Acquisitions	9 Months Ended
Sep. 30, 2013
Acquisitions [Abstract]
Property, Plant and Equipment, Schedule of Significant Acquisitions and Disposals [Table Text Block]	Acquisition Agreements Hydro Transaction On September 26, 2013, we entered into an agreement with PPL Montana, LLC (PPL Montana), a wholly owned subsidiary of PPL Corporation, to purchase PPL Montana's hydro-electric generating facilities and associated assets located in Montana, which includes approximately 633 megawatts of hydro-electric generation capacity, for a purchase price of $900 million (Hydro Transaction). The purchase price will be subject to adjustment for proration of operating expenses, performance of planned capital expenditures, and termination of certain power purchase agreements. If the acquisition is completed, we will own over 60 percent of our average load serving requirements in Montana. We expect to close on this acquisition during the second half of 2014; however, it is subject to customary conditions and approvals, including approval from the Federal Energy Regulatory Commission (FERC), the Montana Public Service Commission (MPSC), other appropriate state and federal agencies and as required by the Hart-Scott-Rodino Antitrust Improvements Act. In addition, the closing is conditioned upon the termination or waiver of certain restrictions applicable to the hydro assets under a sale-leaseback arrangement through which PPL Montana holds its interest in the Colstrip thermal generating facility (which interest we will not be acquiring). Through September 30, 2013 we have expensed approximately $3.3 million of legal and professional fees associated with the transaction. Either party may terminate the agreement if the closing does not occur by September 26, 2014; however, this date will be extended for an additional six months if any governmental approval is still pending. On September 26, 2013, in connection with the Hydro Transaction, we signed a commitment letter with Credit Suisse Securities (USA) LLC and Bank of America, N.A., under which the lenders committed to provide a $900 million 364-day senior bridge term loan facility. The permanent financing for the Hydro Transaction is anticipated to be a combination of long-term debt, new equity issuance and cash flows from operations. Natural Gas Production Assets In May 2013, we entered into an agreement to purchase additional natural gas production interests in northern Montana's Bear Paw Basin for approximately $70 million, subject to purchase price adjustments. This purchase would include an 82 percent interest in the Havre Pipeline Company, LLC (Havre Pipeline), which represents approximately $6 million of the purchase price. Due to the requirements of a previous stipulation with the MPSC, we requested a regulatory waiver in June 2013 to acquire the Havre Pipeline. In addition, approval of the sale of the Havre Pipeline must also be approved by the MPSC. In October 2013, the MPSC approved our waiver request. While the seller does not believe MPSC approval is needed to sell its ownership interests in the Havre Pipeline, they have filed a request for determination, which we expect the MPSC to decide on by October 31, 2013. Either party to the agreement may terminate the transaction if the closing does not occur by October 31, 2013. If the MPSC issues a favorable order on the seller's filing by October 31, 2013, we hope to extend the agreement and close on this transaction during the fourth quarter of 2013. As of January 1, 2013, the amount of net proven developed producing reserves associated with the pending acquisition was estimated to be 64.6 billion cubic feet. We estimate the current annual production associated with this pending acquisition would be approximately 28 percent of our total annual natural gas load in Montana, which would increase our total owned production to approximately 37 percent.

Acquisition Agreements

Hydro Transaction

On September 26, 2013, we entered into an agreement with PPL Montana, LLC (PPL Montana), a wholly owned subsidiary of PPL Corporation, to purchase PPL Montana's hydro-electric generating facilities and associated assets located in Montana, which includes approximately 633 megawatts of hydro-electric generation capacity, for a purchase price of $900 million (Hydro Transaction). The purchase price will be subject to adjustment for proration of operating expenses, performance of planned capital expenditures, and termination of certain power purchase agreements. If the acquisition is completed, we will own over 60 percent of our average load serving requirements in Montana.

We expect to close on this acquisition during the second half of 2014; however, it is subject to customary conditions and approvals, including approval from the Federal Energy Regulatory Commission (FERC), the Montana Public Service Commission (MPSC), other appropriate state and federal agencies and as required by the Hart-Scott-Rodino Antitrust Improvements Act. In addition, the closing is conditioned upon the termination or waiver of certain restrictions applicable to the hydro assets under a sale-leaseback arrangement through which PPL Montana holds its interest in the Colstrip thermal generating facility (which interest we will not be acquiring). Through September 30, 2013 we have expensed approximately $3.3 million of legal and professional fees associated with the transaction.

Either party may terminate the agreement if the closing does not occur by September 26, 2014; however, this date will be extended for an additional six months if any governmental approval is still pending.

On September 26, 2013, in connection with the Hydro Transaction, we signed a commitment letter with Credit Suisse Securities (USA) LLC and Bank of America, N.A., under which the lenders committed to provide a $900 million 364-day senior bridge term loan facility. The permanent financing for the Hydro Transaction is anticipated to be a combination of long-term debt, new equity issuance and cash flows from operations.

Natural Gas Production Assets

In May 2013, we entered into an agreement to purchase additional natural gas production interests in northern Montana's Bear Paw Basin for approximately $70 million, subject to purchase price adjustments. This purchase would include an 82 percent interest in the Havre Pipeline Company, LLC (Havre Pipeline), which represents approximately $6 million of the purchase price. Due to the requirements of a previous stipulation with the MPSC, we requested a regulatory waiver in June 2013 to acquire the Havre Pipeline. In addition, approval of the sale of the Havre Pipeline must also be approved by the MPSC. In October 2013, the MPSC approved our waiver request. While the seller does not believe MPSC approval is needed to sell its ownership interests in the Havre Pipeline, they have filed a request for determination, which we expect the MPSC to decide on by October 31, 2013. Either party to the agreement may terminate the transaction if the closing does not occur by October 31, 2013. If the MPSC issues a favorable order on the seller's filing by October 31, 2013, we hope to extend the agreement and close on this transaction during the fourth quarter of 2013.

As of January 1, 2013, the amount of net proven developed producing reserves associated with the pending acquisition was estimated to be 64.6 billion cubic feet. We estimate the current annual production associated with this pending acquisition would be approximately 28 percent of our total annual natural gas load in Montana, which would increase our total owned production to approximately 37 percent.