NorthWestern (NWE) 10-K 12 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information Document (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 06, 2015	Jun. 30, 2014
Entity Information [Line Items]
Entity Registrant Name	NORTHWESTERN CORPORATION
Entity Central Index Key	73088
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Amendment Flag	FALSE
Entity Common Stock, Shares Outstanding		46,933,953
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$2,042,683,000

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Electric	$877,967	$865,239	$805,554
Gas	326,896	287,605	263,394
Other	0	1,675	1,394
Total Revenues	1,204,863	1,154,519	1,070,342
Operating Expenses
Cost of sales	482,591	479,546	395,434
Operating, general and administrative	305,886	285,569	269,966
Mountain States Transmission Intertie impairment	0	0	24,039
Property and other taxes	114,592	105,540	97,674
Depreciation and depletion	123,776	112,831	106,044
Total Operating Expenses	1,026,845	983,486	893,157
Operating Income	178,018	171,033	177,185
Interest Expense	-77,802	-70,486	-65,062
Other Income	10,198	7,737	4,372
Income Before Income Taxes	110,414	108,284	116,495
Income Tax Benefit (Expense)	10,272	-14,301	-18,089
Net Income	$120,686	$93,983	$98,406
Average Common Shares Outstanding	40,156,177	38,144,852	36,847,427
Basic Earnings per Average Common Share	$3.01	$2.46	$2.67
Diluted Earnings per Average Common Share	$2.99	$2.46	$2.66

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income	$120,686	$93,983	$98,406
Other comprehensive (loss) income, net of tax:
Reclassification of net gains on derivative instruments to net income	-684	-730	-732
Realized loss on cash flow hedging activities	-11,145	0	0
Postretirement medical liability adjustment	82	963	-553
Foreign currency translation	265	166	-54
Total Other Comprehensive Income (Loss)	-11,482	399	-1,339
Comprehensive Income	$109,204	$94,382	$97,067

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
OPERATING ACTIVITIES:
Net Income	$120,686	$93,983	$98,406
Items not affecting cash:
Depreciation and depletion	123,776	112,831	106,044
Amortization of debt issue costs, discount and deferred hedge gain	5,033	2,039	369
Amortization of nonvested shares	3,262	2,404	2,759
Equity portion of allowance for funds used during construction	-6,554	-5,050	-4,846
Gain on disposition of assets	-1,330	-721	-332
Deferred income taxes	-9,612	54,617	51,890
Mountain States Transmission Intertie impairment	0	0	24,039
Gain on CELP arbitration decision	0	0	-47,894
Changes in current assets and liabilities:
Restricted cash	-6,408	-196	6,016
Accounts receivable	12,622	-30,792	3,456
Inventories	747	181	5,371
Other current assets	4,201	-2,940	-1,856
Accounts payable	-9,565	6,235	10,976
Accrued expenses	8,530	1,949	14,149
Regulatory assets	-8,952	-2,846	-6,285
Regulatory liabilities	9,763	-2,019	15,241
Other noncurrent assets	2,853	-43,714	-27,362
Other noncurrent liabilities	987	7,755	1,052
Cash provided by operating activities	250,039	193,716	251,193
INVESTING ACTIVITIES:
Property, plant, and equipment additions	-270,384	-230,454	-219,234
Acquisitions	-903,573	-68,666	-103,241
Proceeds from sale of assets	1,535	3,766	262
Change in restricted cash	-16,358	0	0
Investment in New Market Tax Credit program	-18,169	0	0
Cash used in investing activities	-1,206,949	-295,354	-322,213
FINANCING ACTIVITIES:
Dividends on common stock	-65,019	-57,684	-54,246
Proceeds from issuance of common stock, net	399,207	56,825	28,477
Issuance of long-term debt	505,789	100,000	150,000
Repayment of long-term debt	-90	-149	-3,945
Issuances (repayments) of short-term borrowings, net	126,890	18,016	-44,000
Treasury stock activity	-814	-1,042	-429
Financing costs	-5,248	-7,593	-943
Cash provided by financing activities	960,715	108,373	74,914
Increase in Cash and Cash Equivalents	3,805	6,735	3,894
Cash and Cash Equivalents, beginning of period	16,557	9,822	5,928
Cash and Cash Equivalents, end of period	$20,362	$16,557	$9,822

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current Assets:
Cash and cash equivalents	$20,362	$16,557
Restricted cash	29,662	6,896
Accounts receivable, net	163,479	174,913
Inventories	55,094	55,609
Regulatory assets	47,374	37,719
Deferred income taxes	20,843	14,301
Other	14,071	14,961
Total current assets	350,885	320,956
Property, plant, and equipment, net	3,758,008	2,690,128
Goodwill	355,128	355,128
Regulatory assets	455,757	316,952
Other noncurrent assets	54,165	32,096
Total assets	4,973,943	3,715,260
Current Liabilities:
Current maturities of capital leases	1,730	1,662
Short-term borrowings	267,840	140,950
Accounts payable	81,961	92,957
Accrued expenses	206,882	181,613
Regulatory liabilities	56,169	46,406
Total current liabilities	614,582	463,588
Long-term capital leases	28,162	29,895
Long-term debt	1,662,099	1,155,097
Deferred income taxes	446,600	395,333
Noncurrent regulatory liabilities	362,228	348,053
Other noncurrent liabilities	382,489	292,624
Total liabilities	3,496,160	2,684,590
Commitments and Contingencies (Note 20)
Shareholders' Equity:
Common stock, par value $0.01; authorized 200,000,000 shares; issued and outstanding 50,522,280 and 46,914,811 respectively; Preferred stock, par value $0.01; authorized 50,000,000 shares; none issued	505	423
Treasury stock at cost	-92,558	-91,744
Paid-in capital	1,313,844	910,184
Retained earnings	264,758	209,091
Accumulated other comprehensive income	-8,766	2,716
Total shareholders' equity	1,477,783	1,030,670
Total liabilities and shareholders' equity	$4,973,943	$3,715,260

CONSOLIDATED_BALANCE_SHEETS_PA

CONSOLIDATED BALANCE SHEETS PARENTHETICAL (Parentheticals) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Common stock, par or stated value per share	$0.01	$0.01
Common stock, shares authorized	200,000,000	200,000,000
Common Stock, shares issued	50,522,280	42,340,173
Common Stock, shares outstanding	46,914,811	38,745,624
Preferred stock, par or stated value per share	$0.01	$0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred Stock, shares issued	0	0

CONSOLIDATED_STATEMENTS_OF_COM1

CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $)	Total	Common Stock	Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income
In Thousands, except Share data, unless otherwise specified
Balance at Dec. 31, 2011	$859,112	$398	$816,700	($90,273)	$128,631	$3,656
Shares, Balance at Dec. 31, 2011		39,841,000		3,563,000
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	98,406	0	0	0	98,406	0
Other comprehensive income:
Foreign currency translation adjustment	-54	0	0	0	0	-54
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	-732	0	0	0	0	-732
Realized loss on cash flow hedging activities	0
Postretirement medical liability adjustment	-553	0	0	0	0	-553
Stock based compensation, value	3,133	1	3,925	-793	0	0
Stock based compensation, shares		136,000		22,000
Issuance of shares, value	28,966	9	28,593		0	0
Issuance of shares, shares		815,000
Issuance of shares, value, treasury stock reissued				364
Issuance of shares, shares, treasury stock reissued				-14,000
Dividends on common stock	-54,246	0	0	0	-54,246	0
Dividends per share	$1.48
Balance at Dec. 31, 2012	934,032	408	849,218	-90,702	172,791	2,317
Shares, Balance at Dec. 31, 2012		40,792,000		3,571,000
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	93,983	0	0	0	93,983	0
Other comprehensive income:
Foreign currency translation adjustment	166	0	0	0	0	166
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	-730	0	0	0	0	-730
Realized loss on cash flow hedging activities	0
Postretirement medical liability adjustment	963	0	0	0	0	963
Stock based compensation, value	2,663	1	3,987	-1,325	0	0
Stock based compensation, shares		167,000		35,000
Issuance of shares, value	57,276	14	56,979		0	0
Issuance of shares, shares		1,381,000
Issuance of shares, value, treasury stock reissued				283
Issuance of shares, shares, treasury stock reissued				-11,000
Dividends on common stock	-57,683	0	0	0	-57,683	0
Dividends per share	$1.52
Balance at Dec. 31, 2013	1,030,670	423	910,184	-91,744	209,091	2,716
Shares, Balance at Dec. 31, 2013		42,340,000		3,595,000
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	120,686	0	0	0	120,686	0
Other comprehensive income:
Foreign currency translation adjustment	265	0	0	0	0	265
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	-684	0	0	0	0	-684
Realized loss on cash flow hedging activities	-11,145					-11,145
Postretirement medical liability adjustment	82	0	0	0	0	82
Stock based compensation, value	3,423	0	4,288	-865	0	0
Stock based compensation, shares		119,000		12,000
Issuance of shares, value	399,505	82	399,372		0	0
Issuance of shares, shares		8,063,000
Issuance of shares, value, treasury stock reissued				51
Issuance of shares, shares, treasury stock reissued				0
Dividends on common stock	-65,019	0	0	0	-65,019	0
Dividends per share	$1.60
Balance at Dec. 31, 2014	$1,477,783	$505	$1,313,844	($92,558)	$264,758	($8,766)
Shares, Balance at Dec. 31, 2014		50,522,000		3,607,000

Nature_of_Operations_and_Basis

Nature of Operations and Basis of Consolidation	12 Months Ended
	Dec. 31, 2014
Nature of Operations and Basis of Consolidation [Abstract]
Nature of Operations and Basis of Consolidation	Nature of Operations and Basis of Consolidation
	NorthWestern Corporation, doing business as NorthWestern Energy, provides electricity and natural gas to approximately 692,600 customers in Montana, South Dakota and Nebraska. We have generated and distributed electricity in South Dakota and distributed natural gas in South Dakota and Nebraska since 1923 and have generated and distributed electricity and distributed natural gas in Montana since 2002.
	The Consolidated Financial Statements for the periods included herein have been prepared by NorthWestern Corporation (NorthWestern, we or us), pursuant to the rules and regulations of the SEC. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that may affect the reported amounts of assets, liabilities, revenues and expenses during the reporting period. Actual results could differ from those estimates. The accompanying Consolidated Financial Statements include our accounts together with those of our wholly and majority-owned or controlled subsidiaries. All intercompany balances and transactions have been eliminated from the Consolidated Financial Statements. Events occurring subsequent to December 31, 2014, have been evaluated as to their potential impact to the Consolidated Financial Statements through the date of issuance. Our November 2014 acquisition of hydro generating assets is included in the results of operations for the year ended December 31, 2014, and impacts the comparability of the current year financial statements to prior years. For a further discussion of this acquisition, see Note 3 - Hydro Transaction.
	Variable Interest Entities
	A reporting company is required to consolidate a variable interest entity (VIE) as its primary beneficiary, which means it has a controlling financial interest, when it has both the power to direct the activities of the VIE that most significantly impact the VIE's economic performance, and the obligation to absorb losses or the right to receive benefits from the VIE that could potentially be significant to the VIE. An entity is considered to be a VIE when its total equity investment at risk is not sufficient to permit the entity to finance its activities without additional subordinated financial support, or its equity investors, as a group, lack the characteristics of having a controlling financial interest. The determination of whether a company is required to consolidate an entity is based on, among other things, an entity's purpose and design and a company's ability to direct the activities of the entity that most significantly impact the entity's economic performance.
	Certain long-term purchase power and tolling contracts may be considered variable interests. We have various long-term purchase power contracts with other utilities and certain QF plants. We identified one QF contract that may constitute a VIE. We entered into a power purchase contract in 1984 with this 35 MW coal-fired QF to purchase substantially all of the facility's capacity and electrical output over a substantial portion of its estimated useful life. We absorb a portion of the facility's variability through annual changes to the price we pay per MWH (energy payment). After making exhaustive efforts, we have been unable to obtain the information from the facility necessary to determine whether the facility is a VIE or whether we are the primary beneficiary of the facility. The contract with the facility contains no provision which legally obligates the facility to release this information. We have accounted for this QF contract as an executory contract. Based on the current contract terms with this QF, our estimated gross contractual payments aggregate approximately $262.9 million through 2024. For further discussion of our gross QF liability, see Note 20 - Commitments and Contingencies. During the years ended December 31, 2014, 2013 and 2012 purchases from this QF were approximately $24.4 million, $23.8 million, and $21.0 million, respectively.

Significant_Accounting_Policie

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Estimates are used for such items as long-lived asset values and impairment charges, long-lived asset useful lives, tax provisions, asset retirement obligations, uncollectible accounts, our QF liability, environmental costs, unbilled revenues and actuarially determined benefit costs. We revise the recorded estimates when we receive better information or when we can determine actual amounts. Those revisions can affect operating results.
	Revenue Recognition
	Customers are billed monthly on a cycle basis. To match revenues with associated expenses, we accrue unbilled revenues for electrical and natural gas services delivered to customers, but not yet billed at month-end.
	Cash Equivalents
	We consider all highly liquid investments with maturities of three months or less at the time of purchase to be cash equivalents.
	Restricted Cash
	Restricted cash consists primarily of funds held in trust accounts to satisfy the requirements of certain stipulation agreements and insurance reserve requirements.
	Accounts Receivable, Net
	Accounts receivable are net of allowances for uncollectible accounts of $4.3 million and $4.5 million at December 31, 2014 and December 31, 2013, respectively. Receivables include unbilled revenues of $70.3 million and $74.3 million at December 31, 2014 and December 31, 2013, respectively.
	Inventories
	Inventories are stated at average cost. Inventory consisted of the following (in thousands):
		December 31,
		2014		2013
	Materials and supplies	$	30,672	$	28,263
	Storage gas and fuel	24,422		27,346
		$	55,094	$	55,609
	Regulation of Utility Operations
	Our regulated operations are subject to the provisions of ASC 980. Regulated accounting is appropriate provided that (i) rates are established by or subject to approval by independent, third-party regulators, (ii) rates are designed to recover the specific enterprise's cost of service, and (iii) in view of demand for service, it is reasonable to assume that rates are set at levels that will recover costs and can be charged to and collected from customers.
	Our Consolidated Financial Statements reflect the effects of the different rate making principles followed by the jurisdictions regulating us. The economic effects of regulation can result in regulated companies recording costs that have been, or are expected to be, allowed in the ratemaking process in a period different from the period in which the costs would be charged to expense by an unregulated enterprise. When this occurs, costs are deferred as regulatory assets and recorded as expenses in the periods when those same amounts are reflected in rates. Additionally, regulators can impose liabilities upon a regulated company for amounts previously collected from customers and for amounts that are expected to be refunded to customers (regulatory liabilities).
	If we were required to terminate the application of these provisions to our regulated operations, all such deferred amounts would be recognized in the Consolidated Income Statements at that time. This would result in a charge to earnings, net of applicable income taxes, which could be material. In addition, we would determine any impairment to the carrying costs of deregulated plant and inventory assets.
	Derivative Financial Instruments
	We account for derivative instruments in accordance with ASC 815, Derivatives and Hedging. All derivatives are recognized in the Consolidated Balance Sheets at their fair value unless they qualify for certain exceptions, including the normal purchases and normal sales exception. Additionally, derivatives that qualify and are designated for hedge accounting are classified as either hedges of the fair value of a recognized asset or liability or of an unrecognized firm commitment (fair-value hedge) or hedges of a forecasted transaction or the variability of cash flows to be received or paid related to a recognized asset or liability (cash-flow hedge). For fair-value hedges, changes in fair values for both the derivative and the underlying hedged exposure are recognized in earnings each period. For cash-flow hedges, the portion of the derivative gain or loss that is effective in offsetting the change in the cost or value of the underlying exposure is deferred in accumulated other comprehensive income (AOCI) and later reclassified into earnings when the underlying transaction occurs. Gains and losses from the ineffective portion of any hedge are recognized in earnings immediately. For other derivative contracts that do not qualify or are not designated for hedge accounting, changes in the fair value of the derivatives are recognized in earnings each period. Cash inflows and outflows related to derivative instruments are included as a component of operating, investing or financing cash flows in the Consolidated Statements of Cash Flows, depending on the underlying nature of the hedged items.
	Revenues and expenses on contracts that qualify are designated as normal purchases and normal sales and are recognized when the underlying physical transaction is completed. While these contracts are considered derivative financial instruments, they are not required to be recorded at fair value, but on an accrual basis of accounting. Normal purchases and normal sales are contracts where physical delivery is probable, quantities are expected to be used or sold in the normal course of business over a reasonable period of time, and price is not tied to an unrelated underlying derivative. As part of our regulated electric and gas operations, we enter into contracts to buy and sell energy to meet the requirements of our customers. These contracts include short-term and long-term commitments to purchase and sell energy in the retail and wholesale markets with the intent and ability to deliver or take delivery. If it were determined that a transaction designated as a normal purchase or a normal sale no longer met the exceptions, the fair value of the related contract would be reflected as an asset or liability and immediately recognized through earnings. See Note 9, Risk Management and Hedging Activities for further discussion of our derivative activity.
	Property, Plant and Equipment
	Property, plant and equipment are stated at original cost, including contracted services, direct labor and material, AFUDC, and indirect charges for engineering, supervision and similar overhead items. All expenditures for maintenance and repairs of utility property, plant and equipment are charged to the appropriate maintenance expense accounts. A betterment or replacement of a unit of property is accounted for as an addition and retirement of utility plant. At the time of such a retirement, the accumulated provision for depreciation is charged with the original cost of the property retired and also for the net cost of removal. Also included in plant and equipment are assets under capital lease, which are stated at the present value of minimum lease payments.
	AFUDC represents the cost of financing construction projects with borrowed funds and equity funds. While cash is not realized currently from such allowance, it is realized under the ratemaking process over the service life of the related property through increased revenues resulting from a higher rate base and higher depreciation expense. The component of AFUDC attributable to borrowed funds is included as a reduction to interest expense, while the equity component is included in other income. We determine the rate used to compute AFUDC in accordance with a formula established by the FERC. This rate averaged 8.0%, 8.1%, and 8.0%, for Montana and South Dakota for 2014, 2013, and 2012, respectively. AFUDC capitalized totaled $10.8 million for the year ended December 31, 2014, $8.2 million for the year ended December 31, 2013 and $7.9 million for the year ended December 31, 2012 for Montana and South Dakota combined.
	We record provisions for depreciation at amounts substantially equivalent to calculations made on a straight-line method by applying various rates based on useful lives of the various classes of properties (ranging from three to 50 years) determined from engineering studies. As a percentage of the depreciable utility plant at the beginning of the year, our provision for depreciation of utility plant was approximately 2.9%, 3.2%, and 3.3% for 2014, 2013, and 2012, respectively.
	Depreciation rates include a provision for our share of the estimated costs to decommission our jointly owned plants at the end of the useful life. The annual provision for such costs is included in depreciation expense, while the accumulated provisions are included in noncurrent regulatory liabilities.
	Other Noncurrent Liabilities
	Other noncurrent liabilities consisted of the following (in thousands):
		December 31,
		2014		2013
	Pension and other employee benefits	$	137,377	$	57,140
	Future QF obligation, net	136,893		136,448
	Environmental	28,060		28,194
	Customer advances	30,001		27,371
	Other	50,158		43,471
		$	382,489	$	292,624
	Income Taxes
	Exposures exist related to various tax filing positions, which may require an extended period of time to resolve and may result in income tax adjustments by taxing authorities. We have reduced deferred tax assets or established liabilities based on our best estimate of future probable adjustments related to these exposures. On a quarterly basis, we evaluate exposures in light of any additional information and make adjustments as necessary to reflect the best estimate of the future outcomes. We believe our deferred tax assets and established liabilities are appropriate for estimated exposures; however, actual results may differ from these estimates. The resolution of tax matters in a particular future period could have a material impact on our Consolidated Income Statements and provision for income taxes.
	Environmental Costs
	We record environmental costs when it is probable we are liable for the costs and we can reasonably estimate the liability. We may defer costs as a regulatory asset if there is precedent for recovering similar costs from customers in rates. Otherwise, we expense the costs. If an environmental cost is related to facilities we currently use, such as pollution control equipment, then we may capitalize and depreciate the costs over the remaining life of the asset, assuming the costs are recoverable in future rates or future cash flows.
	Our remediation cost estimates are based on the use of an environmental consultant, our experience, our assessment of the current situation and the technology currently available for use in the remediation. We regularly adjust the recorded costs as we revise estimates and as remediation proceeds. If we are one of several designated responsible parties, then we estimate and record only our share of the cost.
	Business Combination
	Our November 2014 acquisition of hydro generating assets was accounted for using business combination accounting. Under this method, the purchase price paid by the acquirer is allocated to the assets acquired and liabilities assumed as of the acquisition date based on their fair value. For additional information see Note 3 - Hydro Transaction.
	Accounting Standards Issued
	In May 2014, the Financial Accounting Standards Board (FASB) issued accounting guidance on the recognition of revenue from contracts with customers, which will supersede nearly all existing revenue recognition guidance under GAAP. Under the new standard, entities will recognize revenue to depict the transfer of goods and services to customers in amounts that reflect the payment to which the entity expects to be entitled in exchange for those goods or services. The guidance also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows from an entity’s contracts with customers. The new guidance will be effective for us in our first quarter of 2017. Early adoption is not permitted. We are currently evaluating the impact of adoption of this new guidance on our Financial Statements and disclosures.
	In January 2015, the FASB issued guidance which eliminates from GAAP the concept of an extraordinary item. As a result, an entity will no longer (1) segregate an extraordinary item from the results of ordinary operations; (2) separately present an extraordinary item on its income statement, net of tax, after income from continuing operations; and (3) disclose income taxes and earnings-per-share data applicable to an extraordinary item. The new guidance will be effective for us in our first quarter of 2016 and early adoption is permitted. We do not expect the adoption of this standard to have a material effect on our reporting and disclosure.
	Accounting Standards Adopted
	There have been no new accounting pronouncements or changes in accounting pronouncements adopted during the period that are of significance, or potential significance, to us.
	Supplemental Cash Flow Information
		Year Ended December 31,
		2014		2013		2012
				(in thousands)
	Cash paid for:
	Income taxes	$	35	$	50	$	2,944
	Interest	63,482		57,789		51,271
	Significant non-cash transactions:
	Capital expenditures included in trade accounts payable	8,555		12,025		13,136

Hydro_Acquisition_Regulated_Op

Hydro Acquisition Regulated Operations (Notes)	12 Months Ended
	Dec. 31, 2014
Regulated Operations [Abstract]
Schedule of Significant Events, Acquisitions and Disposals [Text Block]	Hydro Transaction
	In November 2014, we completed the purchase of hydroelectric generating facilities and associated assets located in Montana for an adjusted purchase price of approximately $904 million (Hydro Transaction). The addition of hydroelectric generation is intended to provide long-term supply diversity to our portfolio and reduce risks associated with variable fuel prices. We expect the Hydro Transaction to allow us to reduce our reliance on third party power purchase agreements and spot market purchases, more closely matching our electric generation resources with forecasted customer demand. With reduced amounts of purchased power, we believe we will be less exposed to market volatility and will be better positioned to control the cost of supplying electricity to our customers.
	The facilities acquired include eleven hydro-electric plants and one storage reservoir (each a ‘‘Facility’’ and together the ‘‘Facilities’’) located in central and western Montana along the Missouri, Flathead, Clark Fork and Madison Rivers and Rosebud Creek. The net aggregate generating capacity of the Facilities is 633 MWs, which includes the Kerr Project, a 194 MW hydroelectric generating facility that we expect to transfer to the Confederated Salish and Kootenai Tribes of the Flathead Reservation (CSKT) in September 2015. See further discussion below. Eight of the Facilities, along with the storage reservoir, are collectively licensed as the Missouri-Madison Project, by the FERC. Each of the remaining three Facilities is licensed by FERC as a separate project.
	With the addition of these generating assets and assuming ownership of the Kerr Project is transferred as discussed below, we own generation facilities that provide approximately 60% of our average electric load serving requirements in Montana. The following chart provides an overview of the facilities by name, net capacity in MWs, commercial operation date (COD), river source, FERC license expiration date and average capacity factor. We are the sole direct owner of each facility.
	Plant	COD	River	FERC	Net
			Source	License	Capacity
				Expiration	(MW) (1)
	Black Eagle	1927	Missouri	2040	21
	Cochrane	1958	Missouri	2040	69
	Hauser	1911	Missouri	2040	19
	Holter	1918	Missouri	2040	48
	Madison	1906	Madison	2040	8
	Morony	1930	Missouri	2040	48
	Mystic	1925	West Rosebud	2050	12
			Creek
	Rainbow	1910/2013	Missouri	2040	60
	Ryan	1915	Missouri	2040	60
	Thompson Falls	1915	Clark Fork	2025	94
	Subtotal				439
	Kerr	1938	Flathead	2035	194
	Total				633
	(1) Hebgen facility (0 MW net capacity) excluded from figures. These are run-of-river dams except for Kerr and Mystic, which are storage generation.
	The purchase price was allocated based on the estimated fair values of the assets acquired and liabilities assumed at the date of the acquisition as follows:
	Purchase Price Allocation	(in millions)
	Assets Acquired
	Inventory	$	0.2
	Property Plant and Equipment	899.6
	Other Prepayments	4.5
	Total Assets Acquired	$	904.3
	Liabilities Assumed
	Other Current Liabilities	$	0.4
	Other Deferred Credits	0.4
	Total Liabilities Assumed	$	0.8
	Total Purchase Price	$	903.5
	We expect to finalize the purchase price allocation, including analysis of environmental matters and potential removal obligations, during the first half of 2015. Pro forma adjustments to our revenues and earnings prior to the date of acquisition would not be meaningful. Prior to the acquisition, the Facilities were nonregulated with output sold to third parties. These Facilities are now part of our regulated fleet used to serve our customers.
	Regulatory Approvals - On September 26, 2014, the MPSC issued a final order (MPSC Order) approving the application, subject to certain conditions, including the following:
	•	Inclusion of $870 million of the $904 million purchase price for the hydro assets in our Montana jurisdictional rate base with a 50-year life;
	•	Return on equity of 9.8%, a cost of debt of 4.25%, and a capital structure of 52% debt and 48% equity, resulting in an associated first year annual retail revenue requirement of approximately $117 million;
	•	A final compliance filing in December 2015 to reflect post-closing adjustments, the conveyance of the Kerr Project as discussed below and the actual property tax expense for the Hydroelectric facilities; and
	•	Tracking of revenue credits on a portfolio basis through our electricity supply cost tracker.
	Financing - We financed the Hydro Transaction with a combination of $450 million of long-term debt, $400 million of equity and cash flows from operations. See Note 12 - Long-Term Debt and Capital Leases and Note 18 - Common Stock for further detail on these transactions.
	Kerr Project - The Hydro Transaction includes the Kerr Project, a 194 MW hydro-electric generating facility that we expect will be transferred to the Confederated Salish and Kootenai Tribes of the Flathead Reservation (CSKT) in September 2015, in accordance with its FERC license, which gives the CSKT the right to acquire the project between September 2015 and September 2025. The CSKT have formally provided notice of their intent to acquire the Kerr Project and designated September 5, 2015, as the date for conveyance to occur. PPL Montana and the CSKT previously conducted an arbitration over the conveyance price of the Kerr Project. In March 2014, an arbitration panel set an estimated conveyance price of approximately $18.3 million. Under our agreement with PPL Montana, the purchase price for the Hydro Transaction includes a $30 million reference price for the Kerr Project. If the CSKT complete the acquisition and pay $18.3 million for the Kerr Project, PPL Montana will pay the difference of $11.7 million to us. We expect to sell any excess generation from the Kerr Project in the market and provide revenue credits to our Montana retail customers until the CSKT exercises their right to acquire the Kerr Project. The MPSC Order provides that customers will have no financial risk related to our temporary ownership of the Kerr Project, with a compliance filing required upon completion of the transfer to CSKT.
	During the twelve months ended December 31, 2014, we incurred approximately $9.5 million of legal and professional fees associated with the Hydro Transaction, which are included in operating, general and administrative expense, and approximately $5.8 million of expenses related to the bridge credit facility included in interest expense.

Regulatory_Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2014
Regulated Operations [Abstract]
Regulatory Matters	Regulatory Matters
	Hydro Transaction
	See Note 3 - Hydro Transaction.
	South Dakota Electric Rate Filing
	In December 2014, we filed a request with the SDPUC for an annual increase to electric rates totaling approximately $26.5 million . Our request was based on a return on equity of 10%, a capital structure consisting of 46% debt and 54% equity and rate base of $447.4 million. The SDPUC has not yet issued a procedural schedule and we do not anticipate implementing new rates until at least July 2015.
	We have not filed an electric rate case in South Dakota since 1980. This filing requests recovery of capital expenditures related to improvements to our transmission and distribution delivery systems over time, the Aberdeen Generating Station, and additions (including estimated 2015 additions) to comply with additional emission reduction requirements at two of our jointly owned electric generating units that serve our South Dakota customers.
	Dave Gates Generating Station at Mill Creek (DGGS)
	FERC Filing - In April 2014, the FERC issued an order affirming a FERC Administrative Law Judge's (ALJ) initial decision in September 2012, regarding cost allocation at DGGS between retail and wholesale customers. This decision concluded we should allocate only a fraction of the costs we believe, based on facts and the law, should be allocated to FERC jurisdictional customers. We have been recognizing revenue consistent with the ALJ's initial decision. As of December 31, 2014, we have cumulative deferred revenue of approximately $27.3 million, which is subject to refund and recorded within current regulatory liabilities in the Consolidated Balance Sheets. The order included a requirement to issue customer refunds (included in deferred revenue) within 30 days.
	In May 2014, we filed a request for rehearing, which remains pending. In our request for rehearing, we have argued that no refunds are due even if the cost allocation method is modified prospectively. There is no deadline by which FERC must act on our rehearing petition, but it could occur during the first quarter of 2015. Customer refunds, if any, will not be due until 30 days after a FERC order on rehearing. If unsuccessful on rehearing, we may appeal to a United States Circuit Court of Appeals. The time line for any such appeal could, depending on when the FERC issues a rehearing order, extend into 2016 or beyond.
	The FERC order was assessed as a triggering event as to whether an impairment charge should be recorded with respect to DGGS. We continue to evaluate options to use DGGS in combination with other generation resources, including our newly acquired hydro facilities, to ensure cost recovery. Any alternative use of DGGS would be subject to regulatory approval and we cannot provide assurance of such approval. We do not believe an impairment loss is probable at this time; however, we will continue to evaluate recovery of this asset in the future as facts and circumstances change.
	Montana Electric Tracker Filings
	Each year we submit an electric tracker filing for recovery of supply costs for the 12-month period ended June 30 and for the projected supply costs for the next 12-month period. The MPSC reviews such filings and makes its cost recovery determination based on whether or not our electric supply procurement activities were prudent.
	In May 2014, we filed our annual electric supply tracker filing for the 2013/2014 tracker period. The MPSC approved this filing on an interim basis and consolidated it with our pending electric supply filing for the 2012/2013 tracker period. Our 2014 electric tracker filing includes market purchases made between July 2013 and January 2014 for replacement power during an outage at Colstrip Unit 4. Inclusion of these costs in the tracker filing is consistent with the treatment of replacement power during previous outages. During a June 2014 MPSC work session, approximately $11 million of these incremental market purchases related to the Colstrip Unit 4 outage were identified by the MPSC for additional prudency review. In July 2014, the Montana Environmental Information Center and Sierra Club filed a petition to intervene in the consolidated 2013 and 2014 tracker dockets to challenge our recovery of costs associated with Colstrip Unit 4, particularly the costs incurred as a result of the outage, as imprudent. A procedural schedule has not yet been established for the consolidated electric supply tracker docket.
	Montana Lost Revenue Adjustment Mechanism
	Demand-side management (DSM) lowers our sales to customers. In 2005, the MPSC created a Lost Revenue Adjustment Mechanism (LRAM) by which we collect revenue that we would have collected without any DSM. In an order issued in October 2013, which was related to our 2011 / 2012 electric supply tracker, the MPSC required us to lower our LRAM revenue recovery and imposed a new burden of proof on us for future LRAM recovery. We appealed the October 2013 order to Montana District Court. The appeal is pending. The District Court approved a partial settlement of our appeal, in which the MPSC agreed to remove from the October 2013 order the sentence that imposed the new burden and to initiate a separate docket to review lost revenue policy issues. The MPSC initiated the new proceeding regarding LRAM in June 2014, discovery is currently in process and a hearing is scheduled for May 2015.
	Based on the MPSC's October 2013 order, we have recognized $7.1 million of DSM lost revenues for each annual electric supply tracker period. However, since the 2012/2013 and 2013/2014 annual electric tracker filings are still subject to final approval, the MPSC may ultimately require us to refund a portion of the DSM lost revenues we have recognized since July 2012.
	Montana Natural Gas Tracker Filings
	Each year we submit a natural gas tracker filing for recovery of supply costs for the 12-month period ended June 30 and for the projected supply costs for the next 12-month period. The MPSC reviews such filings and makes its cost recovery determination based on whether or not our natural gas supply procurement activities were prudent.
	In May 2014, we filed our annual natural gas supply tracker filing for the 2013/2014 tracker period. During June 2014, the MPSC approved this filing on an interim basis and consolidated it with our pending natural gas filing for the 2012/2013 tracker period. During December 2014, we filed supplemental testimony to correct an allocation error in our initial tracker filing related to our owned production. The financial impact of this correction was not material for any period presented. Discovery is currently in process and a hearing is scheduled for May 2015.
	Natural Gas Production Assets
	In 2012 and 2013, we purchased natural gas production interests in northern Montana's Bear Paw Basin (Bear Paw). We are collecting the cost of service for natural gas produced from these assets, including a return on our investment, through our natural gas supply tracker on an interim basis. As a result, we do not expect to file an application with the MPSC to place these assets in natural gas rate base until our next natural gas rate case. We are recognizing Bear Paw related revenue based on the precedent established by the MPSC's approval of Battle Creek in the fourth quarter of 2012. Since acquisition, we have recognized approximately $29.3 million of revenue, a portion of which may be subject to refund.

Regulatory_Assets_and_Liabilit

Regulatory Assets and Liabilities	12 Months Ended
	Dec. 31, 2014
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets and Liabilities	Regulatory Assets and Liabilities
	We prepare our consolidated financial statements in accordance with the provisions of ASC 980, as discussed in Note 2 - Significant Accounting Policies. Pursuant to this guidance, certain expenses and credits, normally reflected in income as incurred, are deferred and recognized when included in rates and recovered from or refunded to the customers. Regulatory assets and liabilities are recorded based on management's assessment that it is probable that a cost will be recovered or that an obligation has been incurred. Accordingly, we have recorded the following major classifications of regulatory assets and liabilities that will be recognized in expenses and revenues in future periods when the matching revenues are collected or refunded. Of these regulatory assets and liabilities, energy supply costs are the only items earning a rate of return. The remaining regulatory items have corresponding assets and liabilities that will be paid for or refunded in future periods.
		Note Reference	Remaining Amortization Period	December 31,
				2014		2013
				(in thousands)
	Pension	16	Undetermined	$	139,050	$	58,474
	Employee related benefits	16	Undetermined	19,080		17,700
	Distribution infrastructure projects		3 Years	9,407		12,543
	Environmental clean-up	20	Various	13,741		14,924
	Supply costs		1 Year	29,200		17,875
	Income taxes	13	Plant Lives	263,764		201,808
	Deferred financing costs		Various	12,151		13,919
	State & local taxes & fees		Various	5,319		6,582
	Other	—	Various	11,419		10,846
	Total regulatory assets			$	503,131	$	354,671
	Removal cost	7	Various	$	351,676	$	336,613
	Gas storage sales		25 Years	10,410		10,831
	Supply costs		1 Year	14,569		11,493
	Deferred revenue	4	1 Year	36,592		33,400
	Environmental clean-up		Various	2,501		1,194
	State & local taxes & fees		1 Year	511		551
	Other		Various	2,138		377
	Total regulatory liabilities			$	418,397	$	394,459
	Pension and Employee Related Benefits
	We recognize the unfunded portion of plan benefit obligations in the Consolidated Balance Sheets, which is remeasured at each year end, with a corresponding adjustment to regulatory assets/liabilities as the costs associated with these plans are recovered in rates. The portion of the regulatory asset related to our Montana pension plan will amortize as cash funding amounts exceed accrual expense under GAAP. The SDPUC allows recovery of pension costs on an accrual basis. The MPSC allows recovery of postretirement benefit costs on an accrual basis. The MPSC allows recovery of other employee related benefits on a cash basis.
	Montana Distribution System Infrastructure Project (DSIP)
	We have an accounting order to defer certain incremental operating and maintenance expenses associated with DSIP. Pursuant to the order, we deferred expenses incurred during 2011 and 2012 as a regulatory asset associated with the phase-in portion of the DSIP. These costs are being amortized into expense over five years, which began in 2013.
	Supply Costs
	The MPSC, SDPUC and NPSC have authorized the use of electric and natural gas supply cost trackers that enable us to track actual supply costs and either recover the under collection or refund the over collection to our customers. Accordingly, we have recorded a regulatory asset and liability to reflect the future recovery of under collections and refunding of over collections through the ratemaking process. We earn interest on electric and natural gas supply costs under collected, or apply interest in an over collection, of 7.5%, in Montana; 10.6% and 7.8%, respectively, in South Dakota; and 8.5% for natural gas in Nebraska.
	Deferred Revenue
	We have deferred revenue associated with DGGS and DSM, which may be subject to refund as we have open regulatory proceedings. See Note 4 - Regulatory Matters, for further information regarding these items.
	Environmental clean-up
	Environmental clean-up costs are the estimated costs of investigating and cleaning up contaminated sites we own. We discuss the specific sites and clean-up requirements further in Note 20 - Commitments and Contingencies. Environmental clean-up costs are typically recoverable in customer rates when they are actually incurred. We record changes in the regulatory asset consistent with changes in our environmental liabilities. When cost projections become known and measurable, we coordinate with the appropriate regulatory authority to determine a recovery period.
	Income Taxes
	Tax assets primarily reflect the effects of plant related temporary differences such as flow-through of depreciation, repairs related deductions, removal costs, capitalized interest and contributions in aid of construction that we will recover or refund in future rates. We amortize these amounts as temporary differences reverse.
	Deferred Financing Costs
	Consistent with our historical regulatory treatment, a regulatory asset has been established to reflect the remaining deferred financing costs on long-term debt that has been replaced through the issuance of new debt. These amounts are amortized over the life of the new debt.
	State & Local Taxes & Fees (Montana Property Tax Tracker)
	The MPSC has authorized recovery in the property tax tracker of approximately 60% of the estimated increase as compared with the related amount included in rates during our last rate case.
	Removal Cost
	The anticipated costs of removing assets upon retirement are provided for over the life of those assets as a component of depreciation expense. Our depreciation method, including cost of removal, is established by the respective regulatory commissions. Therefore, consistent with this regulated treatment, we reflect this accrual of removal costs for our regulated assets by increasing our regulatory liability. See Note 7 - Asset Retirement Obligations, for further information regarding this item.
	Gas Storage Sales
	A regulatory liability was established in 2000 and 2001 based on gains on cushion gas sales in Montana. This gain is being flowed to customers over a period that matches the depreciable life of surface facilities that were added to maintain deliverability from the field after the withdrawal of the gas. This regulatory liability is a reduction of rate base.

Property_Plant_and_Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment
	The following table presents the major classifications of our property, plant and equipment (in thousands):
		Estimated Useful Life		December 31,
				2014				2013
		(years)		(in thousands)
	Land, land rights and easements	54 – 96		$	130,816			$	128,123
	Building and improvements	27 – 64		168,041				163,852
	Transmission, distribution, and storage	15 – 85		2,579,861				2,448,821
	Generation	25 – 50		1,044,764				533,450
	Plant acquisition adjustment	34 – 50		654,835				204,754
	Other	2 – 45		326,211				308,345
	Construction work in process	–—		221,868				104,891
				5,126,396				3,892,236
	Less accumulated depreciation			(1,368,388		)		(1,202,108		)
				$	3,758,008			$	2,690,128
	In 2014, we acquired hydro generating assets which resulted in an increase of approximately $870 million in property, plant and equipment. We recorded the plant assets at original cost, less accumulated depreciation with an acquisition adjustment in accordance with FERC rules. The plant acquisition adjustment balance above also includes an amount related to the inclusion of our interest in Colstrip Unit 4 in rate base in 2009. The acquisition adjustment is being amortized on a straight-line basis over the estimated remaining useful life in depreciation expense. Plant and equipment under capital lease were $23.4 million and $25.6 million as of December 31, 2014 and 2013, respectively, which included $23.1 million and $25.1 million as of December 31, 2014 and 2013, respectively, related to a long-term power supply contract with the owners of a natural gas fired peaking plant, which has been accounted for as a capital lease.
	Jointly Owned Electric Generating Plant
	We have an ownership interest in four base-load electric generating plants, all of which are coal fired and operated by other companies. We have an undivided interest in these facilities and are responsible for our proportionate share of the capital and operating costs while being entitled to our proportionate share of the power generated. Our interest in each plant is reflected in the Consolidated Balance Sheets on a pro rata basis and our share of operating expenses is reflected in the Consolidated Statements of Income. The participants each finance their own investment.
	Information relating to our ownership interest in these facilities is as follows (in thousands):
		Big Stone				Neal #4				Coyote			Colstrip Unit 4 (MT)
		(SD)				(IA)				(ND)
	December 31, 2014
	Ownership percentages	23.4		%		8.7		%		10		%	30		%
	Plant in service	$	61,628			$	59,579			$	46,045		$	292,806
	Accumulated depreciation	46,741				27,742				36,649			72,976
	December 31, 2013
	Ownership percentages	23.4		%		8.7		%		10		%	30		%
	Plant in service	$	61,186			$	57,633			$	46,003		$	290,163
	Accumulated depreciation	45,792				29,841				36,076			70,072

Asset_Retirement_Obligation

Asset Retirement Obligation	12 Months Ended
	Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligation	Asset Retirement Obligations
	We are obligated to dispose of certain long-lived assets upon their abandonment. We recognize a liability for the legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event. We measure the liability at fair value when incurred and capitalize a corresponding amount as part of the book value of the related assets, which increases our property, plant and equipment and other noncurrent liabilities. The increase in the capitalized cost is included in determining depreciation expense over the estimated useful life of these assets. Since the fair value of the asset retirement obligation (ARO) is determined using a present value approach, accretion of the liability due to the passage of time is recognized each period and recorded as a regulatory asset until the settlement of the liability. Revisions to estimated ARO can result from changes in retirement cost estimates, revisions to estimated inflation rates, and changes in the estimated timing of abandonment. If the obligation is settled for an amount other than the carrying amount of the liability, we will recognize a gain or loss on settlement.
	Our AROs relate to the reclamation and removal costs at our jointly-owned coal-fired generation facilities, Department of Transportation requirements to cut, purge and cap retired natural gas pipeline segments, and our obligation to plug and abandon oil and gas wells at the end of their life. The following table presents the change in our gross conditional ARO (in thousands):
		December 31,
		2014			2013
	Liability at January 1,	$	20,886		$	9,283
	Accretion expense	1,073			745
	Liabilities incurred	552			8,829
	Liabilities settled	(85		)	(27		)
	Revisions to cash flows	(991		)	2,056
	Liability at December 31,	$	21,435		$	20,886
	In addition, we have identified removal liabilities related to our electric and natural gas transmission and distribution assets that have been installed on easements over property not owned by us. The easements are generally perpetual and only require remediation action upon abandonment or cessation of use of the property for the specified purpose. The ARO liability is not estimable for such easements as we intend to utilize these properties indefinitely. In the event we decide to abandon or cease the use of a particular easement, an ARO liability would be recorded at that time. We also identified AROs associated with our Hydro Transaction; however, due to the indeterminate removal date, the fair value of the associated liabilities currently cannot be estimated and no amounts are recognized in the consolidated financial statements
	We collect removal costs in rates for certain transmission and distribution assets that do not have associated AROs. Generally, the accrual of future non-ARO removal obligations is not required; however, long-standing ratemaking practices approved by applicable state and federal regulatory commissions have allowed provisions for such costs in historical depreciation rates. The recorded amounts of estimated future removal costs are considered regulatory liabilities and do not represent legal retirement obligations. See Note 5 - Regulatory Assets and Liabilities for removal costs recorded as regulatory liabilities on the consolidated balance sheets as of December 31, 2014 and 2013.

Goodwill

Goodwill	12 Months Ended
	Dec. 31, 2014
Goodwill [Abstract]
Goodwill	Goodwill
	We completed our annual goodwill impairment test as of April 1, 2014 and no impairment was identified. We calculate the fair value of our reporting units by considering various factors, including valuation studies based primarily on a discounted cash flow analysis, with published industry valuations and market data as supporting information. Key assumptions in the determination of fair value include the use of an appropriate discount rate and estimated future cash flows. In estimating cash flows, we incorporate expected long-term growth rates in our service territory, regulatory stability, and commodity prices (where appropriate), as well as other factors that affect our revenue, expense and capital expenditure projections.
	There were no changes in our goodwill during the year ended December 31, 2014. Goodwill by segment is as follows (in thousands):
		December 31,
		2014		2013
	Electric	$	241,100	$	241,100
	Natural gas	114,028		114,028
		$	355,128	$	355,128

Risk_Management_and_Hedging_Ac

Risk Management and Hedging Activities	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Risk Management and Hedging Activities	Risk Management and Hedging Activities
	Nature of Our Business and Associated Risks
	We are exposed to certain risks related to the ongoing operations of our business, including the impact of market fluctuations in the price of electricity and natural gas commodities and changes in interest rates. We rely on market purchases to fulfill a portion of our electric and natural gas supply requirements within the Montana market. Several factors influence price levels and volatility. These factors include, but are not limited to, seasonal changes in demand, weather conditions, available generating assets within regions, transportation availability and reliability within and between regions, fuel availability, market liquidity, and the nature and extent of current and potential federal and state regulations.
	Objectives and Strategies for Using Derivatives
	To manage our exposure to fluctuations in commodity prices we routinely enter into derivative contracts, such as fixed-price forward purchase and sales contracts. The objective of these transactions is to fix the price for a portion of anticipated energy purchases to supply our customers. These types of contracts are included in our electric and natural gas supply portfolios and are used to manage price volatility risk by taking advantage of fluctuations in market prices. While individual contracts may be above or below market value, the overall portfolio approach is intended to provide greater price stability for consumers. These commodity costs are included in our cost tracking mechanisms and are recoverable from customers subject to prudence reviews by the applicable state regulatory commissions. We do not maintain a trading portfolio, and our derivative transactions are only used for risk management purposes consistent with regulatory guidelines.
	In addition, we may use interest rate swaps to manage our interest rate exposures associated with new debt issuances or to manage our exposure to fluctuations in interest rates on variable rate debt.
	Accounting for Derivative Instruments
	We evaluate new and existing transactions and agreements to determine whether they are derivatives. The permitted accounting treatments include: normal purchase normal sale; cash flow hedge; fair value hedge; and mark-to-market. Mark-to-market accounting is the default accounting treatment for all derivatives unless they qualify, and we specifically designate them, for one of the other accounting treatments. Derivatives designated for any of the elective accounting treatments must meet specific, restrictive criteria both at the time of designation and on an ongoing basis. The changes in the fair value of recognized derivatives are recorded each period in current earnings or other comprehensive income, depending on whether a derivative is designated as part of a hedge transaction and the type of hedge transaction.
	Normal Purchases and Normal Sales
	We have applied the NPNS exception to our contracts involving the physical purchase and sale of gas and electricity at fixed prices in future periods. During our normal course of business, we enter into full-requirement energy contracts, power purchase agreements and physical capacity contracts, which qualify for NPNS. All of these contracts are accounted for using the accrual method of accounting; therefore, there were no amounts recorded in the Consolidated Financial Statements at December 31, 2014 and 2013. Revenues and expenses from these contracts are reported on a gross basis in the appropriate revenue and expense categories as the commodities are received or delivered.
	Credit Risk
	Credit risk is the potential loss resulting from counterparty non-performance under an agreement. We manage credit risk with policies and procedures for, among other things, counterparty analysis and exposure measurement, monitoring and mitigation. We limit credit risk in our commodity and interest rate derivative activities by assessing the creditworthiness of potential counterparties before entering into transactions and continuing to evaluate their creditworthiness on an ongoing basis.
	We are exposed to credit risk through buying and selling electricity and natural gas to serve customers. We may request collateral or other security from our counterparties based on the assessment of creditworthiness and expected credit exposure. It is possible that volatility in commodity prices could cause us to have material credit risk exposures with one or more counterparties. We enter into commodity master enabling agreements with our counterparties to mitigate credit exposure, as these agreements reduce the risk of default by allowing us or our counterparty the ability to make net payments. The agreements generally are: (1) Western Systems Power Pool agreements - standardized power purchase and sales contracts in the electric industry; (2) International Swaps and Derivatives Association agreements - standardized financial gas and electric contracts; (3) North American Energy Standards Board agreements - standardized physical gas contracts; and (4) Edison Electric Institute Master Purchase and Sale Agreements - standardized power sales contracts in the electric industry.
	Many of our forward purchase contracts contain provisions that require us to maintain an investment grade credit rating from each of the major credit rating agencies. If our credit rating were to fall below investment grade, the counterparties could require immediate payment or demand immediate and ongoing full overnight collateralization on contracts in net liability positions.
	Interest Rate Swaps Designated as Cash Flow Hedges
	In September 2014, we entered into two forward starting swaps of $225 million each at 3.217% and 3.227% to hedge the risk of changes in the interest payments attributable to changes in the benchmark interest rate during the period from the effective date of the swap to the anticipated date of the debt issuance of $450 million associated with the Hydro Transaction. These forward starting interest rate swaps were designated as cash flow hedges at the time the agreements were executed. In November 2014, the interest rate swap agreements were terminated and the settlement resulted in a $18.4 million loss recorded as a component of AOCI.
	Amounts are reclassified from AOCI into interest expense during the periods in which the hedged interest payments occur. The following table shows the effect of the interest rate swaps terminated in November 2014 and interest rate swaps previously terminated on the Consolidated Financial Statements (in thousands):
	Cash Flow Hedges	Location of Amount Reclassified from AOCI to Income	Amount Reclassified from AOCI into Income during the Year Ended
			31-Dec-14
	Interest rate contracts	Interest Expense	$	1,111
	A net loss of approximately $13.8 million is remaining in AOCI as of December 31, 2014, and we expect to reclassify approximately $0.6 million of net pre-tax gains from AOCI into interest expense during the next twelve months. These amounts relate to terminated swaps, and we have no interest rate swaps outstanding.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements
	Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (i.e., an exit price). Measuring fair value requires the use of market data or assumptions that market participants would use in pricing the asset or liability, including assumptions about risk and the risks inherent in the inputs to the valuation technique. These inputs can be readily observable, corroborated by market data, or generally unobservable. Valuation techniques are required to maximize the use of observable inputs and minimize the use of unobservable inputs.
	Applicable accounting guidance establishes a hierarchy that prioritizes the inputs used to measure fair value, and requires fair value measurements to be categorized based on the observability of those inputs. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 inputs) and the lowest priority to unobservable inputs (Level 3 inputs). The three levels of the fair value hierarchy are as follows:
	•	Level 1 – Unadjusted quoted prices available in active markets at the measurement date for identical assets or liabilities;
	•	Level 2 – Pricing inputs, other than quoted prices included within Level 1, which are either directly or indirectly observable as of the reporting date; and
	•	Level 3 – Significant inputs that are generally not observable from market activity.
	We classify assets and liabilities within the fair value hierarchy based on the lowest level of input that is significant to the fair value measurement of each individual asset and liability taken as a whole. The table below sets forth by level within the fair value hierarchy the gross components of our assets and liabilities measured at fair value on a recurring basis. Normal purchases and sales transactions are not included in the fair values by source table as they are not recorded at fair value. See Note 9 - Risk Management and Hedging Activities for further discussion.
	We record transfers between levels of the fair value hierarchy, if necessary, at the end of the reporting period. There were no transfers between levels for the periods presented.
	December 31, 2014		Quoted Prices in			Significant Other			Significant			Margin Cash Collateral		Total Net Fair Value
			Active Markets for			Observable Inputs			Unobservable Inputs			Offset
			Identical Assets or			(Level 2)			(Level 3)
			Liabilities (Level 1)
			(in thousands)
	Restricted cash		$	13,140		$	—		$	—		$	—	$	13,140
	Rabbi trust investments		21,594			—			—			—		21,594
	Total		$	34,734		$	—		$	—		$	—	$	34,734
	December 31, 2013
	Restricted cash		$	6,650		$	—		$	—		$	—	$	6,650
	Rabbi trust investments		16,477			—			—			—		16,477
	Total		$	23,127		$	—		$	—		$	—	$	23,127
	Restricted cash represents amounts held in money market mutual funds. Rabbi trust assets represent assets held for non-qualified deferred compensation plans, which consist of our common stock and actively traded mutual funds with quoted prices in active markets.
	Financial Instruments
	The estimated fair value of financial instruments is summarized as follows (in thousands):
		December 31, 2014						December 31, 2013
		Carrying Amount			Fair Value			Carrying Amount			Fair Value
	Liabilities:
	Long-term debt	$	1,662,099		$	1,817,642		$	1,155,097		$	1,237,151
	Short-term borrowings consist of commercial paper and are not included in the table above as carrying value approximates fair value. The estimated fair value amounts have been determined using available market information and appropriate valuation methodologies; however, considerable judgment is required in interpreting market data to develop estimates of fair value. Accordingly, the estimates presented herein are not necessarily indicative of the amounts that we would realize in a current market exchange.
	We determined fair value for long-term debt based on interest rates that are currently available to us for issuance of debt with similar terms and remaining maturities, except for publicly traded debt, for which fair value is based on market prices for the same or similar issues or upon the quoted market prices of U.S. treasury issues having a similar term to maturity, adjusted for our bond issuance rating and the present value of future cash flows. These are significant other observable inputs, or level 2 inputs, in the fair value hierarchy.

ShortTerm_Borrowings_and_Credi

Short-Term Borrowings and Credit Arrangements	12 Months Ended
	Dec. 31, 2014
Short-term Debt [Abstract]
Short-Term Debt	Short-Term Borrowings and Credit Arrangements
	Short-Term Borrowings
	Short-term borrowings and the corresponding weighted average interest rates as of December 31 were as follows (dollars in millions, except for percentages):
			2014					2013
	Short-Term Debt		Balance			Interest Rate		Balance		Interest Rate
	Commercial Paper		$	267.8		0.5	%	$	141	0.41	%
	The following information relates to commercial paper for the years ended December 31 (dollars in millions):
		2014			2013
	Maximum short-term debt outstanding	$	276.9		$	199.9
	Average short-term debt outstanding	$	132.5		$	69
	Weighted-average interest rate	0.39		%	0.4		%
	In the fourth quarter of 2014, we increased the size of our commercial paper program from $250 million to $340 million. Under the program we may issue unsecured commercial paper notes on a private placement basis to provide an additional financing source for our short-term liquidity needs. The maturities of the commercial paper issuances will vary, but may not exceed 270 days from the date of issue. Commercial paper issuances are supported by available capacity under our unsecured revolving credit facility.
	Unsecured Revolving Line of Credit
	In the fourth quarter of 2014, we exercised the accordion feature under our $300 million unsecured revolving credit facility to increase the size to $350 million. The facility does not amortize and is scheduled to expire on November 5, 2018. The facility bears interest at the Eurodollar rate plus a credit spread, ranging from 0.88% to 1.75%, or a base rate, plus a margin of 0.0% to 0.75%. A total of eight banks participate in the facility, with no one bank providing more than 21% of the total availability. There were no direct borrowings or letters of credit outstanding as of December 31, 2014. Commitment fees for the unsecured revolving line of credit were $0.4 million and $0.5 million for the years ended December 31, 2014 and 2013, respectively.
	The credit facility includes covenants that require us to meet certain financial tests, including a maximum debt to capitalization ratio not to exceed 65%. The facility also contains covenants which, among other things, limit our ability to engage in any consolidation or merger or otherwise liquidate or dissolve, dispose of property, and enter into transactions with affiliates. A default on the South Dakota or Montana First Mortgage Bonds would trigger a cross default on the credit facility; however a default on the credit facility would not trigger a default on any other obligations.
	Bridge Facility
	In November 2013, in connection with the Hydro Transaction, we entered into a $900 million 364-day senior bridge credit facility. The bridge facility was not drawn upon and cancelled in November 2014.

LongTerm_Debt_and_Capital_Leas

Long-Term Debt and Capital Leases	12 Months Ended
	Dec. 31, 2014
Long-term Debt and Capital Lease Obligations [Abstract]
Long-term Debt And Capital Leases	Long-Term Debt and Capital Leases
	Long-term debt and capital leases consisted of the following (in thousands):
			December 31,
		Due	2014			2013
	Unsecured Debt:
	Unsecured Revolving Line of Credit	2018	$	—		$	—
	Secured Debt:
	Mortgage bonds—
	South Dakota—6.05%	2018	55,000			55,000
	South Dakota—5.01%	2025	64,000			64,000
	South Dakota—4.15%	2042	30,000			30,000
	South Dakota—4.30%	2052	20,000			20,000
	South Dakota—4.85%	2043	50,000			50,000
	South Dakota—4.22%	2044	30,000			—
	Montana—6.04%	2016	150,000			150,000
	Montana—6.34%	2019	250,000			250,000
	Montana—5.71%	2039	55,000			55,000
	Montana—5.01%	2025	161,000			161,000
	Montana—4.15%	2042	60,000			60,000
	Montana—4.30%	2052	40,000			40,000
	Montana—4.85%	2043	15,000			15,000
	Montana—3.99%	2028	35,000			35,000
	Montana—4.176%	2044	450,000			—
	Pollution control obligations—
	Montana—4.65%	2023	170,205			170,205
	Other Long Term Debt:
	New Market Tax Credit Financing—1.146%	2046	26,977			—
	Discount on Notes and Bonds	—	(83		)	(108		)
			1,662,099			1,155,097
	Less current maturities		—			—
			$	1,662,099		$	1,155,097
	Capital Leases:
	Total Capital Leases	Various	$	29,892		$	31,557
	Less current maturities		(1,730		)	(1,662		)
			$	28,162		$	29,895
	Secured Debt
	First Mortgage Bonds and Pollution Control Obligations
	The South Dakota Mortgage Bonds are a series of general obligation bonds issued under our South Dakota indenture. All of such bonds are secured by substantially all of our South Dakota and Nebraska electric and natural gas assets.
	The Montana First Mortgage Bonds and Montana Pollution Control Obligations are secured by substantially all of our Montana electric and natural gas assets.
	In December 2014, we issued $30 million aggregate principal amount of South Dakota First Mortgage Bonds at a fixed interest rate of 4.22% maturing in 2044. The bonds are secured by our electric and natural gas assets in South Dakota and were issued in a transaction exempt from the registration requirements of the Securities Act of 1933, as amended. Proceeds were used to fund a portion of our investment growth opportunities.
	Hydro Transaction Issuance - In November 2014, we issued $450 million aggregate principal amount of Montana First Mortgage Bonds at a fixed interest rate of 4.176% maturing in 2044 as a portion of the permanent financing of the Hydro Transaction. The bonds are secured by our electric and natural gas assets in Montana.
	As of December 31, 2014, we are in compliance with our financial debt covenants.
	Other Long-Term Debt
	During 2014 we entered into a New Market Tax Credit (NMTC) financing agreement, pursuant to Section 45D of the Internal Revenue Code of 1986 as amended, to take advantage of a tax credit program related to the development and construction of a new office building in Butte, Montana. This financing agreement was structured with unrelated third party financial institutions (the Investor) and their wholly-owned community development entities (CDEs) in connection with our participation in qualified transactions under the NMTC program. Upon closing of this transaction, we entered into two loans totaling $27.0 million payable to the CDEs sponsoring the project, and provided an $18.2 million investment. The loans have a term of thirty years with an interest rate of approximately 1.146%. In exchange for substantially all of the benefits derived from the tax credits, the Investor contributed approximately $8.8 million to the project. The NMTC is subject to recapture for a period of seven years. If the expected tax benefits are delivered without risk of recapture to the Investor and our performance obligation is relieved, we expect $7.9 million of the loan to be forgiven in July 2021. If we do not meet the conditions for loan forgiveness, we would be required to repay $27.0 million and would concurrently receive the return of our $18.2 million investment. As we are the primary beneficiary of the entities created in relation to the NMTC transaction, they have been consolidated as variable interest entities. The loans of $27.0 million are recorded in long-term debt and the investment of $18.2 million is recorded in other noncurrent assets in the Consolidated Balance Sheets.
	Maturities of Long-Term Debt
	The aggregate minimum principal maturities of long-term debt and capital leases, during the next five years are $1.7 million in 2015, $151.8 million in 2016, $2.0 million in 2017, $57.1 million in 2018 and $252.3 million in 2019.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	Income tax (benefit) expense is comprised of the following (in thousands):
		Year Ended December 31,
		2014				2013				2012
	Federal
	Current	$	(405	)		$	108			$	5,358
	Deferred	(5,658		)		18,150				13,197
	Investment tax credits	(273		)		(335		)		(376		)
	State
	Current	18				83				(1,411		)
	Deferred	(3,954		)		(3,705		)		1,321
		$	(10,272	)		$	14,301			$	18,089
	The following table reconciles our effective income tax rate to the federal statutory rate:
		Year Ended December 31,
		2014			2013			2012
	Federal statutory rate	35	%		35	%		35	%
	State income, net of federal provisions	(1.8	)		(2.8	)		0.9
	Flow-through repairs deductions	(22.9	)		(16.4	)		(14.0	)
	Recognition of unrecognized tax benefit	(11.4	)		—			—
	Prior year permanent return to accrual adjustments	(4.7	)		0.5			(1.6	)
	Production tax credits	(2.8	)		(2.9	)		—
	Plant and depreciation of flow through items	0.1			(0.5	)		(1.1	)
	Recognition of state net operating loss benefit / valuation allowance release	—			—			(2.1	)
	Other, net	(0.8	)		0.3			(1.6	)
		(9.3	)%		13.2	%		15.5	%
	The following table summarizes the significant differences in income tax (benefit) expense based on the differences between our effective tax rate and the federal statutory rate (in thousands):
		Year Ended December 31,
		2014				2013				2012
	Income Before Income Taxes	$	110,414			$	108,284			$	116,495
	Income tax calculated at 35% federal statutory rate	38,645				37,899				40,774
	Permanent or flow through adjustments:
	State income, net of federal provisions	(1,969		)		(3,082		)		1,078
	Flow-through repairs deductions	(25,268		)		(17,763		)		(16,350		)
	Recognition of unrecognized tax benefit	(12,607		)		—				—
	Prior year permanent return to accrual adjustments	(5,172		)		541				(1,901		)
	Production tax credits	(3,136		)		(3,171		)		—
	Plant and depreciation of flow through items	74				(584		)		(1,281		)
	Recognition of state net operating loss benefit / valuation allowance release	—				—				(2,398		)
	Other, net	(839		)		461				(1,833		)
		$	(48,917	)		$	(23,598	)		$	(22,685	)
	Income tax (benefit) expense	$	(10,272	)		$	14,301			$	18,089
	Our effective tax rate typically differs from the federal statutory tax rate of 35% primarily due to the regulatory impact of flowing through federal and state tax benefits of repairs deductions, state tax benefit of bonus depreciation deductions and production tax credits. The regulatory accounting treatment of these deductions requires immediate income recognition for temporary tax differences of this type, which is referred to as the flow-through method. When the flow-through method of accounting for temporary differences is reflected in regulated revenues, we record deferred income taxes and establish related regulatory assets and liabilities.
	The income tax benefit for 2014 reflects the release of approximately $12.6 million of unrecognized tax benefits, including approximately $0.4 million of accrued interest and penalties due to the lapse of statutes of limitation in the third quarter of 2014.
	In September 2013, the IRS issued final tangible property regulations, which included guidance on a safe harbor method for determining the tax treatment of repair costs related to electric transmission and distribution property. The regulations were effective January 1, 2014. During the third quarter of 2014, we elected the safe harbor method and recorded an income tax benefit of approximately $4.3 million for the cumulative adjustment for years prior to 2014, which is included in the prior year permanent return to accrual adjustment in the table above.
	Deferred income taxes relate primarily to the difference between book and tax methods of depreciating property, amortizing tax-deductible goodwill, the difference in the recognition of revenues and expenses for book and tax purposes, certain natural gas and electric costs which are deferred for book purposes but expensed currently for tax purposes, and NOL carry forwards. We have elected under Internal Revenue Code 46(f)(2) to defer investment tax credit benefits and amortize them against expense and customer billing rates over the book life of the underlying plant.
	The components of the net deferred income tax liability recognized in our Consolidated Balance Sheets are related to the following temporary differences (in thousands):
		December 31,
		2014				2013
	Pension / postretirement benefits	$	51,817			$	20,522
	NOL carryforward	42,787				16,758
	Unbilled revenue	19,863				18,136
	Compensation accruals	17,315				10,409
	Customer advances	11,817				10,781
	AMT credit carryforward	10,357				10,357
	Environmental liability	8,968				9,026
	Production tax credit	6,452				3,171
	Interest rate hedges	6,251				—
	QF obligations	2,162				2,066
	Reserves and accruals	1,772				12,097
	Property taxes	881				796
	Regulatory liabilities	975				659
	Regulatory assets	—				7,248
	Other, net	4,415				2,827
	Deferred Tax Asset	185,832				124,853
Excess tax depreciation	(349,428		)		(304,071		)
Goodwill amortization	(137,090		)		(122,798		)
Flow through depreciation	(103,677		)		(79,016		)
Regulatory assets	(21,394		)		—
Deferred Tax Liability	(611,589		)		(505,885		)
Deferred Tax Liability, net	$	(425,757	)		$	(381,032	)
At December 31, 2014 we estimate our total federal NOL carryforward to be approximately $351 million prior to consideration of unrecognized tax benefits. If unused, our federal NOL carryforwards will expire as follows: $16.3 million in 2025; $95.5 million in 2028; $23.8 million in 2029; $127.5 million in 2031; $13.3 million in 2033 and $74.9 million in 2034. We estimate our state NOL carryforward as of December 31, 2014 is approximately $264.0 million. If unused, our state NOL carryforwards will expire as follows: $74.0 million in 2015; $18.6 million in 2016; $101.2 million in 2018; $10.5 million in 2020 and $59.7 million in 2021. We believe it is more likely than not that sufficient taxable income will be generated to utilize these NOL carryforwards.
Uncertain Tax Positions
We recognize tax positions that meet the more-likely-than-not threshold as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. The change in unrecognized tax benefits is as follows (in thousands):
	2014				2013				2012
Unrecognized Tax Benefits at January 1	$	113,466			$	113,291			$	131,949
Gross increases - tax positions in prior period	—				—				—
Gross decreases - tax positions in prior period	—				—				(1,766		)
Gross increases - tax positions in current period	909				518				2,391
Gross decreases - tax positions in current period	(5,597		)		(343		)		(19,283		)
Lapse of statute of limitations	(12,849		)		—				—
Unrecognized Tax Benefits at December 31	$	95,929			$	113,466			$	113,291
Our unrecognized tax benefits include approximately $62.4 million and $79.0 million related to tax positions as of December 31, 2014 and 2013, respectively, that if recognized, would impact our annual effective tax rate. We do not anticipate that total unrecognized tax benefits will significantly change due to the settlement of audits or the expiration of statutes of limitation within the next twelve months.
Our policy is to recognize interest and penalties related to uncertain tax positions in income tax expense. As discussed above, during the year ended December 31, 2014, we released approximately $0.4 million of accrued interest in the Consolidated Statements of Income. As of December 31, 2014, we do not have any amounts accrued in the Consolidated Balance Sheets. During the year ended December 31, 2013, we recognized approximately $0.4 million of interest in the Consolidated Statements of Income. As of December 31, 2013, we had $0.4 million of interest accrued in the Consolidated Balance Sheets.
Our federal tax returns from 2000 forward remain subject to examination by the IRS.

Comprehensive_Income_Loss

Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Comprehensive Income (Loss) Note [Text Block]	Other Comprehensive Income (Loss)
	The following tables display the components of Other Comprehensive Income (Loss), after-tax, and the related tax effects (in thousands):
		December 31,
		2014												2013										2012
		Before-Tax Amount				Tax Benefit				Net-of-Tax Amount				Before-Tax Amount				Tax Benefit			Net-of-Tax Amount			Before-Tax Amount			Tax Benefit		Net-of-Tax Amount
	Foreign currency translation adjustment	$	265			$	—			$	265			$	166			—			$	166		$	(54	)	$	—	$	(54	)
	Reclassification of net gains on derivative instruments	(1,110		)		426				(684		)		(1,188		)		458			(730		)	(1,188		)	456		(732		)
	Realized loss on cash flow hedging derivatives	(18,388		)		7,243				(11,145		)		—				—			—			—			—		—
	Pension and postretirement medical liability adjustment	134				(52		)		82				1,568				(605		)	963			(897		)	344		(553		)
	Other comprehensive income (loss)	$	(19,099	)		$	7,617			$	(11,482	)		$	546			$	(147	)	$	399		$	(2,139	)	$	800	$	(1,339	)
	Balances by classification included within AOCI on the Consolidated Balance Sheets are as follows, net of tax (in thousands):
		December 31, 2014				December 31, 2013
	Foreign currency translation	$	797			$	532
	Derivative instruments designated as cash flow hedges	(8,316		)		3,513
	Pension and postretirement medical plans	(1,247		)		(1,329		)
	Accumulated other comprehensive income	(8,766		)		2,716
	The following table displays the changes in AOCI by component, net of tax (in thousands):
				31-Dec-14
				Twelve Months Ended
		Affected Line Item in the Consolidated Statements of Income		Interest Rate Derivative Instruments Designated as Cash Flow Hedges				Pension and Postretirement Medical Plans				Foreign Currency Translation				Total
	Beginning balance			$	3,513			$	(1,329	)		$	532			$	2,716
	Other comprehensive (loss) income before reclassifications			(11,145		)		—				265				$	(10,880	)
	Amounts reclassified from accumulated other comprehensive income	Interest Expense		(684		)		—				—				$	(684	)
	Amounts reclassified from accumulated other comprehensive income			—				82				—				$	82
	Net current-period other comprehensive (loss) income			(11,829		)		82				265				(11,482		)
	Ending balance			$	(8,316	)		$	(1,247	)		$	797			$	(8,766	)
				31-Dec-13
				Twelve Months Ended
		Affected Line Item in the Consolidated Statements of Income		Interest Rate Derivative Instruments Designated as Cash Flow Hedges				Pension and Postretirement Medical Plans				Foreign Currency Translation				Total
	Beginning balance			$	4,243			$	(2,292	)		$	366			$	2,317
	Other comprehensive income before reclassifications			—				—				166				$	166
	Amounts reclassified from accumulated other comprehensive income	Interest Expense		(730		)		—				—				$	(730	)
	Amounts reclassified from accumulated other comprehensive income			—				963				—				$	963
	Net current-period other comprehensive (loss) income			(730		)		963				166				399
	Ending balance			$	3,513			$	(1,329	)		$	532			$	2,716

Operating_Leases

Operating Leases	12 Months Ended
	Dec. 31, 2014
Leases [Abstract]
Operating Leases	Operating Leases
	We lease vehicles, office equipment and facilities under various long-term operating leases. At December 31, 2014 future minimum lease payments for the next five years under non-cancelable lease agreements are as follows (in thousands):
	2015	1,996
	2016	1,484
	2017	671
	2018	70
	2019	61
	Lease and rental expense incurred was $2.2 million, $2.0 million and $2.2 million for the years ended December 31, 2014, 2013 and 2012, respectively.

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Employee Benefit Plans	Employee Benefit Plans
	Pension and Other Postretirement Benefit Plans
	We sponsor and/or contribute to pension and postretirement health care and life insurance benefit plans for eligible employees, which includes two cash balance pension plans. The plan for our South Dakota and Nebraska employees is referred to as the NorthWestern Corporation pension plan, and the plan for our Montana employees is referred to as the NorthWestern Energy pension plan. We utilize a number of accounting mechanisms that reduce the volatility of reported pension costs. Differences between actuarial assumptions and actual plan results are deferred and are recognized into earnings only when the accumulated differences exceed 10% of the greater of the projected benefit obligation or the market-related value of plan assets. If necessary, the excess is amortized over the average remaining service period of active employees. The Plan’s funded status is recognized as an asset or liability in our financial statements. See Note 5 - Regulatory Assets and Liabilities, for further discussion on how these costs are recovered through rates charged to our customers.
	Benefit Obligation and Funded Status
	Following is a reconciliation of the changes in plan benefit obligations and fair value of plan assets, and a statement of the funded status (in thousands):
		Pension Benefits								Other Postretirement Benefits
		December 31,								December 31,
		2014				2013				2014				2013
	Change in Benefit Obligation:
	Obligation at beginning of period	$	567,866			$	609,643			$	30,084			$	34,040
	Service cost	10,830				13,465				465				541
	Interest cost	26,147				22,719				859				877
	Actuarial loss (gain)	107,023				(54,671		)		958				(3,156		)
	Settlements	—				—				690				—
	Benefits paid	(23,422		)		(23,290		)		(3,052		)		(2,218		)
	Benefit obligation at end of period	$	688,444			$	567,866			$	30,004			$	30,084
	Change in Fair Value of Plan Assets:
	Fair value of plan assets at beginning of period	$	516,352			$	472,936			$	18,183			$	15,893
	Return on plan assets	52,921				55,006				1,391				2,662
	Employer contributions	10,200				11,700				1,518				1,846
	Benefits paid	(23,422		)		(23,290		)		(3,052		)		(2,218		)
	Fair value of plan assets at end of period	$	556,051			$	516,352			$	18,040			$	18,183
	Funded Status	$	(132,393	)		$	(51,514	)		$	(11,964	)		$	(11,901	)
	Amounts recognized in the balance sheet consist of:
	Current liability	—				—				(1,169		)		(1,178		)
	Noncurrent liability	(132,393		)		(51,514		)		(10,795		)		(10,723		)
	Net amount recognized	$	(132,393	)		$	(51,514	)		$	(11,964	)		$	(11,901	)
	Amounts recognized in regulatory assets consist of:
	Prior service (cost) credit	(502		)		(748		)		17,098				19,247
	Net actuarial loss	(153,268		)		(71,777		)		(4,945		)		(4,807		)
	Amounts recognized in AOCI consist of:
	Prior service cost	—				—				(1,151		)		(1,302		)
	Net actuarial gain	—				—				(409		)		(971		)
	Total	$	(153,770	)		$	(72,525	)		$	10,593			$	12,167
	The total projected benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets were as follows (in millions):
		Pension Benefits
		December 31,
		2014				2013
	Projected benefit obligation	$	688.4			$	567.9
	Accumulated benefit obligation	685				565
	Fair value of plan assets	556.1				516.4
	Net Periodic Cost (Credit)
	The components of the net costs (credits) for our pension and other postretirement plans are as follows (in thousands):
		Pension Benefits												Other Postretirement Benefits
		December 31,												December 31,
		2014				2013				2012				2014				2013			2012
	Components of Net Periodic Benefit Cost
	Service cost	$	10,830			$	13,465			$	11,488			$	465			$	541		$	541
	Interest cost	26,147				22,719				23,823				859				877			1,167
	Expected return on plan assets	(29,506		)		(32,491		)		(29,996		)		(981		)		(1,019		)	(1,021		)
	Amortization of prior service cost (credit)	246				246				246				(1,998		)		(1,998		)	(1,998		)
	Recognized actuarial loss	2,118				11,648				8,646				348				1,271			790
	Settlement loss recognized	—				—				—				690				—			—
	Net Periodic Benefit Cost (Credit)	$	9,835			$	15,587			$	14,207			$	(617	)		$	(328	)	$	(521	)
	For purposes of calculating the expected return on pension plan assets, the market-related value of assets is used, which is based upon fair value. The difference between actual plan asset returns and estimated plan asset returns are amortized equally over a period not to exceed five years.
	We estimate amortizations from regulatory assets into net periodic benefit cost during 2015 will be as follows (in thousands):
		Pension Benefits				Other
						Postretirement
						Benefits
	Prior service credit (cost)	$	(246	)		$	1,998
	Accumulated loss	(10,470		)		(325		)
	Actuarial Assumptions
	The measurement dates used to determine pension and other postretirement benefit measurements for the plans are December 31, 2014 and 2013. The actuarial assumptions used to compute net periodic pension cost and postretirement benefit cost are based upon information available as of the beginning of the year, specifically, market interest rates, past experience and management's best estimate of future economic conditions. Changes in these assumptions may impact future benefit costs and obligations. In computing future costs and obligations, we must make assumptions about such things as employee mortality and turnover, expected salary and wage increases, discount rate, expected return on plan assets, and expected future cost increases. Two of these assumptions have the most impact on the level of cost: (1) discount rate and (2) expected rate of return on plan assets.
	For 2014 and 2013, we set the discount rate using a yield curve analysis, which projects benefit cash flows into the future and then discounts those cash flows to the measurement date using a yield curve. This is done by constructing a hypothetical bond portfolio whose cash flow from coupons and maturities matches the year-by-year, projected benefit cash flow from our plans. The decrease in discount rate during 2014 increased our projected benefit obligation by approximately $73.6 million.
In determining the expected long-term rate of return on plan assets, we review historical returns, the future expectations for returns for each asset class weighted by the target asset allocation of the pension and postretirement portfolios, and long-term inflation assumptions. Based on the target asset allocation for our pension assets and future expectations for asset returns, we are keeping our long term rate of return on assets assumption at 5.80% for 2015.
During 2014, we also updated our mortality assumptions to adopt the Society of Actuaries mortality table (RP-2014) and mortality projection scale (MP-2014) released in October 2014. This change in mortality assumption increased our projected benefit obligation by approximately $33.8 million.
The weighted-average assumptions used in calculating the preceding information are as follows:
	Pension Benefits						Other Postretirement Benefits
	December 31,						December 31,
	2014		2013		2012		2014		2013		2012
Discount rate	3.75-3.90	%	4.55-4.75	%	3.55-3.80	%	3.20-3.40	%	3.75-4.20	%	2.25-3.20	%
Expected rate of return on assets	5.8		7		7		5.8		7		7
Long-term rate of increase in compensation levels (nonunion)	3.58		3.58		3.58		3.58		3.58		3.58
Long-term rate of increase in compensation levels (union)	3.5		3.5		3.5		3.5		3.5		3.5
The postretirement benefit obligation is calculated assuming that health care costs increased by 8.25% in 2014 and the rate of increase in the per capita cost of covered health care benefits thereafter was assumed to decrease gradually by 0.25% per year to an ultimate trend of 4.5% by the year 2029. The company contribution toward the premium cost is capped, therefore future health care cost trend rates are expected to have a minimal impact on company costs and the accumulated postretirement benefit obligation.
Investment Strategy
Our investment goals with respect to managing the pension and other postretirement assets are to meet current and future benefit payment needs while maximizing total investment returns (income and appreciation) after inflation within the constraints of diversification, prudent risk taking, and the Prudent Man Rule of the Employee Retirement Income Security Act of 1974. Each plan is diversified across asset classes to achieve optimal balance between risk and return and between income and growth through capital appreciation. Our investment philosophy is based on the following:
•	Each plan should be substantially fully invested as long-term cash holdings reduce long-term rates of return;
•	It is prudent to diversify each plan across the major asset classes;
•	Equity investments provide greater long-term returns than fixed income investments, although with greater short-term volatility;
•	Fixed income investments of the plans should strongly correlate with the interest rate sensitivity of the plan’s aggregate liabilities in order to hedge the risk of change in interest rates negatively impacting the overall funded status;
•	Allocation to foreign equities increases the portfolio diversification and thereby decreases portfolio risk while providing for the potential for enhanced long-term returns;
•	Active management can reduce portfolio risk and potentially add value through security selection strategies;
•	A portion of plan assets should be allocated to passive, indexed management funds to provide for greater diversification and lower cost; and
•	It is appropriate to retain more than one investment manager, provided that such managers offer asset class or style diversification.
Investment risk is measured and monitored on an ongoing basis through quarterly investment portfolio reviews, annual liability measurements, and periodic asset/liability studies.
The most important component of an investment strategy is the portfolio asset mix, or the allocation between the various classes of securities available. The mix of assets is based on an optimization study that identifies asset allocation targets in order to achieve the maximum return for an acceptable level of risk, while minimizing the expected contributions and pension and postretirement expense. In the optimization study, assumptions are formulated about characteristics, such as expected asset class investment returns, volatility (risk), and correlation coefficients among the various asset classes, and making adjustments to reflect future conditions expected to prevail over the study period. Based on this, the target asset allocation established, within an allowable range of plus or minus 5%, is as follows:
	Pension Benefits						Other Benefits
	December 31,						December 31,
	2014			2013			2014			2013
Domestic debt securities	55	%		60	%		40	%		40	%
International debt securities	5			5			—			—
Domestic equity securities	34			30			50			50
International equity securities	6			5			10			10
The actual allocation by plan is as follows:
	NorthWestern Energy Pension						NorthWestern Corporation Pension						NorthWestern Energy
													Health and Welfare
	December 31,						December 31,						December 31,
	2014			2013			2014			2013			2014			2013
Cash and cash equivalents	—	%		—	%		0.1	%		0.1	%		0.2	%		1.8	%
Domestic debt securities	56			58.6			65.6			64.7			37.2			38.6
International debt securities	4.4			4.9			4.5			4.9			—			0.3
Domestic equity securities	34.1			31.4			25.1			25.3			53.9			50.1
International equity securities	5.5			5.1			4.7			5			8.7			9.2
	100	%		100	%		100	%		100	%		100	%		100	%
Generally, the asset mix will be rebalanced to the target mix as individual portfolios approach their minimum or maximum levels. Debt securities consist of U.S. and international instruments. Core domestic portfolios can be invested in government, corporate, asset-backed and mortgage-backed obligation securities. While the portfolio may invest in high yield securities, the average quality must be rated at least “investment grade" by rating agencies. Performance of fixed income investments is measured by both traditional investment benchmarks as well as relative changes in the present value of the plan's liabilities. Equity investments consist primarily of U.S. stocks including large, mid and small cap stocks, which are diversified across investment styles such as growth and value. We also invest in international equities with exposure to developing and emerging markets. Derivatives, options and futures are permitted for the purpose of reducing risk but may not be used for speculative purposes.
Our plan assets are primarily invested in common collective trusts (CCTs), which are invested in equity and fixed income securities. In accordance with our investment policy, these pooled investment funds must have an adequate asset base relative to their asset class and be invested in a diversified manner and have a minimum of three years of verified investment performance experience or verified portfolio manager investment experience in a particular investment strategy and have management and oversight by an investment advisor registered with the SEC. Investments in a collective investment vehicle are valued by multiplying the investee company’s net asset value per share with the number of units or shares owned at the valuation date. Net asset value per share is determined by the trustee. Investments held by the CCT, including collateral invested for securities on loan, are valued on the basis of valuations furnished by a pricing service approved by the CCT’s investment manager, which determines valuations using methods based on quoted closing market prices on national securities exchanges, or at fair value as determined in good faith by the CCT’s investment manager if applicable. The funds do not contain any redemption restrictions. The direct holding of NorthWestern Corporation stock is not permitted; however, any holding in a diversified mutual fund or collective investment fund is permitted. In addition, the NorthWestern Corporation pension plan assets also include a participating group annuity contract in the John Hancock General Investment Account, which consists primarily of fixed-income securities. The participating group annuity contract is valued based on discounted cash flows of current yields of similar contracts with comparable duration based on the underlying fixed income investments.
The fair value of our plan assets at December 31, 2014, by asset category are as follows (in thousands):
Asset Category	Total				Quoted Market				Significant Observable Inputs				Significant Unobservable Inputs
					Prices in Active				Level 2				Level 3
					Markets for
					Identical Assets
					Level 1
Pension Plan Assets
Cash and cash equivalents	$	126			$	—			$	126			$	—
Equity securities: (1)
US small/mid cap growth	16,605				—				16,605				—
US small/mid cap value	15,264				—				15,264				—
US large cap growth	48,560				—				48,560				—
US large cap value	48,785				—				48,785				—
US large cap passive	54,775				—				54,775				—
Non-US core	22,634				—				22,634				—
Emerging markets	7,650				—				7,650				—
Fixed income securities:(2)
US core	23,177				—				23,177				—
US passive	12,269				—				12,269				—
Long duration	41,451				—				41,451				—
Long duration investment grade	52,767				—				52,767				—
Long duration passive	41,475				—				41,475				—
Opportunistic	5,692				—				5,692				—
Non-US passive	24,504				—				24,504				—
Active long corporate	133,160				—				133,160				—
Participating group annuity contract	7,157				—				7,157				—
	$	556,051			$	—			$	556,051			$	—
Other Postretirement Benefit Plan Assets
Cash and cash equivalents	$	44			$	—			$	44			$	—
Equity securities: (1)									—
US small/mid cap growth	752				—				752				—
US small/mid cap value	721				—				721				—
S&P 500 index	8,234				—				8,234				—
US large cap growth	6				—				6				—
US large cap value	6				—				6				—
US large cap passive	7				—				7				—
Non-US core	1,495				—				1,495				—
Emerging markets	72				—				72				—
Fixed income securities: (2)
Passive bond market	1,992				—				1,992				—
US core	4,435				—				4,435				—
US passive	1				—				1				—
Long duration	5				—				5				—
Long duration investment grade	6				—				6				—
Long duration passive	5				—				5				—
Opportunistic	240				—				240				—
Non-US passive	3				—				3				—
Active long corporate	16				—				16				—
	$	18,040			$	—			$	18,040			$	—
The fair value of our plan assets at December 31, 2013, by asset category are as follows (in thousands):
Asset Category	Total				Quoted Market				Significant Observable Inputs				Significant Unobservable Inputs
					Prices in Active				Level 2				Level 3
					Markets for
					Identical Assets
					Level 1
Pension Plan Assets
Cash and cash equivalents	$	168			$	—			$	168			$	—
Equity securities: (1)
US small/mid cap growth	13,764				—				13,764				—
US small/mid cap value	13,664				—				13,664				—
US large cap growth	42,094				—				42,094				—
US large cap value	42,102				—				42,102				—
US large cap passive	47,227				—				47,227				—
Non-US core	20,015				—				20,015				—
Emerging markets	6,250				—				6,250				—
Fixed income securities:(2)
US core	82,639				—				82,639				—
US passive	44,762				—				44,762				—
Long duration	24,401				—				24,401				—
Long duration investment grade	32,700				—				32,700				—
Long duration passive	24,122				—				24,122				—
Opportunistic	5,876				—				5,876				—
Non-US passive	25,150				—				25,150				—
Active long corporate	83,147				—				83,147				—
Participating group annuity contract	8,271				—				8,271				—
	$	516,352			$	—			$	516,352			$	—
Other Postretirement Benefit Plan Assets
Cash and cash equivalents	$	318			—				$	318			—
Equity securities: (1)
US small/mid cap growth	751				—				751				—
US small/mid cap value	736				—				736				—
S&P 500 index	7,321				—				7,321				—
US large cap growth	98				—				98				—
US large cap value	98				—				98				—
US large cap passive	110				—				110				—
Non-US core	1,595				—				1,595				—
Emerging markets	85				—				85				—
Fixed income securities: (2)
Passive bond market	1,880				—				1,880				—
US core	4,390				—				4,390				—
US passive	107				—				107				—
Long duration	55				—				55				—
Long duration investment grade	79				—				79				—
Long duration passive	55				—				55				—
Opportunistic	261				—				261				—
Non-US passive	57				—				57				—
Active long corporate	187				—				187				—
	$	18,183			$	—			$	18,183			$	—
_________________
-1	This category consists of active and passive managed equity funds, which are invested in multiple strategies to diversify risks and reduce volatility.
(2) This category consists of investment grade bonds of issuers from diverse industries, debt securities issued by international, national, state and local governments, and asset-backed securities. This includes both active and passive managed funds.
For further discussion of the three levels of the fair value hierarchy see Note 10 - Fair Value Measurements.
Cash Flows
In accordance with the Pension Protection Act of 2006 (PPA), and the relief provisions of the Worker, Retiree, and Employer Recovery Act of 2008 (WRERA), we are required to meet minimum funding levels in order to avoid required contributions and benefit restrictions. We have elected to use asset smoothing provided by the WRERA, which allows the use of asset averaging, including expected returns (subject to certain limitations), for a 24-month period in the determination of funding requirements.
Based on the assumptions allowed under the PPA, WRERA, Treasury guidance and IRS guidance, we estimate that our minimum annual required contribution for 2015 will be approximately $10.2 million. Additional legislative or regulatory measures, as well as fluctuations in financial market conditions, may impact these funding requirements.
Due to the regulatory treatment of pension costs in Montana, pension expense for 2014 and 2013 was based on actual contributions to the plan, while 2012 pension expense was calculated using the average of our actual and estimated funding amounts from 2005 through 2012. Annual contributions to each of the pension plans are as follows (in thousands):
	2014				2013				2012
NorthWestern Energy Pension Plan (MT)	$	9,000			$	10,500			$	10,500
NorthWestern Pension Plan (SD)	1,200				1,200				1,200
	$	10,200			$	11,700			$	11,700
We estimate the plans will make future benefit payments to participants as follows (in thousands):
	Pension Benefits				Other Postretirement Benefits
2015	$	27,652			$	3,516
2016	29,905				3,516
2017	31,172				3,387
2018	33,142				3,282
2019	34,660				3,026
2020-2024	194,065				11,923
Defined Contribution Plan
Our defined contribution plan permits employees to defer receipt of compensation as provided in Section 401(k) of the Internal Revenue Code. Under the plan, employees may elect to direct a percentage of their gross compensation to be contributed to the plan. We contribute various percentage amounts of the employee's gross compensation contributed to the plan. Matching contributions for the year ended December 31, 2014, 2013 and 2012 were $8.7 million, $7.8 million, and $7.2 million

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation
	We grant stock-based awards through our Amended and Restated Equity Compensation Plan (ECP), which includes restricted stock awards and performance share awards. In 2014, an additional 600,000 shares of common stock were authorized by the shareholders for issuance under the ECP. As of December 31, 2014, there were 1,124,798 shares of common stock remaining available for grants. The remaining vesting period for awards previously granted ranges from one to five years if the service and/or performance requirements are met. Nonvested shares do not receive dividend distributions. The long-term incentive plan provides for accelerated vesting in the event of a change in control.
	We account for our share-based compensation arrangements by recognizing compensation costs for all share-based awards over the respective service period for employee services received in exchange for an award of equity or equity-based compensation. The compensation cost is based on the fair value of the grant on the date it was awarded.
	Performance Share Awards
	Performance share awards are granted annually under the ECP. With these awards, shares will vest if, at the end of the three-year performance period, we have achieved certain performance goals and the individual remains employed by us. The exact number of shares issued will vary from 0% to 200% of the target award, depending on actual company performance relative to the performance goals. These awards contain both a market and performance based component. For our outstanding performance share awards which were granted in 2012 and 2013, the performance goals are independent of each other and equally weighted, and are based on two metrics: (i) cumulative net income and average return on equity; and (ii) total shareholder return (TSR) relative to a peer group. For the awards granted in 2014, our Board added an earnings per share metric and removed the net income metric, while retaining the average return on equity and TSR metrics.
	Fair value is determined for each component of the performance share awards. The fair value of the net income / earnings per share component is estimated based upon the closing market price of our common stock as of the date of grant less the present value of expected dividends, multiplied by an estimated performance multiple determined on the basis of historical experience, which is subsequently trued up at vesting based on actual performance. The fair value of the TSR portion is estimated using a statistical model that incorporates the probability of meeting performance targets based on historical returns relative to the peer group. The following summarizes the significant assumptions used to determine the fair value of performance shares and related compensation expense as well as the resulting estimated fair value of performance shares granted:
		2014		2013
	Risk-free interest rate	0.67	%	0.44	%
	Expected life, in years	3		3
	Expected volatility	15.5% to 23.3%		16.3% to 25.4%
	Dividend yield	3.3	%	3.9	%
	The risk-free interest rate was based on the U.S. Treasury yield of a three-year bond at the time of grant. The expected term of the performance shares is three years based on the performance cycle. Expected volatility was based on the historical volatility for the peer group. Both performance goals are measured over the three-year vesting period and are charged to compensation expense over the vesting period based on the number of shares expected to vest.
	A summary of nonvested shares as of and changes during the year ended December 31, 2014, are as follows:
		Performance Share Awards
		Shares		Weighted-Average Grant-Date
				Fair Value
	Beginning nonvested grants	173,646		$	29.14
	Granted	96,193		38.33
	Vested	(84,652	)	25.19
	Forfeited	(4,615	)	33.55
	Remaining nonvested grants	180,572		$	35.77
	We recognized compensation expense of $3.1 million, $2.4 million, and $2.8 million for the years ended December 31, 2014, 2013, and 2012, respectively, and a related income tax benefit of $0.1 million, $1.5 million, and $0.4 million for the years ended December 31, 2014, 2013, and 2012, respectively. As of December 31, 2014, we had $3.6 million of unrecognized compensation cost related to the nonvested portion of outstanding awards, which is reflected as nonvested stock as a portion of additional paid in capital in our Statements of Common Shareholders' Equity. The cost is expected to be recognized over a weighted-average period of 2.0 years. The total fair value of shares vested was $2.1 million, $2.2 million, and $2.0 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	Retirement/Retention Restricted Share Awards
	In December 2011, an executive retirement / retention program was established that provides for the annual grant of restricted share units. These awards are subject to a five-year performance and vesting period. The performance measure for these awards requires net income for the calendar year of at least three of the five full calendar years during the performance period to exceed net income for the calendar year the awards are granted. Once vested, the awards will be paid out in shares of common stock in five equal annual installments after a recipient has separated from service. The fair value of these awards is measured based upon the closing market price of our common stock as of the date of grant less the present value of expected dividends.
	A summary of nonvested shares as of and changes during the year ended December 31, 2014, are as follows:
		Shares		Weighted-Average Grant-Date
				Fair Value
	Beginning nonvested grants	26,628		$	30.24
	Granted	15,092		43.79
	Vested	—		—
	Forfeited	—		—
	Remaining nonvested grants	41,720		$	35.14
	Director's Deferred Compensation
	Nonemployee directors may elect to defer up to 100% of any qualified compensation that would be otherwise payable to him or her, subject to compliance with our 2005 Deferred Compensation Plan for Nonemployee Directors and Section 409A of the Internal Revenue Code. The deferred compensation may be invested in NorthWestern stock or in designated investment funds. Compensation deferred in a particular month is recorded as a deferred stock unit (DSU) on the first of the following month based on the closing price of NorthWestern stock or the designated investment fund. The DSUs are marked-to-market on a quarterly basis with an adjustment to director’s compensation expense. Based on the election of the nonemployee director, following separation from service on the Board, other than on account of death, he or she shall be paid a distribution either in a lump sum or in approximately equal installments over a designated number of years (not to exceed 10 years). During the years ended December 31, 2014, 2013 and 2012, DSUs issued to members of our Board totaled 26,460, 33,837 and 31,801, respectively. Total compensation expense attributable to the DSUs during the years ended December 31, 2014, 2013 and 2012 was approximately $2.3 million, $3.6 million and $0.9 million, respectively.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2014
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Common Stock	Common Stock
	We have 250,000,000 shares authorized consisting of 200,000,000 shares of common stock with a $0.01 par value and 50,000,000 shares of preferred stock with a $0.01 par value. Of these shares, 2,865,957 shares of common stock are reserved for the incentive plan awards. For further detail of grants under this plan see Note 17 - Stock-Based Compensation.
	Hydro Transaction Issuance - In November 2014, we issued 7,766,990 shares of our common stock at $51.50 per share, for aggregate net proceeds of $386 million.
	Equity Distribution Agreement - In April 2012, we entered into an Equity Distribution Agreement pursuant to which we could offer and sell shares of our common stock from time to time, having an aggregate gross sales price of up to $100 million. During the first quarter of 2014, we issued 295,979 shares of our common stock at an average price of $45.65 per share, for net proceeds of $13.4 million, which are net of sales commissions of approximately $147,000 and other fees. This concluded our sales pursuant to the Equity Distribution Agreement. Total shares issued under the Equity Distribution Agreement were 2,492,889 shares at an average price of $40.11, for net proceeds of $98.7 million.
	Repurchase of Common Stock
	Shares tendered by employees to us to satisfy the employees' tax withholding obligations in connection with the vesting of restricted stock awards totaled 23,630 and 34,552 during the years ended December 31, 2014 and 2013, respectively, and are reflected in treasury stock. These shares were credited to treasury stock based on their fair market value on the vesting date.

Earnings_Per_Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share
	Basic earnings per share are computed by dividing earnings applicable to common stock by the weighted average number of common shares outstanding for the period. Diluted earnings per share reflect the potential dilution of common stock equivalent shares that could occur if unvested shares were to vest. Common stock equivalent shares are calculated using the treasury stock method, as applicable. The dilutive effect is computed by dividing earnings applicable to common stock by the weighted average number of common shares outstanding plus the effect of the outstanding unvested restricted stock and performance share awards. Average shares used in computing the basic and diluted earnings per share are as follows:
		December 31,
		2014	2013
	Basic computation	40,156,177	38,144,852
	Dilutive effect of
	Performance share awards (1)	275,774	82,223
	Diluted computation	40,431,951	38,227,075
	_____________________
	(1)           Performance share awards are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the award.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies
	Qualifying Facilities Liability
	Our QF liability primarily consists of unrecoverable costs associated with three contracts covered under the PURPA. The QFs require us to purchase minimum amounts of energy at prices ranging from $74 to $136 per MWH through 2029. Our estimated gross contractual obligation related to the QFs is approximately $1.0 billion through 2029. A portion of the costs incurred to purchase this energy is recoverable through rates, totaling approximately $0.8 billion through 2029. The present value of the remaining QF liability is recorded in our Consolidated Balance Sheets as a regulatory disallowance liability pursuant to ASC 980. The following summarizes the change in the QF liability (in thousands):
		December 31,
		2014			2013
	Beginning QF liability	$	136,448		$	136,652
	Unrecovered amount	(10,128		)	(10,647		)
	Interest expense	10,573			10,443
	Ending QF liability	$	136,893		$	136,448
	The following summarizes the estimated gross contractual obligation less amounts recoverable through rates (in thousands):
		Gross			Recoverable			Net
		Obligation			Amounts
	2015	$	69,606		$	56,598		$	13,008
	2016	71,598			57,188			14,410
	2017	73,622			57,789			15,833
	2018	75,688			58,401			17,287
	2019	77,791			59,020			18,771
	Thereafter	646,783			508,195			138,588
	Total	$	1,015,088		$	797,191		$	217,897
	Long Term Supply and Capacity Purchase Obligations
	We have entered into various commitments, largely purchased power, coal and natural gas supply and natural gas transportation contracts. These commitments range from one to 27 years. Costs incurred under these contracts were approximately $402.3 million, $379.4 million and $340.8 million for the years ended December 31, 2014, 2013, and 2012, respectively. As of December 31, 2014, our commitments under these contracts are $206.5 million in 2015, $161.1 million in 2016, $134.9 million in 2017, $107.3 million in 2018, $103.5 million in 2019, and $922.7 million thereafter. These commitments are not reflected in our Consolidated Financial Statements.
	Hydroelectric License Commitments
	With the Hydro Transaction, we assumed two Memoranda of Understanding (MOUs) existing with state, federal and private entities. The MOUs are periodically updated and renewed and require us to implement plans to mitigate the impact of the projects on fish, wildlife and their habitats, and to increase recreational opportunities. The MOUs were created to maximize collaboration between the parties and enhance the possibility to receive matching funds from relevant federal agencies. Under these MOUs, we have a remaining commitment to spend approximately $26.0 million between 2015 and 2040.
	Environmental Matters
	The operation of electric generating, transmission and distribution facilities, and gas gathering, transportation and distribution facilities, along with the development (involving site selection, environmental assessments, and permitting) and construction of these assets, are subject to extensive federal, state, and local environmental and land use laws and regulations. Our activities involve compliance with diverse laws and regulations that address emissions and impacts to the environment, including air and water, protection of natural resources, avian and wildlife. We monitor federal, state, and local environmental initiatives to determine potential impacts on our financial results. As new laws or regulations are implemented, our policy is to assess their applicability and implement the necessary modifications to our facilities or their operation to maintain ongoing compliance.
	Our environmental exposure includes a number of components, including remediation expenses related to the cleanup of current or former properties, and costs to comply with changing environmental regulations related to our operations. At present, the majority of our environmental reserve relates to the remediation of former manufactured gas plant sites owned by us. We use a combination of site investigations and monitoring to formulate an estimate of environmental remediation costs for specific sites. Our monitoring procedures and development of actual remediation plans depend not only on site specific information but also on coordination with the different environmental regulatory agencies in our respective jurisdictions; therefore, while remediation exposure exists, it may be many years before costs are incurred.
	Our liability for environmental remediation obligations is estimated to range between $26.4 million to $35.0 million, primarily for manufactured gas plants discussed below. As of December 31, 2014, we have a reserve of approximately $29.7 million, which has not been discounted. Environmental costs are recorded when it is probable we are liable for the remediation and we can reasonably estimate the liability. Over time, as costs become determinable, we may seek authorization to recover such costs in rates or seek insurance reimbursement as applicable; therefore, although we cannot guarantee regulatory recovery, we do not expect these costs to have a material effect on our consolidated financial position or results of operations.
	Manufactured Gas Plants - Approximately $22.4 million of our environmental reserve accrual is related to manufactured gas plants. A formerly operated manufactured gas plant located in Aberdeen, South Dakota, has been identified on the Federal Comprehensive Environmental Response, Compensation, and Liability Information System list as contaminated with coal tar residue. We are currently conducting feasibility studies and implementing remedial actions at the Aberdeen site pursuant to work plans approved by the South Dakota Department of Environment and Natural Resources (DENR). Our current reserve for remediation costs at this site is approximately $10.8 million, and we estimate that approximately $8.0 million of this amount will be incurred during the next five years.
	We also own sites in North Platte, Kearney and Grand Island, Nebraska on which former manufactured gas facilities were located. We are currently working independently to fully characterize the nature and extent of potential impacts associated with these Nebraska sites. Our reserve estimate includes assumptions for site assessment and remedial action work. At present, we cannot determine with a reasonable degree of certainty the nature and timing of any risk-based remedial action at our Nebraska locations.
	In addition, we own or have responsibility for sites in Butte, Missoula and Helena, Montana on which former manufactured gas plants were located. An investigation conducted at the Missoula site did not require remediation activities, but required preparation of a groundwater monitoring plan. The Butte and Helena sites were placed into the Montana Department of Environmental Quality (MDEQ) voluntary remediation program for cleanup due to soil and groundwater impacts. Voluntary soil and coal tar removals were conducted in the past at the Butte and Helena locations in accordance with MDEQ requirements. We have conducted additional groundwater monitoring at the Butte and Missoula sites and, at this time, we believe natural attenuation should address the conditions at these sites; however, additional groundwater monitoring will be necessary and additional monitoring wells will be installed at the Butte site. Monitoring of groundwater at the Helena site is ongoing and will be necessary for an extended period of time. At this time, we cannot estimate with a reasonable degree of certainty the nature and timing of risk-based remedial action at the Helena site or if any additional actions beyond monitored natural attenuation will be required.
	Global Climate Change - National and international actions have been initiated to address global climate change and the contribution of emissions of greenhouse gases (GHG) including, most significantly, carbon dioxide. These actions include legislative proposals, Executive and Environmental Protection Agency (EPA) actions at the federal level, actions at the state level, and private party litigation relating to GHG emissions. Coal-fired plants have come under particular scrutiny due to their level of GHG emissions. We have joint ownership interests in four electric generating plants, all of which are coal fired and operated by other companies. We have undivided interests in these facilities and are responsible for our proportionate share of the capital and operating costs while being entitled to our proportionate share of the power generated.
	While numerous bills have been introduced that address climate change from different perspectives, including through direct regulation of GHG emissions, the establishment of cap and trade programs and the establishment of Federal renewable portfolio standards, Congress has not passed any federal climate change legislation and we cannot predict the timing or form of any potential legislation. In the absence of such legislation, EPA is presently regulating GHG emissions of the very largest emitters, including large power plants, under the Clean Air Act, and specifically under the Prevention of Significant Deterioration (PSD) pre-construction permit, the Title V operating permit programs and the New Source Performance Standards (NSPS).
	In January, 2014, the EPA reproposed NSPS specifying permissible levels of GHG emissions for newly-constructed fossil fuel-fired electric generating units and in June 2014 proposed performance standards for modified and reconstructed power plants. Also in June, 2014, the EPA proposed the Clean Power Plan (CPP) rule to control carbon dioxide emissions from existing fossil fuel fired electric generating units. The rule proposes the establishment of statewide GHG emission standards for individual states based on the state's potential to shift generation to existing natural gas combined cycle plants, to develop new renewable energy, to achieve demand-side management savings, and to improve performance at existing coal-fired units. Under the proposed CPP, States would be required to submit individual plans for achieving GHG emission standards to EPA by summer, 2016, although under certain circumstances additional time to summer, 2018, would be permitted. The initial performance period for compliance would commence in 2020, with full implementation by 2030. The EPA has indicated that it intends to issue final rules on the NSPS, the performance standards for modified and reconstructed plants and the CPP by midsummer, 2015.
	On June 23, 2014, the U.S. Supreme Court struck down the EPA's Tailoring Rule, which limited the sources subject to GHG permitting requirements to the largest fossil-fueled power plants, indicating that EPA had exceeded its authority under the Clean Air Act by "rewriting unambiguous statutory terms." However, the decision affirmed EPA's ability to regulate GHG emissions from sources already subject to regulation under the PSD program, which includes most electric generating units.
	Requirements to reduce GHG emissions from stationary sources could cause us to incur material costs of compliance and increase our costs of procuring electricity. Although there continues to be changes in legislation and regulations that affect GHG emissions from power plants, technology to efficiently capture, remove and/or sequester such emissions may not be available within a timeframe consistent with the implementation of such requirements. In addition, physical impacts of climate change may present potential risks for severe weather, such as droughts, floods and tornadoes, in the locations where we operate or have interests. We cannot predict with any certainty whether these risks will have a material impact on our operations.
	Coal Combustion Residuals (CCRs) - In December 2014, the EPA issued a final rule regulating the disposal and management of CCRs as a solid waste under Subtitle D of the Resource Conservation and Recovery Act (RCRA). CCRs include fly ash, bottom ash and scrubber wastes. The rule imposes some additional recordkeeping and operating requirements, but does not regulate the beneficial use of CCRs. We continue to review the potential costs of complying with the new CCR rule and cannot currently estimate such costs. Legal challenges to the final rule and EPA’s determination that CCR is not a hazardous waste are expected and legislation has been introduced in Congress to regulate coal ash. We cannot predict at this time the final outcome of any appeal of the CCR regulations or legislation and what impact, if any, they would have on us.
	Water Intakes and Discharges - Section 316(b) of the Federal Clean Water Act (CWA) requires that the location, design, construction and capacity of any cooling water intake structure reflect the “best technology available (BTA)” for minimizing environmental impacts. In May, 2014, the EPA issued a final rule applicable to facilities that withdraw at least 2 million gallons per day of cooling water from waters of the US and use at least 25 percent of the water exclusively for cooling purposes. The final rule gives options for meeting BTA, and provides a flexible compliance approach. In August 2014, EPA published the final rule establishing national requirements applicable to cooling water intake structures, which became effective in October, 2014. Under the rule, permits required for existing facilities will be developed by the individual states and additional capital and/or increased operating costs may be required to comply with future water permit requirements. Challenges to the final cooling water intake rule have been filed by industry groups and by environmental groups in various appellate courts.
	In April 2013, the EPA proposed CWA regulations to address mercury, arsenic, lead, and selenium in water discharged from power plants. The proposed regulations include a variety of options for whether and how these different waste streams should be treated. The EPA is reviewing public comments on these options prior to enacting final regulations. Under the proposed approach, new requirements for existing power plants would be phased in between 2017 and 2022. The EPA is under a modified consent decree to take final action by September 30, 2015. The EPA estimates that over half of the existing power plants will not incur costs under any of the proposed options because many power plants already have the technology and procedures in place to meet the proposed pollution control standards; however, it is too early to determine whether the impacts of these rules will be material.
	Clean Air Act Rules and Associated Emission Control Equipment Expenditures
	The EPA has proposed or issued a number of rules under different provisions of the Clean Air Act that could require the installation of emission control equipment at the generation plants where we have joint ownership.
	The Clean Air Visibility Rule was issued by the EPA in June 2005, to address regional haze in national parks and wilderness areas across the United States. The Clean Air Visibility Rule requires the installation and operation of Best Available Retrofit Technology (BART) to achieve emissions reductions from designated sources (including certain electric generating units) that are deemed to cause or contribute to visibility impairment in such 'Class I' areas.
	In December 2011, the EPA issued a final rule relating to Mercury and Air Toxics Standards (MATS). Among other things, the MATS set stringent emission limits for acid gases, mercury, and other hazardous air pollutants from new and existing electric generating units. Facilities that are subject to the MATS must come into compliance by April 2015, unless a one year extension is granted on a case-by-case basis. In April 2014, the U.S. Court of Appeals for the D.C. Circuit upheld the MATS rule. The decision was appealed by 23 states and industry groups to the Supreme Court, and in November, 2014 the Court agreed to hear the case. Oral argument will likely be scheduled for the spring and the Supreme Court is expected to issue a ruling by June, 2015.
	In July 2011, the EPA finalized the Cross-State Air Pollution Rule (CSAPR) to reduce emissions from electric generating units that interfere with the ability of downwind states to achieve ambient air quality standards. Under CSAPR, significant reductions in emissions of nitrogen oxide (NOx) and sulfur dioxide (SO2) were to be required in certain states beginning in 2012. In April 2014 the Supreme Court reversed and remanded the 2012 decision of the U.S. Court of Appeals for the D.C. Circuit that had vacated the CSAPR. Litigation of the remaining CSAPR lawsuits continues, with briefings and oral argument set for 2015.
	In October 2013, the Supreme Court denied certiorari in Luminant Generation Co v. EPA, which challenged the EPA’s current approach to regulating air emissions during startup, shutdown and malfunction (SSM) events. As a result, fossil fuel power plants may need to address SSM in their permits to reduce the risk of enforcement or citizen actions.
	In September 2012, a final Federal Implementation Plan for Montana was published in the Federal Register to address regional haze. As finalized, Colstrip Unit 4 does not have to improve removal efficiency for pollutants that contribute to regional haze. By 2018, Montana, or EPA, must develop a revised Plan that demonstrates reasonable progress toward eliminating man made emissions of visibility impairing pollutants, which could impact Colstrip Unit 4. In November 2012, PPL Montana, the operator of Colstrip, as well as environmental groups (National Parks Conservation Association, Montana Environmental Information Center, and Sierra Club) jointly filed a petition for review of the Federal Implementation Plan in the U.S. Court of Appeals for the Ninth Circuit. Montana Environmental Information Center and Sierra Club have challenged the EPA's decision not to require any emissions reductions from Colstrip Units 3 and 4. The Ninth Circuit held oral argument on the petition on May 16, 2014, but no decision has been issued and at this time, we cannot predict or determine the timing or outcome of this petition.
	We have joint ownership in generation plants located in South Dakota, North Dakota, Iowa and Montana that are or may become subject to various regulations that have been issued or proposed under the Clean Air Act, as discussed below.
	South Dakota. The South Dakota DENR determined that the Big Stone Plant, of which we have a 23.4% ownership, is subject to the BART requirements of the Regional Haze Rule. South Dakota DENR's State Implementation Plan (SIP) was approved by the EPA in May 2012. Under the SIP, the Big Stone plant must install and operate a new BART compliant air quality control system (AQCS) to reduce SO2, NOx and particulate emissions as expeditiously as practicable, but no later than five years after the EPA's approval of the SIP. The estimated total project cost for the AQCS at the Big Stone plant is approximately $384 million (our share is 23.4%). As of December 31, 2014, we have capitalized costs of approximately $71.8 million related to this project, which is expected to be operational by the end of 2015.
	Our incremental capital expenditure projections include amounts related to our share of the BART at Big Stone based on current estimates. We could, however, face additional capital or financing costs. We will seek to recover any such costs through the regulatory process. The SDPUC has historically allowed timely recovery of the costs of environmental improvements; however, there is no precedent on a project of this size.
	Based on the finalized MATS, Big Stone will meet the requirements by installing the AQCS system and using activated carbon injection for mercury control. In August 2013, the South Dakota DENR granted Big Stone a one year extension to comply with MATS, such that the new compliance deadline is April 16, 2016. New mercury emissions monitoring equipment will also be required.
	North Dakota. The North Dakota Regional Haze SIP requires the Coyote generating facility, of which we have 10% ownership, to reduce its NOx emissions. Coyote must install control equipment to limit its NOx emissions to 0.5 pounds per million Btu as calculated on a 30-day rolling average basis, including periods of start-up and shutdown, beginning on July 1, 2018. The current estimate of the total cost of the project is approximately $9.0 million (our share is 10.0%).
	Based on the finalized MATS, Coyote will meet the requirements by using activated carbon injection for mercury control.
	Iowa. The Neal #4 generating facility, of which we have an 8.7% ownership, installed a scrubber, a baghouse, activated carbon injection and a selective non-catalytic reduction system to comply with national ambient air quality standards and the MATS. The project was substantially completed in 2013.
	Montana. Colstrip Unit 4, a coal fired generating facility in which we have a 30% interest, is currently controlling emissions of mercury under regulations issued by the State of Montana, which are stricter than the Federal MATS. The owners do not believe additional equipment will be necessary to meet the MATS for mercury, and anticipate meeting all other expected MATS emissions limitations required by the rule without additional costs except those costs related to increased monitoring frequency. These additional costs are not expected to be significant.
	See 'Legal Proceedings - Colstrip Litigation' below for discussion of Sierra Club litigation.
	Other - We continue to manage equipment containing polychlorinated biphenyl (PCB) oil in accordance with the EPA's Toxic Substance Control Act regulations. We will continue to use certain PCB-contaminated equipment for its remaining useful life and will, thereafter, dispose of the equipment according to pertinent regulations that govern the use and disposal of such equipment.
	We routinely engage the services of a third-party environmental consulting firm to assist in performing a comprehensive evaluation of our environmental reserve. Based upon information available at this time, we believe that the current environmental reserve properly reflects our remediation exposure for the sites currently and previously owned by us. The portion of our environmental reserve applicable to site remediation may be subject to change as a result of the following uncertainties:
	•	We may not know all sites for which we are alleged or will be found to be responsible for remediation; and
	•	Absent performance of certain testing at sites where we have been identified as responsible for remediation, we cannot estimate with a reasonable degree of certainty the total costs of remediation.
	LEGAL PROCEEDINGS
	Colstrip Litigation
	On March 6, 2013, the Sierra Club and the MEIC (Plaintiffs) filed suit in the United States District Court for the District of Montana (Court) against the six individual owners of Colstrip, including us, as well as the operator or managing agent of the station (Defendants). On September 27, 2013, Plaintiffs filed an Amended Complaint for Injunctive and Declaratory Relief. The original complaint included 39 claims for relief based upon alleged violations of the Clean Air Act and the Montana State Implementation Plan. The Amended Complaint dropped claims associated with projects completed before 2001, the Title V claims and the opacity claims. The Amended Complaint alleged a total of 23 claims covering 64 projects.
	In the Amended Complaint, Plaintiffs identified physical changes made at Colstrip between 2001 and 2012, that Plaintiffs allege (a) have increased emissions of SO2, NOx and particulate matter and (b) were “major modifications” subject to permitting requirements under the Clean Air Act. They also alleged violations of the requirements related to Part 70 Operating Permits.
	On May 3, 2013, the Colstrip owners and operator filed a partial motion to dismiss, seeking dismissal of 36 of the 39 claims asserted in the original complaint. The motion was not ruled upon and the Colstrip owners filed a second motion to dismiss the Amended Complaint on October 11, 2013, incorporating parts of the first motion and supplementing it with new authorities and with regard to new claims contained in the Amended Complaint.
	On September 12, 2013, Plaintiffs filed a motion for partial summary judgment as to the applicable method for calculating emissions increases from modifications.
The parties filed a joint notice (Notice) on April 21, 2014 that advised the Court of Plaintiffs’ intent to file a Second Amended Complaint which dropped claims relating to 52 projects, and added one additional project. At the joint request of the parties, the Court extended various deadlines previously set and set a bench trial date for the liability portion of the case for June 8, 2015.
On May 6, 2014, the Court held oral argument on Defendants' motion to dismiss and on Plaintiffs’ motion for summary judgment on the applicable legal standard. On May 22, 2014, the Magistrate issued findings and recommendations, which denied Plaintiffs’ motion for summary judgment and denied most of the Colstrip owners’ motion to dismiss, but dismissed seven of Plaintiffs’ “best available control technology” claims and dismissed two of Plaintiffs' claims for injunctive relief. The Plaintiffs filed an objection to the Magistrate's findings and recommendations with the U.S. Federal District Court Judge, and on August 13, 2014, the Court adopted the Magistrate's findings and conclusions.
On August 27, 2014, the Plaintiffs filed their Second Amended Complaint, which alleges a total of 13 claims covering eight projects and seeks injunctive and declaratory relief, civil penalties (including $100,000 of civil penalties to be used for beneficial environmental projects), and recovery of their attorney fees. Defendants filed their Answer to the Second Amended Complaint on September 26, 2014. Since filing the Second Amended Complaint, Plaintiffs have indicated that they are no longer pursuing four claims involving two projects thereby reducing their total claims to nine relating to six projects. The four dropped claims have not yet been formally dismissed by the Court. A Bench Trial has been set for November 16, 2015.
We intend to vigorously defend this lawsuit. Due to the preliminary nature of the lawsuit, at this time, we cannot predict an outcome, nor is it reasonably possible to estimate the amount or range of loss, if any, that would be associated with an adverse decision.
Billings Refinery Outage Claim
In August 2014, we received a demand letter from a refinery in Billings claiming that it had sustained damages of approximately $48.5 million as a result of a January 2014 electrical outage. We dispute the claim and intend to vigorously defend against it. We have reported the refinery's claim to our insurance carrier under our primary insurance policy, which has a $2 million retention. This matter is in the initial stages and we cannot predict an outcome or estimate the amount or range of loss, if any, that would be associated with an adverse result.
Other Legal Proceedings
We are also subject to various other legal proceedings, governmental audits and claims that arise in the ordinary course of business. In the opinion of management, the amount of ultimate liability with respect to these other actions will not materially affect our financial position, results of operations, or cash flows.

Segment_and_Related_Informatio

Segment and Related Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment and Related Information	Segment and Related Information
	Our reportable business segments are primarily engaged in the electric and natural gas business. The remainder of our operations are presented as other, which primarily consists of unallocated corporate costs.
	We evaluate the performance of these segments based on gross margin. The accounting policies of the operating segments are the same as the parent except that the parent allocates some of its operating expenses to the operating segments according to a methodology designed by management for internal reporting purposes and involves estimates and assumptions.
	Financial data for the business segments are as follows (in thousands):
	December 31, 2014	Electric			Gas			Other			Eliminations		Total
	Operating revenues	$	877,967		326,896			$	—		$	—	$	1,204,863
	Cost of sales	348,640			133,951			—			—		482,591
	Gross margin	529,327			192,945			—			—		722,272
	Operating, general and administrative	200,186			91,437			14,263			—		305,886
	Property and other taxes	84,759			29,821			12			—		114,592
	Depreciation	94,813			28,930			33			—		123,776
	Operating income (loss)	149,569			42,757			(14,308		)	—		178,018
	Interest expense	(60,424		)	(10,618		)	(6,760		)	—		(77,802		)
	Other income	4,758			1,324			4,116			—		10,198
	Income tax (expense) benefit	(1,490		)	(7,463		)	19,225			—		10,272
	Net income	$	92,413		$	26,000		$	2,273		$	—	$	120,686
	Total assets	$	3,442,659		$	1,522,902		$	8,382		$	—	$	4,973,943
	Capital expenditures	$	233,538		$	36,846		$	—		$	—	$	270,384
	December 31, 2013	Electric			Gas			Other			Eliminations		Total
	Operating revenues	$	865,239		$	287,605		$	1,675		$	—	$	1,154,519
	Cost of sales	358,688			120,858			—			—		479,546
	Gross margin	506,551			166,747			1,675			—		674,973
	Operating, general and administrative	195,100			78,822			11,647			—		285,569
	Property and other taxes	78,536			26,993			11			—		105,540
	Depreciation	89,728			23,070			33			—		112,831
	Operating income (loss)	143,187			37,862			(10,016		)	—		171,033
	Interest expense	(57,920		)	(9,993		)	(2,573		)	—		(70,486		)
	Other income	4,061			1,239			2,437			—		7,737
	Income tax (expense) benefit	(13,905		)	(4,134		)	3,738			—		(14,301		)
	Net income (loss)	$	75,423		$	24,974		$	(6,414	)	$	—	$	93,983
	Total assets	$	2,583,554		$	1,117,861		$	13,845		—		$	3,715,260
	Capital expenditures	$	198,032		$	32,422		$	—		—		$	230,454
	December 31, 2012	Electric			Gas			Other			Eliminations		Total
	Operating revenues	$	805,554		$	263,394		$	1,394		$	—	$	1,070,342
	Cost of sales	277,826			117,608			—			—		395,434
	Gross margin	527,728			145,786			1,394			—		674,908
	Operating, general and administrative	187,599			75,971			6,396			—		269,966
	MSTI impairment	24,039			—			—			—		24,039
	Property and other taxes	72,755			24,907			12			—		97,674
	Depreciation	86,559			19,452			33			—		106,044
	Operating income (loss)	156,776			25,456			(5,047		)	—		177,185
	Interest expense	(55,118		)	(9,063		)	(881		)	—		(65,062		)
	Other income	2,630			1,633			109			—		4,372
	Income tax (expense) benefit	(22,298		)	(692		)	4,901			—		(18,089		)
	Net income	$	81,990		$	17,334		$	(918	)	$	—	$	98,406
	Total assets	2,442,602			$	1,032,259		$	10,672		$	—	$	3,485,533
	Capital expenditures	178,325			$	40,909		$	—		$	—	$	219,234

Quarterly_Financial_Data_Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	Quarterly Financial Data (Unaudited)
	Our quarterly financial information has not been audited, but, in management's opinion, includes all adjustments necessary for a fair presentation. Our business is seasonal in nature with the peak sales periods generally occurring during the summer and winter months. Accordingly, comparisons among quarters of a year may not represent overall trends and changes in operations. Amounts presented are in thousands, except per share data:
	2014	First		Second		Third		Fourth
	Operating revenues	$	369,723	$	270,281	$	251,912	$	312,947
	Operating income	71,350		25,097		30,987		50,584
	Net income	$	45,580	$	7,746	$	30,191	$	37,169
	Average common shares outstanding	38,856		39,137		39,141		43,451
	Income per average common share (basic):
	Net income	$	1.17	$	0.2	$	0.77	$	0.87
	Income per average common share (diluted):
	Net income	$	1.17	$	0.2	$	0.77	$	0.85
	Dividends per share	$	0.4	$	0.4	$	0.4	$	0.4
	Stock price:
	High	$	47.86	$	52.49	$	52.7	$	58.7
	Low	42.64		45.49		45.3		45.14
	Quarter-end close	47.43		52.19		45.36		56.58
	2013	First		Second		Third		Fourth
	Operating revenues	$	313,020	$	260,161	$	262,248	$	319,090
	Operating income	57,010		32,660		31,401		49,962
	Net income	$	37,902	$	14,341	$	15,647	$	26,093
	Average common shares outstanding	37,384		38,092		38,459		38,626
	Income per average common share (basic):					0		0
	Net income	$	1.01	$	0.37	$	0.41	$	0.67
	Income per average common share (diluted):
	Net income	$	1.01	$	0.37	$	0.4	$	0.68
	Dividends per share	$	0.38	$	0.38	$	0.38	$	0.38
	Stock price:
	High	$	40.35	$	43.17	$	45.85	$	47.18
	Low	35.06		38.12		39.08		41.31
	Quarter-end close	39.86		39.9		44.92		43.32

Nature_of_Operations_and_Basis1

Nature of Operations and Basis of Consolidation (Policies)	12 Months Ended
	Dec. 31, 2014
Nature of Operations and Basis of Consolidation [Abstract]
Variable Interest Entities	Variable Interest Entities
	A reporting company is required to consolidate a variable interest entity (VIE) as its primary beneficiary, which means it has a controlling financial interest, when it has both the power to direct the activities of the VIE that most significantly impact the VIE's economic performance, and the obligation to absorb losses or the right to receive benefits from the VIE that could potentially be significant to the VIE. An entity is considered to be a VIE when its total equity investment at risk is not sufficient to permit the entity to finance its activities without additional subordinated financial support, or its equity investors, as a group, lack the characteristics of having a controlling financial interest. The determination of whether a company is required to consolidate an entity is based on, among other things, an entity's purpose and design and a company's ability to direct the activities of the entity that most significantly impact the entity's economic performance.
	Certain long-term purchase power and tolling contracts may be considered variable interests. We have various long-term purchase power contracts with other utilities and certain QF plants. We identified one QF contract that may constitute a VIE. We entered into a power purchase contract in 1984 with this 35 MW coal-fired QF to purchase substantially all of the facility's capacity and electrical output over a substantial portion of its estimated useful life. We absorb a portion of the facility's variability through annual changes to the price we pay per MWH (energy payment). After making exhaustive efforts, we have been unable to obtain the information from the facility necessary to determine whether the facility is a VIE or whether we are the primary beneficiary of the facility. The contract with the facility contains no provision which legally obligates the facility to release this information. We have accounted for this QF contract as an executory contract. Based on the current contract terms with this QF, our estimated gross contractual payments aggregate approximately $262.9 million through 2024. For further discussion of our gross QF liability, see Note 20 - Commitments and Contingencies. During the years ended December 31, 2014, 2013 and 2012 purchases from this QF were approximately $24.4 million, $23.8 million, and $21.0 million, respectively.

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Estimates are used for such items as long-lived asset values and impairment charges, long-lived asset useful lives, tax provisions, asset retirement obligations, uncollectible accounts, our QF liability, environmental costs, unbilled revenues and actuarially determined benefit costs. We revise the recorded estimates when we receive better information or when we can determine actual amounts. Those revisions can affect operating results.
Revenue Recognition	Revenue Recognition
	Customers are billed monthly on a cycle basis. To match revenues with associated expenses, we accrue unbilled revenues for electrical and natural gas services delivered to customers, but not yet billed at month-end.
Cash Equivalents	Cash Equivalents
	We consider all highly liquid investments with maturities of three months or less at the time of purchase to be cash equivalents.
Restricted cash	Restricted Cash
	Restricted cash consists primarily of funds held in trust accounts to satisfy the requirements of certain stipulation agreements and insurance reserve requirements.
Accounts Receivable, Net	Accounts Receivable, Net
	Accounts receivable are net of allowances for uncollectible accounts of $4.3 million and $4.5 million at December 31, 2014 and December 31, 2013, respectively. Receivables include unbilled revenues of $70.3 million and $74.3 million at December 31, 2014 and December 31, 2013, respectively.
Regulation of Utility Operations	Regulation of Utility Operations
	Our regulated operations are subject to the provisions of ASC 980. Regulated accounting is appropriate provided that (i) rates are established by or subject to approval by independent, third-party regulators, (ii) rates are designed to recover the specific enterprise's cost of service, and (iii) in view of demand for service, it is reasonable to assume that rates are set at levels that will recover costs and can be charged to and collected from customers.
	Our Consolidated Financial Statements reflect the effects of the different rate making principles followed by the jurisdictions regulating us. The economic effects of regulation can result in regulated companies recording costs that have been, or are expected to be, allowed in the ratemaking process in a period different from the period in which the costs would be charged to expense by an unregulated enterprise. When this occurs, costs are deferred as regulatory assets and recorded as expenses in the periods when those same amounts are reflected in rates. Additionally, regulators can impose liabilities upon a regulated company for amounts previously collected from customers and for amounts that are expected to be refunded to customers (regulatory liabilities).
	If we were required to terminate the application of these provisions to our regulated operations, all such deferred amounts would be recognized in the Consolidated Income Statements at that time. This would result in a charge to earnings, net of applicable income taxes, which could be material. In addition, we would determine any impairment to the carrying costs of deregulated plant and inventory assets.
Derivative Financial Instruments	Derivative Financial Instruments
	We account for derivative instruments in accordance with ASC 815, Derivatives and Hedging. All derivatives are recognized in the Consolidated Balance Sheets at their fair value unless they qualify for certain exceptions, including the normal purchases and normal sales exception. Additionally, derivatives that qualify and are designated for hedge accounting are classified as either hedges of the fair value of a recognized asset or liability or of an unrecognized firm commitment (fair-value hedge) or hedges of a forecasted transaction or the variability of cash flows to be received or paid related to a recognized asset or liability (cash-flow hedge). For fair-value hedges, changes in fair values for both the derivative and the underlying hedged exposure are recognized in earnings each period. For cash-flow hedges, the portion of the derivative gain or loss that is effective in offsetting the change in the cost or value of the underlying exposure is deferred in accumulated other comprehensive income (AOCI) and later reclassified into earnings when the underlying transaction occurs. Gains and losses from the ineffective portion of any hedge are recognized in earnings immediately. For other derivative contracts that do not qualify or are not designated for hedge accounting, changes in the fair value of the derivatives are recognized in earnings each period. Cash inflows and outflows related to derivative instruments are included as a component of operating, investing or financing cash flows in the Consolidated Statements of Cash Flows, depending on the underlying nature of the hedged items.
	Revenues and expenses on contracts that qualify are designated as normal purchases and normal sales and are recognized when the underlying physical transaction is completed. While these contracts are considered derivative financial instruments, they are not required to be recorded at fair value, but on an accrual basis of accounting. Normal purchases and normal sales are contracts where physical delivery is probable, quantities are expected to be used or sold in the normal course of business over a reasonable period of time, and price is not tied to an unrelated underlying derivative. As part of our regulated electric and gas operations, we enter into contracts to buy and sell energy to meet the requirements of our customers. These contracts include short-term and long-term commitments to purchase and sell energy in the retail and wholesale markets with the intent and ability to deliver or take delivery. If it were determined that a transaction designated as a normal purchase or a normal sale no longer met the exceptions, the fair value of the related contract would be reflected as an asset or liability and immediately recognized through earnings. See Note 9, Risk Management and Hedging Activities for further discussion of our derivative activity.
Property, Plant and Equipment	Property, Plant and Equipment
	Property, plant and equipment are stated at original cost, including contracted services, direct labor and material, AFUDC, and indirect charges for engineering, supervision and similar overhead items. All expenditures for maintenance and repairs of utility property, plant and equipment are charged to the appropriate maintenance expense accounts. A betterment or replacement of a unit of property is accounted for as an addition and retirement of utility plant. At the time of such a retirement, the accumulated provision for depreciation is charged with the original cost of the property retired and also for the net cost of removal. Also included in plant and equipment are assets under capital lease, which are stated at the present value of minimum lease payments.
	AFUDC represents the cost of financing construction projects with borrowed funds and equity funds. While cash is not realized currently from such allowance, it is realized under the ratemaking process over the service life of the related property through increased revenues resulting from a higher rate base and higher depreciation expense. The component of AFUDC attributable to borrowed funds is included as a reduction to interest expense, while the equity component is included in other income. We determine the rate used to compute AFUDC in accordance with a formula established by the FERC. This rate averaged 8.0%, 8.1%, and 8.0%, for Montana and South Dakota for 2014, 2013, and 2012, respectively. AFUDC capitalized totaled $10.8 million for the year ended December 31, 2014, $8.2 million for the year ended December 31, 2013 and $7.9 million for the year ended December 31, 2012 for Montana and South Dakota combined.
	We record provisions for depreciation at amounts substantially equivalent to calculations made on a straight-line method by applying various rates based on useful lives of the various classes of properties (ranging from three to 50 years) determined from engineering studies. As a percentage of the depreciable utility plant at the beginning of the year, our provision for depreciation of utility plant was approximately 2.9%, 3.2%, and 3.3% for 2014, 2013, and 2012, respectively.
	Depreciation rates include a provision for our share of the estimated costs to decommission our jointly owned plants at the end of the useful life. The annual provision for such costs is included in depreciation expense, while the accumulated provisions are included in noncurrent regulatory liabilities.
Income Taxes	Income Taxes
	Exposures exist related to various tax filing positions, which may require an extended period of time to resolve and may result in income tax adjustments by taxing authorities. We have reduced deferred tax assets or established liabilities based on our best estimate of future probable adjustments related to these exposures. On a quarterly basis, we evaluate exposures in light of any additional information and make adjustments as necessary to reflect the best estimate of the future outcomes. We believe our deferred tax assets and established liabilities are appropriate for estimated exposures; however, actual results may differ from these estimates. The resolution of tax matters in a particular future period could have a material impact on our Consolidated Income Statements and provision for income taxes.
Environmental Costs	Environmental Costs
	We record environmental costs when it is probable we are liable for the costs and we can reasonably estimate the liability. We may defer costs as a regulatory asset if there is precedent for recovering similar costs from customers in rates. Otherwise, we expense the costs. If an environmental cost is related to facilities we currently use, such as pollution control equipment, then we may capitalize and depreciate the costs over the remaining life of the asset, assuming the costs are recoverable in future rates or future cash flows.
	Our remediation cost estimates are based on the use of an environmental consultant, our experience, our assessment of the current situation and the technology currently available for use in the remediation. We regularly adjust the recorded costs as we revise estimates and as remediation proceeds. If we are one of several designated responsible parties, then we estimate and record only our share of the cost.
Business Combinations	Business Combination
	Our November 2014 acquisition of hydro generating assets was accounted for using business combination accounting. Under this method, the purchase price paid by the acquirer is allocated to the assets acquired and liabilities assumed as of the acquisition date based on their fair value. For additional information see Note 3 - Hydro Transaction.

Significant_Accounting_Policie2

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Inventories	Inventories are stated at average cost. Inventory consisted of the following (in thousands):
		December 31,
		2014		2013
	Materials and supplies	$	30,672	$	28,263
	Storage gas and fuel	24,422		27,346
		$	55,094	$	55,609
Other Noncurrent Liabilties	Other noncurrent liabilities consisted of the following (in thousands):
		December 31,
		2014		2013
	Pension and other employee benefits	$	137,377	$	57,140
	Future QF obligation, net	136,893		136,448
	Environmental	28,060		28,194
	Customer advances	30,001		27,371
	Other	50,158		43,471
		$	382,489	$	292,624
Supplemental Cash Flow Information	Supplemental Cash Flow Information
		Year Ended December 31,
		2014		2013		2012
				(in thousands)
	Cash paid for:
	Income taxes	$	35	$	50	$	2,944
	Interest	63,482		57,789		51,271
	Significant non-cash transactions:
	Capital expenditures included in trade accounts payable	8,555		12,025		13,136

Hydro_Acquisition_Tables

Hydro Acquisition (Tables)	12 Months Ended
	Dec. 31, 2014
Purchase Price Allocation [Abstract]
Purchase price allocation [Table Text Block]	The purchase price was allocated based on the estimated fair values of the assets acquired and liabilities assumed at the date of the acquisition as follows:
	Purchase Price Allocation	(in millions)
	Assets Acquired
	Inventory	$	0.2
	Property Plant and Equipment	899.6
	Other Prepayments	4.5
	Total Assets Acquired	$	904.3
	Liabilities Assumed
	Other Current Liabilities	$	0.4
	Other Deferred Credits	0.4
	Total Liabilities Assumed	$	0.8
	Total Purchase Price	$	903.5

Regulatory_Assets_and_Liabilit1

Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Regulatory Assets and Liabilities Disclosure [Abstract]
Schedule of Regulatory Assets And Liabilities
		Note Reference	Remaining Amortization Period	December 31,
				2014		2013
				(in thousands)
	Pension	16	Undetermined	$	139,050	$	58,474
	Employee related benefits	16	Undetermined	19,080		17,700
	Distribution infrastructure projects		3 Years	9,407		12,543
	Environmental clean-up	20	Various	13,741		14,924
	Supply costs		1 Year	29,200		17,875
	Income taxes	13	Plant Lives	263,764		201,808
	Deferred financing costs		Various	12,151		13,919
	State & local taxes & fees		Various	5,319		6,582
	Other	—	Various	11,419		10,846
	Total regulatory assets			$	503,131	$	354,671
	Removal cost	7	Various	$	351,676	$	336,613
	Gas storage sales		25 Years	10,410		10,831
	Supply costs		1 Year	14,569		11,493
	Deferred revenue	4	1 Year	36,592		33,400
	Environmental clean-up		Various	2,501		1,194
	State & local taxes & fees		1 Year	511		551
	Other		Various	2,138		377
	Total regulatory liabilities			$	418,397	$	394,459

Property_Plant_and_Equipment_T

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Major classifications of property, plant and equipment	The following table presents the major classifications of our property, plant and equipment (in thousands):
		Estimated Useful Life		December 31,
				2014				2013
		(years)		(in thousands)
	Land, land rights and easements	54 – 96		$	130,816			$	128,123
	Building and improvements	27 – 64		168,041				163,852
	Transmission, distribution, and storage	15 – 85		2,579,861				2,448,821
	Generation	25 – 50		1,044,764				533,450
	Plant acquisition adjustment	34 – 50		654,835				204,754
	Other	2 – 45		326,211				308,345
	Construction work in process	–—		221,868				104,891
				5,126,396				3,892,236
	Less accumulated depreciation			(1,368,388		)		(1,202,108		)
				$	3,758,008			$	2,690,128
Schedule of jointly owned utility plants	Information relating to our ownership interest in these facilities is as follows (in thousands):
		Big Stone				Neal #4				Coyote			Colstrip Unit 4 (MT)
		(SD)				(IA)				(ND)
	December 31, 2014
	Ownership percentages	23.4		%		8.7		%		10		%	30		%
	Plant in service	$	61,628			$	59,579			$	46,045		$	292,806
	Accumulated depreciation	46,741				27,742				36,649			72,976
	December 31, 2013
	Ownership percentages	23.4		%		8.7		%		10		%	30		%
	Plant in service	$	61,186			$	57,633			$	46,003		$	290,163
	Accumulated depreciation	45,792				29,841				36,076			70,072

Asset_Retirement_Obligation_Ta

Asset Retirement Obligation (Tables)	12 Months Ended
	Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]
Schedule of Change in Asset Retirement Obligation	The following table presents the change in our gross conditional ARO (in thousands):
		December 31,
		2014			2013
	Liability at January 1,	$	20,886		$	9,283
	Accretion expense	1,073			745
	Liabilities incurred	552			8,829
	Liabilities settled	(85		)	(27		)
	Revisions to cash flows	(991		)	2,056
	Liability at December 31,	$	21,435		$	20,886

Goodwill_Tables

Goodwill (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill [Abstract]
Schedule of Goodwill	Goodwill by segment is as follows (in thousands):
		December 31,
		2014		2013
	Electric	$	241,100	$	241,100
	Natural gas	114,028		114,028
		$	355,128	$	355,128

Risk_Management_and_Hedging_Ac1

Risk Management and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss)	The following table shows the effect of the interest rate swaps terminated in November 2014 and interest rate swaps previously terminated on the Consolidated Financial Statements (in thousands):
	Cash Flow Hedges	Location of Amount Reclassified from AOCI to Income	Amount Reclassified from AOCI into Income during the Year Ended
			31-Dec-14
	Interest rate contracts	Interest Expense	$	1,111

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The table below sets forth by level within the fair value hierarchy the gross components of our assets and liabilities measured at fair value on a recurring basis. Normal purchases and sales transactions are not included in the fair values by source table as they are not recorded at fair value. See Note 9 - Risk Management and Hedging Activities for further discussion.
	We record transfers between levels of the fair value hierarchy, if necessary, at the end of the reporting period. There were no transfers between levels for the periods presented.
	December 31, 2014		Quoted Prices in			Significant Other			Significant			Margin Cash Collateral		Total Net Fair Value
			Active Markets for			Observable Inputs			Unobservable Inputs			Offset
			Identical Assets or			(Level 2)			(Level 3)
			Liabilities (Level 1)
			(in thousands)
	Restricted cash		$	13,140		$	—		$	—		$	—	$	13,140
	Rabbi trust investments		21,594			—			—			—		21,594
	Total		$	34,734		$	—		$	—		$	—	$	34,734
	December 31, 2013
	Restricted cash		$	6,650		$	—		$	—		$	—	$	6,650
	Rabbi trust investments		16,477			—			—			—		16,477
	Total		$	23,127		$	—		$	—		$	—	$	23,127
Schedule of Estimated Fair Value of Financial Instruments	The estimated fair value of financial instruments is summarized as follows (in thousands):
		December 31, 2014						December 31, 2013
		Carrying Amount			Fair Value			Carrying Amount			Fair Value
	Liabilities:
	Long-term debt	$	1,662,099		$	1,817,642		$	1,155,097		$	1,237,151

ShortTerm_Borrowings_and_Credi1

Short-Term Borrowings and Credit Arrangements (Tables)	12 Months Ended
	Dec. 31, 2014
Short-term Debt [Abstract]
Schedule of Short-term Debt	Short-term borrowings and the corresponding weighted average interest rates as of December 31 were as follows (dollars in millions, except for percentages):
			2014					2013
	Short-Term Debt		Balance			Interest Rate		Balance		Interest Rate
	Commercial Paper		$	267.8		0.5	%	$	141	0.41	%
	The following information relates to commercial paper for the years ended December 31 (dollars in millions):
		2014			2013
	Maximum short-term debt outstanding	$	276.9		$	199.9
	Average short-term debt outstanding	$	132.5		$	69
	Weighted-average interest rate	0.39		%	0.4		%

LongTerm_Debt_and_Capital_Leas1

Long-Term Debt and Capital Leases (Tables)	12 Months Ended
	Dec. 31, 2014
Long-term Debt and Capital Lease Obligations [Abstract]
Schedule of Debt and Capital Leases	Long-term debt and capital leases consisted of the following (in thousands):
			December 31,
		Due	2014			2013
	Unsecured Debt:
	Unsecured Revolving Line of Credit	2018	$	—		$	—
	Secured Debt:
	Mortgage bonds—
	South Dakota—6.05%	2018	55,000			55,000
	South Dakota—5.01%	2025	64,000			64,000
	South Dakota—4.15%	2042	30,000			30,000
	South Dakota—4.30%	2052	20,000			20,000
	South Dakota—4.85%	2043	50,000			50,000
	South Dakota—4.22%	2044	30,000			—
	Montana—6.04%	2016	150,000			150,000
	Montana—6.34%	2019	250,000			250,000
	Montana—5.71%	2039	55,000			55,000
	Montana—5.01%	2025	161,000			161,000
	Montana—4.15%	2042	60,000			60,000
	Montana—4.30%	2052	40,000			40,000
	Montana—4.85%	2043	15,000			15,000
	Montana—3.99%	2028	35,000			35,000
	Montana—4.176%	2044	450,000			—
	Pollution control obligations—
	Montana—4.65%	2023	170,205			170,205
	Other Long Term Debt:
	New Market Tax Credit Financing—1.146%	2046	26,977			—
	Discount on Notes and Bonds	—	(83		)	(108		)
			1,662,099			1,155,097
	Less current maturities		—			—
			$	1,662,099		$	1,155,097
	Capital Leases:
	Total Capital Leases	Various	$	29,892		$	31,557
	Less current maturities		(1,730		)	(1,662		)
			$	28,162		$	29,895

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule Of Income Tax Expense Domestic	Income tax (benefit) expense is comprised of the following (in thousands):
		Year Ended December 31,
		2014				2013				2012
	Federal
	Current	$	(405	)		$	108			$	5,358
	Deferred	(5,658		)		18,150				13,197
	Investment tax credits	(273		)		(335		)		(376		)
	State
	Current	18				83				(1,411		)
	Deferred	(3,954		)		(3,705		)		1,321
		$	(10,272	)		$	14,301			$	18,089
Schedule of Effective Income Tax Rate Reconciliation	The following table reconciles our effective income tax rate to the federal statutory rate:
		Year Ended December 31,
		2014			2013			2012
	Federal statutory rate	35	%		35	%		35	%
	State income, net of federal provisions	(1.8	)		(2.8	)		0.9
	Flow-through repairs deductions	(22.9	)		(16.4	)		(14.0	)
	Recognition of unrecognized tax benefit	(11.4	)		—			—
	Prior year permanent return to accrual adjustments	(4.7	)		0.5			(1.6	)
	Production tax credits	(2.8	)		(2.9	)		—
	Plant and depreciation of flow through items	0.1			(0.5	)		(1.1	)
	Recognition of state net operating loss benefit / valuation allowance release	—			—			(2.1	)
	Other, net	(0.8	)		0.3			(1.6	)
		(9.3	)%		13.2	%		15.5	%
	The following table summarizes the significant differences in income tax (benefit) expense based on the differences between our effective tax rate and the federal statutory rate (in thousands):
		Year Ended December 31,
		2014				2013				2012
	Income Before Income Taxes	$	110,414			$	108,284			$	116,495
	Income tax calculated at 35% federal statutory rate	38,645				37,899				40,774
	Permanent or flow through adjustments:
	State income, net of federal provisions	(1,969		)		(3,082		)		1,078
	Flow-through repairs deductions	(25,268		)		(17,763		)		(16,350		)
	Recognition of unrecognized tax benefit	(12,607		)		—				—
	Prior year permanent return to accrual adjustments	(5,172		)		541				(1,901		)
	Production tax credits	(3,136		)		(3,171		)		—
	Plant and depreciation of flow through items	74				(584		)		(1,281		)
	Recognition of state net operating loss benefit / valuation allowance release	—				—				(2,398		)
	Other, net	(839		)		461				(1,833		)
		$	(48,917	)		$	(23,598	)		$	(22,685	)
	Income tax (benefit) expense	$	(10,272	)		$	14,301			$	18,089
Schedule of Deferred Tax Assets and Liabilities	The components of the net deferred income tax liability recognized in our Consolidated Balance Sheets are related to the following temporary differences (in thousands):
		December 31,
		2014				2013
	Pension / postretirement benefits	$	51,817			$	20,522
	NOL carryforward	42,787				16,758
	Unbilled revenue	19,863				18,136
	Compensation accruals	17,315				10,409
	Customer advances	11,817				10,781
	AMT credit carryforward	10,357				10,357
	Environmental liability	8,968				9,026
	Production tax credit	6,452				3,171
	Interest rate hedges	6,251				—
	QF obligations	2,162				2,066
	Reserves and accruals	1,772				12,097
	Property taxes	881				796
	Regulatory liabilities	975				659
	Regulatory assets	—				7,248
	Other, net	4,415				2,827
	Deferred Tax Asset	185,832				124,853
	Excess tax depreciation	(349,428		)		(304,071		)
	Goodwill amortization	(137,090		)		(122,798		)
	Flow through depreciation	(103,677		)		(79,016		)
	Regulatory assets	(21,394		)		—
	Deferred Tax Liability	(611,589		)		(505,885		)
	Deferred Tax Liability, net	$	(425,757	)		$	(381,032	)
Summary of Income Tax Contingencies	The change in unrecognized tax benefits is as follows (in thousands):
		2014				2013				2012
	Unrecognized Tax Benefits at January 1	$	113,466			$	113,291			$	131,949
	Gross increases - tax positions in prior period	—				—				—
	Gross decreases - tax positions in prior period	—				—				(1,766		)
	Gross increases - tax positions in current period	909				518				2,391
	Gross decreases - tax positions in current period	(5,597		)		(343		)		(19,283		)
	Lapse of statute of limitations	(12,849		)		—				—
	Unrecognized Tax Benefits at December 31	$	95,929			$	113,466			$	113,291

Comprehensive_Income_Loss_Tabl

Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Schedule of Comprehensive Income (Loss)	The following tables display the components of Other Comprehensive Income (Loss), after-tax, and the related tax effects (in thousands):
		December 31,
		2014												2013										2012
		Before-Tax Amount				Tax Benefit				Net-of-Tax Amount				Before-Tax Amount				Tax Benefit			Net-of-Tax Amount			Before-Tax Amount			Tax Benefit		Net-of-Tax Amount
	Foreign currency translation adjustment	$	265			$	—			$	265			$	166			—			$	166		$	(54	)	$	—	$	(54	)
	Reclassification of net gains on derivative instruments	(1,110		)		426				(684		)		(1,188		)		458			(730		)	(1,188		)	456		(732		)
	Realized loss on cash flow hedging derivatives	(18,388		)		7,243				(11,145		)		—				—			—			—			—		—
	Pension and postretirement medical liability adjustment	134				(52		)		82				1,568				(605		)	963			(897		)	344		(553		)
	Other comprehensive income (loss)	$	(19,099	)		$	7,617			$	(11,482	)		$	546			$	(147	)	$	399		$	(2,139	)	$	800	$	(1,339	)
Schedule of Accumulated Comprehensive Income (Loss)	Balances by classification included within AOCI on the Consolidated Balance Sheets are as follows, net of tax (in thousands):
		December 31, 2014				December 31, 2013
	Foreign currency translation	$	797			$	532
	Derivative instruments designated as cash flow hedges	(8,316		)		3,513
	Pension and postretirement medical plans	(1,247		)		(1,329		)
	Accumulated other comprehensive income	(8,766		)		2,716
Reclassification out of Accumulated Other Comprehensive Income	The following table displays the changes in AOCI by component, net of tax (in thousands):
				31-Dec-14
				Twelve Months Ended
		Affected Line Item in the Consolidated Statements of Income		Interest Rate Derivative Instruments Designated as Cash Flow Hedges				Pension and Postretirement Medical Plans				Foreign Currency Translation				Total
	Beginning balance			$	3,513			$	(1,329	)		$	532			$	2,716
	Other comprehensive (loss) income before reclassifications			(11,145		)		—				265				$	(10,880	)
	Amounts reclassified from accumulated other comprehensive income	Interest Expense		(684		)		—				—				$	(684	)
	Amounts reclassified from accumulated other comprehensive income			—				82				—				$	82
	Net current-period other comprehensive (loss) income			(11,829		)		82				265				(11,482		)
	Ending balance			$	(8,316	)		$	(1,247	)		$	797			$	(8,766	)
				31-Dec-13
				Twelve Months Ended
		Affected Line Item in the Consolidated Statements of Income		Interest Rate Derivative Instruments Designated as Cash Flow Hedges				Pension and Postretirement Medical Plans				Foreign Currency Translation				Total
	Beginning balance			$	4,243			$	(2,292	)		$	366			$	2,317
	Other comprehensive income before reclassifications			—				—				166				$	166
	Amounts reclassified from accumulated other comprehensive income	Interest Expense		(730		)		—				—				$	(730	)
	Amounts reclassified from accumulated other comprehensive income			—				963				—				$	963
	Net current-period other comprehensive (loss) income			(730		)		963				166				399
	Ending balance			$	3,513			$	(1,329	)		$	532			$	2,716

Operating_Leases_Tables

Operating Leases (Tables)	12 Months Ended
	Dec. 31, 2014
Leases [Abstract]
Schedule of Future Minimum Payments for Operating Leases	We lease vehicles, office equipment and facilities under various long-term operating leases. At December 31, 2014 future minimum lease payments for the next five years under non-cancelable lease agreements are as follows (in thousands):
	2015	1,996
	2016	1,484
	2017	671
	2018	70
	2019	61

Employee_Benefit_Plans_Tables

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Schedule of Changes in Projected Benefit Obligations	Following is a reconciliation of the changes in plan benefit obligations and fair value of plan assets, and a statement of the funded status (in thousands):
		Pension Benefits								Other Postretirement Benefits
		December 31,								December 31,
		2014				2013				2014				2013
	Change in Benefit Obligation:
	Obligation at beginning of period	$	567,866			$	609,643			$	30,084			$	34,040
	Service cost	10,830				13,465				465				541
	Interest cost	26,147				22,719				859				877
	Actuarial loss (gain)	107,023				(54,671		)		958				(3,156		)
	Settlements	—				—				690				—
	Benefits paid	(23,422		)		(23,290		)		(3,052		)		(2,218		)
	Benefit obligation at end of period	$	688,444			$	567,866			$	30,004			$	30,084
	Change in Fair Value of Plan Assets:
	Fair value of plan assets at beginning of period	$	516,352			$	472,936			$	18,183			$	15,893
	Return on plan assets	52,921				55,006				1,391				2,662
	Employer contributions	10,200				11,700				1,518				1,846
	Benefits paid	(23,422		)		(23,290		)		(3,052		)		(2,218		)
	Fair value of plan assets at end of period	$	556,051			$	516,352			$	18,040			$	18,183
	Funded Status	$	(132,393	)		$	(51,514	)		$	(11,964	)		$	(11,901	)
	Amounts recognized in the balance sheet consist of:
	Current liability	—				—				(1,169		)		(1,178		)
	Noncurrent liability	(132,393		)		(51,514		)		(10,795		)		(10,723		)
	Net amount recognized	$	(132,393	)		$	(51,514	)		$	(11,964	)		$	(11,901	)
	Amounts recognized in regulatory assets consist of:
	Prior service (cost) credit	(502		)		(748		)		17,098				19,247
	Net actuarial loss	(153,268		)		(71,777		)		(4,945		)		(4,807		)
	Amounts recognized in AOCI consist of:
	Prior service cost	—				—				(1,151		)		(1,302		)
	Net actuarial gain	—				—				(409		)		(971		)
	Total	$	(153,770	)		$	(72,525	)		$	10,593			$	12,167
Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets	The total projected benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets were as follows (in millions):
		Pension Benefits
		December 31,
		2014				2013
	Projected benefit obligation	$	688.4			$	567.9
	Accumulated benefit obligation	685				565
	Fair value of plan assets	556.1				516.4
Schedule of Defined Benefit Plans Disclosures	The components of the net costs (credits) for our pension and other postretirement plans are as follows (in thousands):
		Pension Benefits												Other Postretirement Benefits
		December 31,												December 31,
		2014				2013				2012				2014				2013			2012
	Components of Net Periodic Benefit Cost
	Service cost	$	10,830			$	13,465			$	11,488			$	465			$	541		$	541
	Interest cost	26,147				22,719				23,823				859				877			1,167
	Expected return on plan assets	(29,506		)		(32,491		)		(29,996		)		(981		)		(1,019		)	(1,021		)
	Amortization of prior service cost (credit)	246				246				246				(1,998		)		(1,998		)	(1,998		)
	Recognized actuarial loss	2,118				11,648				8,646				348				1,271			790
	Settlement loss recognized	—				—				—				690				—			—
	Net Periodic Benefit Cost (Credit)	$	9,835			$	15,587			$	14,207			$	(617	)		$	(328	)	$	(521	)
Schedule of Estimated Amortization of Regulatory Assets Into Net Periodic Benefit Costs	We estimate amortizations from regulatory assets into net periodic benefit cost during 2015 will be as follows (in thousands):
		Pension Benefits				Other
						Postretirement
						Benefits
	Prior service credit (cost)	$	(246	)		$	1,998
	Accumulated loss	(10,470		)		(325		)
Schedule of Assumptions Used	The weighted-average assumptions used in calculating the preceding information are as follows:
		Pension Benefits						Other Postretirement Benefits
		December 31,						December 31,
		2014		2013		2012		2014		2013		2012
	Discount rate	3.75-3.90	%	4.55-4.75	%	3.55-3.80	%	3.20-3.40	%	3.75-4.20	%	2.25-3.20	%
	Expected rate of return on assets	5.8		7		7		5.8		7		7
	Long-term rate of increase in compensation levels (nonunion)	3.58		3.58		3.58		3.58		3.58		3.58
	Long-term rate of increase in compensation levels (union)	3.5		3.5		3.5		3.5		3.5		3.5
Schedule of Pension And Postretirement Benefits Investment Strategy	Based on this, the target asset allocation established, within an allowable range of plus or minus 5%, is as follows:
		Pension Benefits						Other Benefits
		December 31,						December 31,
		2014			2013			2014			2013
	Domestic debt securities	55	%		60	%		40	%		40	%
	International debt securities	5			5			—			—
	Domestic equity securities	34			30			50			50
	International equity securities	6			5			10			10
Schedule of Allocation of Plan Assets	The actual allocation by plan is as follows:
		NorthWestern Energy Pension						NorthWestern Corporation Pension						NorthWestern Energy
														Health and Welfare
		December 31,						December 31,						December 31,
		2014			2013			2014			2013			2014			2013
	Cash and cash equivalents	—	%		—	%		0.1	%		0.1	%		0.2	%		1.8	%
	Domestic debt securities	56			58.6			65.6			64.7			37.2			38.6
	International debt securities	4.4			4.9			4.5			4.9			—			0.3
	Domestic equity securities	34.1			31.4			25.1			25.3			53.9			50.1
	International equity securities	5.5			5.1			4.7			5			8.7			9.2
		100	%		100	%		100	%		100	%		100	%		100	%
Schedule of Fair Value of Plan Assets by Category	The fair value of our plan assets at December 31, 2014, by asset category are as follows (in thousands):
	Asset Category	Total				Quoted Market				Significant Observable Inputs				Significant Unobservable Inputs
						Prices in Active				Level 2				Level 3
						Markets for
						Identical Assets
						Level 1
	Pension Plan Assets
	Cash and cash equivalents	$	126			$	—			$	126			$	—
	Equity securities: (1)
	US small/mid cap growth	16,605				—				16,605				—
	US small/mid cap value	15,264				—				15,264				—
	US large cap growth	48,560				—				48,560				—
	US large cap value	48,785				—				48,785				—
	US large cap passive	54,775				—				54,775				—
	Non-US core	22,634				—				22,634				—
	Emerging markets	7,650				—				7,650				—
	Fixed income securities:(2)
	US core	23,177				—				23,177				—
	US passive	12,269				—				12,269				—
	Long duration	41,451				—				41,451				—
	Long duration investment grade	52,767				—				52,767				—
	Long duration passive	41,475				—				41,475				—
	Opportunistic	5,692				—				5,692				—
	Non-US passive	24,504				—				24,504				—
	Active long corporate	133,160				—				133,160				—
	Participating group annuity contract	7,157				—				7,157				—
		$	556,051			$	—			$	556,051			$	—
	Other Postretirement Benefit Plan Assets
	Cash and cash equivalents	$	44			$	—			$	44			$	—
	Equity securities: (1)									—
	US small/mid cap growth	752				—				752				—
	US small/mid cap value	721				—				721				—
	S&P 500 index	8,234				—				8,234				—
	US large cap growth	6				—				6				—
	US large cap value	6				—				6				—
	US large cap passive	7				—				7				—
	Non-US core	1,495				—				1,495				—
	Emerging markets	72				—				72				—
	Fixed income securities: (2)
	Passive bond market	1,992				—				1,992				—
	US core	4,435				—				4,435				—
	US passive	1				—				1				—
	Long duration	5				—				5				—
	Long duration investment grade	6				—				6				—
	Long duration passive	5				—				5				—
	Opportunistic	240				—				240				—
	Non-US passive	3				—				3				—
	Active long corporate	16				—				16				—
		$	18,040			$	—			$	18,040			$	—
	The fair value of our plan assets at December 31, 2013, by asset category are as follows (in thousands):
	Asset Category	Total				Quoted Market				Significant Observable Inputs				Significant Unobservable Inputs
						Prices in Active				Level 2				Level 3
						Markets for
						Identical Assets
						Level 1
	Pension Plan Assets
	Cash and cash equivalents	$	168			$	—			$	168			$	—
	Equity securities: (1)
	US small/mid cap growth	13,764				—				13,764				—
	US small/mid cap value	13,664				—				13,664				—
	US large cap growth	42,094				—				42,094				—
	US large cap value	42,102				—				42,102				—
	US large cap passive	47,227				—				47,227				—
	Non-US core	20,015				—				20,015				—
Emerging markets	6,250				—				6,250				—
Fixed income securities:(2)
US core	82,639				—				82,639				—
US passive	44,762				—				44,762				—
Long duration	24,401				—				24,401				—
Long duration investment grade	32,700				—				32,700				—
Long duration passive	24,122				—				24,122				—
Opportunistic	5,876				—				5,876				—
Non-US passive	25,150				—				25,150				—
Active long corporate	83,147				—				83,147				—
Participating group annuity contract	8,271				—				8,271				—
	$	516,352			$	—			$	516,352			$	—
Other Postretirement Benefit Plan Assets
Cash and cash equivalents	$	318			—				$	318			—
Equity securities: (1)
US small/mid cap growth	751				—				751				—
US small/mid cap value	736				—				736				—
S&P 500 index	7,321				—				7,321				—
US large cap growth	98				—				98				—
US large cap value	98				—				98				—
US large cap passive	110				—				110				—
Non-US core	1,595				—				1,595				—
Emerging markets	85				—				85				—
Fixed income securities: (2)
Passive bond market	1,880				—				1,880				—
US core	4,390				—				4,390				—
US passive	107				—				107				—
Long duration	55				—				55				—
Long duration investment grade	79				—				79				—
Long duration passive	55				—				55				—
Opportunistic	261				—				261				—
Non-US passive	57				—				57				—
Active long corporate	187				—				187				—
	$	18,183			$	—			$	18,183			$	—
_________________
-1	This category consists of active and passive managed equity funds, which are invested in multiple strategies to diversify risks and reduce volatility.
(2) This category consists of investment grade bonds of issuers from diverse industries, debt securities issued by international, national, state and local governments, and asset-backed securities. This includes both active and passive managed funds.
Schedule of Pension Contributions	Annual contributions to each of the pension plans are as follows (in thousands):
		2014				2013				2012
	NorthWestern Energy Pension Plan (MT)	$	9,000			$	10,500			$	10,500
	NorthWestern Pension Plan (SD)	1,200				1,200				1,200
		$	10,200			$	11,700			$	11,700
Schedule of Expected Benefit Payments	We estimate the plans will make future benefit payments to participants as follows (in thousands):
		Pension Benefits				Other Postretirement Benefits
	2015	$	27,652			$	3,516
	2016	29,905				3,516
	2017	31,172				3,387
	2018	33,142				3,282
	2019	34,660				3,026
	2020-2024	194,065				11,923

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions	The following summarizes the significant assumptions used to determine the fair value of performance shares and related compensation expense as well as the resulting estimated fair value of performance shares granted:
		2014		2013
	Risk-free interest rate	0.67	%	0.44	%
	Expected life, in years	3		3
	Expected volatility	15.5% to 23.3%		16.3% to 25.4%
	Dividend yield	3.3	%	3.9	%
Schedule of Nonvested Share Activity	A summary of nonvested shares as of and changes during the year ended December 31, 2014, are as follows:
		Performance Share Awards
		Shares		Weighted-Average Grant-Date
				Fair Value
	Beginning nonvested grants	173,646		$	29.14
	Granted	96,193		38.33
	Vested	(84,652	)	25.19
	Forfeited	(4,615	)	33.55
	Remaining nonvested grants	180,572		$	35.77
Share-based Compensation Arrangement by Share-based Payment Award
Schedule of Nonvested Restricted Stock Units Activity	A summary of nonvested shares as of and changes during the year ended December 31, 2014, are as follows:
		Shares		Weighted-Average Grant-Date
				Fair Value
	Beginning nonvested grants	26,628		$	30.24
	Granted	15,092		43.79
	Vested	—		—
	Forfeited	—		—
	Remaining nonvested grants	41,720		$	35.14

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule of Weighted Average Number of Shares	Average shares used in computing the basic and diluted earnings per share are as follows:
		December 31,
		2014	2013
	Basic computation	40,156,177	38,144,852
	Dilutive effect of
	Performance share awards (1)	275,774	82,223
	Diluted computation	40,431,951	38,227,075
	_____________________
	(1)           Performance share awards are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the award.

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of Changes In Qualifying Facility Liability	The following summarizes the change in the QF liability (in thousands):
		December 31,
		2014			2013
	Beginning QF liability	$	136,448		$	136,652
	Unrecovered amount	(10,128		)	(10,647		)
	Interest expense	10,573			10,443
	Ending QF liability	$	136,893		$	136,448
Schedule of Estimated Gross Contractual Obligation Less Amounts Recoverable Through Rates	The following summarizes the estimated gross contractual obligation less amounts recoverable through rates (in thousands):
		Gross			Recoverable			Net
		Obligation			Amounts
	2015	$	69,606		$	56,598		$	13,008
	2016	71,598			57,188			14,410
	2017	73,622			57,789			15,833
	2018	75,688			58,401			17,287
	2019	77,791			59,020			18,771
	Thereafter	646,783			508,195			138,588
	Total	$	1,015,088		$	797,191		$	217,897

Segment_and_Related_Informatio1

Segment and Related Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment	Financial data for the business segments are as follows (in thousands):
	December 31, 2014	Electric			Gas			Other			Eliminations		Total
	Operating revenues	$	877,967		326,896			$	—		$	—	$	1,204,863
	Cost of sales	348,640			133,951			—			—		482,591
	Gross margin	529,327			192,945			—			—		722,272
	Operating, general and administrative	200,186			91,437			14,263			—		305,886
	Property and other taxes	84,759			29,821			12			—		114,592
	Depreciation	94,813			28,930			33			—		123,776
	Operating income (loss)	149,569			42,757			(14,308		)	—		178,018
	Interest expense	(60,424		)	(10,618		)	(6,760		)	—		(77,802		)
	Other income	4,758			1,324			4,116			—		10,198
	Income tax (expense) benefit	(1,490		)	(7,463		)	19,225			—		10,272
	Net income	$	92,413		$	26,000		$	2,273		$	—	$	120,686
	Total assets	$	3,442,659		$	1,522,902		$	8,382		$	—	$	4,973,943
	Capital expenditures	$	233,538		$	36,846		$	—		$	—	$	270,384
	December 31, 2013	Electric			Gas			Other			Eliminations		Total
	Operating revenues	$	865,239		$	287,605		$	1,675		$	—	$	1,154,519
	Cost of sales	358,688			120,858			—			—		479,546
	Gross margin	506,551			166,747			1,675			—		674,973
	Operating, general and administrative	195,100			78,822			11,647			—		285,569
	Property and other taxes	78,536			26,993			11			—		105,540
	Depreciation	89,728			23,070			33			—		112,831
	Operating income (loss)	143,187			37,862			(10,016		)	—		171,033
	Interest expense	(57,920		)	(9,993		)	(2,573		)	—		(70,486		)
	Other income	4,061			1,239			2,437			—		7,737
	Income tax (expense) benefit	(13,905		)	(4,134		)	3,738			—		(14,301		)
	Net income (loss)	$	75,423		$	24,974		$	(6,414	)	$	—	$	93,983
	Total assets	$	2,583,554		$	1,117,861		$	13,845		—		$	3,715,260
	Capital expenditures	$	198,032		$	32,422		$	—		—		$	230,454
	December 31, 2012	Electric			Gas			Other			Eliminations		Total
	Operating revenues	$	805,554		$	263,394		$	1,394		$	—	$	1,070,342
	Cost of sales	277,826			117,608			—			—		395,434
	Gross margin	527,728			145,786			1,394			—		674,908
	Operating, general and administrative	187,599			75,971			6,396			—		269,966
	MSTI impairment	24,039			—			—			—		24,039
	Property and other taxes	72,755			24,907			12			—		97,674
	Depreciation	86,559			19,452			33			—		106,044
	Operating income (loss)	156,776			25,456			(5,047		)	—		177,185
	Interest expense	(55,118		)	(9,063		)	(881		)	—		(65,062		)
	Other income	2,630			1,633			109			—		4,372
	Income tax (expense) benefit	(22,298		)	(692		)	4,901			—		(18,089		)
	Net income	$	81,990		$	17,334		$	(918	)	$	—	$	98,406
	Total assets	2,442,602			$	1,032,259		$	10,672		$	—	$	3,485,533
	Capital expenditures	178,325			$	40,909		$	—		$	—	$	219,234

Quarterly_Financial_Data_Unaud1

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	Amounts presented are in thousands, except per share data:
	2014	First		Second		Third		Fourth
	Operating revenues	$	369,723	$	270,281	$	251,912	$	312,947
	Operating income	71,350		25,097		30,987		50,584
	Net income	$	45,580	$	7,746	$	30,191	$	37,169
	Average common shares outstanding	38,856		39,137		39,141		43,451
	Income per average common share (basic):
	Net income	$	1.17	$	0.2	$	0.77	$	0.87
	Income per average common share (diluted):
	Net income	$	1.17	$	0.2	$	0.77	$	0.85
	Dividends per share	$	0.4	$	0.4	$	0.4	$	0.4
	Stock price:
	High	$	47.86	$	52.49	$	52.7	$	58.7
	Low	42.64		45.49		45.3		45.14
	Quarter-end close	47.43		52.19		45.36		56.58
	2013	First		Second		Third		Fourth
	Operating revenues	$	313,020	$	260,161	$	262,248	$	319,090
	Operating income	57,010		32,660		31,401		49,962
	Net income	$	37,902	$	14,341	$	15,647	$	26,093
	Average common shares outstanding	37,384		38,092		38,459		38,626
	Income per average common share (basic):					0		0
	Net income	$	1.01	$	0.37	$	0.41	$	0.67
	Income per average common share (diluted):
	Net income	$	1.01	$	0.37	$	0.4	$	0.68
	Dividends per share	$	0.38	$	0.38	$	0.38	$	0.38
	Stock price:
	High	$	40.35	$	43.17	$	45.85	$	47.18
	Low	35.06		38.12		39.08		41.31
	Quarter-end close	39.86		39.9		44.92		43.32

Nature_of_Operations_and_Basis2

Nature of Operations and Basis of Consolidation (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	watts
	customers
Number of customers	692,600
Number of megawatts of qualifying facility	35
Estimated aggregate gross contractual payments through 2024	$262.90
Variable interest entity, measure of activity, purchases	$24.40	$23.80	$21

Significant_Accounting_Policie3

Significant Accounting Policies Inventory (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accounting Policies [Abstract]
Materials and supplies	$30,672	$28,263
Storage gas and fuel	24,422	27,346
Total	$55,094	$55,609

Significant_Accounting_Policie4

Significant Accounting Policies Property plant equipment (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Interest costs, capitalized during period	$10.80	$8.20	$7.90
Property, plant and equipment, disclosure of composite depreciation rate for plants in service	2.90%	3.20%	3.30%
MONTANA
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	8.00%	8.10%	8.00%

Significant_Accounting_Policie5

Significant Accounting Policies Other Noncurrent Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accounting Policies [Abstract]
Pension and other employee benefits	$137,377	$57,140
Future QF obligation, net	136,893	136,448
Environmental	28,060	28,194
Customer advances	30,001	27,371
Other	50,158	43,471
Total	$382,489	$292,624

Significant_Accounting_Policie6

Significant Accounting Policies Supplemental Cash Flows (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Income taxes, cash paid (received)	$35	$50	$2,944
Interest, cash paid (received)	63,482	57,789	51,271
Capital expenditures included in trade accounts payable	$8,555	$12,025	$13,136

Significant_Accounting_Policie7

Significant Accounting Policies Narrative (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
	days
Allowance for doubtful accounts receivable, current	$4.30	$4.50
Unbilled receivables,current	$70.30	$74.30
Number of days or less of maturity to be considered cash equivalent	90

Hydro_Acquisition_Purchase_pri

Hydro Acquisition Purchase price allocation (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Assets Acquired [Abstract]
Inventory	$0.20
Property, plant, and equipment	899.6
Other prepayments	4.5
Total assets acquired	904.3
Liabilities Assumed [Abstract]
Other current liabilities	0.4
Other deferred credits	0.4
Total liabilities assumed	0.8
Total purchase price	$903.50

Hydro_Acquisition_Hydroelectri

Hydro Acquisition Hydro-electric (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Nov. 30, 2014	Sep. 26, 2014
		watts
Significant Acquisitions and Disposals [Line Items]
Megawatts of generation		633
Purchase price		$904
Owned average load serving requirement	60.00%
Approved rate base, Hydro Electric			870
Estimated Useful Life	50 years
Approved rate increase, amount	117
Common stock aggregate gross sales price, maximum	400
Debt issuance for Hydro transaction	450
Transaction costs	9.5
Debt related commitment fees	5.8
Hydro-electric Assets [Member]
Significant Acquisitions and Disposals [Line Items]
Approved return on equity, percentage	9.80%
Authorized cost of debt	4.25%
Approved debt capital structure, percentage	52.00%
Approved equity capital structure, percentage	48.00%
Kerr Project [Member]
Significant Acquisitions and Disposals [Line Items]
Megawatts of generation		194
Purchase price		30
Estimated reference price less conveyance price		11.7
Estimated conveyance price of hydroelectric facility		$18.30

Regulatory_Matters_Details

Regulatory Matters (Details) (USD $)	3 Months Ended	12 Months Ended	27 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2014
Regulatory Assets [Line Items]
Demand side management lost revenue expected recovery	$7.10	$7.10	$7.10
South Dakota Electric Rate Filing [Member]
Regulatory Assets [Line Items]
Requested rate increase	26.5
Approved return on equity, percentage	10.00%
Capital structure,percentage of debt	46.00%
Capitalized structure, percentage of equity	54.00%
Rate base	447.4	447.4	447.4
Regulatory Reviews of Filings [Member]
Regulatory Assets [Line Items]
CU4 incremental market purchases identified for further review		11
Revenue Subject to Refund [Member] | Dave Gates Generating Station [Member]
Regulatory Assets [Line Items]
Deferred revenue subject to refund	27.3	27.3	27.3
Revenue Subject to Refund [Member] | Montana Natural Gas Production Assets [Member]
Regulatory Assets [Line Items]
Revenue of acquiree since acquisition date, Actual			$29.30

Regulatory_Assets_and_Liabilit2

Regulatory Assets and Liabilities (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$503,131	$354,671
Regulatory Assets [Member] | Pension [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	139,050	58,474
Regulatory Assets [Member] | Employee related benefits [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	19,080	17,700
Regulatory Assets [Member] | Distribution infrastructure projects
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	9,407	12,543
Regulatory assets, remaining amortization period	P3Y
Regulatory Assets [Member] | Environmental clean-up [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	13,741	14,924
Regulatory Assets [Member] | Supply costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	29,200	17,875
Regulatory assets, remaining amortization period	P1Y
Regulatory Assets [Member] | Income taxes [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	263,764	201,808
Regulatory Assets [Member] | Deferred financing costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	12,151	13,919
Regulatory Assets [Member] | State And Local Taxes And Fees [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	5,319	6,582
Regulatory Assets [Member] | Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	11,419	10,846
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	418,397	394,459
Regulatory Liabilities [Member] | Removal cost
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	351,676	336,613
Regulatory Liabilities [Member] | Gas storage sales
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	10,410	10,831
Regulatory liability, remaining amortization period	P25Y
Regulatory Liabilities [Member] | Supply costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	14,569	11,493
Regulatory liability, remaining amortization period	P1Y
Regulatory Liabilities [Member] | Deferred revenue [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	36,592	33,400
Regulatory liability, remaining amortization period	P1Y
Regulatory Liabilities [Member] | Environmental clean-up [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	2,501	1,194
Regulatory Liabilities [Member] | State And Local Taxes And Fees [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	511	551
Regulatory liability, remaining amortization period	P1Y
Regulatory Liabilities [Member] | Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$2,138	$377

Regulatory_Assets_and_Liabilit3

Regulatory Assets and Liabilities Narrative (Details)	12 Months Ended
	Dec. 31, 2014
State And Local Taxes And Fees [Member]
Regulatory Assets And Liabilities [Line Items]
Percentage of estimated increase In local taxes and fees authorized for recovery by MPSC	60.00%
Electric Supply Costs [Member] | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	10.60%
Natural Gas Supply Costs [Member] | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.80%
Natural Gas Supply Costs [Member] | Nebraska
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	8.50%
Supply costs [Member] | Montana
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.50%

Property_Plant_and_Equipment_D

Property, Plant and Equipment (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Land, land rights and easements	$130,816,000	$128,123,000
Buildings and improvements	168,041,000	163,852,000
Transmission, distribution and storage	2,579,861,000	2,448,821,000
Generation	1,044,764,000	533,450,000
Plant acquisition adjustment	654,835,000	204,754,000
Other	326,211,000	308,345,000
Construction work in progress	221,868,000	104,891,000
Property, plant and equipment, gross	5,126,396,000	3,892,236,000
Less accumulated depreciation	-1,368,388,000	-1,202,108,000
Property, plant, and equipment, net	3,758,008,000	2,690,128,000
Estimated Useful Life	50 years
Property, plant, and equipment under capiltal leases	23,400,000	25,600,000
Increase in PP&E due to hydro acquisition	870,000,000
Land and improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	54 years
Land and improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	96 years
Building and improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	27 years
Building and improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	64 years
Tranmission, distribution and storage[Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	15 years
Tranmission, distribution and storage[Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	85 years
Generation [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	25 years
Generation [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	50 years
Plant Acquisition adjustment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	34 years
Plant Acquisition adjustment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	50 years
Other [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	2 years
Other [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	45 years
Basin Capital Lease [Member]
Property, Plant and Equipment [Line Items]
Property, plant, and equipment under capiltal leases	$23,100,000	$25,100,000

Property_Plant_and_Equipment_J

Property, Plant and Equipment Joint Ownership (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Jointly Owned Utility Plant Interests [Line Items]
Number of joint ownership interests in electric generating plants	4
Big Stone Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	23.40%	23.40%
Plant in service	$61,628	$61,186
Accumulated depreciation	46,741	45,792
Neal 4 Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	8.70%	8.70%
Plant in service	59,579	57,633
Accumulated depreciation	27,742	29,841
Coyote Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	10.00%	10.00%
Plant in service	46,045	46,003
Accumulated depreciation	36,649	36,076
Colstrip Unit 4 [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	30.00%	30.00%
Plant in service	292,806	290,163
Accumulated depreciation	$72,976	$70,072

Asset_Retirement_Obligation_Ro

Asset Retirement Obligation Rollforward (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liability at January 1,	$20,886	$9,283
Accretion expense	1,073	745
Liabilities incurred	552	8,829
Liabilities settled	-85	-27
Revision to cash flows	-991	2,056
Liability at December 31,	$21,435	$20,886

Goodwill_Details

Goodwill (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Line Items]
Goodwill	$355,128	$355,128
Goodwill, Period Increase (Decrease)	0	0
Electric
Goodwill [Line Items]
Goodwill	241,100	241,100
Natural gas
Goodwill [Line Items]
Goodwill	$114,028	$114,028

Risk_Management_and_Hedging_Ac2

Risk Management and Hedging Activities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014
				swaps
Derivative [Line Items]
Number of forward starting swaps				2
Amount of hedged item				$225,000,000
Debt issuance for Hydro transaction	450,000,000
Realized loss on cash flow hedging derivatives	18,388,000	0	0
Physical purchase and sale of gas and electricity at fixed prices	0	0
Pre-tax gain on cash flow hedge from AOCI to be reclassified during next 12 months	600,000
No swaps outstanding, interest rate fair value derivatives	0	0
Cash Flow Hedging [Member]
Derivative [Line Items]
Realized loss on cash flow hedging derivatives	18,400,000
Interest rate swap at 3.217% [Member]
Derivative [Line Items]
Derivative, fixed interest rate				3.22%
Interest rate swap at 3.227% [Member] [Member]
Derivative [Line Items]
Derivative, fixed interest rate				3.23%
Interest Rate Swap [Member]
Derivative [Line Items]
Pre-tax loss on cash flow hedges remaining in AOCI	13,800,000
Interest Rate Swap [Member] | Interest Expense [Member]
Derivative [Line Items]
Interest rate contracts, amount of gain reclassified from AOCI into income	1,111,000
Commodity Contract [Member] | Forward Contracts [Member]
Derivative [Line Items]
None of the forward purchase contracts that do not qualify for NPNS contain credit risk-related contingent features	$0

Fair_Value_Measurements_Fair_V

Fair Value Measurements Fair Value Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Level 1 to level 2 asset transfers, amount	$0	$0
Level 2 to level 1 assets, transfers, amount	0	0
Level 1 to level 2 liabilities transfers, amount	0	0
Level 2 to level 1 liabilities, transfers, amount	0	0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	0	0
Rabbi trust investments	0	0
Total	0	0
Fair Value, Measurements, Recurring [Member] | Total Net Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	13,140	6,650
Rabbi trust investments	21,594	16,477
Total	34,734	23,127
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets for Identical Assets or Liabilities, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	13,140	6,650
Rabbi trust investments	21,594	16,477
Total	34,734	23,127
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	0	0
Rabbi trust investments	0	0
Total	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	0	0
Rabbi trust investments	0	0
Total	$0	$0

Fair_Value_Measurements_Fair_V1

Fair Value Measurements Fair Value Financial Instruments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	$1,662,099	$1,155,097
Long-term debt, fair value	$1,817,642	$1,237,151

ShortTerm_Borrowings_and_Credi2

Short-Term Borrowings and Credit Arrangements (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Commercial Paper, Balance	$267,840,000	$140,950,000
Maximum borrowing capacity under commercial paper program	340,000,000
Short term borrowings maximum days outstanding	270 days
Short-term Debt [Member]
Short-term Debt [Line Items]
Commercial Paper, Balance	267,800,000	141,000,000
Commercial Paper, Interest Rate	0.50%	0.41%
Maximum short-term debt outstanding	276,900,000	199,900,000
Average short-term debt outstanding	$132,500,000	$69,000,000
Weighted average interest rate	0.39%	0.40%

ShortTerm_Borrowings_and_Credi3

Short-Term Borrowings and Credit Arrangements Unsecured Revolving Line of Credit (Details) (Unsecured Revolving Line Of Credit [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
	numberofbanks
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$350
Number of institutions participating in the credit facility	8
Maximum number of institutions participating in the credit faciltiy pertaining to maximum contributory percentage	1
Line of Credit Facility, maximum percentage of total availability provided by a single lender	21.00%
Commitment fees	0.4	0.5
Letters of credit outstanding, amount	$0
Maximum ratio of indebtedness to net capital threshold percentage	65.00%
Eurodollar [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.88%
Eurodollar [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Base Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Base Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%

ShortTerm_Borrowings_and_Credi4

Short-Term Borrowings and Credit Arrangements Bridge Loan (Details) (Bridge Loan [Member], USD $)	Nov. 01, 2013
In Millions, unless otherwise specified	days
Bridge Loan [Member]
Short-term Debt [Line Items]
Bridge term loan facility	$900
Bridge term loan facility agreement term	364

LongTerm_Debt_and_Capital_Leas2

Long-Term Debt and Capital Leases Schedule of Debt (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-term debt	$1,662,099	$1,155,097
Less current maturities	0	0
Long-term debt, excluding current maturities	1,662,099	1,155,097
Capital Lease
Total Capital Leases	29,892	31,557
Less current maturities	-1,730	-1,662
Capital lease obligations, noncurrent	28,162	29,895
Unsecured Debt | Unsecured Revolving Line Of Credit
Debt Instrument [Line Items]
Long-term debt	0	0
Maturity date	5-Nov-18
Secured Debt | South Dakota, 6.05%, Due 2018
Debt Instrument [Line Items]
Long-term debt	55,000	55,000
Interest rate, stated percentage	6.05%
Maturity date	1-May-18
Secured Debt | South Dakota, 5.01%, Due 2025
Debt Instrument [Line Items]
Long-term debt	64,000	64,000
Interest rate, stated percentage	5.01%
Maturity date	1-May-25
Secured Debt | South Dakota, 4.15%, Due 2042
Debt Instrument [Line Items]
Long-term debt	30,000	30,000
Interest rate, stated percentage	4.15%
Maturity date	10-Aug-42
Secured Debt | South Dakota, 4.30%, Due 2052
Debt Instrument [Line Items]
Long-term debt	20,000	20,000
Interest rate, stated percentage	4.30%
Maturity date	10-Aug-52
Secured Debt | South Dakota, 4.85% Due 2043
Debt Instrument [Line Items]
Long-term debt	50,000	50,000
Interest rate, stated percentage	4.85%
Maturity date	19-Dec-43
Secured Debt | Secured Debt South Dakota Due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	30,000	0
Interest rate, stated percentage	4.22%
Maturity date	19-Dec-44
Secured Debt | Montana, 6.04%, Due 2016
Debt Instrument [Line Items]
Long-term debt	150,000	150,000
Interest rate, stated percentage	6.04%
Maturity date	1-Sep-16
Secured Debt | Montana, 6.34%, Due 2019
Debt Instrument [Line Items]
Long-term debt	250,000	250,000
Interest rate, stated percentage	6.34%
Maturity date	1-Apr-19
Secured Debt | Montana, 5.71%, Due 2039
Debt Instrument [Line Items]
Long-term debt	55,000	55,000
Interest rate, stated percentage	5.71%
Maturity date	15-Oct-39
Secured Debt | Montana, 5.01%, Due 2025
Debt Instrument [Line Items]
Long-term debt	161,000	161,000
Interest rate, stated percentage	5.01%
Maturity date	1-May-25
Secured Debt | Montana, 4.15%, Due 2042
Debt Instrument [Line Items]
Long-term debt	60,000	60,000
Interest rate, stated percentage	4.15%
Maturity date	10-Aug-42
Secured Debt | Montana, 4.30%, Due 2052
Debt Instrument [Line Items]
Long-term debt	40,000	40,000
Interest rate, stated percentage	4.30%
Maturity date	10-Aug-52
Secured Debt | Montana 4.85%, Due 2043
Debt Instrument [Line Items]
Long-term debt	15,000	15,000
Interest rate, stated percentage	4.85%
Maturity date	19-Dec-43
Secured Debt | Montana 3.99% Due 2028
Debt Instrument [Line Items]
Long-term debt	35,000	35,000
Interest rate, stated percentage	3.99%
Maturity date	19-Dec-28
Secured Debt | Montana 4.176% Due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	450,000	0
Interest rate, stated percentage	4.18%
Maturity date	15-Nov-44
Secured Debt | Montana, 4.65%, Due 2023
Debt Instrument [Line Items]
Long-term debt	170,205	170,205
Interest rate, stated percentage	4.65%
Maturity date	1-Aug-23
Secured Debt | New Market Tax Credit Financing [Member]
Debt Instrument [Line Items]
Long-term debt	26,977	0
Interest rate, stated percentage	1.15%
Maturity date	1-Jul-46
Unamortized Discount Premium, Net | Discount on Notes and Bonds
Debt Instrument [Line Items]
Long-term debt	($83)	($108)

LongTerm_Debt_and_Capital_Leas3

Long-Term Debt and Capital Leases Schedule of Long-Term Debt (Details) (Secured Debt, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Secured Debt South Dakota Due 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	19-Dec-14
Debt instrument, face amount	$30
Interest rate, stated percentage	4.22%
Maturity date	19-Dec-44
Montana 4.176% Due 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	15-Nov-14
Debt instrument, face amount	$450
Interest rate, stated percentage	4.18%
Maturity date	15-Nov-44

LongTerm_Debt_and_Capital_Leas4

Long-Term Debt and Capital Leases Other Long-term Debt (Details) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2021
Debt Instrument [Line Items]
Other Long-term Debt	$450,000,000
Investment in New Market Tax Credit Program	18,169,000	0	0
New Market Tax Credit [Member]
Debt Instrument [Line Items]
Other Long-term Debt	27,000,000
New Market Tax Credit Financing [Member]
Debt Instrument [Line Items]
Investments	18,200,000
Debt Instrument, term	30 years
Interest rate, stated percentage	1.15%
Investment in New Market Tax Credit Program	8,800,000
Scenario, Forecast [Member] | New Market Tax Credit Financing [Member]
Debt Instrument [Line Items]
Debt instrument, forgiveness				$7,900,000

LongTerm_Debt_and_Capital_Leas5

Long-Term Debt and Capital Leases (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Maturities of Long-term Debt [Abstract]
2015	$1.70
2016	151.8
2017	2
2018	57.1
2019	$252.30

Income_Taxes_Narrative_Details

Income Taxes Narrative (Details) (Internal Revenue Service (IRS) [Member])	12 Months Ended
	Dec. 31, 2014
Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Earliest year subject to examination	2000

Income_Taxes_Domestic_Tax_Comp

Income Taxes Domestic Tax Components (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Federal
Current	($405)	$108	$5,358
Deferred	-5,658	18,150	13,197
Investment tax credits	-273	-335	-376
State
Current	18	83	-1,411
Deferred	-3,954	-3,705	1,321
Income tax expense (benefit)	($10,272)	$14,301	$18,089

Income_Taxes_Effective_Rate_Re

Income Taxes Effective Rate Reconciliation (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
State income, net of federal provisions	-1.80%	-2.80%	0.90%
Flow-through repair deduction	-22.90%	-16.40%	-14.00%
Release of unrecognized tax benefit	-11.40%	0.00%	0.00%
Prior year permanent return to accrual adjustments	-4.70%	0.50%	-1.60%
Production tax credits	-2.80%	-2.90%	0.00%
Plant and depreciation of flow through items	0.10%	-0.50%	-1.10%
Recognition of state net operating loss benefit / valuation allowance release	0.00%	0.00%	-2.10%
Other, net	-0.80%	0.30%	-1.60%
Effective income tax rate	-9.30%	13.20%	15.50%
Income (Loss) Before Income Taxes	$110,414	$108,284	$116,495
Income tax calculated at 35% federal statutory rate	38,645	37,899	40,774
State income, net of federal provisions	-1,969	-3,082	1,078
Flow-through repairs deductions	-25,268	-17,763	-16,350
Release of unrecognized tax benefit	-12,607	0	0
Prior year permanent return to accrual adjustments	-5,172	541	-1,901
Production tax credits	-3,136	-3,171	0
Plant and depreciation of flow through items	74	-584	-1,281
Recognition of state net operating loss benefit / valuation allowance release	0	0	-2,398
Other, net	-839	461	-1,833
Total reconciling items	-48,917	-23,598	-22,685
Income tax expense (benefit)	($10,272)	$14,301	$18,089

Income_Taxes_Deferred_Tax_Liab

Income Taxes Deferred Tax Liability (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred Tax Assets, [Abstract]
Pension / postretirement benefits	$51,817	$20,522
NOL carryforward	42,787	16,758
Unbilled revenue	19,863	18,136
Compensation accruals	17,315	10,409
Customer advances	11,817	10,781
AMT credit carryforwards	10,357	10,357
Environmental liability	8,968	9,026
Production tax credits	6,452	3,171
Interest rate hedges	6,251	0
QF obligations	2,162	2,066
Reserves and accruals	1,772	12,097
Property taxes	881	796
Regulatory liabilities	975	659
Regulatory assets	0	7,248
Other, net	4,415	2,827
Deferred Tax Asset	185,832	124,853
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation	-349,428	-304,071
Goodwill amortization	-137,090	-122,798
Flow through depreciation	-103,677	-79,016
Regulatory assets	-21,394	0
Deferred Tax Liability	-611,589	-505,885
Deferred Tax Liability, net	($425,757)	($381,032)

Income_Taxes_Operating_Loss_De

Income Taxes Operating Loss (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforward	$351
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforward	264
Year 2025 [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	16.3
Year 2028 [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	95.5
Year 2029 [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	23.8
Year 2031 [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	127.5
Year 2033 [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	13.3
Year 2034 [Member] [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	74.9
Year 2015 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	74
Year 2016 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	18.6
Year 2018 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	101.2
Year 2020 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	10.5
Year 2021 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
NOL carryforwards, subject to expiration	$59.70

Income_Taxes_Uncertain_Tax_Pos

Income Taxes Uncertain Tax Positions (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Contingency [Line Items]
Unrecognized tax benefit more likely than not percentage threshold	50.00%
Unrecognized tax benefits that would impact effective tax rate	$62,400,000	$79,000,000
Interest and penalty expense	400,000	400,000
Accrual for interest and penalties		400,000
Safe harbor adoption	4,300,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits at January 1	113,466,000	113,291,000	131,949,000
Gross increases - tax positions in prior period	0	0	0
Gross decreases - tax positions in prior periods	0	0	-1,766,000
Gross increases - tax positions in current period	909,000	518,000	2,391,000
Gross decreases - tax positions in current period	-5,597,000	-343,000	-19,283,000
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	-12,849,000	0
Unrecognized Tax Benefits at December 31	$95,929,000	$113,466,000	$113,291,000

Comprehensive_Income_Loss_Deta

Comprehensive Income (Loss) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Loss, Before Tax [Abstract]
Foreign currency translation adjustment	$265	$166	($54)
Reclassification of net gains on derivative instruments to net income	-1,110	-1,188	-1,188
Realized loss on cash flow hedging derivatives	-18,388	0	0
Pension and postretirement medical liability adjustment	134	1,568	-897
Other comprehensive loss, before tax	-19,099	546	-2,139
Other Comprehensive Loss, Tax [Abstract]
Foreign currency translation adjustment	0	0	0
Reclassification of net gains on derivative instruments to net income	426	458	456
Realized loss on cash flow hedging derivatives	7,243	0	0
Pension and postretirement medical liability adjustment	-52	-605	344
Other comprehensive loss, tax	7,617	-147	800
Other Comprehensive Loss, Net of Tax [Abstract]
Foreign currency translation adjustment	265	166	-54
Reclassification of net gains on derivative instruments to net income	-684	-730	-732
Realized loss on cash flow hedging activities	-11,145	0	0
Postretirement medical liability adjustment	82	963	-553
Other comprehensive loss, net of tax	-11,482	399	-1,339
Accumulated Other Comprehensive Income, Net of Tax [Abstract]
Foreign currency translation	797	532
Derivative instruments designated as cash flow hedges	-8,316	3,513
Pension and postretirement medical plans	-1,247	-1,329
Accumulated other comprehensive income	-8,766	2,716
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Other Comprehensive Loss, Net of Tax [Abstract]
Other comprehensive loss, net of tax	-11,829	-730
Accumulated Other Comprehensive Income, Net of Tax [Abstract]
Accumulated other comprehensive income	-8,316	3,513	4,243
Accumulated Defined Benefit Plans Adjustment [Member]
Other Comprehensive Loss, Net of Tax [Abstract]
Other comprehensive loss, net of tax	82	963
Accumulated Other Comprehensive Income, Net of Tax [Abstract]
Accumulated other comprehensive income	($1,247)	($1,329)	($2,292)

Comprehensive_Income_Loss_Comp

Comprehensive Income (Loss) Components of AOCI (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	$2,716
Net current-period other comprehensive (loss) income	-11,482	399	-1,339
Ending balance	-8,766	2,716
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	3,513	4,243
Other comprehensive income before reclassifications	-11,145	0
Amounts reclassified from accumulated other comprehensive income	0	0
Net current-period other comprehensive (loss) income	-11,829	-730
Ending balance	-8,316	3,513
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income	-684	-730
Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	-1,329	-2,292
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	82	963
Net current-period other comprehensive (loss) income	82	963
Ending balance	-1,247	-1,329
Accumulated Defined Benefit Plans Adjustment [Member] | Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income	0	0
Foreign Currency Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	532	366
Other comprehensive income before reclassifications	265	166
Amounts reclassified from accumulated other comprehensive income	0	0
Net current-period other comprehensive (loss) income	265	166
Ending balance	797	532
Foreign Currency Gain (Loss) [Member] | Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income	0	0
Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	2,716	2,317
Other comprehensive income before reclassifications	-10,880	166
Amounts reclassified from accumulated other comprehensive income	82	963
Net current-period other comprehensive (loss) income	-11,482	399
Ending balance	-8,766	2,716
Other Comprehensive Income (Loss) [Member] | Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income	($684)	($730)

Operating_Leases_Details

Operating Leases (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	$1,996
2016	1,484
2017	671
2018	70
2019	$61

Operating_Leases_Narrative_Det

Operating Leases Narrative (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Leases [Abstract]
Lease and rental expense	$2.20	$2	$2.20

Employee_Benefit_Plans_Benefit

Employee Benefit Plans Benefit Obligation And Funded Status (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plan [Member]
Change in Benefit Obligation:
Obligation beginning of period	$567,866,000	$609,643,000
Service cost	10,830,000	13,465,000
Interest cost	26,147,000	22,719,000
Actuarial (gain) loss	107,023,000	-54,671,000
Settlements	0	0
Benefits paid	-23,422,000	-23,290,000
Benefit obligation end of period	688,444,000	567,866,000	609,643,000
Change in Fair Value of Plan Assets:
Employer contributions	8,700,000	7,800,000	7,200,000
Benefits paid	-23,422,000	-23,290,000
Amounts recognized in balance sheet consist of:
Current liability	0	0
Noncurrent liability	-132,393,000	-51,514,000
Net amount recognized	-132,393,000	-51,514,000
Amounts recognized in AOCI consist of:
Prior service cost	0	0
Net actuarial gain	0	0
Total	-153,770,000	-72,525,000
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	688,400,000	567,900,000
Accumulated benefit obligation	685,000,000	565,000,000
Fair value of plan assets	556,100,000	516,400,000
Pension Plan [Member] | Pension Costs [Member]
Amounts recognized in regulatory assets consist of:
Prior service (cost) credit	-502,000	-748,000
Net actuarial loss	-153,268,000	-71,777,000
Pension Plan [Member] | Changes Measurement [Member]
Change in Benefit Obligation:
Benefits paid	-23,422,000	-23,290,000
Change in Fair Value of Plan Assets:
Fair value of plan assets at beginning of period	516,352,000	472,936,000
Return on plan assets	52,921,000	55,006,000
Employer contributions	10,200,000	11,700,000
Benefits paid	-23,422,000	-23,290,000
Fair value of plan assets at end of period	556,051,000	516,352,000
Funded Status	-132,393,000	-51,514,000
Other Postretirement Benefit Plan [Member]
Change in Benefit Obligation:
Obligation beginning of period	30,084,000	34,040,000
Service cost	465,000	541,000
Interest cost	859,000	877,000
Actuarial (gain) loss	958,000	-3,156,000
Settlements	690,000	0
Benefits paid	-3,052,000	-2,218,000
Benefit obligation end of period	30,004,000	30,084,000
Change in Fair Value of Plan Assets:
Benefits paid	-3,052,000	-2,218,000
Amounts recognized in balance sheet consist of:
Current liability	-1,169,000	-1,178,000
Noncurrent liability	-10,795,000	-10,723,000
Net amount recognized	-11,964,000	-11,901,000
Amounts recognized in AOCI consist of:
Prior service cost	-1,151,000	-1,302,000
Net actuarial gain	-409,000	-971,000
Total	10,593,000	12,167,000
Other Postretirement Benefit Plan [Member] | Pension Costs [Member]
Amounts recognized in regulatory assets consist of:
Prior service (cost) credit	17,098,000	19,247,000
Net actuarial loss	-4,945,000	-4,807,000
Other Postretirement Benefit Plan [Member] | Changes Measurement [Member]
Change in Benefit Obligation:
Benefits paid	-3,052,000	-2,218,000
Change in Fair Value of Plan Assets:
Fair value of plan assets at beginning of period	18,183,000	15,893,000
Return on plan assets	1,391,000	2,662,000
Employer contributions	1,518,000	1,846,000
Benefits paid	-3,052,000	-2,218,000
Fair value of plan assets at end of period	18,040,000	18,183,000
Funded Status	($11,964,000)	($11,901,000)

Employee_Benefit_Plans_Net_Per

Employee Benefit Plans Net Periodic Costs (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Pension Plan [Member]
Components of Net Periodic Benefit Cost
Service cost	$10,830	$13,465
Interest cost	26,147	22,719
Recognized actuarial loss	-107,023	54,671
Settlements	0	0
Net Periodic Costs [Member] | Pension Plan [Member]
Components of Net Periodic Benefit Cost
Service cost	10,830	13,465	11,488
Interest cost	26,147	22,719	23,823
Expected return on plan assets	-29,506	-32,491	-29,996
Amortization of prior service cost (credit)	246	246	246
Recognized actuarial loss	2,118	11,648	8,646
Settlements	0	0	0
Net Periodic Benefit Cost (Credit)	9,835	15,587	14,207
Net Periodic Costs [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Components of Net Periodic Benefit Cost
Service cost	465	541	541
Interest cost	859	877	1,167
Expected return on plan assets	-981	-1,019	-1,021
Amortization of prior service cost (credit)	-1,998	-1,998	-1,998
Recognized actuarial loss	348	1,271	790
Settlements	690	0	0
Net Periodic Benefit Cost (Credit)	-617	-328	-521
Scenario, Forecast [Member] | Net Periodic Costs [Member] | Pension Plan [Member]
Components of Net Periodic Benefit Cost
Amortization of prior service cost (credit)				-246
Scenario, Forecast [Member] | Net Periodic Costs [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Components of Net Periodic Benefit Cost
Amortization of prior service cost (credit)				1,998
Scenario, Forecast [Member] | Pension Costs [Member] | Pension Plan [Member]
Components of Net Periodic Benefit Cost
Accumulated gain				-10,470
Scenario, Forecast [Member] | Pension Costs [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Components of Net Periodic Benefit Cost
Accumulated gain				($325)

Employee_Benefit_Plans_Actuari

Employee Benefit Plans Actuarial Assumptions (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	5.00%
Discount rate change [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Actuarial Gain (Loss)	73,600
Change in mortality rate assumption [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Actuarial Gain (Loss)	33,800
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Actuarial Gain (Loss)	107,023	-54,671
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Expected rate of return on plan assets	5.80%	7.00%	7.00%
Pension Plan [Member] | Nonunion [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Long-term rate of increase in compensation levels	3.58%	3.58%	3.58%
Pension Plan [Member] | Union [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Long-term rate of increase in compensation levels	3.50%	3.50%	3.50%
Pension Plan [Member] | Minimum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.75%	4.55%	3.55%
Pension Plan [Member] | Maximum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.90%	4.75%	3.80%
Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Actuarial Gain (Loss)	958	($3,156)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Expected rate of return on plan assets	5.80%	7.00%	7.00%
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
Health care cost trend rate assumed for current year	8.25%
Health care cost trend rate decrease assumed for next fiscal year	0.25%
Ultimate health care cost trend rate	4.50%
Year that rate reaches ultimate trend rate	2029
Other Postretirement Benefit Plan [Member] | Nonunion [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Long-term rate of increase in compensation levels	3.58%	3.58%	3.58%
Other Postretirement Benefit Plan [Member] | Union [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Long-term rate of increase in compensation levels	3.50%	3.50%	3.50%
Other Postretirement Benefit Plan [Member] | Minimum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.20%	3.75%	2.25%
Other Postretirement Benefit Plan [Member] | Maximum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.40%	4.20%	3.20%
Domestic [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations	34.00%	30.00%
Domestic [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations	50.00%	50.00%
Domestic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations	55.00%	60.00%
Domestic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations	40.00%	40.00%
Scenario, Forecast [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Expected rate of return on plan assets				5.80%

Employee_Benefit_Plans_Investm

Employee Benefit Plans Investment Strategy (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation allowable range of plus or minus	5.00%
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%		100.00%
Cash and Cash Equivalents [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.20%		1.80%
Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	55.00%		60.00%
Debt Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	5.00%		5.00%
Debt Securities [Member] | Other Postretirement Benefit Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	40.00%		40.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	37.20%		38.60%
Debt Securities [Member] | Other Postretirement Benefit Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	0.00%		0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%		0.30%
Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	34.00%		30.00%
Equity Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	6.00%		5.00%
Equity Securities [Member] | Other Postretirement Benefit Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	50.00%		50.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	53.90%		50.10%
Equity Securities [Member] | Other Postretirement Benefit Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	10.00%		10.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	8.70%		9.20%
SOUTH DAKOTA | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%		100.00%
SOUTH DAKOTA | Cash and Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.10%		0.10%
SOUTH DAKOTA | Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	65.60%		64.70%
SOUTH DAKOTA | Debt Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.50%		4.90%
SOUTH DAKOTA | Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	25.10%		25.30%
SOUTH DAKOTA | Equity Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.70%		5.00%
MONTANA | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%		100.00%
MONTANA | Cash and Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%		0.00%
MONTANA | Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	56.00%		58.60%
MONTANA | Debt Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.40%		4.90%
MONTANA | Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	34.10%		31.40%
MONTANA | Equity Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	5.50%		5.10%
Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	126		168
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	556,051		516,352
Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	44		318
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	18,040		18,183
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16,605	[1]	13,764
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	752	[1]	751
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	15,264	[1]	13,664
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	721	[1]	736
Fair Value, Inputs, Level 2 [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	8,234	[1]	7,321
Fair Value, Inputs, Level 2 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,560	[1]	42,094
Fair Value, Inputs, Level 2 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98
Fair Value, Inputs, Level 2 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,785	[1]	42,102
Fair Value, Inputs, Level 2 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98
Fair Value, Inputs, Level 2 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	54,775	[1]	47,227
Fair Value, Inputs, Level 2 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7	[1]	110
Fair Value, Inputs, Level 2 [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	22,634	[1]	20,015
Fair Value, Inputs, Level 2 [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,495	[1]	1,595
Fair Value, Inputs, Level 2 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,650	[1]	6,250
Fair Value, Inputs, Level 2 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	72	[1]	85
Fair Value, Inputs, Level 2 [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,992	[2]	1,880
Fair Value, Inputs, Level 2 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	23,177	[2]	82,639
Fair Value, Inputs, Level 2 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	4,435	[2]	4,390
Fair Value, Inputs, Level 2 [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	12,269	[2]	44,762
Fair Value, Inputs, Level 2 [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1	[2]	107
Fair Value, Inputs, Level 2 [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,451	[2]	24,401
Fair Value, Inputs, Level 2 [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55
Fair Value, Inputs, Level 2 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	52,767		32,700
Fair Value, Inputs, Level 2 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[2]	79
Fair Value, Inputs, Level 2 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,475		24,122
Fair Value, Inputs, Level 2 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55
Fair Value, Inputs, Level 2 [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5,692		5,876
Fair Value, Inputs, Level 2 [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	240	[2]	261
Fair Value, Inputs, Level 2 [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	24,504	[2]	25,150
Fair Value, Inputs, Level 2 [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	3	[2]	57
Fair Value, Inputs, Level 2 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	133,160	[2]	83,147
Fair Value, Inputs, Level 2 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16	[2]	187
Fair Value, Inputs, Level 2 [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,157	[2]	8,271
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Estimate of Fair Value Measurement [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	126		168
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	556,051		516,352
Estimate of Fair Value Measurement [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	44		318
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	18,040		18,183
Estimate of Fair Value Measurement [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16,605	[1]	13,764	[1]
Estimate of Fair Value Measurement [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	752	[1]	751	[1]
Estimate of Fair Value Measurement [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	15,264	[1]	13,664	[1]
Estimate of Fair Value Measurement [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	721	[1]	736	[1]
Estimate of Fair Value Measurement [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	8,234	[1]	7,321	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,560	[1]	42,094	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,785	[1]	42,102	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	54,775	[1]	47,227	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7	[1]	110	[1]
Estimate of Fair Value Measurement [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	22,634	[1]	20,015	[1]
Estimate of Fair Value Measurement [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,495	[1]	1,595	[1]
Estimate of Fair Value Measurement [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,650	[1]	6,250
Estimate of Fair Value Measurement [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	72	[1]	85
Estimate of Fair Value Measurement [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,992	[2]	1,880	[2]
Estimate of Fair Value Measurement [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	23,177	[2]	82,639	[2]
Estimate of Fair Value Measurement [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	4,435	[2]	4,390	[2]
Estimate of Fair Value Measurement [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	12,269	[2]	44,762	[2]
Estimate of Fair Value Measurement [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1	[2]	107	[2]
Estimate of Fair Value Measurement [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,451	[2]	24,401	[2]
Estimate of Fair Value Measurement [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	52,767	[2]	32,700	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[2]	79	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,475	[2]	24,122	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55	[2]
Estimate of Fair Value Measurement [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5,692	[2]	5,876	[2]
Estimate of Fair Value Measurement [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	240	[2]	261	[2]
Estimate of Fair Value Measurement [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	24,504	[2]	25,150	[2]
Estimate of Fair Value Measurement [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	3	[2]	57	[2]
Estimate of Fair Value Measurement [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	133,160	[2]	83,147	[2]
Estimate of Fair Value Measurement [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16	[2]	187	[2]
Estimate of Fair Value Measurement [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,157	[2]	8,271	[2]
[1]	This category consists of active and passive managed equity funds, which are invested in multiple strategies to diversify risks and reduce volatility.
[2]	This category consists of investment grade bonds of issuers from diverse industries, debt securities issued by international, national, state and local governments, and asset-backed securities. This includes both active and passive managed funds.

Employee_Benefit_Plans_Fair_Va

Employee Benefit Plans Fair Value (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	$44		$318
Available-for-sale Securities	18,040		18,183
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	752	[1]	751	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	721	[1]	736	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | S&P 500 Index [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	8,234	[1]	7,321	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	6	[1]	98	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	6	[1]	98	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	7	[1]	110	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	1,495	[1]	1,595	[1]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	72	[1]	85
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Passive Bond Market [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	1,992	[2]	1,880	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	4,435	[2]	4,390	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	1	[2]	107	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	5	[2]	55	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	6	[2]	79	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	5	[2]	55	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	240	[2]	261	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	3	[2]	57	[2]
Other Postretirement Benefit Plan [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	16	[2]	187	[2]
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | S&P 500 Index [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Passive Bond Market [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	44		318
Available-for-sale Securities	18,040		18,183
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	752	[1]	751
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	721	[1]	736
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | S&P 500 Index [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	8,234	[1]	7,321
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	6	[1]	98
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	6	[1]	98
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	7	[1]	110
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	1,495	[1]	1,595
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	72	[1]	85
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Passive Bond Market [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	1,992	[2]	1,880
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	4,435	[2]	4,390
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	1	[2]	107
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	5	[2]	55
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	6	[2]	79
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	5	[2]	55
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	240	[2]	261
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	3	[2]	57
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	16	[2]	187
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | S&P 500 Index [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Passive Bond Market [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Defined Contribution Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	8,700		7,800		7,200
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	126		168
Available-for-sale Securities	556,051		516,352
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	16,605	[1]	13,764	[1]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	15,264	[1]	13,664	[1]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	48,560	[1]	42,094	[1]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	48,785	[1]	42,102	[1]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	54,775	[1]	47,227	[1]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	22,634	[1]	20,015	[1]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	7,650	[1]	6,250
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	23,177	[2]	82,639	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	12,269	[2]	44,762	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	41,451	[2]	24,401	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	52,767	[2]	32,700	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	41,475	[2]	24,122	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	5,692	[2]	5,876	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	24,504	[2]	25,150	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	133,160	[2]	83,147	[2]
Defined Contribution Pension [Member] | Estimate of Fair Value Measurement [Member] | Debt Securities [Member] | Participating Group Annuity Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	7,157	[2]	8,271	[2]
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Available-for-sale Securities	0		0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] | Participating Group Annuity Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	126		168
Available-for-sale Securities	556,051		516,352
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	16,605	[1]	13,764
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	15,264	[1]	13,664
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	48,560	[1]	42,094
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	48,785	[1]	42,102
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	54,775	[1]	47,227
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	22,634	[1]	20,015
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	7,650	[1]	6,250
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	23,177	[2]	82,639
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	12,269	[2]	44,762
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	41,451	[2]	24,401
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	52,767		32,700
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	41,475		24,122
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	5,692		5,876
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	24,504	[2]	25,150
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	133,160	[2]	83,147
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Participating Group Annuity Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	7,157	[2]	8,271
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Available-for-sale Securities	0		0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Small Midcap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Small Midcap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Large Cap Growth [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Large Cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | US Large Cap Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | Non US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] | Emerging Markets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[1]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | US Core [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Long Duration [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Long Duration Investment Grade [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Long Duration Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Opportunistic [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Non US Passive [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0		0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Active Long Corporate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	0	[2]	0
Defined Contribution Pension [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] | Participating Group Annuity Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	$0	[2]	$0
[1]	This category consists of active and passive managed equity funds, which are invested in multiple strategies to diversify risks and reduce volatility.
[2]	This category consists of investment grade bonds of issuers from diverse industries, debt securities issued by international, national, state and local governments, and asset-backed securities. This includes both active and passive managed funds.

Employee_Benefit_Plans_Cash_Fl

Employee Benefit Plans Cash Flows (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension contributions	$10,200,000	$11,700,000	$11,700,000
MONTANA | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension contributions	9,000,000	10,500,000	10,500,000
SOUTH DAKOTA | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension contributions	1,200,000	1,200,000	1,200,000
Scenario, Forecast [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected contributions to pension plans				$10,200,000

Employee_Benefit_Plans_Estimat

Employee Benefit Plans Estimated Payments (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Pension Plan [Member]
Estimated Future Benefit Payments
2015	$27,652
2016	29,905
2017	31,172
2018	33,142
2019	34,660
2020-2024	194,065
Other Pension Plan, Postretirement or Supplemental Plans [Member]
Estimated Future Benefit Payments
2015	3,516
2016	3,516
2017	3,387
2018	3,282
2019	3,026
2020-2024	$11,923

Employee_Benefit_Plans_Narrati

Employee Benefit Plans Narrative (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan percentage threshold of differences between actuarial assumptions and actual plan results that are greater than projected benefit or market value	10.00%
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Matching employer contributions	$8,700	$7,800	$7,200

StockBased_Compensation_Detail

Stock-Based Compensation (Details) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$3.10	$2.40	$2.80
Compensation expense tax benefit	0.1	1.5	0.4
Compensation expense not yet recognized for nonvested awards	3.6
Nonvested awards, total compensation cost not yet recognized, period for recognition	1 year 11 months 13 days
Shares vested in period, total fair value	2.1	2.2	2
Share-based Compensation, Significant Assumptions
Risk-free interest rate	0.67%	0.44%
Expected life, in years	3 years	3 years
Expected volatility, minimum	15.50%	16.30%
Expected volatility, maximum	23.30%	25.40%
Dividend yield	3.30%	3.90%
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and vesting period	3 years
Performance Shares [Member] | Minimum
Share-based Compensation, Significant Assumptions
Percent of shares issued based on company performance	0.00%
Performance Shares [Member] | Maximum
Share-based Compensation, Significant Assumptions
Percent of shares issued based on company performance	200.00%
Stock Compensation Plan [Member] | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and vesting period	1 year
Stock Compensation Plan [Member] | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and vesting period	5 years
Executive retirement/retention program [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and vesting period	5 years
Deferred Stock Unit [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of compensation to be deferred	100.00%
Maximum number of years for distribution payments	10
Deferred stock units issued during period, shares	26,460	33,837	31,801
Deferred stock units total compensation expense	$2.30	$3.60	$0.90
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	600,000
Number of shares available for grant	1,124,798

StockBased_Compensation_Nonves

Stock-Based Compensation Nonvested (Details) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Beginning nonvested grants (shares)	173,646
Granted (shares)	96,193
Vested (shares)	-84,652
Forfeited (shares)	-4,615
Remaining nonvested grants (shares)	180,572
Beginning nonvested (weighted-average grant date fair value)	$29.14
Granted (weighted-average grant date fair value)	$38.33
Vested (weighted-average grant date fair value)	$25.19
Forfeited (weighted-average grant date fair value)	($33.55)
Remaining nonvested (weighted-average grant date fair value)	$35.77
Performance and vesting period	3 years
Executive retirement/retention program [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Beginning nonvested grants (shares)	26,628
Granted (shares)	15,092
Vested (shares)	0
Forfeited (shares)	0
Remaining nonvested grants (shares)	41,720
Beginning nonvested (weighted-average grant date fair value)	$30.24
Granted (weighted-average grant date fair value)	$43.79
Vested (weighted-average grant date fair value)	$0
Forfeited (weighted-average grant date fair value)	$0
Remaining nonvested (weighted-average grant date fair value)	$35.14
Performance and vesting period	5 years

Common_Stock_Common_Stock_Deta

Common Stock Common Stock (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended	24 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2014	Mar. 31, 2014	Mar. 31, 2014	Jun. 30, 2012
Class of Stock [Line Items]
Combined common and preferred stock, shares authorized	250,000,000
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par or stated value per share	$0.01	$0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, par or stated value per share	$0.01	$0.01
Common stock reserved for incentive plan awards	2,865,957
Common stock aggregate gross sales price, maximum	$400,000,000
Net proceeds from sale of stock	399,505,000	57,276,000	28,966,000
Shares paid for tax withholding	23,630	34,552
Hydro-electric Assets [Member]
Class of Stock [Line Items]
Stock issued during period, shares				7,766,990
Shares issued, price per share	$51.50
Net proceeds from sale of stock				386,000,000
Equity Distribution Agreement [Member]
Class of Stock [Line Items]
Common stock aggregate gross sales price, maximum							100,000,000
Net proceeds from sale of stock					13,400,000	98,700,000
Sales commission and other fees					$147,000
Number of shares issued during the period under the Equity Distribution Agreement					295,979	2,492,889
Common stock average share price					$45.65	$40.11

Earnings_Per_Share_Details

Earnings Per Share (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Basic computation	43,451,000	39,141,000	39,137,000	38,856,000	38,626,000	38,459,000	38,092,000	37,384,000	40,156,177		38,144,852	36,847,427
Restricted stock and performance share awards (1)									275,774	[1]	82,223
Diluted computation									40,431,951		38,227,075
[1]	Performance share awards are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the award.

Commitments_and_Contingencies_1

Commitments and Contingencies Qualifying Facilities Liability (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Ending QF liability	$136,893,000	$136,448,000
Qualifying Facility Contracts [Member]
Beginning QF liability	136,448,000	136,652,000
Unrecovered amount	-10,128,000	-10,647,000
Interest expense	10,573,000	10,443,000
Ending QF liability	136,893,000	136,448,000
Qualifying Facility Contracts [Member] | Gross Obligation [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	69,606,000
2016	71,598,000
2017	73,622,000
2018	75,688,000
2019	77,791,000
Thereafter	646,783,000
Contractual obligation related to QF's	1,015,088,000
Qualifying Facility Contracts [Member] | Recoverable Amounts [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	56,598,000
2016	57,188,000
2017	57,789,000
2018	58,401,000
2019	59,020,000
Thereafter	508,195,000
Contractual obligation related to QF's	797,191,000
Qualifying Facility Contracts [Member] | Net Amount [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	13,008,000
2016	14,410,000
2017	15,833,000
2018	17,287,000
2019	18,771,000
Thereafter	138,588,000
Contractual obligation related to QF's	217,897,000
Qualifying Facility Contracts [Member] | Year 2029 [Member] | Gross Obligation [Member]
Recorded unconditional purchase obligation portion recoverable through rates	800,000,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Contractual obligation related to QF's	1,000,000,000
Qualifying Facility Contracts [Member] | Minimum [Member] | Year 2029 [Member]
Price per MWH of energy required to be purchased per QF agreement	74
Qualifying Facility Contracts [Member] | Maximum [Member] | Year 2029 [Member]
Price per MWH of energy required to be purchased per QF agreement	$136

Commitments_and_Contingencies_2

Commitments and Contingencies Long Term Supply and Capacity Purchase Obligations (Details) (Purchased Coal and Natural Gas Supply And Natural Gas Transportation Contracts [Member], USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-term Purchase Commitment [Line Items]
Long Term Purchase Committments Costs Incurred	$402,300,000	$379,400,000	$340,800,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	206,500,000
2016	161,100,000
2017	134,900,000
2018	107,300,000
2019	103,500,000
Thereafter	$922,700,000
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitments Term In Years	1 year
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitments Term In Years	27

Commitments_and_Contingencies_3

Commitments and Contingencies Environmental Liabilities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jul. 01, 2018
Other Commitment	$26
Colstrip Unit 4 [Member]
Jointly owned utility plant, proportionate ownership share	30.00%	30.00%
Neal 4 Generating Facility [Member]
Jointly owned utility plant, proportionate ownership share	8.70%	8.70%
Coyote Generating Facility [Member]
Environmental obligation, estimated capital expenditures	9
Jointly owned utility plant, proportionate ownership share	10.00%	10.00%
Big Stone Generating Facility [Member]
Environmental obligation, estimated capital expenditures	384
Jointly owned utility plant, proportionate ownership share	23.40%	23.40%
Capitalized costs, jointly owned utility plant	71.8
Environmental remediation obligations [Member]
Environmental remediation obligation, minimum	26.4
Environmental remediation obligation, maximum	35
Accrual for environmental loss contingencies	29.7
Manufactured Gas Plants [Member] | Combined Manufacturing Sites [Member]
Accrual for environmental loss contingencies	22.4
Manufactured Gas Plants [Member] | Aberdeen South Dakota Site [Member]
Accrual for environmental loss contingencies	10.8
Environmental remediation obligation, to be incurred during next 5 years	$8
Number of years for environmental remediation obligation to be incurred	5 years
Scenario, Forecast [Member] | Coyote Generating Facility [Member]
NOx emissions per million Btu			0.5

Commitments_and_Contingencies_4

Commitments and Contingencies Legal Proceedings (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Damages sought,value	$48.50
Retention amount	$2

Segment_and_Related_Informatio2

Segment and Related Information (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$312,947	$251,912	$270,281	$369,723	$319,090	$262,248	$260,161	$313,020	$1,204,863	$1,154,519	$1,070,342
Cost of sales									482,591	479,546	395,434
Gross margin									722,272	674,973	674,908
Operating, general and administrative									305,886	285,569	269,966
MSTI impairment									0	0	24,039
Property and other taxes									114,592	105,540	97,674
Depreciation and depletion									123,776	112,831	106,044
Operating Income	50,584	30,987	25,097	71,350	49,962	31,401	32,660	57,010	178,018	171,033	177,185
Interest expense									-77,802	-70,486	-65,062
Other income									10,198	7,737	4,372
Income tax (expense) benefit									10,272	-14,301	-18,089
Net Income	37,169	30,191	7,746	45,580	26,093	15,647	14,341	37,902	120,686	93,983	98,406
Total assets	4,973,943				3,715,260				4,973,943	3,715,260	3,485,533
Capital expenditures									270,384	230,454	219,234
Electric
Segment Reporting Information [Line Items]
Operating revenues									877,967	865,239	805,554
Cost of sales									348,640	358,688	277,826
Gross margin									529,327	506,551	527,728
Operating, general and administrative									200,186	195,100	187,599
MSTI impairment											24,039
Property and other taxes									84,759	78,536	72,755
Depreciation and depletion									94,813	89,728	86,559
Operating Income									149,569	143,187	156,776
Interest expense									-60,424	-57,920	-55,118
Other income									4,758	4,061	2,630
Income tax (expense) benefit									-1,490	-13,905	-22,298
Net Income									92,413	75,423	81,990
Total assets	3,442,659				2,583,554				3,442,659	2,583,554	2,442,602
Capital expenditures									233,538	198,032	178,325
Gas
Segment Reporting Information [Line Items]
Operating revenues									326,896	287,605	263,394
Cost of sales									133,951	120,858	117,608
Gross margin									192,945	166,747	145,786
Operating, general and administrative									91,437	78,822	75,971
MSTI impairment											0
Property and other taxes									29,821	26,993	24,907
Depreciation and depletion									28,930	23,070	19,452
Operating Income									42,757	37,862	25,456
Interest expense									-10,618	-9,993	-9,063
Other income									1,324	1,239	1,633
Income tax (expense) benefit									-7,463	-4,134	-692
Net Income									26,000	24,974	17,334
Total assets	1,522,902				1,117,861				1,522,902	1,117,861	1,032,259
Capital expenditures									36,846	32,422	40,909
Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	1,675	1,394
Cost of sales									0	0	0
Gross margin									0	1,675	1,394
Operating, general and administrative									14,263	11,647	6,396
MSTI impairment											0
Property and other taxes									12	11	12
Depreciation and depletion									33	33	33
Operating Income									-14,308	-10,016	-5,047
Interest expense									-6,760	-2,573	-881
Other income									4,116	2,437	109
Income tax (expense) benefit									19,225	3,738	4,901
Net Income									2,273	-6,414	-918
Total assets	8,382				13,845				8,382	13,845	10,672
Capital expenditures									0	0	0
Eliminations
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
Cost of sales									0	0	0
Gross margin									0	0	0
Operating, general and administrative									0	0	0
MSTI impairment											0
Property and other taxes									0	0	0
Depreciation and depletion									0	0	0
Operating Income									0	0	0
Interest expense									0	0	0
Other income									0	0	0
Income tax (expense) benefit									0	0	0
Net Income									0	0	0
Total assets	0				0				0	0	0
Capital expenditures									$0	$0	$0

Quarterly_Financial_Data_Unaud2

Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues	$312,947	$251,912	$270,281	$369,723	$319,090	$262,248	$260,161	$313,020	$1,204,863	$1,154,519	$1,070,342
Operating income	50,584	30,987	25,097	71,350	49,962	31,401	32,660	57,010	178,018	171,033	177,185
Net Income	$37,169	$30,191	$7,746	$45,580	$26,093	$15,647	$14,341	$37,902	$120,686	$93,983	$98,406
Average Common Shares Outstanding	43,451,000	39,141,000	39,137,000	38,856,000	38,626,000	38,459,000	38,092,000	37,384,000	40,156,177	38,144,852	36,847,427
Dividends per share	$0.40	$0.40	$0.40	$0.40	$0.38	$0.38	$0.38	$0.38	$1.60	$1.52	$1.48
Quarter-end close	$56.58	$45.36	$52.19	$47.43	$43.32	$44.92	$39.90	$39.86	$56.58	$43.32
Income per average common share, (basic)
Net income (loss)	$0.87	$0.77	$0.20	$1.17	$0.67	$0.41	$0.37	$1.01	$3.01	$2.46	$2.67
Income per average common share (diluted)
Net income (loss)	$0.85	$0.77	$0.20	$1.17	$0.68	$0.40	$0.37	$1.01	$2.99	$2.46	$2.66
Maximum
Stock price	$58.70	$52.70	$52.49	$47.86	$47.18	$45.85	$43.17	$40.35
Minimum
Stock price	$45.14	$45.30	$45.49	$42.64	$41.31	$39.08	$38.12	$35.06