollforward (Details) (USD $)	9 Months Ended				12 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	$636,039,000	[1]	$634,931,000	[1]	$634,931,000	[1]
Goodwill acquired during the year	0		1,108,000	[2]	1,108,000	[2]
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	[2]	0	[2]
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	[2]	0	[2]
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	[2]	0	[2]
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		103,168,000		103,168,000
Goodwill acquired during the year	0		1,108,000	[2]	1,108,000	[2]
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.
[2]	Includes contingent consideration that was not material related to an acquisition in a prior period.

Other_intangible_assets_Detail

Other intangible assets (Details 2) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, net (excluding goodwill)	$14,092,000	$18,015,000	$14,092,000	$18,015,000	$17,129,000
Amortization of intangible assets	1,200,000	1,000,000	3,000,000	2,900,000	'
Estimated amortization expense for amortizable intangible assets [Abstract]	'	'	'	'	'
2013	3,700,000	'	3,700,000	'	'
2014	3,400,000	'	3,400,000	'	'
2015	2,600,000	'	2,600,000	'	'
2016	2,200,000	'	2,200,000	'	'
2017	1,900,000	'	1,900,000	'	'
Thereafter	3,300,000	'	3,300,000	'	'
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	-13,221,000	-11,192,000	-13,221,000	-11,192,000	-11,701,000
Intangible assets, net (excluding goodwill)	8,089,000	10,118,000	8,089,000	10,118,000	9,609,000
Noncompete agreements [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	6,186,000	7,236,000	6,186,000	7,236,000	7,236,000
Intangible assets, accumulated amortization	-4,706,000	-5,198,000	-4,706,000	-5,198,000	-5,326,000
Intangible assets, net (excluding goodwill)	1,480,000	2,038,000	1,480,000	2,038,000	1,910,000
Other intangible assets [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	10,995,000	10,979,000	10,995,000	10,979,000	10,979,000
Intangible assets, accumulated amortization	-6,472,000	-5,120,000	-6,472,000	-5,120,000	-5,369,000
Intangible assets, net (excluding goodwill)	$4,523,000	$5,859,000	$4,523,000	$5,859,000	$5,610,000

Derivative_instruments_Details

Derivative instruments (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013	Dec. 31, 2012
	MMBTU		MMBTU
Derivative instruments [Line Items]	'	'	'	'	'
Natural gas swap agreement total forward notional volumes in MMBtu	31,000	'	31,000	'	'
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets	($175,000)	'	($384,000)	'	'
Derivative, net liability position, aggregate fair value	53,000	'	53,000	'	'
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered	53,000	'	53,000	'	'
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	'	'15 months	'	'	'
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	'	28,000	'	'	'
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	9,900,000	9,900,000	9,900,000	'	6,300,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value	9,900,000	9,900,000	9,900,000	'	6,300,000
Oil swap and\or collar [Member]	'	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'	'
Derivative, nonmonetary notional amount	3,300,000	3,900,000	3,300,000	'	2,600,000
Natural gas swap and\or collar [Member]	'	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'	'
Derivative, nonmonetary notional amount	8,200,000	16,500,000	8,200,000	'	11,000,000
Natural gas basis swap [Member]	'	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'	'
Derivative, nonmonetary notional amount	874,000	'	874,000	'	'
Interest rate swap [Member]	'	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'	'
Derivative, notional amount	$60,000,000	'	$60,000,000	'	$50,000,000

Gains_and_losses_on_derivative

Gains and losses on derivatives instruments (Details 2) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$0	($9,125)	($5,594)	$7,962
Amount of gain reclassified from accumulated other comprehensive loss (effective portion), net of tax	-510	-7,782	-3,678	-7,029
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	-8,622	-6,153	9,822
Amount of gain reclassified from accumulated other comprehensive loss (effective portion), net of tax	-634	-7,788	-4,349	-7,056
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	-529	-1,422	-917
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	-503	559	-1,860
Amount of gain reclassified from accumulated other comprehensive loss (effective portion), net of tax	124	6	671	27
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	0	-769	0
Commodity derivatives not designated as hedging instruments:	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in operating revenues, before tax	($12,594)	($654)	($3,957)	$394

Derivative_instruments_Details1

Derivative instruments (Details 3) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'	'
Derivative asset, fair value, gross asset	$6,144	$18,304	$22,719
Derivative liability fair value gross liability	9,889	6,255	9,934
Designated as hedging instrument [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Derivative instruments in hedges, assets, at fair value	0	18,084	22,082
Derivative instruments in hedges, liabilities, at fair value	0	6,255	9,820
Designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current assets [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Derivative instruments in hedges, assets, at fair value	0	18,084	18,619
Designated as hedging instrument [Member] | Commodity contract [Member] | Other assets noncurrent [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Derivative instruments in hedges, assets, at fair value	0	0	3,463
Designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current liabilities [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Derivative instruments in hedges, liabilities, at fair value	0	0	1,958
Designated as hedging instrument [Member] | Commodity contract [Member] | Other liabilities noncurrent [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Derivative instruments in hedges, liabilities, at fair value	0	0	83
Designated as hedging instrument [Member] | Interest rate swap [Member] | Other accrued liabilities [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Derivative instruments in hedges, liabilities, at fair value	0	6,255	7,779
Not designated as hedging instrument [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments assets at fair value	6,144	220	637
Other derivatives not designated as hedging instruments liabilities at fair value	9,889	0	114
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	4,373	220	574
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	1,771	0	63
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	9,740	0	114
Not designated as hedging instrument [Member] | Commodity contract [Member] | Other liabilities noncurrent [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments liabilities at fair value	$149	$0	$0

Subject_to_master_netting_agre

Subject to master netting agreements (Details 4) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
In Thousands, unless otherwise specified
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	$6,144	$18,304	$22,719
Derivative liability fair value gross liability	9,889	6,255	9,934
Commodity derivatives [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	6,144	18,304	22,719
Gross amount not offset	-4,939	0	-2,102
Derivative asset, fair value, net	1,205	18,304	20,617
Total assets [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	6,144	18,304	22,719
Gross amount not offset	-4,939	0	-2,102
Derivative asset, fair value, net	1,205	18,304	20,617
Commodity derivatives - liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	9,889	'	2,155
Gross amount not offset	-4,939	'	-2,102
Derivative liability, fair value, net	4,950	'	53
Interest rate derivatives - liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	'	6,255	7,779
Gross amount not offset	'	0	0
Derivative liability, fair value, net	'	6,255	7,779
Total liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	9,889	6,255	9,934
Gross amount not offset	-4,939	0	-2,102
Derivative liability, fair value, net	$4,950	$6,255	$7,832

Availableforsale_securities_De

Available-for-sale securities (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Available-for-sale securities [Line Items]	'	'	'	'	'
Investments used to satisfy obligations under nonqualified benefit plans	$58,100,000	$48,400,000	$58,100,000	$48,400,000	$48,900,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	4,100,000	2,400,000	9,200,000	4,700,000	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	9,963,000	10,149,000	9,963,000	10,149,000	9,817,000
Gross unrealized gains	'	'	85,000	222,000	187,000
Gross unrealized losses	'	'	-24,000	-2,000	-3,000
Fair value	10,024,000	10,369,000	10,024,000	10,369,000	10,001,000
Mortgage backed securities [Member]	'	'	'	'	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	8,051,000	8,391,000	8,051,000	8,391,000	8,054,000
Gross unrealized gains	'	'	70,000	175,000	144,000
Gross unrealized losses	'	'	-20,000	-2,000	-3,000
Fair value	8,101,000	8,564,000	8,101,000	8,564,000	8,195,000
US Treasury securities [Member]	'	'	'	'	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	1,912,000	1,758,000	1,912,000	1,758,000	1,763,000
Gross unrealized gains	'	'	15,000	47,000	43,000
Gross unrealized losses	'	'	-4,000	0	0
Fair value	$1,923,000	$1,805,000	$1,923,000	$1,805,000	$1,806,000

Fair_value_measurements_Detail

Fair value measurements (Details 2) (USD $)	Sep. 30, 2013		Jun. 30, 2013	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012
Concentration risks, percentage [Abstract]	'		'	'		'		'
Percentage investment in common stock of mid-cap companies	29.00%		'	28.00%		28.00%		'
Percentage investment in common stock of small-cap companies	28.00%		'	28.00%		28.00%		'
Percentage investment in common stock of large-cap companies	28.00%		'	29.00%		29.00%		'
Percentage in fixed-income and other investments	15.00%		'	15.00%		15.00%		'
Assets, fair value disclosure, nonrecurring	'		$9,700,000	'		'		$2,500,000
Fair value, measurements, recurring [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	95,329,000		'	101,443,000		103,288,000		'
Liabilities, fair value disclosure	9,889,000		'	6,255,000		9,934,000		'
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Liabilities, fair value disclosure	9,889,000		'	'		2,155,000		'
Fair value, measurements, recurring [Member] | Interest rate swap [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Liabilities, fair value disclosure	'		'	6,255,000		7,779,000		'
Fair value, measurements, recurring [Member] | Money market funds [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	21,019,000		'	24,240,000		21,816,000		'
Fair value, measurements, recurring [Member] | Insurance contract [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	58,142,000	[1]	'	48,898,000	[2]	48,384,000	[2]	'
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	8,101,000		'	8,195,000		8,564,000		'
Fair value, measurements, recurring [Member] | US Treasury securities [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	1,923,000		'	1,806,000		1,805,000		'
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	6,144,000		'	18,304,000		22,719,000		'
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		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Interest rate swap [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Liabilities, fair value disclosure	'		'	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	[2]	'
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	95,329,000		'	101,443,000		103,288,000		'
Liabilities, fair value disclosure	9,889,000		'	6,255,000		9,934,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Liabilities, fair value disclosure	9,889,000		'	'		2,155,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Liabilities, fair value disclosure	'		'	6,255,000		7,779,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	21,019,000		'	24,240,000		21,816,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	58,142,000	[1]	'	48,898,000	[2]	48,384,000	[2]	'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	8,101,000		'	8,195,000		8,564,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Assets, fair value disclosure	1,923,000		'	1,806,000		1,805,000		'
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	6,144,000		'	18,304,000		22,719,000		'
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		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Interest rate swap [Member]	'		'	'		'		'
Fair value measurements [Line Items]	'		'	'		'		'
Liabilities, fair value disclosure	'		'	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	[2]	'
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 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 and other investments.
[2]	The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.

Fair_value_measurements_Detail1

Fair value measurements (Details 3) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
In Thousands, unless otherwise specified
Carrying amount [Member]	'	'	'
Fair value, balance sheet grouping [Line Items]	'	'	'
Long-term debt	$2,011,896	$1,744,975	$1,742,977
Fair value [Member]	'	'	'
Fair value, balance sheet grouping [Line Items]	'	'	'
Long-term debt	$2,106,887	$1,888,135	$1,906,673

Income_taxes_Details

Income taxes (Details) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2012	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'	'
Income tax expense for unrecognized tax positions	$2.20	'
Reduction in deferred income tax expense due to deferred income tax rate reduction	2.5	'
Unrecognized tax benefits to be settled within 12 months	'	$14.90
Anticipated settlement of federal and state audits	'	'12 months

Business_segment_data_Details

Business segment data (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	$1,285,782	$1,173,518	$3,277,980	$2,994,288
Intersegment operating revenues	0	0	0	0
Earnings on common stock	84,285	-29,842	186,970	59,719
Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	192,103	184,863	843,670	784,399
Intersegment operating revenues	4,906	7,046	31,623	36,393
Earnings on common stock	5,523	5,491	42,319	55,175
Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	1,093,679	988,655	2,434,310	2,209,889
Intersegment operating revenues	23,092	19,366	70,283	42,251
Earnings on common stock	79,964	-35,333	147,910	4,544
Electric [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	68,314	63,492	189,949	174,410
Intersegment operating revenues	0	0	0	0
Earnings on common stock	11,417	11,000	25,652	22,977
Natural gas distribution [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	77,417	80,069	536,756	504,805
Intersegment operating revenues	0	0	0	0
Earnings on common stock	-11,204	-8,782	15,420	10,314
Pipeline and energy services [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	46,372	41,302	116,965	105,184
Intersegment operating revenues	4,906	7,046	31,623	36,393
Earnings on common stock	5,310	3,273	1,247	21,884
Exploration and production [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	119,234	100,380	371,648	289,106
Intersegment operating revenues	10,714	8,076	33,083	25,114
Earnings on common stock	17,434	-87,748	70,713	-56,860
Construction materials and contracting [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	706,982	641,500	1,287,305	1,229,731
Intersegment operating revenues	7,422	8,508	24,673	11,756
Earnings on common stock	49,159	41,889	38,602	24,748
Construction services [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	267,038	246,358	774,103	688,368
Intersegment operating revenues	3,097	834	7,011	1,078
Earnings on common stock	12,154	9,863	36,733	29,951
Other [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	425	417	1,254	2,684
Intersegment operating revenues	1,859	1,948	5,516	4,303
Earnings on common stock	1,217	663	1,862	6,705
Intersegment Eliminations [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	0	0	0	0
Intersegment operating revenues	-27,998	-26,412	-101,906	-78,644
Earnings on common stock	($1,202)	$0	($3,259)	$0

Employee_benefit_plans_Details

Employee benefit plans (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension benefits [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Service cost	$39	$349	$116	$1,044
Interest cost	4,062	4,407	12,186	13,223
Expected return on assets	-4,979	-5,865	-14,937	-17,596
Amortization of prior service cost (credit)	18	-22	54	-64
Amortization of net actuarial loss	1,793	1,887	5,373	5,670
Amortization of net transition obligation	0	0	0	0
Curtailment gain	0	-1,023	0	-1,023
Net periodic benefit cost, including amount capitalized	933	-267	2,792	1,254
Less amount capitalized	157	185	425	615
Net periodic benefit cost (credit)	776	-452	2,367	639
Other postretirement benefits [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Service cost	419	437	1,257	1,310
Interest cost	804	943	2,411	3,124
Expected return on assets	-1,086	-1,222	-3,258	-3,667
Amortization of prior service cost (credit)	-364	-534	-1,092	-1,078
Amortization of net actuarial loss	327	356	1,405	1,769
Amortization of net transition obligation	0	531	0	1,594
Curtailment gain	0	0	0	0
Net periodic benefit cost, including amount capitalized	100	511	723	3,052
Less amount capitalized	47	314	137	635
Net periodic benefit cost (credit)	53	197	586	2,417
Supplemental employee retirement plans [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Net periodic benefit cost (credit)	$1,800	$2,000	$5,500	$6,100

Regulatory_matters_and_revenue1

Regulatory matters and revenues subject to refund (Details) (USD $)	0 Months Ended
	Apr. 12, 2013	Sep. 26, 2012	Oct. 23, 2013	Jun. 19, 2013	Dec. 12, 2012	Oct. 23, 2013	Sep. 30, 2013	Sep. 18, 2013	Oct. 31, 2013
	MTPSC [Member]	MTPSC [Member]	SDPUC [Member]	SDPUC [Member]	SDPUC [Member]	NDPSC [Member]	NDPSC [Member]	NDPSC [Member]	FERC [Member]
Regulatory matters and revenues subject to refund [Line Items]	'	'	'	'	'	'	'	'	'
Total gas rate increase requested	'	$3,500,000	'	'	$1,500,000	'	'	$6,800,000	'
Percent above current rates requested	'	5.90%	'	'	3.30%	'	'	6.40%	'
Amount of requested interim increase in annual gas rates	'	1,700,000	'	1,500,000	'	'	'	4,500,000	'
Requested percentage of interim gas rate increase	'	2.90%	'	'	'	'	'	4.20%	'
Interim rate increase (decrease), amount	850,000	'	'	'	'	4,300,000	'	'	'
Settlement stipulation	'	'	900,000	'	'	'	'	'	'
Settlement stipulation percent	'	'	2.00%	'	'	'	'	'	'
Estimated project cost	'	'	'	'	'	'	27,700,000	'	'
Settlement agreement, amount	'	'	'	'	'	4,300,000	'	'	'
Settlement agreement, percent	'	'	'	'	'	4.00%	'	'	'
Rate change application annual increase, amount	'	'	'	'	'	'	'	'	28,900,000
Increased investment	'	'	'	'	'	'	'	'	$312,000,000

Litigation_Details

Litigation (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Aug. 31, 2011	Jun. 30, 2013	Jun. 30, 2012	Oct. 31, 2010	Sep. 30, 2010	Jan. 13, 2012	Dec. 31, 2009
				Litigation related to construction materials [Member]	Natural gas gathering operations [Member]	Natural gas gathering operations [Member]	Natural gas gathering operations [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	$30,800,000	$22,500,000	$41,600,000	'	'	'	'	'	'	'
Amount of compensatory damages in LPP's notice of demand	'	'	'	'	'	'	'	'	'	149,700,000
Arbitration award	'	'	'	'	'	'	26,600,000	'	22,000,000	'
Estimated damages in pending litigation, low estimate	'	'	'	'	16,100,000	'	'	'	'	'
Estimated damages in pending litigation, high estimate	'	'	'	'	22,600,000	'	'	'	'	'
Loss contingency, estimate of possible loss	'	'	'	3,700,000	'	'	'	'	'	'
Additional amount per day of potential penalties	'	'	'	5,000	'	'	'	'	'	'
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, before tax	'	'	'	'	'	-24,100,000	'	26,600,000	'	'
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, after tax	'	'	'	'	'	-15,000,000	'	16,500,000	'	'
Revised damage calculation	'	'	'	'	$1,000,000	'	'	'	'	'

Enviromental_matters_Details_2

Enviromental matters (Details 2) (USD $)	0 Months Ended
	Sep. 30, 2013
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	1,300,000
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	12,500,000
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 $)	Sep. 30, 2013
Guarantor Obligations [Line Items]	'
Amount of hedging obligations guaranteed reflected on balance sheet	$6,300,000
Guarantor obligations, maximum exposure, undiscounted	63,600,000
Fixed maximum amounts guaranteed by year 2013	13,000,000
Fixed maximum amounts guaranteed by year 2014	30,900,000
Fixed maximum amounts guaranteed by year 2015	1,600,000
Fixed maximum amounts guaranteed by year 2016	300,000
Fixed maximun amounts guaranteed by year 2017	100,000
Fixed maximum amounts guaranteed by year 2019	200,000
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	300,000
Letters of credit	38,200,000
Letters of credit set to expire - 2013	7,700,000
Letters of credit set to expire - 2014	30,500,000
Outstanding letters of credit	0
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	5,000,000
Amount outstanding by a related party under the guarantee	900,000
Amount of surety bonds outstanding	$552,000,000

Commitment_and_contingencies_v

Commitment and contingencies variable interest entities (Details 4) (USD $)	Sep. 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
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	23,146,000
Dakota Prairie Refining, LLC [Member] | Accounts receivable [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, assets	1,000
Dakota Prairie Refining, LLC [Member] | Other current assets [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, assets	25,000
Dakota Prairie Refining, LLC [Member] | Total current assets [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, assets	23,172,000
Dakota Prairie Refining, LLC [Member] | Net property, plant and equipment [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, assets	123,297,000
Dakota Prairie Refining, LLC [Member] | Total assets [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, assets	146,469,000
Dakota Prairie Refining, LLC [Member] | Accounts Payable [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, liabilities	20,313,000
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
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, liabilities	363,000
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, liabilities	23,676,000
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, liabilities	72,000,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member]	'
Variable Interest Entities [Line Items]	'
Limited liability entity, consolidated, carrying amount, liabilities	95,676,000
Fuel contract [Member]	'
Variable Interest Entities [Line Items]	'
Percent of ownership	25.00%
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$7,400,000

Subsquent_events_Details

Subsquent events (Details) (Subsequent event [Member], USD $)	0 Months Ended
In Millions, unless otherwise specified	Oct. 01, 2013	Oct. 30, 2013	Oct. 30, 2013	Nov. 01, 2013
	MDU Energy Capital [Member]	Cascade Natural Gas Co. [Member]	Intermountain Gas Company [Member]	Centennial Energy Holdings, Inc [Member]
Subsequent Event [Line Items]	'	'	'	'
Credit agreement	$30	'	'	'
Senior notes issued	'	$50	$50	$25
Senior notes weighted average interest rate	'	4.24%	4.21%	4.84%