MDU Resources (MDU) 10-Q 7 Nov 14 2014 Q3 Quarterly report Financial data

Document_And_Entity_Informatio

Document And Entity Information	9 Months Ended
	Sep. 30, 2014	Oct. 31, 2014
Entity Information [Line Items]	'	'
Entity registrant name	'MDU RESOURCES GROUP INC	'
Entity central index key	'0000067716	'
Current fiscal year end date	'--12-31	'
Entity filer category	'Large Accelerated Filer	'
Entity common stock, shares outstanding	'	194,106,937
Document fiscal year focus	'2014	'
Document fiscal period focus	'Q3	'
Document type	'10-Q	'
Amendment flag	'false	'
Document period end date	30-Sep-14	'

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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating revenues:	'	'	'	'
Electric, natural gas distribution and pipeline and energy services	$211,536	$192,103	$957,769	$843,670
Exploration and production, construction materials and contracting, construction services and other	1,158,919	1,093,679	2,549,585	2,434,310
Total operating revenues	1,370,455	1,285,782	3,507,354	3,277,980
Operating expenses:	'	'	'	'
Fuel and purchased power	19,236	19,983	66,826	59,760
Purchased natural gas sold	47,718	35,826	377,024	305,268
Operation and maintenance:	'	'	'	'
Electric, natural gas distribution and pipeline and energy services	79,848	64,078	225,180	206,808
Exploration and production, construction materials and contracting, construction services and other	897,887	870,252	2,002,884	1,925,762
Depreciation, depletion and amortization	103,497	99,966	306,180	288,816
Taxes, other than income	45,504	45,804	150,657	145,784
Total operating expenses	1,193,690	1,135,909	3,128,751	2,932,198
Operating income	176,765	149,873	378,603	345,782
Loss from equity method investments	-97	-61	-343	-380
Other income	2,644	2,326	7,552	5,003
Interest expense	22,425	21,012	64,912	63,312
Income before income taxes	156,887	131,126	320,900	287,093
Income taxes	54,769	46,576	109,818	99,559
Income from continuing operations	102,118	84,550	211,082	187,534
Income (loss) from discontinued operations, net of tax (Note 12)	3	-118	506	-254
Net income	102,121	84,432	211,588	187,280
Net loss attributable to noncontrolling interest	-1,088	-24	-2,390	-204
Dividends declared on preferred stocks	171	171	514	514
Earnings on common stock	$103,038	$84,285	$213,464	$186,970
Earnings per common share - basic:	'	'	'	'
Earnings before discontinued operations	$0.53	$0.45	$1.11	$0.99
Discontinued operations, net of tax	$0	$0	$0	$0
Earnings per common share - basic	$0.53	$0.45	$1.11	$0.99
Earnings per common share - diluted:	'	'	'	'
Earnings before discontinued operations	$0.53	$0.44	$1.11	$0.99
Discontinued operations, net of tax	$0	$0	$0	$0
Earnings per common share - diluted	$0.53	$0.44	$1.11	$0.99
Dividends declared per common share	$0.18	$0.17	$0.53	$0.52
Weighted average common shares outstanding - basic	193,949	188,831	191,958	188,831
Weighted average common shares outstanding - diluted	194,300	189,638	192,307	189,634

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Consolidated Statements of Comprehensive Income [Abstract]	'	'	'	'
Net income	$102,121	$84,432	$211,588	$187,280
Net unrealized gain (loss) on derivative instruments qualifying as hedges:	'	'	'	'
Net unrealized loss on derivative instruments arising during the period, net of tax of $0 and $0 for the three months ended and $0 and $(3,116) for the nine months ended in 2014 and 2013, respectively	0	0	0	-5,594
Reclassification adjustment for (gain) loss on derivative instruments included in net income, net of tax of $50 and $(297) for the three months ended and $264 and $(2,246) for the nine months ended in 2014 and 2013, respectively	82	-510	439	-3,678
Net unrealized gain (loss) on derivative instruments qualifying as hedges	82	-510	439	-9,272
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $159 and $166 for the three months ended and $477 and $1,027 for the nine months ended in 2014 and 2013, respectively	261	271	781	1,344
Foreign currency translation adjustment recognized during the period, net of tax of $(89) and $(12) for the three months ended and $(36) and $(209) for the nine months ended in 2014 and 2013, respectively	-146	-20	-58	-351
Reclassification adjustment for loss on foreign currency translation adjustment included in net income, net of tax of $0 and $70 for the three months ended and $0 and $70 for the nine months ended in 2014 and 2013, respectively	0	115	0	143
Foreign currency translation adjustment	-146	95	-58	-208
Net unrealized gain (loss) on available-for-sale investments:	'	'	'	'
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(33)and $(5) for the three months ended and $(48) and $(106) for the nine months ended in 2014 and 2013, respectively	-62	-10	-89	-197
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $16 and $20 for the three months ended and $54 and $63 for the nine months ended in 2014 and 2013, respectively	31	38	100	117
Net unrealized gain (loss) on available-for-sale investments	-31	28	11	-80
Other comprehensive income (loss)	166	-116	1,173	-8,216
Comprehensive income	102,287	84,316	212,761	179,064
Comprehensive loss attributable to noncontrolling interest	-1,088	-24	-2,390	-204
Comprehensive income attributable to common stockholders	$103,375	$84,340	$215,151	$179,268

Consolidated_Statements_of_Com1

Consolidated Statements of Comprehensive Income - Parenthetical (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net unrealized gain (loss) on derivative instruments arising during the period, tax	$0	$0	$0	($3,116)
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax	50	-297	264	-2,246
Amortization of postretirement liability losses included in net periodic benefit cost, tax	159	166	477	1,027
Foreign currency translation adjustments arising during the period, tax	-89	-12	-36	-209
Reclassification adjustment for foreign currency translation (gain) loss included in net income, tax	0	70	0	70
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	-33	-5	-48	-106
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income, tax	$16	$20	$54	$63

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
In Thousands, unless otherwise specified
Current assets:	'		'		'
Cash and cash equivalents	$233,676		$45,225		$66,174
Receivables, net	784,028		713,067		787,311
Inventories	302,705		282,391		314,571
Deferred income taxes	13,041		25,048		26,284
Commodity derivative instruments	11,322		1,447		4,373
Prepayments and other current assets	72,900		49,510		56,257
Total current assets	1,417,672		1,116,688		1,254,970
Investments	115,656		112,939		108,664
Property, plant and equipment	9,438,609		8,803,866		8,651,334
Less accumulated depreciation, depletion and amortization	4,092,017		3,872,487		3,796,052
Net property, plant and equipment	5,346,592		4,931,379		4,855,282
Deferred charges and other assets:	'		'		'
Goodwill	636,039	[1]	636,039	[1]	636,039	[1]
Other intangible assets, net	10,596		13,099		14,092
Other	247,539		251,188		298,061
Total deferred charges and other assets	894,174		900,326		948,192
Total assets	7,774,094		7,061,332		7,167,108
Current liabilities:	'		'		'
Short-term borrowings	0		11,500		7,000
Long-term debt due within one year	149,101		12,277		44,024
Accounts payable	410,382		404,961		437,740
Taxes payable	105,027		74,175		80,392
Dividends payable	34,607		33,737		32,745
Accrued compensation	66,119		69,661		62,746
Commodity derivative instruments	44		7,483		9,740
Other accrued liabilities	173,247		171,106		171,420
Total current liabilities	938,527		784,900		845,807
Long-term debt	2,061,456		1,842,286		1,967,872
Deferred credits and other liabilities:	'		'		'
Deferred income taxes	887,807		859,306		808,011
Other liabilities	727,801		718,938		794,928
Total deferred credits and other liabilities	1,615,608		1,578,244		1,602,939
Equity:	'		'		'
Preferred stocks	15,000		15,000		15,000
Common stockholders' equity:	'		'		'
Authorized - 500,000,000 shares, $1.00 par value. Shares issued - 194,548,389 at September 30, 2014, 189,369,450 at September 30, 2013 and 189,868,780 at December 31, 2013	194,548		189,869		189,369
Other paid-in-capital	1,200,591		1,056,996		1,041,787
Retained earnings	1,713,774		1,603,130		1,546,000
Accumulated other comprehensive loss	-37,032		-38,205		-56,937
Treasury stock at cost - 538,921 shares	-3,626		-3,626		-3,626
Total common stockholders' equity	3,068,255		2,808,164		2,716,593
Total stockholders' equity	3,083,255		2,823,164		2,731,593
Noncontrolling interest	75,248		32,738		18,897
Total equity	3,158,503		2,855,902		2,750,490
Total liabilities and equity	$7,774,094		$7,061,332		$7,167,108
[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, 2014	Dec. 31, 2013	Sep. 30, 2013
Stockholders' equity:	'	'	'
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, par value	$1	$1	$1
Common stock, shares issued	194,548,389	189,868,780	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, 2014	Sep. 30, 2013
Operating activities:	'	'
Net income	$211,588	$187,280
Income (loss) from discontinued operations, net of tax	506	-254
Income from continuing operations	211,082	187,534
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation, depletion and amortization	306,180	288,816
Loss, net of distributions, from equity method investments	401	1,736
Deferred income taxes	37,006	46,212
Unrealized (gain) loss on commodity derivatives	-16,847	5,379
Excess tax benefit on stock-based compensation	-4,729	0
Changes in current assets and liabilities, net of acquisitions:	'	'
Receivables	-73,596	-107,482
Inventories	-20,153	1,562
Other current assets	-20,416	-15,397
Accounts payable	-22,007	25,817
Other current liabilities	32,767	18,680
Other noncurrent changes	-26,915	-24,149
Net cash provided by continuing operations	402,773	428,708
Net cash provided by discontinued operations	541	254
Net cash provided by operating activities	403,314	428,962
Investing activities:	'	'
Capital expenditures	-638,731	-648,465
Acquisitions, net of cash acquired	-208,945	0
Net proceeds from sale or disposition of property and other	203,386	40,985
Investments	792	218
Proceeds from sale of equity method investment	0	1,896
Net cash used in continuing operations	-643,498	-605,366
Net cash provided by discontinued operations	0	0
Net cash used in investing activities	-643,498	-605,366
Financing activities:	'	'
Issuance of short-term borrowings	0	5,000
Repayment of short-term borrowings	-11,500	0
Issuance of long-term debt	672,351	497,318
Repayment of long-term debt	-318,991	-255,980
Proceeds from issuance of common stock	144,868	0
Dividends paid	-102,105	-65,660
Excess tax benefit on stock-based compensation	4,729	0
Tax withholding on performance shares	-5,564	0
Contribution from noncontrolling interest	44,900	13,000
Net cash provided by continuing operations	428,688	193,678
Net cash provided by discontinued operations	0	0
Net cash provided by financing activities	428,688	193,678
Effect of exchange rate changes on cash and cash equivalents	-53	-142
Increase in cash and cash equivalents	188,451	17,132
Cash and cash equivalents - beginning of year	45,225	49,042
Cash and cash equivalents - end of period	$233,676	$66,174

Basis_of_presentation

Basis of presentation	9 Months Ended
	Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Basis of presentation	'
	Basis of presentation
	The accompanying consolidated interim financial statements were prepared in conformity with the basis of presentation reflected in the consolidated financial statements included in the Company's 2013 Annual Report, and the standards of accounting measurement set forth in the interim reporting guidance in the ASC and any amendments thereto adopted by the FASB. Interim financial statements do not include all disclosures provided in annual financial statements and, accordingly, these financial statements should be read in conjunction with those appearing in the 2013 Annual Report. The information is unaudited but includes all adjustments that are, in the opinion of management, necessary for a fair presentation of the accompanying consolidated interim financial statements and are of a normal recurring nature. Depreciation, depletion and amortization expense is reported separately on the Consolidated Statements of Income and therefore is excluded from the other line items within operating expenses. Management has also evaluated the impact of events occurring after September 30, 2014, up to the date of issuance of these consolidated interim financial statements.

Seasonality_of_operations

Seasonality of operations	9 Months Ended
	Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Seasonality of operations	'
	Seasonality of operations
	Some of the Company's operations are highly seasonal and revenues from, and certain expenses for, such operations may fluctuate significantly among quarterly periods. Accordingly, the interim results for particular businesses, and for the Company as a whole, may not be indicative of results for the full fiscal year.

Accounts_receivable_and_allowa

Accounts receivable and allowance for doubtful accounts	9 Months Ended
	Sep. 30, 2014
Receivables [Abstract]	'
Accounts receivable and allowance for doubtful accounts	'
	Accounts receivable and allowance for doubtful accounts
	Accounts receivable consist primarily of trade receivables from the sale of goods and services which are recorded at the invoiced amount net of allowance for doubtful accounts, and costs and estimated earnings in excess of billings on uncompleted contracts. The total balance of receivables past due 90 days or more was $29.8 million, $31.1 million and $36.4 million at September 30, 2014 and 2013, and December 31, 2013, respectively.
	The allowance for doubtful accounts is determined through a review of past due balances and other specific account data. Account balances are written off when management determines the amounts to be uncollectible. The Company's allowance for doubtful accounts at September 30, 2014 and 2013, and December 31, 2013, was $8.9 million, $9.6 million and $10.1 million, respectively.

Inventories_and_natural_gas_in

Inventories and natural gas in storage	9 Months Ended
	Sep. 30, 2014
Inventory Disclosure [Abstract]	'
Inventories and natural gas in storage	'
	Inventories and natural gas in storage
	Inventories, other than natural gas in storage for the Company's regulated operations, are stated at the lower of average cost or market value. Natural gas in storage for the Company's regulated operations is generally carried at average cost, or cost using the last-in, first-out method. The portion of the cost of natural gas in storage expected to be used within one year is included in inventories. Inventories consisted of:
		September 30,		September 30,		December 31,
		2014		2013		2013
		(In thousands)
	Aggregates held for resale	$	106,623	$	104,784	$	101,568
	Asphalt oil	33,551		43,078		38,099
	Materials and supplies	71,515		71,370		69,808
	Merchandise for resale	24,566		23,713		21,720
	Natural gas in storage (current)	29,979		37,689		16,417
	Other	36,471		33,937		34,779
	Total	$	302,705	$	314,571	$	282,391
	The remainder of natural gas in storage, which largely represents the cost of gas required to maintain pressure levels for normal operating purposes, is included in other assets and was $47.4 million, $48.6 million and $48.3 million at September 30, 2014 and 2013, and December 31, 2013, respectively.

Oil_and_natural_gas_properties

Oil and natural gas properties disposition	9 Months Ended
	Sep. 30, 2014
Property, Plant and Equipment Impairment or Disposal [Abstract]	'
Oil and natural gas properties disposition	'
	Oil and natural gas properties disposition
	Fidelity entered into a purchase and sale agreement on July 17, 2014, to sell certain oil and natural gas properties in Mountrail County, North Dakota. Proceeds from the sale were $184.4 million, subject to final adjustments. The effective date of the disposition was May 1, 2014, with the closing date occurring on September 30, 2014.

Impairment_of_longlived_assets

Impairment of long-lived assets	9 Months Ended
	Sep. 30, 2014
Asset Impairment Charges [Abstract]	'
Impairment of long-lived assets	'
	Impairment of long-lived assets
	During the second quarter of 2013, the Company recognized an impairment of coalbed natural gas gathering assets at the pipeline and energy services segment of $14.5 million ($9.0 million after tax), which is recorded in operation and maintenance expense on the Consolidated Statements of Income. The impairment is related to coalbed natural gas gathering assets located in Wyoming and Montana where there has been a significant decline in natural gas development and production activity largely due to low natural gas prices. The coalbed natural gas gathering assets were written down to fair value that was determined using the income approach. For more information on this nonrecurring fair value measurement, see Note 16.

Earnings_per_common_share

Earnings per common share	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Earnings per common share	'
	Earnings per common share
	Basic earnings per common share were computed by dividing earnings on common stock by the weighted average number of shares of common stock outstanding during the applicable period. Diluted earnings per common share were computed by dividing earnings on common stock by the total of the weighted average number of shares of common stock outstanding during the applicable period, plus the effect of outstanding performance share awards. Common stock outstanding includes issued shares less shares held in treasury. Net income was the same for both the basic and diluted earnings per share calculations. A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows:
		Three Months Ended		Nine Months Ended
		September 30,		September 30,
		2014	2013	2014	2013
		(In thousands)
	Weighted average common shares outstanding - basic	193,949	188,831	191,958	188,831
	Effect of dilutive performance share awards	351	807	349	803
	Weighted average common shares outstanding - diluted	194,300	189,638	192,307	189,634
	Shares excluded from the calculation of diluted earnings per share	—	—	—	—

Cash_flow_information

Cash flow information	9 Months Ended
	Sep. 30, 2014
Supplemental Cash Flow Information [Abstract]	'
Cash flow information	'
	Cash flow information
	Cash expenditures for interest and income taxes were as follows:
		Nine Months Ended
		September 30,
		2014		2013
		(In thousands)
	Interest, net of amounts capitalized and AFUDC - borrowed of $8.6 million and $6.3 million in 2014 and 2013, respectively	$	61,690	$	60,281
	Income taxes paid	$	44,166	$	30,262
	Noncash investing transactions were as follows:
		September 30,
		2014		2013
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	96,373	$	85,646

New_accounting_standards

New accounting standards	9 Months Ended
	Sep. 30, 2014
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]	'
New accounting standard	'
	New Accounting Standard
	Revenue from Contracts with Customers In May 2014, the FASB issued guidance on accounting for revenue from contracts with customers. The guidance provides for a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry specific guidance. This guidance will be effective for the Company on January 1, 2017. Entities will have the option of using either a full retrospective or modified retrospective approach to adopting the guidance. Under the modified approach, an entity would recognize the cumulative effect of initially applying the guidance with an adjustment to the opening balance of retained earnings in the period of adoption. In addition, the modified approach will require additional disclosures. The Company is evaluating the effects the adoption of the new revenue guidance will have on its results of operations, financial position, cash flows and disclosures, as well as its method of adoption.

Comprehensive_income_loss

Comprehensive income (loss)	9 Months Ended
	Sep. 30, 2014
Comprehensive income (loss) [Abstract]	'
Comprehensive income (loss)	'
	Comprehensive income (loss)
	The after-tax changes in the components of accumulated other comprehensive loss were as follows:
	Three Months Ended September 30, 2014	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,408	)	$	(33,287	)	$	(579	)	$	76		$	(37,198	)
	Other comprehensive income (loss) before reclassifications	—			—			(146		)	(62		)	(208		)
	Amounts reclassified from accumulated other comprehensive loss	82			261			—			31			374
	Net current-period other comprehensive income (loss)	82			261			(146		)	(31		)	166
	Balance at end of period	$	(3,326	)	$	(33,026	)	$	(725	)	$	45		$	(37,032	)
	Three Months Ended September 30, 2013	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(2,744	)	$	(53,275	)	$	(813	)	$	11		$	(56,821	)
	Other comprehensive income (loss) before reclassifications	—			—			(20		)	(10		)	(30		)
	Amounts reclassified from accumulated other comprehensive loss	(510		)	272			114			38			(86		)
	Net current-period other comprehensive income (loss)	(510		)	272			94			28			(116		)
	Balance at end of period	$	(3,254	)	$	(53,003	)	$	(719	)	$	39		$	(56,937	)
	Nine Months Ended September 30, 2014	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,765	)	$	(33,807	)	$	(667	)	$	34		$	(38,205	)
	Other comprehensive income (loss) before reclassifications	—			—			(58		)	(89		)	(147		)
	Amounts reclassified from accumulated other comprehensive loss	439			781			—			100			1,320
	Net current-period other comprehensive income (loss)	439			781			(58		)	11			1,173
	Balance at end of period	$	(3,326	)	$	(33,026	)	$	(725	)	$	45		$	(37,032	)
	Nine Months Ended September 30, 2013	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	6,018		$	(54,347	)	$	(511	)	$	119		$	(48,721	)
	Other comprehensive income (loss) before reclassifications	(5,594		)	—			(351		)	(197		)	(6,142		)
	Amounts reclassified from accumulated other comprehensive loss	(3,678		)	1,344			143			117			(2,074		)
	Net current-period other comprehensive income (loss)	(9,272		)	1,344			(208		)	(80		)	(8,216		)
	Balance at end of period	$	(3,254	)	$	(53,003	)	$	(719	)	$	39		$	(56,937	)
	Reclassifications out of accumulated other comprehensive loss were as follows:
		Three Months Ended						Nine Months Ended						Location on Consolidated Statements of Income
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands)
	Reclassification adjustment for gain (loss) on derivative instruments included in net income:
	Commodity derivative instruments	$	28		$	1,007		$	(223	)	$	6,903		Operating revenues
	Interest rate derivative instruments	(160		)	(200		)	(480		)	(979		)	Interest expense
		(132		)	807			(703		)	5,924
		50			(297		)	264			(2,246		)	Income taxes
		(82		)	510			(439		)	3,678
	Amortization of postretirement liability losses included in net periodic benefit cost	(420		)	(437		)	(1,258		)	(2,371		)	(a)
		159			166			477			1,027			Income taxes
		(261		)	(271		)	(781		)	(1,344		)
	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		)
	Reclassification adjustment for loss on available-for-sale investments included in net income	(47		)	(58		)	(154		)	(180		)	Other income
		16			20			54			63			Income taxes
		(31		)	(38		)	(100		)	(117		)
	Total reclassifications	$	(374	)	$	86		$	(1,320	)	$	2,074
(a) Included in net periodic benefit cost (credit). For more information, see Note 20.

Acquisition

Acquisition	9 Months Ended
	Sep. 30, 2014
Business Combinations [Abstract]	'
Acquisition	'
	Acquisition
	On February 10, 2014, the Company entered into agreements to purchase working interests and leasehold positions in oil and natural gas production assets in the southern Powder River Basin of Wyoming. The effective date of the acquisition was October 1, 2013, and the closing occurred on March 6, 2014. The purchase price was $208.9 million, including purchase price adjustments.
	The acquisition was accounted for under the acquisition method of accounting and, accordingly, the acquired assets and liabilities assumed have been recorded at their respective fair values as of the date of acquisition. The results of operations of the acquired properties are included in the financial statements since the date of the acquisition. Pro forma financial amounts reflecting the effects of the acquisition are not presented, as such acquisition was not material to the Company's financial position or results of operations.

Discontinued_operations

Discontinued operations	9 Months Ended
	Sep. 30, 2014
Discontinued Operations and Disposal Groups [Abstract]	'
Discontinued operations	'
	Discontinued operations
	In 2007, Centennial Resources sold CEM to Bicent. In connection with the sale, Centennial Resources agreed to indemnify Bicent and its affiliates from certain third party claims arising out of or in connection with Centennial Resources' ownership or operation of CEM prior to the sale. In addition, Centennial had previously guaranteed CEM's obligations under a construction contract. The Company incurred legal expenses and had a benefit related to the resolution of this matter in the second quarter of 2014, which are reflected in discontinued operations in the consolidated financial statements and accompanying notes. Discontinued operations are included in the Other category.

Equity_method_investments

Equity method investments	9 Months Ended
	Sep. 30, 2014
Equity Method Investments and Joint Ventures [Abstract]	'
Equity method investments	'
	Equity method investments
	Investments in companies in which the Company has the ability to exercise significant influence over operating and financial policies are accounted for using the equity method. At September 30, 2014 and 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 interests in the Brazilian Transmission Lines. In November 2010, the Company completed the sale of its entire ownership interest in ENTE and ERTE and 59.96 percent of the Company's ownership interest in ECTE. The Company's remaining interest in ECTE is being sold over a four-year period. In August 2013 and 2012, and November 2011, the Company completed the sale of one-fourth of the remaining interest in each year. The Company recognized an immaterial gain in 2013. The Company's remaining ownership interest in ECTE is being accounted for under the cost method.

Goodwill_and_other_intangible_

Goodwill and other intangible assets	9 Months Ended
	Sep. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Goodwill and other intangible assets	'
	Goodwill and other intangible assets
	The changes in the carrying amount of goodwill were as follows:
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-14	as of			Acquired			as of
		January 1,			During			September 30, 2014*
		2014*			the Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-13	as of			Acquired			as of
		January 1,			During the			September 30, 2013*
		2013*			Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Year Ended	Balance			Goodwill			Balance
	31-Dec-13	as of			Acquired			as of
		January 1,			During the			December 31,
		2013*			Year			2013*
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Other amortizable intangible assets were as follows:
		September 30,			September 30,			December 31,
		2014			2013			2013
		(In thousands)
	Customer relationships	$	21,310		$	21,310		$	21,310
	Accumulated amortization	(15,116		)	(13,221		)	(13,726		)
		6,194			8,089			7,584
	Noncompete agreements	5,080			6,186			6,186
	Accumulated amortization	(4,021		)	(4,706		)	(4,840		)
		1,059			1,480			1,346
	Other	10,921			10,995			10,995
	Accumulated amortization	(7,578		)	(6,472		)	(6,826		)
		3,343			4,523			4,169
	Total	$	10,596		$	14,092		$	13,099
	Amortization expense for amortizable intangible assets for the three and nine months ended September 30, 2014, was $700,000 and $2.5 million, respectively. 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. Estimated amortization expense for amortizable intangible assets is $3.3 million in 2014, $2.5 million in 2015, $2.2 million in 2016, $1.9 million in 2017, $1.0 million in 2018 and $2.2 million thereafter.

Derivative_instruments

Derivative instruments	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Derivative Instruments	'
	Derivative instruments
	The Company's policy allows the use of derivative instruments as part of an overall energy price, foreign currency and interest rate risk management program to efficiently manage and minimize commodity price, foreign currency and interest rate risk. As of September 30, 2014, the Company had no outstanding foreign currency or interest rate hedges.
	The fair value of derivative instruments must be estimated as of the end of each reporting period and is recorded on the Consolidated Balance Sheets as an asset or a liability.
	Fidelity
	At September 30, 2014 and 2013, and December 31, 2013, Fidelity held oil swap and collar agreements with total forward notional volumes of 1.4 million, 3.9 million and 2.9 million Bbl, respectively, and natural gas swap agreements with total forward notional volumes of 7.3 million, 16.5 million and 18.3 million MMBtu, respectively. Fidelity utilizes these derivative instruments to manage a portion of the market risk associated with fluctuations in the price of oil and natural gas on its forecasted sales of oil and natural gas production.
	Effective April 1, 2013, Fidelity elected to de-designate all commodity derivative contracts previously designated as cash flow hedges and elected to discontinue hedge accounting prospectively for all of its commodity derivative instruments. When the criteria for hedge accounting is not met or when hedge accounting is not elected, realized gains and losses and unrealized gains and losses are both recorded in operating revenues on the Consolidated Statements of Income. As a result of discontinuing hedge accounting on commodity derivative instruments, gains and losses on the oil and natural gas derivative instruments remain in accumulated other comprehensive income (loss) as of the de-designation date and are reclassified into earnings in future periods as the underlying hedged transactions affect earnings. At April 1, 2013, accumulated other comprehensive income (loss) included $1.8 million of unrealized gains, representing the mark-to-market value of the Company's commodity derivative instruments that qualified as cash flow hedges as of the balance sheet date. The Company expects to reclassify into earnings from accumulated other comprehensive income (loss) the remaining value related to de-designating commodity derivative instruments over the next 3 months.
	Prior to April 1, 2013, changes in the fair value attributable to the effective portion of the hedging instruments, net of tax, were recorded in stockholders' equity as a component of accumulated other comprehensive income (loss). To the extent that the hedges were not effective or did not qualify for hedge accounting, the ineffective portion of the changes in fair market value was recorded directly in earnings. Gains and losses on the oil and natural gas derivative instruments were reclassified from accumulated other comprehensive income (loss) into operating revenues on the Consolidated Statements of Income at the date the oil and natural gas quantities were settled.
	There were no components of the derivative instruments' gain or loss excluded from the assessment of hedge effectiveness. Gains and losses must be reclassified into earnings as a result of the discontinuance of cash flow hedges if it is probable that the original forecasted transactions will not occur, and there were no such reclassifications.
	Certain of Fidelity's derivative instruments contain cross-default provisions that state if Fidelity or any of its affiliates fails to make payment with respect to certain indebtedness, in excess of specified amounts, the counterparties could require early settlement or termination of the derivative instruments in liability positions. The aggregate fair value of Fidelity's derivative instruments with credit-risk-related contingent features that are in a liability position at September 30, 2014 and 2013, and December 31, 2013, were $44,000, $9.9 million and $7.5 million, respectively. The aggregate fair value of assets that would have been needed to settle the instruments immediately if the credit-risk-related contingent features were triggered on September 30, 2014 and 2013, and December 31, 2013, were $44,000, $9.9 million and $7.5 million, respectively.
	Centennial
	Centennial has historically entered into interest rate derivative instruments to manage a portion of its interest rate exposure on the forecasted issuance of long-term debt. As of September 30, 2014 and 2013, and December 31, 2013, Centennial had no outstanding interest rate swap agreements.
	Fidelity and Centennial
	The gains and losses on derivative instruments were as follows:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of loss recognized in accumulated other comprehensive loss (effective portion), net of tax	$	—		$	—		$	—		$	(6,153	)
	Amount of (gain) loss reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(18		)	(634		)	140			(4,349		)
	Amount of loss recognized in operating revenues (ineffective portion), before tax	—			—			—			(1,422		)
	Interest rate derivatives designated as cash flow hedges:
	Amount of gain recognized in accumulated other comprehensive loss (effective portion), net of tax	—			—			—			559
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	100			124			299			671
	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	28,755			(12,594		)	16,847			(3,957		)
	Over the next 12 months net losses of approximately $553,000 (after tax) are estimated to be reclassified from accumulated other comprehensive income (loss) into earnings, as the hedged transactions affect earnings.
	The location and fair value of the gross amount of the Company's derivative instruments on the Consolidated Balance Sheets were as follows:
	Asset	Location on	Fair Value at September 30, 2014			Fair Value at September 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	11,322		$	4,373		$	1,447
		Other assets - noncurrent	259			1,771			503
	Total asset derivatives		$	11,581		$	6,144		$	1,950
	Liability	Location on	Fair Value at September 30, 2014			Fair Value at September 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	44		$	9,740		$	7,483
		Other liabilities - noncurrent	—			149			—
	Total liability derivatives		$	44		$	9,889		$	7,483
	All of the Company's commodity derivative instruments at September 30, 2014 and 2013, and December 31, 2013, were subject to legally enforceable master netting agreements. However, the Company's policy is to not offset fair value amounts for derivative instruments and, as a result, the Company's derivative assets and liabilities are presented gross on the Consolidated Balance Sheets. The gross derivative assets and liabilities (excluding settlement receivables and payables that may be subject to the same master netting agreements) presented on the Consolidated Balance Sheets and the amount eligible for offset under the master netting agreements is presented in the following table:
	30-Sep-14	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	11,581		$	(44	)	$	11,537
	Total assets	$	11,581		$	(44	)	$	11,537
	Liabilities:
	Commodity derivatives	$	44		$	(44	)	$	—
	Total liabilities	$	44		$	(44	)	$	—
	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
	31-Dec-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
	(In thousands)
Assets:
Commodity derivatives	$	1,950		$	(1,950	)	$	—
Total assets	$	1,950		$	(1,950	)	$	—
Liabilities:
Commodity derivatives	$	7,483		$	(1,950	)	$	5,533
Total liabilities	$	7,483		$	(1,950	)	$	5,533

Fair_value_measurements

Fair value measurements	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair value measurements	'
	Fair value measurements
	The Company measures its investments in certain fixed-income and equity securities at fair value with changes in fair value recognized in income. The Company anticipates using these investments, which consist of an insurance contract, to satisfy its obligations under its unfunded, nonqualified benefit plans for executive officers and certain key management employees, and invests in these fixed-income and equity securities for the purpose of earning investment returns and capital appreciation. These investments, which totaled $63.6 million, $58.1 million and $62.4 million, at September 30, 2014 and 2013, and December 31, 2013, respectively, are classified as Investments on the Consolidated Balance Sheets. The net unrealized loss on these investments was $800,000 for the three months ended September 30, 2014, and the net unrealized gain on these investments was $1.2 million for the nine months ended September 30, 2014, respectively. 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 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, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,838	$	71	$	(8	)	$	7,901
	U.S. Treasury securities	2,368		8		(2		)	2,374
	Total	$	10,206	$	79	$	(10	)	$	10,275
	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
	December 31, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,151	$	69	$	(27	)	$	8,193
	U.S. Treasury securities	1,906		15		(4		)	1,917
	Total	$	10,057	$	84	$	(31	)	$	10,110
	The fair value of the Company's money market funds approximates cost.
	Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the measurement date. The ASC establishes a hierarchy for grouping assets and liabilities, based on the significance of inputs.
	The estimated fair values of the Company's assets and liabilities measured on a recurring basis are determined using the market approach.
	The Company's Level 2 money market funds consist of investments in short-term unsecured promissory notes and the value is based on comparable market transactions taking into consideration the credit quality of the issuer. The estimated fair value of the Company's Level 2 mortgage-backed securities and U.S. Treasury securities are based on comparable market transactions, other observable inputs or other sources, including pricing from outside sources.
	The estimated fair value of the Company's Level 2 insurance contract is based on contractual cash surrender values that are determined primarily by investments in managed separate accounts of the insurer. These amounts approximate fair value. The managed separate accounts are valued based on other observable inputs or corroborated market data.
	The estimated fair value of the Company's Level 2 commodity derivative instruments is based upon futures prices, volatility and time to maturity, among other things. Counterparty statements are utilized to determine the value of the commodity derivative instruments and are reviewed and corroborated using various methodologies and significant observable inputs. The Company's and the counterparties' nonperformance risk is also evaluated.
	Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value. For the nine months ended September 30, 2014 and 2013, there were no transfers between Levels 1 and 2.
	The Company's assets and liabilities measured at fair value on a recurring basis were as follows:
		Fair Value Measurements at September 30, 2014, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2014
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	19,687	$	—		$	19,687
	Insurance contract*	—		63,578		—			63,578
	Available-for-sale securities:
	Mortgage-backed securities	—		7,901		—			7,901
	U.S. Treasury securities	—		2,374		—			2,374
	Commodity derivative instruments	—		11,581		—			11,581
	Total assets measured at fair value	$	—	$	105,121	$	—		$	105,121
	Liabilities:
	Commodity derivative instruments	$	—	$	44	$	—		$	44
	Total liabilities measured at fair value	$	—	$	44	$	—		$	44
	*  The insurance contract invests approximately 21 percent in common stock of mid-cap companies, 17 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
		Fair Value Measurements at September 30, 2013, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2013
		(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 investments.
		Fair Value Measurements at December 31, 2013, Using
		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2013
						(Level 3)
		(In thousands)
Assets:
Money market funds	$	—	$	19,227	$	—		$	19,227
Insurance contract*	—		62,370		—			62,370
Available-for-sale securities:
Mortgage-backed securities	—		8,193		—			8,193
U.S. Treasury securities	—		1,917		—			1,917
Commodity derivative instruments	—		1,950		—			1,950
Total assets measured at fair value	$	—	$	93,657	$	—		$	93,657
Liabilities:
Commodity derivative instruments	$	—	$	7,483	$	—		$	7,483
Total liabilities measured at fair value	$	—	$	7,483	$	—		$	7,483
* The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.
The Company applies the provisions of the fair value measurement standard to its nonrecurring, non-financial measurements, including long-lived asset impairments. These assets are not measured at fair value on an ongoing basis but are subject to fair value adjustments only in certain circumstances. The Company reviews the carrying value of its long-lived assets, excluding goodwill and oil and natural gas properties, whenever events or changes in circumstances indicate that such carrying amounts may not be recoverable. During the second quarter of 2013, coalbed natural gas gathering assets were reviewed for impairment and found to be impaired and were written down to their estimated fair value using the income approach. Under this approach, fair value is determined by using the present value of future estimated cash flows. The factors used to determine the estimated future cash flows include, but are not limited to, internal estimates of gathering revenue, future commodity prices and operating costs and equipment salvage values. The estimated cash flows are discounted using a rate that approximates the weighted average cost of capital of a market participant. These fair value inputs are not typically observable. At June 30, 2013, coalbed natural gas gathering assets were written down to the nonrecurring fair value measurement of $9.7 million. The fair value of these coalbed natural gas gathering assets have been categorized as Level 3 (Significant Unobservable Inputs) in the fair value hierarchy.
The Company's long-term debt is not measured at fair value on the Consolidated Balance Sheets and the fair value is being provided for disclosure purposes only. The fair value was based on discounted future cash flows using current market interest rates. The estimated fair value of the Company's Level 2 long-term debt was as follows:
	Carrying		Fair
	Amount		Value
	(In thousands)
Long-term debt at September 30, 2014	$	2,210,557	$	2,332,887
Long-term debt at September 30, 2013	$	2,011,896	$	2,106,887
Long-term debt at December 31, 2013	$	1,854,563	$	1,912,590
The carrying amounts of the Company's remaining financial instruments included in current assets and current liabilities approximate their fair values.

Longterm_debt

Long-term debt	9 Months Ended
	Sep. 30, 2014
Debt Disclosure [Abstract]	'
Long-term debt	'
	Long-term debt
	On May 8, 2014, the Company amended its revolving credit agreement to increase the borrowing limit to $175.0 million and extend the termination date to May 8, 2019.
	The Company entered into a $150.0 million note purchase agreement on January 28, 2014, and issued $50.0 million of Senior Notes on April 15, 2014, with a due date of April 15, 2044, at an interest rate of 5.2 percent. The remaining $100.0 million of Senior Notes was issued on July 15, 2014, with due dates ranging from July 15, 2024 to July 15, 2026, at a weighted average interest rate of 4.3 percent.
	On May 8, 2014, Centennial entered into an amended and restated revolving credit agreement which increased the borrowing limit to $650.0 million and extended the termination date to May 8, 2019. The credit agreement contains customary covenants and provisions, including a covenant of Centennial not to permit, as of the end of any fiscal quarter, the ratio of total consolidated debt to total consolidated capitalization to be greater than 65 percent. Other covenants include restrictions on the sale of certain assets, limitations on subsidiary indebtedness and the making of certain loans and investments.
	Centennial's revolving credit agreement contains cross-default provisions. These provisions state that if Centennial or any subsidiary of Centennial fails to make any payment with respect to any indebtedness or contingent obligation, in excess of a specified amount, under any agreement that causes such indebtedness to be due prior to its stated maturity or the contingent obligation to become payable, then Centennial will be in default under the revolving credit agreement.
	Centennial entered into two separate two year $125.0 million term loan agreements with variable interest rates on March 31, 2014 and April 2, 2014, respectively. These agreements contain customary covenants and default provisions, including covenants not to permit, as of the end of any fiscal quarter, the ratio of Centennial's total debt to total capitalization to be greater than 65 percent. The covenants also include certain limitations on subsidiary indebtedness and restrictions on the sale of certain assets and on the making of certain loans and investments. On August 6, 2014, Centennial paid all of the outstanding borrowings under one of the two year term loan agreements and all the outstanding borrowings under the remaining two year term loan agreement were paid on October 2, 2014.
	In addition, borrowings outstanding that were classified as long-term debt under the Company’s and Centennial’s commercial paper programs totaled $293.5 million at September 30, 2014, compared to $153.9 million at December 31, 2013, respectively.

Equity

Equity	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Equity	'
	Equity
	A summary of the changes in equity was as follows:
	Nine Months Ended September 30, 2014	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2013	$	2,823,164		$	32,738		$	2,855,902
	Net income (loss)	213,978			(2,390		)	211,588
	Other comprehensive income	1,173			—			1,173
	Dividends declared on preferred stocks	(514		)	—			(514		)
	Dividends declared on common stock	(102,461		)	—			(102,461		)
	Stock-based compensation	4,257			—			4,257
	Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(5,564		)	—			(5,564		)
	Net tax benefit on stock-based compensation	4,729			—			4,729
	Issuance of common stock	144,493			—			144,493
	Contribution from noncontrolling interest	—			44,900			44,900
	Balance at September 30, 2014	$	3,083,255		$	75,248		$	3,158,503
	Nine Months Ended September 30, 2013	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2012	$	2,648,248		$	—		$	2,648,248
	Net income (loss)	187,484			(204		)	187,280
	Other comprehensive loss	(8,216		)	—			(8,216		)
	Dividends declared on preferred stocks	(514		)	—			(514		)
	Dividends declared on common stock	(97,720		)	—			(97,720		)
	Stock-based compensation	3,730			—			3,730
	Net tax deficit on stock-based compensation	(1,419		)	—			(1,419		)
	Contribution from noncontrolling interest	—			19,101			19,101
	Balance at September 30, 2013	$	2,731,593		$	18,897		$	2,750,490

Business_segment_data

Business segment data	9 Months Ended
	Sep. 30, 2014
Segment Reporting [Abstract]	'
Business segment data	'
	Business segment data
	The Company's reportable segments are those that are based on the Company's method of internal reporting, which generally segregates the strategic business units due to differences in products, services and regulation. The internal reporting of these operating segments is defined based on the reporting and review process used by the Company's chief executive officer. The vast majority of the Company's operations are located within the United States. The Company also has an investment in a foreign country, which consists of Centennial Resources' investment in ECTE.
	The electric segment generates, transmits and distributes electricity in Montana, North Dakota, South Dakota and Wyoming. The natural gas distribution segment distributes natural gas in those states as well as in Idaho, Minnesota, Oregon and Washington. These operations also supply related value-added services.
	The pipeline and energy services segment provides natural gas transportation, underground storage, processing and gathering services, as well as oil gathering, through regulated and nonregulated pipeline systems and processing facilities primarily in the Rocky Mountain and northern Great Plains regions of the United States. This segment is constructing Dakota Prairie Refinery in conjunction with Calumet to refine crude oil and also provides cathodic protection and other energy-related services.
	The exploration and production segment is engaged in oil and natural gas development and production activities in the Rocky Mountain and Mid-Continent/Gulf States regions of the United States. For more information regarding this segment, see Note 23.
	The construction materials and contracting segment mines aggregates and markets crushed stone, sand, gravel and related construction materials, including ready-mixed concrete, cement, asphalt, liquid asphalt and other value-added products. It also performs integrated contracting services. This segment operates in the central, southern and western United States and Alaska and Hawaii.
	The construction services segment specializes in constructing and maintaining electric and communication lines, gas pipelines, fire suppression systems, and external lighting and traffic signalization equipment. This segment also provides utility excavation services and inside electrical wiring, cabling and mechanical services, sells and distributes electrical materials, and manufactures and distributes specialty equipment.
	The Other category includes the activities of Centennial Capital, which insures various types of risks as a captive insurer for certain of the Company's subsidiaries. The function of the captive insurer is to fund the deductible layers of the insured companies' general liability, automobile liability and pollution liability coverages. Centennial Capital also owns certain real and personal property. The Other category also includes Centennial Resources' investment in ECTE.
	The information below follows the same accounting policies as described in Note 1 of the Company's Notes to Consolidated Financial Statements in the 2013 Annual Report. Information on the Company's businesses was as follows:
	Three Months Ended September 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	68,936	$	—		$	9,162
	Natural gas distribution	96,185		—			(12,252		)
	Pipeline and energy services	46,415		4,334			5,060
		211,536		4,334			1,970
	Exploration and production	147,677		8,130			34,750
	Construction materials and contracting	740,496		6,322			55,218
	Construction services	270,313		16,420			9,876
	Other	433		2,601			2,746
		1,158,919		33,473			102,590
	Intersegment eliminations	—		(37,807		)	(1,522		)
	Total	$	1,370,455	$	—		$	103,038
	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
	Nine Months Ended September 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	207,732	$	—		$	28,018
	Natural gas distribution	616,496		—			10,516
	Pipeline and energy services	133,541		30,497			15,198
		957,769		30,497			53,732
	Exploration and production	393,653		39,269			74,869
	Construction materials and contracting	1,339,371		18,445			42,199
	Construction services	815,313		27,431			40,751
	Other	1,248		6,069			4,618
		2,549,585		91,214			162,437
	Intersegment eliminations	—		(121,711		)	(2,705		)
	Total	$	3,507,354	$	—		$	213,464
	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
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, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]	'
Pension and other postretirement benefit plans	'
	Employee benefit plans
	The Company has noncontributory defined benefit pension plans and other postretirement benefit plans for certain eligible employees. Components of net periodic benefit cost for the Company's pension and other postretirement benefit plans were as follows:
								Other
								Postretirement
		Pension Benefits						Benefits
	Three Months Ended September 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	32		$	39		$	379		$	419
	Interest cost	4,420			4,062			919			804
	Expected return on assets	(5,304		)	(4,979		)	(1,154		)	(1,086		)
	Amortization of prior service cost (credit)	18			18			(348		)	(364		)
	Amortization of net actuarial loss	1,217			1,793			162			327
	Net periodic benefit cost (credit), including amount capitalized	383			933			(42		)	100
	Less amount capitalized	27			157			(65		)	47
	Net periodic benefit cost	$	356		$	776		$	23		$	53
								Other
								Postretirement
		Pension Benefits						Benefits
	Nine Months Ended September 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	96		$	116		$	1,138		$	1,257
	Interest cost	13,265			12,186			2,701			2,411
	Expected return on assets	(15,913		)	(14,937		)	(3,463		)	(3,258		)
	Amortization of prior service cost (credit)	54			54			(1,044		)	(1,092		)
	Amortization of net actuarial loss	3,651			5,373			486			1,405
	Net periodic benefit cost (credit), including amount capitalized	1,153			2,792			(182		)	723
	Less amount capitalized	195			425			(55		)	137
	Net periodic benefit cost (credit)	$	958		$	2,367		$	(127	)	$	586
	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, 2014, was $1.7 million and $5.0 million, respectively. 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.

Regulatory_matters_and_revenue

Regulatory matters and revenues subject to refund	9 Months Ended
	Sep. 30, 2014
Regulated Operations [Abstract]	'
Regulatory matters and revenues subject to refund	'
	Regulatory matters and revenues subject to refund
	On April 8, 2014, Montana-Dakota submitted a request to the NDPSC to update the environmental cost recovery rider to reflect actual costs incurred through February 2014 and projected costs through June 2015 related to the recovery of Montana-Dakota's share of the costs resulting from the environmental retrofit required to be installed at the Big Stone Station. The NDPSC approved the proposed rider on July 10, 2014, reflecting an annual amount of $8.6 million to be recovered under the rider. The rider was effective with service rendered on and after July 15, 2014.
	On February 27, 2014, Montana-Dakota filed an application with the NDPSC for approval of an electric generation resource recovery rider for recovery of Montana-Dakota's investment in the recently constructed 88-MW simple-cycle natural gas turbine and associated facilities near Mandan, ND. Montana-Dakota requested recovery of $7.4 million annually or approximately 4.6 percent above current rates. Advance determination of prudence and a certificate of public convenience and necessity were received from the NDPSC on April 11, 2012. On March 12, 2014, the NDPSC suspended the filing pending further review. The NDPSC held a hearing regarding this matter on May 28, 2014. On August 20, 2014, the NDPSC approved a settlement agreement reached with the NDPSC staff on May 20, 2014, as amended July 25, 2014, which provides for establishing a generation resource recovery rider and a provision to recover costs associated with a pipeline to the facility through the fuel and purchased power adjustment mechanism. As part of the settlement, the requested recovery for the 88-MW simple-cycle natural gas turbine was established with Montana-Dakota withdrawing the initial rate adjustment and allowing Montana-Dakota the right to file and implement an adjustment within 30 days of filing upon a reasonable showing that the expected return is below a specified return on equity. The settlement also provides for sharing of earnings received by Montana-Dakota in 2014 over a specified return on equity.
	On August 11, 2014, Montana-Dakota filed an application with the MTPSC for a natural gas rate increase. Montana-Dakota requested a total increase of approximately $3.0 million annually or approximately 3.6 percent above current rates. The requested increase includes the costs associated with the increased investment in facilities, including ongoing investment in new and replacement distribution facilities, depreciation and taxes associated with the increased investment as well as an increase in Montana-Dakota's operation and maintenance expenses. Montana-Dakota requested an interim increase, subject to refund, of $2.2 million or approximately 2.6 percent, which is pending before the MTPSC.
	On October 3, 2014, Montana-Dakota filed an application with the WYPSC for a natural gas rate increase. Montana-Dakota requested a total increase of approximately $788,000 annually or approximately 4.1 percent above current rates. The requested increase includes the costs associated with the increased investment in facilities, including ongoing investment in new and replacement distribution facilities and the associated operation and maintenance expenses, depreciation and taxes associated with the increase in investment.
	On October 31, 2013, WBI Energy Transmission filed a general natural gas rate change application with the FERC based on an increase in investments of $312 million, increased operating costs, and the effect of lower storage and off system volumes. On April 30, 2014, WBI Energy Transmission reached a settlement in principle with FERC Trial Staff and all active parties to resolve the rate case. WBI Energy Transmission filed settlement rates to take effect on an interim basis, effective May 1, 2014, pending final approval of the settlement. On June 4, 2014, WBI Energy Transmission submitted to the FERC an Uncontested Offer of Settlement. On June 11, 2014, the Presiding Administrative Law Judge issued a Certification of Uncontested Settlement recommending FERC approval of the settlement without modification. On August 11, 2014, the FERC issued an order approving the settlement without modification and the resulting rates were approved to be effective May 1, 2014. No parties to the proceeding requested rehearing of the order, which is now final. Based on the adjusted base period volumes filed in the case, the annual increase in revenues is approximately $11.5 million.

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Contingencies	'
	Contingencies
	The Company is party to claims and lawsuits arising out of its business and that of its consolidated subsidiaries. The Company accrues a liability for those contingencies when the incurrence of a loss is probable and the amount can be reasonably estimated. If a range of amounts can be reasonably estimated and no amount within the range is a better estimate than any other amount, then the minimum of the range is accrued. The Company does not accrue liabilities when the likelihood that the liability has been incurred is probable but the amount cannot be reasonably estimated or when the liability is believed to be only reasonably possible or remote. For contingencies where an unfavorable outcome is probable or reasonably possible and which are material, the Company discloses the nature of the contingency and, in some circumstances, an estimate of the possible loss. The Company had accrued liabilities of $31.9 million, $30.8 million and $29.5 million for contingencies, including litigation, production taxes, royalty claims and environmental matters, at September 30, 2014 and 2013, and December 31, 2013, respectively, which include amounts that may have been accrued for matters discussed in Litigation and Environmental matters within this note.
	Litigation
	Construction Materials Until the fall of 2011 when it discontinued active mining operations at the pit, JTL operated the Target Range Gravel Pit in Missoula County, Montana under a 1975 reclamation contract pursuant to the Montana Opencut Mining Act. In September 2009, the Montana DEQ sent a letter asserting JTL was in violation of the Montana Opencut Mining Act by conducting mining operations outside a permitted area. JTL filed a complaint in Montana First Judicial District Court in June 2010, seeking a declaratory order that the reclamation contract is a valid permit under the Montana Opencut Mining Act. The Montana DEQ filed an answer and counterclaim to the complaint in August 2011, alleging JTL was in violation of the Montana Opencut Mining Act and requesting imposition of penalties of not more than $3.7 million plus not more than $5,000 per day from the date of the counterclaim. The Company believes the operation of the Target Range Gravel Pit was conducted under a valid permit; however, the imposition of civil penalties is reasonably possible. The Company filed an application for amendment of its opencut mining permit and intends to resolve this matter through settlement or continuation of the Montana First Judicial District Court litigation.
	Former Employee Litigation On August 6, 2012, a former employee and his spouse filed actions against Connolly-Pacific and others in California Superior Court alleging the former employee contracted acute myelogenous leukemia from exposure to substances while employed as a seaman by the defendants. The plaintiffs request compensatory damages of approximately $23.8 million plus punitive damages, costs and interest. Connolly-Pacific is contesting the claims and believes it has meritorious defenses to them. Connolly-Pacific will seek insurance coverage for defense costs and any liability incurred in the litigation.
	Natural Gas Gathering Operations In January 2010, SourceGas filed an application with the Colorado State District Court to compel WBI Energy Midstream to arbitrate a dispute regarding operating pressures under a natural gas gathering contract on one of WBI Energy Midstream's pipeline gathering systems in Montana. WBI Energy Midstream resisted the application and sought a declaratory order interpreting the gathering contract. In May 2010, the Colorado State District Court granted the application and ordered WBI Energy Midstream into arbitration. In October 2010, the arbitration panel issued an award in favor of SourceGas for approximately $26.6 million. The Colorado Court of Appeals issued a decision on May 24, 2012, reversing the Colorado State District Court order compelling arbitration, vacating the final award and remanding the case to the Colorado State District Court to determine SourceGas's claims and WBI Energy Midstream's counterclaims. On remand of the matter to the Colorado State District Court, SourceGas may assert claims similar to those asserted in the arbitration proceeding.
	In a related matter, Omimex filed a complaint against WBI Energy Midstream in Montana Seventeenth Judicial District Court in July 2010 alleging WBI Energy Midstream breached a separate gathering contract with Omimex as a result of the increased operating pressures demanded by SourceGas on the same natural gas gathering system. In December 2011, Omimex filed an amended complaint alleging WBI Energy Midstream breached obligations to operate its gathering system as a common carrier under United States and Montana law. WBI Energy Midstream removed the action to the United States District Court for the District of Montana. The parties subsequently settled the breach of contract claim and, subject to final determination on liability, stipulated to the damages on the common carrier claim, for amounts that are not material. A trial on the common carrier claim was held during July 2013, but a decision has not been issued.
	Exploration and Production During the ordinary course of its business, Fidelity is subject to audit for various production related taxes by certain state and federal tax authorities for varying periods as well as claims for royalty obligations under lease agreements for oil and gas production. Disputes may exist regarding facts and questions of law relating to the tax and royalty obligations.
	On May 15, 2013, Austin Holdings, LLC filed an action against Fidelity in Wyoming State District Court alleging Fidelity violated the Wyoming Royalty Payment Act and implied lease covenants by deducting production costs from and by failing to properly report and pay royalties for coalbed methane gas production in Wyoming. The plaintiff, in addition to declaratory and injunctive relief, seeks class certification for similarly situated persons and an unspecified amount of monetary damages on behalf of the class for unpaid royalties, interest, reporting violations and attorney fees. Fidelity believes it has meritorious defenses against class certification and the claims. The Company intends to resolve this matter through settlement or continuation of the Wyoming State District Court litigation.
	The Company also is subject to other litigation, and actual and potential claims in the ordinary course of its business which may include, but are not limited to, matters involving property damage, personal injury, and environmental, contractual, statutory and regulatory obligations. Accruals are based on the best information available but actual losses in future periods are affected by various factors making them uncertain. After taking into account liabilities accrued for the foregoing matters, management believes that the outcomes with respect to the above issues and other probable and reasonably possible losses in excess of the amounts accrued, while uncertain, will not have a material effect upon the Company's financial position, results of operations or cash flows.
	Environmental matters
	Portland Harbor Site In December 2000, Knife River - Northwest was named by the EPA as a PRP in connection with the cleanup of a riverbed site adjacent to a commercial property site acquired by Knife River - Northwest from Georgia-Pacific West, Inc. in 1999. The riverbed site is part of the Portland, Oregon, Harbor Superfund Site. The EPA wants responsible parties to share in the cleanup of sediment contamination in the Willamette River. To date, costs of the overall remedial investigation and feasibility study of the harbor site are being recorded, and initially paid, through an administrative consent order by the LWG, a group of several entities, which does not include Knife River - Northwest or Georgia-Pacific West, Inc. Investigative costs are indicated to be in excess of $70 million. It is not possible to estimate the cost of a corrective action plan until the remedial investigation and feasibility study have been completed, the EPA has decided on a strategy and a ROD has been published. Corrective action will be taken after the development of a proposed plan and ROD on the harbor site is issued. Knife River - Northwest also received notice in January 2008 that the Portland Harbor Natural Resource Trustee Council intends to perform an injury assessment to natural resources resulting from the release of hazardous substances at the Harbor Superfund Site. The Portland Harbor Natural Resource Trustee Council indicates the injury determination is appropriate to facilitate early settlement of damages and restoration for natural resource injuries. It is not possible to estimate the costs of natural resource damages until an assessment is completed and allocations are undertaken.
	Based upon a review of the Portland Harbor sediment contamination evaluation by the Oregon DEQ and other information available, Knife River - Northwest does not believe it is a Responsible Party. In addition, Knife River - Northwest has notified Georgia-Pacific West, Inc., that it intends to seek indemnity for liabilities incurred in relation to the above matters pursuant to the terms of their sale agreement. Knife River - Northwest has entered into an agreement tolling the statute of limitations in connection with the LWG's potential claim for contribution to the costs of the remedial investigation and feasibility study. By letter in March 2009, LWG stated its intent to file suit against Knife River - Northwest and others to recover LWG's investigation costs to the extent Knife River - Northwest cannot demonstrate its non-liability for the contamination or is unwilling to participate in an alternative dispute resolution process that has been established to address the matter. At this time, Knife River - Northwest has agreed to participate in the alternative dispute resolution process.
	The Company believes it is not probable that it will incur any material environmental remediation costs or damages in relation to the above referenced administrative action.
	Manufactured Gas Plant Sites There are three claims against Cascade for cleanup of environmental contamination at manufactured gas plant sites operated by Cascade's predecessors.
	The first claim is for contamination at a site in Eugene, Oregon which was received in 1995. There are PRPs in addition to Cascade that may be liable for cleanup of the contamination. Some of these PRPs have shared in the investigation costs. It is expected that these and other PRPs will share in the cleanup costs. Several alternatives for cleanup have been identified, with preliminary cost estimates ranging from approximately $500,000 to $11.0 million. The Oregon DEQ released a staff report in September 2014, which recommends a cleanup alternative for the site. It is not known at this time what share of the cleanup costs will actually be borne by Cascade; however, Cascade anticipates its proportional share could be approximately 50 percent. Cascade has accrued $1.7 million for remediation of this site. In January 2013, the OPUC approved Cascade's application to defer environmental remediation costs at the Eugene site for a period of 12 months starting November 30, 2012. Cascade received an order reauthorizing the deferred accounting for the 12 months starting November 30, 2013.
	The second claim is for contamination at a site in Bremerton, Washington which was received in 1997. A preliminary investigation has found soil and groundwater at the site contain contaminants requiring further investigation and cleanup. EPA conducted a Targeted Brownfields Assessment of the site and released a report summarizing the results of that assessment in August 2009. The assessment confirms that contaminants have affected soil and groundwater at the site, as well as sediments in the adjacent Port Washington Narrows. Alternative remediation options have been identified with preliminary cost estimates ranging from $340,000 to $6.4 million. Data developed through the assessment and previous investigations indicates the contamination likely derived from multiple, different sources and multiple current and former owners of properties and businesses in the vicinity of the site may be responsible for the contamination. In April 2010, the Washington Department of Ecology issued notice it considered Cascade a PRP for hazardous substances at the site. In May 2012, the EPA added the site to the National Priorities List of Superfund sites. Cascade has entered into an administrative settlement agreement and consent order with the EPA regarding the scope and schedule for a remedial investigation and feasibility study for the site. Cascade has accrued $12.6 million for the remedial investigation, feasibility study and remediation of this site. In April 2010, Cascade filed a petition with the WUTC for authority to defer the costs, which are included in other noncurrent assets, incurred in relation to the environmental remediation of this site until the next general rate case. The WUTC approved the petition in September 2010, subject to conditions set forth in the order.
	The third claim is for contamination at a site in Bellingham, Washington. Cascade received notice from a party in May 2008 that Cascade may be a PRP, along with other parties, for contamination from a manufactured gas plant owned by Cascade and its predecessor from about 1946 to 1962. The notice indicates that current estimates to complete investigation and cleanup of the site exceed $8.0 million. Other PRPs have reached an agreed order and work plan with the Washington Department of Ecology for completion of a remedial investigation and feasibility study for the site. A report documenting the initial phase of the remedial investigation was completed in June 2011. There is currently not enough information available to estimate the potential liability to Cascade associated with this claim although Cascade believes its proportional share of any liability will be relatively small in comparison to other PRPs. The plant manufactured gas from coal between approximately 1890 and 1946. In 1946, shortly after Cascade's predecessor acquired the plant, it converted the plant to a propane-air gas facility. There are no documented wastes or by-products resulting from the mixing or distribution of propane-air gas.
	Cascade has received notices from and entered into agreement with certain of its insurance carriers that they will participate in defense of Cascade for these contamination claims subject to full and complete reservations of rights and defenses to insurance coverage. To the extent these claims are not covered by insurance, Cascade will seek recovery through the OPUC and WUTC of remediation costs in its natural gas rates charged to customers. The accruals related to these matters are reflected in regulatory assets.
	Guarantees
	In connection with the sale of the Brazilian Transmission Lines, as discussed in Note 13, Centennial has agreed to guarantee payment of any indemnity obligations of certain of the Company's indirect wholly owned subsidiaries who are the sellers in three purchase and sale agreements for periods ranging up to 10 years from the date of sale. The guarantees were required by the buyers as a condition to the sale of the Brazilian Transmission Lines.
	WBI Holdings has guaranteed certain of Fidelity's oil and natural gas swap agreement obligations. There is no fixed maximum amount guaranteed in relation to the oil and natural gas swap agreements as the amount of the obligation is dependent upon oil and natural gas commodity prices. The amount of derivative activity entered into by the subsidiary is limited by corporate policy. The guarantees of the oil and natural gas swap agreements at September 30, 2014, expire in the years ranging from 2014 to 2015; however, Fidelity continues to enter into additional derivative instruments and, as a result, WBI Holdings from time to time may issue additional guarantees on these derivative instruments. There were no amounts outstanding by Fidelity at September 30, 2014. In the event Fidelity defaults under its obligations, WBI Holdings would be required to make payments under its guarantees.
	Certain subsidiaries of the Company have outstanding guarantees to third parties that guarantee the performance of other subsidiaries of the Company. These guarantees are related to construction contracts, natural gas transportation and sales agreements, gathering contracts and certain other guarantees. At September 30, 2014, the fixed maximum amounts guaranteed under these agreements aggregated $114.3 million. The amounts of scheduled expiration of the maximum amounts guaranteed under these agreements aggregate $19.4 million in 2014; $75.1 million in 2015; $700,000 in 2016; $600,000 in 2017; $500,000 in 2018; $500,000 in 2019; $13.5 million, which is subject to expiration on a specified number of days after the receipt of written notice; and $4.0 million, which has no scheduled maturity date. The amount outstanding by subsidiaries of the Company under the above guarantees was $200,000 and was reflected on the Consolidated Balance Sheet at September 30, 2014. In the event of default under these guarantee obligations, the subsidiary issuing the guarantee for that particular obligation would be required to make payments under its guarantee.
	Certain subsidiaries have outstanding letters of credit to third parties related to insurance policies and other agreements, some of which are guaranteed by other subsidiaries of the Company. At September 30, 2014, the fixed maximum amounts guaranteed under these letters of credit, aggregated $39.0 million. In 2014 and 2015, $8.2 million and $30.8 million, respectively, of letters of credit are scheduled to expire. There were no amounts outstanding under the above letters of credit at September 30, 2014.
	WBI Holdings has an outstanding guarantee to WBI Energy Transmission. This guarantee is related to a natural gas transportation and storage agreement that guarantees the performance of Prairielands. At September 30, 2014, the fixed maximum amount guaranteed under this agreement was $4.0 million and is scheduled to expire in 2016. In the event of Prairielands' default in its payment obligations, WBI Holdings would be required to make payment under its guarantee. The amount outstanding by Prairielands under the above guarantee was $1.0 million. The amount outstanding under this guarantee was not reflected on the Consolidated Balance Sheet at September 30, 2014, because this intercompany transaction was eliminated in consolidation.
	In addition, Centennial, Knife River and MDU Construction Services have issued guarantees to third parties related to the routine purchase of maintenance items, materials and lease obligations for which no fixed maximum amounts have been specified. These guarantees have no scheduled maturity date. In the event a subsidiary of the Company defaults under these obligations, Centennial, Knife River and MDU Construction Services would be required to make payments under these guarantees. Any amounts outstanding by subsidiaries of the Company for these guarantees were reflected on the Consolidated Balance Sheet at September 30, 2014.
	In the normal course of business, Centennial has surety bonds related to construction contracts and reclamation obligations of its subsidiaries. In the event a subsidiary of Centennial does not fulfill a bonded obligation, Centennial would be responsible to the surety bond company for completion of the bonded contract or obligation. A large portion of the surety bonds is expected to expire within the next 12 months; however, Centennial will likely continue to enter into surety bonds for its subsidiaries in the future. At September 30, 2014, approximately $387.0 million of surety bonds were outstanding, which were not reflected on the Consolidated Balance Sheet.
	Variable interest entities
	The Company evaluates its arrangements and contracts with other entities to determine if they are VIEs and if so, if the Company is the primary beneficiary.
	Dakota Prairie Refining, LLC On February 7, 2013, WBI Energy and Calumet formed a limited liability company, Dakota Prairie Refining, and entered into an operating agreement to develop, build and operate Dakota Prairie Refinery in southwestern North Dakota. WBI Energy and Calumet each have a 50 percent ownership interest in Dakota Prairie Refining. WBI Energy's and Calumet's capital commitments, based on a total project cost of $300.0 million, under the agreement are $150.0 million and $75.0 million, respectively. Capital commitments in excess of $300.0 million are expected to be shared equally between WBI Energy and Calumet. The total project cost is currently estimated at approximately $360 million. Dakota Prairie Refining entered into a term loan for project debt financing of $75.0 million on April 22, 2013. The operating agreement provides for allocation of profits and losses consistent with ownership interests; however, deductions attributable to project financing debt will be allocated to Calumet. Calumet's future cash distributions from Dakota Prairie Refining will be decreased by the principal and interest to be paid on the project debt, while the cash distributions to WBI Energy will not be decreased. Pursuant to the operating agreement, Centennial agreed to guarantee Dakota Prairie Refining's obligation under the term loan.
	Dakota Prairie Refining has been determined to be a VIE, and the Company has determined that it is the primary beneficiary as it has an obligation to absorb losses that could be potentially significant to the VIE through WBI Energy's equity investment and Centennial's guarantee of the third-party term loan. Accordingly, the Company consolidates Dakota Prairie Refining in its financial statements and records a noncontrolling interest for Calumet's ownership interest.
	Construction of Dakota Prairie Refinery began in early 2013 and the plant is not yet operational. Therefore, the results of operations of Dakota Prairie Refining did not have a material effect on the Company's Consolidated Statements of Income. The assets of Dakota Prairie Refining shall be used solely for the benefit of Dakota Prairie Refining. The total assets and liabilities of Dakota Prairie Refining reflected on the Company's Consolidated Balance Sheets were as follows:
		30-Sep-14		30-Sep-13		31-Dec-13
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	16,723	$	23,146	$	4,774
	Accounts receivable	150		1		—
	Other current assets	4,187		25		26
	Total current assets	21,060		23,172		4,800
	Net property, plant and equipment	314,551		123,297		172,073
	Total assets	$	335,611	$	146,469	$	176,873
	LIABILITIES
	Current liabilities:
	Long-term debt due within one year	$	3,000	$	3,000	$	3,000
	Accounts payable	36,541		20,313		8,904
	Taxes payable	323		—		5
	Accrued compensation	617		—		26
	Other accrued liabilities	633		363		461
	Total current liabilities	41,114		23,676		12,396
	Long-term debt	69,000		72,000		72,000
	Total liabilities	$	110,114	$	95,676	$	84,396
	Fuel Contract On October 10, 2012, the Coyote Station entered into a new coal supply agreement with Coyote Creek that will replace a coal supply agreement expiring in May 2016. The new agreement provides for the purchase of coal necessary to supply the coal requirements of the Coyote Station for the period May 2016 through December 2040.
	The new coal supply agreement creates a variable interest in Coyote Creek due to the transfer of all operating and economic risk to the Coyote Station owners, as the agreement is structured so the price of the coal will cover all costs of operations as well as future reclamation costs. The Coyote Station owners are also providing a guarantee of the value of the assets of Coyote Creek as they would be required to buy the assets at book value should they terminate the contract prior to the end of the contract term and are providing a guarantee of the value of the equity of Coyote Creek in that they are required to buy the entity at the end of the contract term at equity value. Although the Company has determined that Coyote Creek is a VIE, the Company has concluded that it is not the primary beneficiary of Coyote Creek because the authority to direct the activities of the entity is shared by the four unrelated owners of the Coyote Station, with no primary beneficiary existing. As a result, Coyote Creek is not required to be consolidated in the Company's financial statements.
	At September 30, 2014, Coyote Creek was not yet operational. The assets and liabilities of Coyote Creek and exposure to loss as a result of the Company's involvement with the VIE, based on the Company's ownership percentage, at September 30, 2014, was $12.0 million.

Subsequent_event

Subsequent event	9 Months Ended
	Sep. 30, 2014
Subsequent Events [Abstract]	'
Subsequent event	'
	Subsequent Events
	On September 24, 2014, Knife River provided notice to the plan administrator under one of the multiemployer pension plans to which Knife River is a party that it was withdrawing from the plan effective October 26, 2014. The plan administrator will determine Knife River's withdrawal liability, which the Company currently estimates at approximately $14 million (approximately $8.4 million after tax). Actual withdrawal liability costs may be significantly different.
	On October 31, 2014, the Company’s board of directors approved a plan to market the Company’s Fidelity assets and potentially exit the oil and natural gas exploration and production business. During the marketing and sales process, Fidelity intends to focus on production and continue to develop its acreage. The Company believes that the potential sale of the Fidelity assets will allow it to focus on growing its utility, pipeline and construction businesses and lower its overall business-risk profile.

Inventories_and_natural_gas_in1

Inventories and natural gas in storage (Tables)	9 Months Ended
	Sep. 30, 2014
Inventory Disclosure [Abstract]	'
Inventories	'
	Inventories consisted of:
		September 30,		September 30,		December 31,
		2014		2013		2013
		(In thousands)
	Aggregates held for resale	$	106,623	$	104,784	$	101,568
	Asphalt oil	33,551		43,078		38,099
	Materials and supplies	71,515		71,370		69,808
	Merchandise for resale	24,566		23,713		21,720
	Natural gas in storage (current)	29,979		37,689		16,417
	Other	36,471		33,937		34,779
	Total	$	302,705	$	314,571	$	282,391

Earnings_per_common_share_Tabl

Earnings per common share (Tables)	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Weighted average common shares outstanding	'
	A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculations was as follows:
		Three Months Ended		Nine Months Ended
		September 30,		September 30,
		2014	2013	2014	2013
		(In thousands)
	Weighted average common shares outstanding - basic	193,949	188,831	191,958	188,831
	Effect of dilutive performance share awards	351	807	349	803
	Weighted average common shares outstanding - diluted	194,300	189,638	192,307	189,634
	Shares excluded from the calculation of diluted earnings per share	—	—	—	—

Cash_flow_information_Tables

Cash flow information (Tables)	9 Months Ended
	Sep. 30, 2014
Supplemental Cash Flow Information [Abstract]	'
Supplemental cash flow information	'
	Cash expenditures for interest and income taxes were as follows:
		Nine Months Ended
		September 30,
		2014		2013
		(In thousands)
	Interest, net of amounts capitalized and AFUDC - borrowed of $8.6 million and $6.3 million in 2014 and 2013, respectively	$	61,690	$	60,281
	Income taxes paid	$	44,166	$	30,262
	Noncash investing transactions were as follows:
		September 30,
		2014		2013
		(In thousands)
	Property, plant and equipment additions in accounts payable	$	96,373	$	85,646

Comprehensive_income_loss_Tabl

Comprehensive income (loss) (Tables)	9 Months Ended
	Sep. 30, 2014
Comprehensive income (loss) [Abstract]	'
Comprehensive income (loss)	'
	The after-tax changes in the components of accumulated other comprehensive loss were as follows:
	Three Months Ended September 30, 2014	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,408	)	$	(33,287	)	$	(579	)	$	76		$	(37,198	)
	Other comprehensive income (loss) before reclassifications	—			—			(146		)	(62		)	(208		)
	Amounts reclassified from accumulated other comprehensive loss	82			261			—			31			374
	Net current-period other comprehensive income (loss)	82			261			(146		)	(31		)	166
	Balance at end of period	$	(3,326	)	$	(33,026	)	$	(725	)	$	45		$	(37,032	)
	Three Months Ended September 30, 2013	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(2,744	)	$	(53,275	)	$	(813	)	$	11		$	(56,821	)
	Other comprehensive income (loss) before reclassifications	—			—			(20		)	(10		)	(30		)
	Amounts reclassified from accumulated other comprehensive loss	(510		)	272			114			38			(86		)
	Net current-period other comprehensive income (loss)	(510		)	272			94			28			(116		)
	Balance at end of period	$	(3,254	)	$	(53,003	)	$	(719	)	$	39		$	(56,937	)
	Nine Months Ended September 30, 2014	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	(3,765	)	$	(33,807	)	$	(667	)	$	34		$	(38,205	)
	Other comprehensive income (loss) before reclassifications	—			—			(58		)	(89		)	(147		)
	Amounts reclassified from accumulated other comprehensive loss	439			781			—			100			1,320
	Net current-period other comprehensive income (loss)	439			781			(58		)	11			1,173
	Balance at end of period	$	(3,326	)	$	(33,026	)	$	(725	)	$	45		$	(37,032	)
	Nine Months Ended September 30, 2013	Net Unrealized Gain (Loss) on Derivative			Postretirement			Foreign Currency Translation Adjustment			Net Unrealized Gain (Loss) on Available-for-sale Investments			Total Accumulated
		Instruments			Liability Adjustment									Other
		Qualifying as Hedges												Comprehensive
														Loss
		(In thousands)
	Balance at beginning of period	$	6,018		$	(54,347	)	$	(511	)	$	119		$	(48,721	)
	Other comprehensive income (loss) before reclassifications	(5,594		)	—			(351		)	(197		)	(6,142		)
	Amounts reclassified from accumulated other comprehensive loss	(3,678		)	1,344			143			117			(2,074		)
	Net current-period other comprehensive income (loss)	(9,272		)	1,344			(208		)	(80		)	(8,216		)
	Balance at end of period	$	(3,254	)	$	(53,003	)	$	(719	)	$	39		$	(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
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands)
	Reclassification adjustment for gain (loss) on derivative instruments included in net income:
	Commodity derivative instruments	$	28		$	1,007		$	(223	)	$	6,903		Operating revenues
	Interest rate derivative instruments	(160		)	(200		)	(480		)	(979		)	Interest expense
		(132		)	807			(703		)	5,924
		50			(297		)	264			(2,246		)	Income taxes
		(82		)	510			(439		)	3,678
	Amortization of postretirement liability losses included in net periodic benefit cost	(420		)	(437		)	(1,258		)	(2,371		)	(a)
		159			166			477			1,027			Income taxes
		(261		)	(271		)	(781		)	(1,344		)
	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		)
	Reclassification adjustment for loss on available-for-sale investments included in net income	(47		)	(58		)	(154		)	(180		)	Other income
		16			20			54			63			Income taxes
		(31		)	(38		)	(100		)	(117		)
	Total reclassifications	$	(374	)	$	86		$	(1,320	)	$	2,074
	(a) Included in net periodic benefit cost (credit). For more information, see Note 20.

Goodwill_and_other_intangible_1

Goodwill and other intangible assets (Tables)	9 Months Ended
	Sep. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Changes in the carrying amount of goodwill	'
	The changes in the carrying amount of goodwill were as follows:
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-14	as of			Acquired			as of
		January 1,			During			September 30, 2014*
		2014*			the Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Nine Months Ended	Balance			Goodwill			Balance
	30-Sep-13	as of			Acquired			as of
		January 1,			During the			September 30, 2013*
		2013*			Year
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
	Year Ended	Balance			Goodwill			Balance
	31-Dec-13	as of			Acquired			as of
		January 1,			During the			December 31,
		2013*			Year			2013*
		(In thousands)
	Natural gas distribution	$	345,736		$	—		$	345,736
	Pipeline and energy services	9,737			—			9,737
	Construction materials and contracting	176,290			—			176,290
	Construction services	104,276			—			104,276
	Total	$	636,039		$	—		$	636,039
	* Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.
Other amortizable intangible assets	'
	Other amortizable intangible assets were as follows:
		September 30,			September 30,			December 31,
		2014			2013			2013
		(In thousands)
	Customer relationships	$	21,310		$	21,310		$	21,310
	Accumulated amortization	(15,116		)	(13,221		)	(13,726		)
		6,194			8,089			7,584
	Noncompete agreements	5,080			6,186			6,186
	Accumulated amortization	(4,021		)	(4,706		)	(4,840		)
		1,059			1,480			1,346
	Other	10,921			10,995			10,995
	Accumulated amortization	(7,578		)	(6,472		)	(6,826		)
		3,343			4,523			4,169
	Total	$	10,596		$	14,092		$	13,099

Derivative_instruments_Tables

Derivative instruments (Tables)	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Schedule of derivative instruments, gain (losses)	'
	The gains and losses on derivative instruments were as follows:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands)
	Commodity derivatives designated as cash flow hedges:
	Amount of loss recognized in accumulated other comprehensive loss (effective portion), net of tax	$	—		$	—		$	—		$	(6,153	)
	Amount of (gain) loss reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(18		)	(634		)	140			(4,349		)
	Amount of loss recognized in operating revenues (ineffective portion), before tax	—			—			—			(1,422		)
	Interest rate derivatives designated as cash flow hedges:
	Amount of gain recognized in accumulated other comprehensive loss (effective portion), net of tax	—			—			—			559
	Amount of loss reclassified from accumulated other comprehensive loss into interest expense (effective portion), net of tax	100			124			299			671
	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	28,755			(12,594		)	16,847			(3,957		)
Derivative instruments	'
	The location and fair value of the gross amount of the Company's derivative instruments on the Consolidated Balance Sheets were as follows:
	Asset	Location on	Fair Value at September 30, 2014			Fair Value at September 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	11,322		$	4,373		$	1,447
		Other assets - noncurrent	259			1,771			503
	Total asset derivatives		$	11,581		$	6,144		$	1,950
	Liability	Location on	Fair Value at September 30, 2014			Fair Value at September 30, 2013			Fair Value at December 31, 2013
	Derivatives	Consolidated
		Balance Sheets
			(In thousands)
	Not designated as hedges:
	Commodity derivatives	Commodity derivative instruments	$	44		$	9,740		$	7,483
		Other liabilities - noncurrent	—			149			—
	Total liability derivatives		$	44		$	9,889		$	7,483
Offsetting assets and liabilities master netting	'
	The gross derivative assets and liabilities (excluding settlement receivables and payables that may be subject to the same master netting agreements) presented on the Consolidated Balance Sheets and the amount eligible for offset under the master netting agreements is presented in the following table:
	30-Sep-14	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	11,581		$	(44	)	$	11,537
	Total assets	$	11,581		$	(44	)	$	11,537
	Liabilities:
	Commodity derivatives	$	44		$	(44	)	$	—
	Total liabilities	$	44		$	(44	)	$	—
	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
	31-Dec-13	Gross Amounts Recognized on the Consolidated Balance Sheets			Gross Amounts Not Offset on the Consolidated Balance Sheets			Net
		(In thousands)
	Assets:
	Commodity derivatives	$	1,950		$	(1,950	)	$	—
	Total assets	$	1,950		$	(1,950	)	$	—
	Liabilities:
	Commodity derivatives	$	7,483		$	(1,950	)	$	5,533
	Total liabilities	$	7,483		$	(1,950	)	$	5,533

Fair_value_measurements_Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Available-for-sale securities	'
	Details of available-for-sale securities were as follows:
	September 30, 2014	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	7,838	$	71	$	(8	)	$	7,901
	U.S. Treasury securities	2,368		8		(2		)	2,374
	Total	$	10,206	$	79	$	(10	)	$	10,275
	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
	December 31, 2013	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
		(In thousands)
	Mortgage-backed securities	$	8,151	$	69	$	(27	)	$	8,193
	U.S. Treasury securities	1,906		15		(4		)	1,917
	Total	$	10,057	$	84	$	(31	)	$	10,110
Assets and liabilities measured at fair value on a recurring basis	'
	The Company's assets and liabilities measured at fair value on a recurring basis were as follows:
		Fair Value Measurements at September 30, 2014, Using
		Quoted Prices in		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2014
		Active Markets
		for Identical Assets (Level 1)
		(In thousands)
	Assets:
	Money market funds	$	—	$	19,687	$	—		$	19,687
	Insurance contract*	—		63,578		—			63,578
	Available-for-sale securities:
	Mortgage-backed securities	—		7,901		—			7,901
	U.S. Treasury securities	—		2,374		—			2,374
	Commodity derivative instruments	—		11,581		—			11,581
	Total assets measured at fair value	$	—	$	105,121	$	—		$	105,121
	Liabilities:
	Commodity derivative instruments	$	—	$	44	$	—		$	44
	Total liabilities measured at fair value	$	—	$	44	$	—		$	44
	*  The insurance contract invests approximately 21 percent in common stock of mid-cap companies, 17 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
		Fair Value Measurements at September 30, 2013, Using
		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)			Balance at September 30, 2013
		(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 investments.
		Fair Value Measurements at December 31, 2013, Using
		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs			Balance at December 31, 2013
						(Level 3)
		(In thousands)
	Assets:
	Money market funds	$	—	$	19,227	$	—		$	19,227
	Insurance contract*	—		62,370		—			62,370
	Available-for-sale securities:
	Mortgage-backed securities	—		8,193		—			8,193
	U.S. Treasury securities	—		1,917		—			1,917
	Commodity derivative instruments	—		1,950		—			1,950
	Total assets measured at fair value	$	—	$	93,657	$	—		$	93,657
	Liabilities:
	Commodity derivative instruments	$	—	$	7,483	$	—		$	7,483
	Total liabilities measured at fair value	$	—	$	7,483	$	—		$	7,483
	* The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.
Fair value of long term debt outstanding	'
	The estimated fair value of the Company's Level 2 long-term debt was as follows:
		Carrying		Fair
		Amount		Value
		(In thousands)
	Long-term debt at September 30, 2014	$	2,210,557	$	2,332,887
	Long-term debt at September 30, 2013	$	2,011,896	$	2,106,887
	Long-term debt at December 31, 2013	$	1,854,563	$	1,912,590

Equity_Tables

Equity (Tables)	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Summary of changes in equity	'
	A summary of the changes in equity was as follows:
	Nine Months Ended September 30, 2014	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2013	$	2,823,164		$	32,738		$	2,855,902
	Net income (loss)	213,978			(2,390		)	211,588
	Other comprehensive income	1,173			—			1,173
	Dividends declared on preferred stocks	(514		)	—			(514		)
	Dividends declared on common stock	(102,461		)	—			(102,461		)
	Stock-based compensation	4,257			—			4,257
	Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings	(5,564		)	—			(5,564		)
	Net tax benefit on stock-based compensation	4,729			—			4,729
	Issuance of common stock	144,493			—			144,493
	Contribution from noncontrolling interest	—			44,900			44,900
	Balance at September 30, 2014	$	3,083,255		$	75,248		$	3,158,503
	Nine Months Ended September 30, 2013	Total Stockholders' Equity			Noncontrolling Interest			Total Equity
		(In thousands)
	Balance at December 31, 2012	$	2,648,248		$	—		$	2,648,248
	Net income (loss)	187,484			(204		)	187,280
	Other comprehensive loss	(8,216		)	—			(8,216		)
	Dividends declared on preferred stocks	(514		)	—			(514		)
	Dividends declared on common stock	(97,720		)	—			(97,720		)
	Stock-based compensation	3,730			—			3,730
	Net tax deficit on stock-based compensation	(1,419		)	—			(1,419		)
	Contribution from noncontrolling interest	—			19,101			19,101
	Balance at September 30, 2013	$	2,731,593		$	18,897		$	2,750,490

Business_segment_data_Tables

Business segment data (Tables)	9 Months Ended
	Sep. 30, 2014
Segment Reporting [Abstract]	'
Information on the Company's businesses	'
	Information on the Company's businesses was as follows:
	Three Months Ended September 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	68,936	$	—		$	9,162
	Natural gas distribution	96,185		—			(12,252		)
	Pipeline and energy services	46,415		4,334			5,060
		211,536		4,334			1,970
	Exploration and production	147,677		8,130			34,750
	Construction materials and contracting	740,496		6,322			55,218
	Construction services	270,313		16,420			9,876
	Other	433		2,601			2,746
		1,158,919		33,473			102,590
	Intersegment eliminations	—		(37,807		)	(1,522		)
	Total	$	1,370,455	$	—		$	103,038
	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
	Nine Months Ended September 30, 2014	External		Inter-			Earnings
		Operating		segment			on Common
		Revenues		Operating			Stock
				Revenues
		(In thousands)
	Electric	$	207,732	$	—		$	28,018
	Natural gas distribution	616,496		—			10,516
	Pipeline and energy services	133,541		30,497			15,198
		957,769		30,497			53,732
	Exploration and production	393,653		39,269			74,869
	Construction materials and contracting	1,339,371		18,445			42,199
	Construction services	815,313		27,431			40,751
	Other	1,248		6,069			4,618
		2,549,585		91,214			162,437
	Intersegment eliminations	—		(121,711		)	(2,705		)
	Total	$	3,507,354	$	—		$	213,464
	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

Employee_benefit_plans_Tables

Employee benefit plans (Tables)	9 Months Ended
	Sep. 30, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]	'
Schedule of net benefit costs	'
	Components of net periodic benefit cost for the Company's pension and other postretirement benefit plans were as follows:
								Other
								Postretirement
		Pension Benefits						Benefits
	Three Months Ended September 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	32		$	39		$	379		$	419
	Interest cost	4,420			4,062			919			804
	Expected return on assets	(5,304		)	(4,979		)	(1,154		)	(1,086		)
	Amortization of prior service cost (credit)	18			18			(348		)	(364		)
	Amortization of net actuarial loss	1,217			1,793			162			327
	Net periodic benefit cost (credit), including amount capitalized	383			933			(42		)	100
	Less amount capitalized	27			157			(65		)	47
	Net periodic benefit cost	$	356		$	776		$	23		$	53
								Other
								Postretirement
		Pension Benefits						Benefits
	Nine Months Ended September 30,	2014			2013			2014			2013
		(In thousands)
	Components of net periodic benefit cost:
	Service cost	$	96		$	116		$	1,138		$	1,257
	Interest cost	13,265			12,186			2,701			2,411
	Expected return on assets	(15,913		)	(14,937		)	(3,463		)	(3,258		)
	Amortization of prior service cost (credit)	54			54			(1,044		)	(1,092		)
	Amortization of net actuarial loss	3,651			5,373			486			1,405
	Net periodic benefit cost (credit), including amount capitalized	1,153			2,792			(182		)	723
	Less amount capitalized	195			425			(55		)	137
	Net periodic benefit cost (credit)	$	958		$	2,367		$	(127	)	$	586

Contingencies_Commitment_and_C

Contingencies Commitment and Contingencies Disclosure (Tables)	9 Months Ended
	Sep. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Limited liability project reflected on consolidated balance sheet	'
	The total assets and liabilities of Dakota Prairie Refining reflected on the Company's Consolidated Balance Sheets were as follows:
		30-Sep-14		30-Sep-13		31-Dec-13
		(In thousands)
	ASSETS
	Current assets:
	Cash and cash equivalents	$	16,723	$	23,146	$	4,774
	Accounts receivable	150		1		—
	Other current assets	4,187		25		26
	Total current assets	21,060		23,172		4,800
	Net property, plant and equipment	314,551		123,297		172,073
	Total assets	$	335,611	$	146,469	$	176,873
	LIABILITIES
	Current liabilities:
	Long-term debt due within one year	$	3,000	$	3,000	$	3,000
	Accounts payable	36,541		20,313		8,904
	Taxes payable	323		—		5
	Accrued compensation	617		—		26
	Other accrued liabilities	633		363		461
	Total current liabilities	41,114		23,676		12,396
	Long-term debt	69,000		72,000		72,000
	Total liabilities	$	110,114	$	95,676	$	84,396

Accounts_receivable_and_allowa1

Accounts receivable and allowance for doubtful accounts (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
In Millions, unless otherwise specified
Receivables [Abstract]	'	'	'
Receivables past due 90 days or more	$29.80	$36.40	$31.10
Allowance for doubtful accounts receivable	$8.90	$10.10	$9.60

Inventories_and_natural_gas_in2

Inventories and natural gas in storage (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Inventory Disclosure [Abstract]	'	'	'
Aggregates held for resale	$106,623,000	$101,568,000	$104,784,000
Asphalt oil	33,551,000	38,099,000	43,078,000
Materials and supplies	71,515,000	69,808,000	71,370,000
Merchandise for resale	24,566,000	21,720,000	23,713,000
Natural gas in storage (current)	29,979,000	16,417,000	37,689,000
Other	36,471,000	34,779,000	33,937,000
Total	302,705,000	282,391,000	314,571,000
Natural gas in storage noncurrent	$47,400,000	$48,300,000	$48,600,000

Oil_and_natural_gas_properties1

Oil and natural gas properties disposition (Details) (USD $)	Sep. 30, 2014
In Millions, unless otherwise specified
Property, Plant and Equipment Impairment or Disposal [Abstract]	'
Sale of oil and natural gas properties	$184.40

Impairment_of_longlived_assets1

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

Earnings_per_common_share_Deta

Earnings per common share (Details)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]	'	'	'	'
Weighted average common shares outstanding - basic	193,949	188,831	191,958	188,831
Effect of dilutive performance share awards	351	807	349	803
Weighted average common shares outstanding - diluted	194,300	189,638	192,307	189,634
Shares excluded from the calculation of diluted earnings per share	0	0	0	0

Cash_flow_information_Details

Cash flow information (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Supplemental Cash Flow Information [Abstract]	'	'
Interest, net of amounts capitalized and AFUDC - borrowed of $8.6 million and $6.3 million in 2014 and 2013, respectively	$61,690	$60,281
Income taxes paid	44,166	30,262
Property, plant and equipment additions in accounts payable	$96,373	$85,646

Cash_flow_information_Cash_flo

Cash flow information Cash flow information - Parenthetical (Details) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Supplemental Cash Flow Information [Abstract]	'	'
Capitalized interest and AFUDC borrowed	$8.60	$6.30

Comprehensive_income_loss_Deta

Comprehensive income (loss) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	($37,198)	($56,821)	($38,205)	($48,721)
Other comprehensive income (loss) before reclassifications	-208	-30	-147	-6,142
Amounts reclassified from accumulated other comprehensive loss	374	-86	1,320	-2,074
Net current-period other comprehensive income (loss)	166	-116	1,173	-8,216
Balance	-37,032	-56,937	-37,032	-56,937
Net unrealized gain (loss) on derivative instruments qualifying as hedges	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	-3,408	-2,744	-3,765	6,018
Other comprehensive income (loss) before reclassifications	0	0	0	-5,594
Amounts reclassified from accumulated other comprehensive loss	82	-510	439	-3,678
Net current-period other comprehensive income (loss)	82	-510	439	-9,272
Balance	-3,326	-3,254	-3,326	-3,254
Postretirement liability adjustment	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	-33,287	-53,275	-33,807	-54,347
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	261	272	781	1,344
Net current-period other comprehensive income (loss)	261	272	781	1,344
Balance	-33,026	-53,003	-33,026	-53,003
Foreign currency translation adjustment	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	-579	-813	-667	-511
Other comprehensive income (loss) before reclassifications	-146	-20	-58	-351
Amounts reclassified from accumulated other comprehensive loss	0	114	0	143
Net current-period other comprehensive income (loss)	-146	94	-58	-208
Balance	-725	-719	-725	-719
Net unrealized gain (loss) on available-for-sale investments	'	'	'	'
Accumulated other comprehensive income (loss) [Roll Forward]	'	'	'	'
Balance	76	11	34	119
Other comprehensive income (loss) before reclassifications	-62	-10	-89	-197
Amounts reclassified from accumulated other comprehensive loss	31	38	100	117
Net current-period other comprehensive income (loss)	-31	28	11	-80
Balance	$45	$39	$45	$39

Reclassification_out_of_accumu

Reclassification out of accumulated other comprehensive income (loss) (Details 2) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Revenues	$1,370,455	$1,285,782	$3,507,354	$3,277,980
Interest expense	-22,425	-21,012	-64,912	-63,312
Other income	2,644	2,326	7,552	5,003
Income before income taxes	156,887	131,126	320,900	287,093
Income taxes	54,769	46,576	109,818	99,559
Loss from equity method investments	-97	-61	-343	-380
Net income (loss)	102,121	84,432	211,588	187,280
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	-132	807	-703	5,924
Income taxes	50	-297	264	-2,246
Net income (loss)	-82	510	-439	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	-420	-437	-1,258	-2,371
Income taxes	159	166	477	1,027
Net income (loss)	-261	-271	-781	-1,344
Reclassification adjustment for loss on foreign currency translation adjustment included in net income	'	'	'	'
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Loss from equity method investments	0	-185	0	-213
Earnings from equity method investments, tax	0	70	0	70
Net income (loss)	0	-115	0	-143
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	-47	-58	-154	-180
Income taxes	16	20	54	63
Net income (loss)	-31	-38	-100	-117
Total reclassifications	'	'	'	'
Reclassification adjustment out of accumulated other comprehensive income [Line Items]	'	'	'	'
Net income (loss)	-374	86	-1,320	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	28	1,007	-223	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	($160)	($200)	($480)	($979)

Acquisition_Details

Acquisition (Details) (USD $)	0 Months Ended
In Millions, unless otherwise specified	Mar. 06, 2014
Business Combinations [Abstract]	'
Business acquisition, cost of acquired entity, purchase price	$208.90

Equity_method_investments_Deta

Equity method investments (Details)	0 Months Ended
	Nov. 30, 2010	Aug. 31, 2013	Aug. 31, 2012	Nov. 30, 2011	Nov. 30, 2010
ECTE [Member]	'	'	'	'	'
Schedule of Equity Method Investments [Line Items]	'	'	'	'	'
Percentage ownership interest in equity method investment sold	'	'	'	'	59.96%
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%	'
ENTE [Member]	'	'	'	'	'
Schedule of Equity Method Investments [Line Items]	'	'	'	'	'
Percentage ownership interest in equity method investment sold	100.00%	'	'	'	100.00%
ERTE [Member]	'	'	'	'	'
Schedule of Equity Method Investments [Line Items]	'	'	'	'	'
Percentage ownership interest in equity method investment sold	100.00%	'	'	'	100.00%

Goodwill_rollforward_Details

Goodwill rollforward (Details) (USD $)	9 Months Ended				12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	$636,039,000	[1]	$636,039,000	[1]	$636,039,000	[1]
Goodwill acquired during the year	0		0		0
Balance as of end of period	636,039,000	[1]	636,039,000	[1]	636,039,000	[1]
Natural gas distribution [Member]	'		'		'
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	345,736,000		345,736,000		345,736,000
Goodwill acquired during the year	0		0		0
Balance as of end of period	345,736,000		345,736,000		345,736,000
Pipeline and energy services [Member]	'		'		'
Goodwill [Line Items]	'		'		'
Goodwill, impaired, accumulated impairment loss	12,300,000		12,300,000		12,300,000
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
Goodwill acquired during the year	0		0		0
Balance as of end of period	9,737,000	[1]	9,737,000	[1]	9,737,000	[1]
Construction materials and contracting [Member]	'		'		'
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	176,290,000		176,290,000		176,290,000
Goodwill acquired during the year	0		0		0
Balance as of end of period	176,290,000		176,290,000		176,290,000
Construction services [Member]	'		'		'
Goodwill [Roll Forward]	'		'		'
Balance as of beginning of period	104,276,000		104,276,000		104,276,000
Goodwill acquired during the year	0		0		0
Balance as of end of period	$104,276,000		$104,276,000		$104,276,000
[1]	Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.

Other_intangible_assets_Detail

Other intangible assets (Details 2) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, net (excluding goodwill)	$10,596,000	$14,092,000	$10,596,000	$14,092,000	$13,099,000
Amortization of intangible assets	700,000	1,200,000	2,500,000	3,000,000	'
Estimated amortization expense for amortizable intangible assets [Abstract]	'	'	'	'	'
2014	3,300,000	'	3,300,000	'	'
2015	2,500,000	'	2,500,000	'	'
2016	2,200,000	'	2,200,000	'	'
2017	1,900,000	'	1,900,000	'	'
2018	1,000,000	'	1,000,000	'	'
Thereafter	2,200,000	'	2,200,000	'	'
Customer relationships [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	21,310,000	21,310,000	21,310,000	21,310,000	21,310,000
Intangible assets, accumulated amortization	-15,116,000	-13,221,000	-15,116,000	-13,221,000	-13,726,000
Intangible assets, net (excluding goodwill)	6,194,000	8,089,000	6,194,000	8,089,000	7,584,000
Noncompete agreements [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	5,080,000	6,186,000	5,080,000	6,186,000	6,186,000
Intangible assets, accumulated amortization	-4,021,000	-4,706,000	-4,021,000	-4,706,000	-4,840,000
Intangible assets, net (excluding goodwill)	1,059,000	1,480,000	1,059,000	1,480,000	1,346,000
Other intangible assets [Member]	'	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Intangible assets, gross	10,921,000	10,995,000	10,921,000	10,995,000	10,995,000
Intangible assets, accumulated amortization	-7,578,000	-6,472,000	-7,578,000	-6,472,000	-6,826,000
Intangible assets, net (excluding goodwill)	$3,343,000	$4,523,000	$3,343,000	$4,523,000	$4,169,000

Derivative_instruments_Details

Derivative instruments (Details) (USD $)	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Mar. 31, 2013
Derivative instruments [Line Items]	'	'	'	'
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI	'	'	'	$1,800,000
Amount of time remaining to transfer accumulated other comprehensive income to earnings	'3 months	'	'	'
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	44,000	7,500,000	9,900,000	'
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments	44,000	7,500,000	9,900,000	'
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	($553,000)	'	'	'
Oil swap and\or collar [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Derivative, nonmonetary notional amount	1,400,000	2,900,000	3,900,000	'
Natural gas swap and\or collar [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Derivative, nonmonetary notional amount	7,300,000	18,300,000	16,500,000	'

Gains_and_losses_on_derivative

Gains and losses on derivative instruments (Details 2) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$0	$0	$0	($5,594)
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	82	-510	439	-3,678
Commodity derivatives designated as cash flow hedges [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	0	0	0	-6,153
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	-18	-634	140	-4,349
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	0	0	-1,422
Interest rate derivatives designated as cash flow hedges [Member]	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	0	0	0	559
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	100	124	299	671
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	0	0	-769
Commodity derivatives not designated as hedging instruments:	'	'	'	'
Derivative instruments [Line Items]	'	'	'	'
Amount of gain (loss) recognized in operating revenues, before tax	$28,755	($12,594)	$16,847	($3,957)

Derivative_instruments_Details1

Derivative instruments (Details 3) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'	'
Derivative asset, fair value, gross asset	$11,581	$1,950	$6,144
Derivative liability fair value gross liability	44	7,483	9,889
Commodity contract [Member] | Not designated as hedging instrument [Member] | Commodity derivative instruments - current assets [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments assets at fair value	11,322	1,447	4,373
Commodity contract [Member] | Not designated as hedging instrument [Member] | Other assets noncurrent [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments assets at fair value	259	503	1,771
Commodity contract [Member] | Not designated as hedging instrument [Member] | Commodity derivative instruments - current liabilities [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments liabilities at fair value	44	7,483	9,740
Commodity contract [Member] | Not designated as hedging instrument [Member] | Other liabilities noncurrent [Member]	'	'	'
Derivatives, Fair Value [Line Items]	'	'	'
Other derivatives not designated as hedging instruments liabilities at fair value	$0	$0	$149

Subject_to_master_netting_agre

Subject to master netting agreements (Details 4) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
In Thousands, unless otherwise specified
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	$11,581	$1,950	$6,144
Derivative liability fair value gross liability	44	7,483	9,889
Commodity derivatives [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	11,581	1,950	6,144
Gross amount not offset	-44	-1,950	-4,939
Derivative asset, fair value, net	11,537	0	1,205
Total assets [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative asset, fair value, gross asset	11,581	1,950	6,144
Gross amount not offset	-44	-1,950	-4,939
Derivative asset, fair value, net	11,537	0	1,205
Commodity derivatives - liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	44	7,483	9,889
Gross amount not offset	-44	-1,950	-4,939
Derivative liability, fair value, net	0	5,533	4,950
Total liabilities [Member]	'	'	'
Derivative instruments [Line Items]	'	'	'
Derivative liability fair value gross liability	44	7,483	9,889
Gross amount not offset	-44	-1,950	-4,939
Derivative liability, fair value, net	$0	$5,533	$4,950

Availableforsale_securities_De

Available-for-sale securities (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Available-for-sale securities [Line Items]	'	'	'	'	'
Investments used to satisfy obligations under nonqualified benefit plans	$63,600,000	$58,100,000	$63,600,000	$58,100,000	$62,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	-800,000	4,100,000	1,200,000	9,200,000	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	10,206,000	9,963,000	10,206,000	9,963,000	10,057,000
Gross unrealized gains	'	'	79,000	85,000	84,000
Gross unrealized losses	'	'	-10,000	-24,000	-31,000
Fair value	10,275,000	10,024,000	10,275,000	10,024,000	10,110,000
Mortgage backed securities [Member]	'	'	'	'	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	7,838,000	8,051,000	7,838,000	8,051,000	8,151,000
Gross unrealized gains	'	'	71,000	70,000	69,000
Gross unrealized losses	'	'	-8,000	-20,000	-27,000
Fair value	7,901,000	8,101,000	7,901,000	8,101,000	8,193,000
US Treasury securities [Member]	'	'	'	'	'
Available-for-sale securities [Abstract]	'	'	'	'	'
Cost	2,368,000	1,912,000	2,368,000	1,912,000	1,906,000
Gross unrealized gains	'	'	8,000	15,000	15,000
Gross unrealized losses	'	'	-2,000	-4,000	-4,000
Fair value	$2,374,000	$1,923,000	$2,374,000	$1,923,000	$1,917,000

Fair_value_measurements_Detail

Fair value measurements (Details 2) (USD $)	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013
Concentration risks, percentage [Abstract]	'		'		'		'
Percentage investment in common stock of mid-cap companies	21.00%		29.00%		29.00%		'
Percentage investment in common stock of small-cap companies	17.00%		28.00%		28.00%		'
Percentage investment in common stock of large-cap companies	29.00%		28.00%		28.00%		'
Percentage in fixed-income and other investments	32.00%		15.00%		15.00%		'
Percentage investment in cash and cash equivalents	1.00%		'		'		'
Assets, fair value disclosure, nonrecurring	'		'		'		$9,700,000
Fair value, measurements, recurring [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	105,121,000		93,657,000		95,329,000		'
Liabilities, fair value disclosure	44,000		7,483,000		9,889,000		'
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Liabilities, fair value disclosure	44,000		7,483,000		9,889,000		'
Fair value, measurements, recurring [Member] | Money market funds [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	19,687,000		19,227,000		21,019,000		'
Fair value, measurements, recurring [Member] | Insurance contract [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	63,578,000	[1]	62,370,000	[2]	58,142,000	[2]	'
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	7,901,000		8,193,000		8,101,000		'
Fair value, measurements, recurring [Member] | US Treasury securities [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	2,374,000		1,917,000		1,923,000		'
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	11,581,000		1,950,000		6,144,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	0		0		0		'
Liabilities, fair value disclosure	0		0		0		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Liabilities, fair value disclosure	0		0		0		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	0		0		0		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	0	[1]	0	[2]	0	[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	105,121,000		93,657,000		95,329,000		'
Liabilities, fair value disclosure	44,000		7,483,000		9,889,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Liabilities, fair value disclosure	44,000		7,483,000		9,889,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	19,687,000		19,227,000		21,019,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	63,578,000	[1]	62,370,000	[2]	58,142,000	[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	7,901,000		8,193,000		8,101,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	2,374,000		1,917,000		1,923,000		'
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	11,581,000		1,950,000		6,144,000		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	0		0		0		'
Liabilities, fair value disclosure	0		0		0		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Liabilities, fair value disclosure	0		0		0		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	0		0		0		'
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract [Member]	'		'		'		'
Fair value measurements [Line Items]	'		'		'		'
Assets, fair value disclosure	0	[1]	0	[2]	0	[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 21 percent in common stock of mid-cap companies, 17 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
[2]	The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.

Fair_value_measurements_Detail1

Fair value measurements (Details 3) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
In Thousands, unless otherwise specified
Carrying amount [Member]	'	'	'
Fair value, balance sheet grouping [Line Items]	'	'	'
Long-term debt	$2,210,557	$1,854,563	$2,011,896
Fair value [Member]	'	'	'
Fair value, balance sheet grouping [Line Items]	'	'	'
Long-term debt, fair value	$2,332,887	$1,912,590	$2,106,887

Longterm_debt_Details

Long-term debt (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Jul. 15, 2014	8-May-14	Apr. 15, 2014	Jan. 28, 2014	Apr. 02, 2014	Mar. 31, 2014	8-May-14
In Millions, unless otherwise specified			The Company [Member]	The Company [Member]	The Company [Member]	The Company [Member]	Centennial Energy Holdings, Inc [Member]	Centennial Energy Holdings, Inc [Member]	Centennial Energy Holdings, Inc [Member]
Line of Credit Facility [Line Items]	'	'	'	'	'	'	'	'	'
Note purchase agreement	'	'	'	'	'	$150	'	'	'
Senior notes issued	'	'	100	'	50	'	'	'	'
Senior notes weighted average interest rate	'	'	4.30%	'	5.20%	'	'	'	'
Facility, maximum borrowing capacity	'	'	'	175	'	'	'	'	650
Ratio of total debt to total capitalization as specified in debt convenants	'	'	'	'	'	'	65.00%	65.00%	65.00%
Debt instrument, term	'	'	'	'	'	'	'2 years	'2 years	'
Term loan agreement	'	'	'	'	'	'	125	125	'
Long-term commercial paper	$293.50	$153.90	'	'	'	'	'	'	'

Equity_Details

Equity (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Balance	'	'	'	'
Total stockholders' equity	'	'	$2,823,164	'
Noncontrolling interest	'	'	32,738	'
Total equity	'	'	2,855,902	2,648,248
Net loss attributable to noncontrolling interest	-1,088	-24	-2,390	-204
Net income (loss)	102,121	84,432	211,588	187,280
Other comprehensive income	166	-116	1,173	-8,216
Dividends declared on preferred stocks	'	'	-514	-514
Dividends declared on common stock	'	'	-102,461	-97,720
Stock-based compensation	'	'	4,257	3,730
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	'	'	-5,564	'
Net tax benefit on stock-based compensation	'	'	4,729	-1,419
Issuance of common stock	'	'	144,493	'
Contribution from noncontrolling interest	'	'	44,900	19,101
Total stockholders' equity	3,083,255	2,731,593	3,083,255	2,731,593
Noncontrolling interest	75,248	18,897	75,248	18,897
Total equity	3,158,503	2,750,490	3,158,503	2,750,490
Total stockholders’ equity [Member]	'	'	'	'
Balance	'	'	'	'
Total stockholders' equity	'	'	2,823,164	2,648,248
Net income (loss) attributable to parent	'	'	213,978	187,484
Other comprehensive income	'	'	1,173	-8,216
Dividends declared on preferred stocks	'	'	-514	-514
Dividends declared on common stock	'	'	-102,461	-97,720
Stock-based compensation	'	'	4,257	3,730
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	'	'	-5,564	'
Net tax benefit on stock-based compensation	'	'	4,729	-1,419
Issuance of common stock	'	'	144,493	'
Contribution from noncontrolling interest	'	'	0	0
Total stockholders' equity	3,083,255	2,731,593	3,083,255	2,731,593
Noncontrolling interest [Member]	'	'	'	'
Balance	'	'	'	'
Noncontrolling interest	'	'	32,738	0
Net loss attributable to noncontrolling interest	'	'	-2,390	-204
Other comprehensive income	'	'	0	0
Dividends declared on preferred stocks	'	'	0	0
Dividends declared on common stock	'	'	0	0
Stock-based compensation	'	'	0	0
Issuance of common stock upon vesting of performance shares, net of shares used for tax withholdings	'	'	0	'
Net tax benefit on stock-based compensation	'	'	0	0
Issuance of common stock	'	'	0	'
Contribution from noncontrolling interest	'	'	44,900	19,101
Noncontrolling interest	$75,248	$18,897	$75,248	$18,897

Business_segment_data_Details

Business segment data (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	$1,370,455	$1,285,782	$3,507,354	$3,277,980
Intersegment operating revenues	0	0	0	0
Earnings on common stock	103,038	84,285	213,464	186,970
Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	211,536	192,103	957,769	843,670
Intersegment operating revenues	4,334	4,906	30,497	31,623
Earnings on common stock	1,970	5,523	53,732	42,319
Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	1,158,919	1,093,679	2,549,585	2,434,310
Intersegment operating revenues	33,473	23,092	91,214	70,283
Earnings on common stock	102,590	79,964	162,437	147,910
Electric [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	68,936	68,314	207,732	189,949
Intersegment operating revenues	0	0	0	0
Earnings on common stock	9,162	11,417	28,018	25,652
Natural gas distribution [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	96,185	77,417	616,496	536,756
Intersegment operating revenues	0	0	0	0
Earnings on common stock	-12,252	-11,204	10,516	15,420
Pipeline and energy services [Member] | Regulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	46,415	46,372	133,541	116,965
Intersegment operating revenues	4,334	4,906	30,497	31,623
Earnings on common stock	5,060	5,310	15,198	1,247
Exploration and production [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	147,677	119,234	393,653	371,648
Intersegment operating revenues	8,130	10,714	39,269	33,083
Earnings on common stock	34,750	17,434	74,869	70,713
Construction materials and contracting [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	740,496	706,982	1,339,371	1,287,305
Intersegment operating revenues	6,322	7,422	18,445	24,673
Earnings on common stock	55,218	49,159	42,199	38,602
Construction services [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	270,313	267,038	815,313	774,103
Intersegment operating revenues	16,420	3,097	27,431	7,011
Earnings on common stock	9,876	12,154	40,751	36,733
Other [Member] | Unregulated Operation [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	433	425	1,248	1,254
Intersegment operating revenues	2,601	1,859	6,069	5,516
Earnings on common stock	2,746	1,217	4,618	1,862
Intersegment eliminations [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	0	0	0	0
Intersegment operating revenues	-37,807	-27,998	-121,711	-101,906
Earnings on common stock	($1,522)	($1,202)	($2,705)	($3,259)

Employee_benefit_plans_Details

Employee benefit plans (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension benefits [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Service cost	$32	$39	$96	$116
Interest cost	4,420	4,062	13,265	12,186
Expected return on assets	-5,304	-4,979	-15,913	-14,937
Amortization of prior service cost (credit)	18	18	54	54
Amortization of net actuarial loss	1,217	1,793	3,651	5,373
Net periodic benefit cost (credit), including amount capitalized	383	933	1,153	2,792
Less amount capitalized	27	157	195	425
Net periodic benefit cost	356	776	958	2,367
Other postretirement benefits [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Service cost	379	419	1,138	1,257
Interest cost	919	804	2,701	2,411
Expected return on assets	-1,154	-1,086	-3,463	-3,258
Amortization of prior service cost (credit)	-348	-364	-1,044	-1,092
Amortization of net actuarial loss	162	327	486	1,405
Net periodic benefit cost (credit), including amount capitalized	-42	100	-182	723
Less amount capitalized	-65	47	-55	137
Net periodic benefit cost	23	53	-127	586
Supplemental employee retirement plans [Member]	'	'	'	'
Defined benefit plan disclosure, net periodic benefit cost [Line Items]	'	'	'	'
Net periodic benefit cost	$1,700	$1,800	$5,000	$5,500

Regulatory_matters_and_revenue1

Regulatory matters and revenues subject to refund (Details) (USD $)	0 Months Ended
	Jul. 10, 2014	Feb. 27, 2014	Aug. 11, 2014	Oct. 31, 2013	Oct. 03, 2014
	NDPSC [Member]	NDPSC [Member]	MTPSC [Member]	FERC [Member]	Subsequent event [Member]
					WYPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]	'	'	'	'	'
Annual amount recovered	$8,600,000	'	'	'	'
Rate increase requested	'	7,400,000	3,000,000	'	788,000
Percent above current rates requested	'	4.60%	3.60%	'	4.10%
Interim rate increase (decrease), amount	'	'	2,200,000	'	'
Interim rate increase, percent	'	'	2.60%	'	'
Increased expenditures covered by change application	'	'	'	312,000,000	'
Approximate annual increase in revenues	'	'	'	$11,500,000	'

Litigation_Details

Litigation (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Aug. 31, 2011	Aug. 06, 2012	Oct. 31, 2010
				Litigation related to construction materials [Member]	Former employee litigation [Member]	Natural gas gathering operations [Member]
Loss Contingencies [Line Items]	'	'	'	'	'	'
Potential liabilities related to litigation and environmental matters	$31,900,000	$29,500,000	$30,800,000	'	'	'
Loss contingency, estimate of possible loss	'	'	'	3,700,000	'	'
Additional amount per day of potential penalties	'	'	'	5,000	'	'
Plaintiffs request compensatory damages	'	'	'	'	23,800,000	'
Arbitration award	'	'	'	'	'	$26,600,000

Enviromental_matters_Details_2

Enviromental matters (Details 2) (USD $)	9 Months Ended
	Sep. 30, 2014
Portland Harbor Site [Member]	'
Site Contingency [Line Items]	'
Environmental matters investigative costs	$70,000,000
Eugene,OR Manufactured Gas Plant Site [Member]	'
Site Contingency [Line Items]	'
Site contingency, low estimate, loss exposure	500,000
Site contingency, high estimate, loss exposure	11,000,000
Estimated proportional share of cleanup liability	50.00%
Accrual for environmental loss contingencies	1,700,000
Bremerton, WA Manufactured Gas Plant Site [Member]	'
Site Contingency [Line Items]	'
Site contingency, low estimate, loss exposure	340,000
Site contingency, high estimate, loss exposure	6,400,000
Accrual for environmental loss contingencies	12,600,000
Bellingham, WA Manufactured Gas Plant Site [Member]	'
Site Contingency [Line Items]	'
Site contingency, loss exposure not accrued, best estimate	$8,000,000

Guarantees_Details_3

Guarantees (Details 3) (USD $)	Sep. 30, 2014
Guarantor Obligations [Line Items]	'
Amount of hedging obligations guaranteed reflected on balance sheet	$0
Guarantor obligations, maximum exposure, undiscounted	114,300,000
Fixed maximum amounts guaranteed by year 2014	19,400,000
Fixed maximum amounts guaranteed by year 2015	75,100,000
Fixed maximum amounts guaranteed by year 2016	700,000
Fixed maximum amounts guaranteed by year 2017	600,000
Fixed maximum amounts guaranteed by year 2018	500,000
Fixed maximum amounts guaranteed by year 2019	500,000
Fixed maximum amounts guaranteed by year expires after certain number of days after written notice	13,500,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	200,000
Letters of credit	39,000,000
Letters of credit set to expire - 2014	8,200,000
Letters of credit set to expire - 2015	30,800,000
Outstanding letters of credit	0
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	4,000,000
Amount outstanding by a related party under the guarantee	1,000,000
Amount of surety bonds outstanding	$387,000,000

Commitment_and_contingencies_v

Commitment and contingencies variable interest entities (Details 4) (USD $)	Sep. 30, 2014	Dec. 31, 2013	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	'	'
Excess capital commitments	300,000,000	'	'
Project cost estimate	360,000,000	'	'
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	16,723,000	4,774,000	23,146,000
Dakota Prairie Refining, LLC [Member] | Accounts receivable [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	150,000	0	1,000
Dakota Prairie Refining, LLC [Member] | Other current assets [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	4,187,000	26,000	25,000
Dakota Prairie Refining, LLC [Member] | Total current assets [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	21,060,000	4,800,000	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	314,551,000	172,073,000	123,297,000
Dakota Prairie Refining, LLC [Member] | Total assets [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, assets	335,611,000	176,873,000	146,469,000
Dakota Prairie Refining, LLC [Member] | Long-term debt due within one year [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	3,000,000	3,000,000	3,000,000
Dakota Prairie Refining, LLC [Member] | Accounts payable [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	36,541,000	8,904,000	20,313,000
Dakota Prairie Refining, LLC [Member] | Taxes payable [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	323,000	5,000	0
Dakota Prairie Refining, LLC [Member] | Accrued compensation [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	617,000	26,000	0
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	633,000	461,000	363,000
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	41,114,000	12,396,000	23,676,000
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	69,000,000	72,000,000	72,000,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Limited liability entity, consolidated, carrying amount, liabilities	110,114,000	84,396,000	95,676,000
Fuel contract [Member]	'	'	'
Variable Interest Entities [Line Items]	'	'	'
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$12,000,000	'	'

Subsequent_event_Details

Subsequent event (Details) (Withdrawal from multiemployer defined benefit plan [Member], Subsequent event [Member], USD $)	Oct. 26, 2014
In Millions, unless otherwise specified
Withdrawal from multiemployer defined benefit plan [Member] | Subsequent event [Member]	'
Subsequent event [Line Items]	'
Estimated multiemployer plan, withdrawal obligation	$14
Estimated multiemployer plan, withdrawal obligation, after tax	$8.40