Regulatory Matters	6 Months Ended
Jun. 30, 2013
Regulated Operations [Abstract]
Public Utilities Disclosure [Text Block]	Regulatory Matters Dave Gates Generating Station at Mill Creek (DGGS) As a result of a Federal Energy Regulatory Commission (FERC) Administrative Law Judge's (ALJ) initial nonbinding decision issued in September 2012, we have cumulative deferred revenue of approximately $20.7 million, which is subject to refund and recorded within current regulatory liabilities in the Condensed Consolidated Balance Sheets. The ALJ concluded we should allocate only a fraction of the costs we believe (based on past practice) should be allocated to FERC jurisdictional customers. The matter has been fully briefed before the FERC. Although we have no assurance as to timing, the FERC is expected to consider the matter and issue a binding decision during 2013. The FERC is not obligated to follow any of the ALJ's findings and conclusions, and the FERC can accept or reject the initial decision in whole or in part. If the FERC upholds the ALJ's decision and a portion of the costs are effectively disallowed, we would be required to assess DGGS for impairment. If we disagree with a decision issued by the FERC, we may pursue full appellate rights through rehearing and appeal to a United States Circuit Court of Appeals, which could extend into 2015. We continue to bill FERC jurisdictional customers interim rates that have been in effect since January 1, 2011. These interim rates are subject to refund plus interest pending final resolution at FERC. Montana Electric and Natural Gas Tracker Filings Each year we submit electric and natural gas tracker filings 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 Montana Public Service Commission (MPSC) reviews such filings and makes its cost recovery determination based on whether or not our electric and natural gas supply procurement activities were prudent. In May 2013, we filed our 2013 annual electric and natural gas supply tracker filings. During June, we received orders from the MPSC approving the electric and natural gas supply tracker filings on an interim basis. A hearing was held in June 2013 related to our 2012 electric supply tracker filing, including our request for demand-side management (DSM) lost revenues, and we expect the MPSC to issue a final order during 2013. As of June 30, 2013, we have deferred revenue of approximately $7.4 million related to DSM lost revenues, which is recorded within current regulatory liabilities in the Condensed Consolidated Balance Sheets. Montana Natural Gas Production Assets During the third quarter of 2012, we completed the purchase of natural gas production interests in northern Montana's Bear Paw Basin, including a 75% interest in two gas gathering systems. 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. We expect to file an application with the MPSC during 2013 to place these assets in natural gas rate base. Since acquisition, we have recognized approximately $3.0 million of revenue that is subject to refund until we receive MPSC approval of our application.

Regulatory Matters

Dave Gates Generating Station at Mill Creek (DGGS)

As a result of a Federal Energy Regulatory Commission (FERC) Administrative Law Judge's (ALJ) initial nonbinding decision issued in September 2012, we have cumulative deferred revenue of approximately $20.7 million, which is subject to refund and recorded within current regulatory liabilities in the Condensed Consolidated Balance Sheets. The ALJ concluded we should allocate only a fraction of the costs we believe (based on past practice) should be allocated to FERC jurisdictional customers. The matter has been fully briefed before the FERC.

Although we have no assurance as to timing, the FERC is expected to consider the matter and issue a binding decision during 2013. The FERC is not obligated to follow any of the ALJ's findings and conclusions, and the FERC can accept or reject the initial decision in whole or in part. If the FERC upholds the ALJ's decision and a portion of the costs are effectively disallowed, we would be required to assess DGGS for impairment. If we disagree with a decision issued by the FERC, we may pursue full appellate rights through rehearing and appeal to a United States Circuit Court of Appeals, which could extend into 2015. We continue to bill FERC jurisdictional customers interim rates that have been in effect since January 1, 2011. These interim rates are subject to refund plus interest pending final resolution at FERC.

Montana Electric and Natural Gas Tracker Filings

Each year we submit electric and natural gas tracker filings 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 Montana Public Service Commission (MPSC) reviews such filings and makes its cost recovery determination based on whether or not our electric and natural gas supply procurement activities were prudent.

In May 2013, we filed our 2013 annual electric and natural gas supply tracker filings. During June, we received orders from the MPSC approving the electric and natural gas supply tracker filings on an interim basis.

A hearing was held in June 2013 related to our 2012 electric supply tracker filing, including our request for demand-side management (DSM) lost revenues, and we expect the MPSC to issue a final order during 2013. As of June 30, 2013, we have deferred revenue of approximately $7.4 million related to DSM lost revenues, which is recorded within current regulatory liabilities in the Condensed Consolidated Balance Sheets.

Montana Natural Gas Production Assets

During the third quarter of 2012, we completed the purchase of natural gas production interests in northern Montana's Bear Paw Basin, including a 75% interest in two gas gathering systems. 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. We expect to file an application with the MPSC during 2013 to place these assets in natural gas rate base. Since acquisition, we have recognized approximately $3.0 million of revenue that is subject to refund until we receive MPSC approval of our application.