Joint Plant Ownership	9 Months Ended
Sep. 30, 2017
Regulated Operations [Abstract]
Joint Plant Ownership	Joint Plant Ownership - MGE Energy and MGE. Columbia. In 2016, MGE and WPL negotiated an amendment to the existing Columbia joint operating agreement, that has been approved by the PSCW, under which MGE will have the option to reduce its obligation to pay certain capital expenditures (other than SCR-related expenditures) at Columbia in exchange for a proportional reduction in MGE's ownership in Columbia. On January 1 of each year, beginning in 2017 and ending June 1, 2020, the ownership percentage will be adjusted, through a partial sale, based on the amount of capital expenditures foregone. In June 2017, the FERC approved the ownership transfer in Columbia, effective January 1, 2017. During 2016, MGE accrued $14.8 million of 2016 capital expenditures that MGE has forgone as part of the ownership transfer agreement with WPL. As of December 31, 2016, MGE classified $14.8 million of Columbia assets as held-for-sale on the consolidated balance sheets. In January 2017, MGE reduced its ownership interest in Columbia from 22.0% to 20.4% through the partial sale of plant assets to WPL. During three and nine months ended September 30, 2017, MGE accrued $1.6 million and $6.7 million, respectively, of 2017 capital expenditures that MGE has forgone subject to the ownership transfer agreement. As of September 30, 2017, MGE classified $6.7 million of Columbia assets as held-for-sale on the consolidated balance sheets. The assets recognized as held-for-sale are subject to a partial sale of plant assets to WPL, expected to occur in January 2018.

Joint Plant Ownership - MGE Energy and MGE.

Columbia.

In 2016, MGE and WPL negotiated an amendment to the existing Columbia joint operating agreement, that has been approved by the PSCW, under which MGE will have the option to reduce its obligation to pay certain capital expenditures (other than SCR-related expenditures) at Columbia in exchange for a proportional reduction in MGE's ownership in Columbia. On January 1 of each year, beginning in 2017 and ending June 1, 2020, the ownership percentage will be adjusted, through a partial sale, based on the amount of capital expenditures foregone. In June 2017, the FERC approved the ownership transfer in Columbia, effective January 1, 2017.

During 2016, MGE accrued $14.8 million of 2016 capital expenditures that MGE has forgone as part of the ownership transfer agreement with WPL. As of December 31, 2016, MGE classified $14.8 million of Columbia assets as held-for-sale on the consolidated balance sheets. In January 2017, MGE reduced its ownership interest in Columbia from 22.0% to 20.4% through the partial sale of plant assets to WPL.

During three and nine months ended September 30, 2017, MGE accrued $1.6 million and $6.7 million, respectively, of 2017 capital expenditures that MGE has forgone subject to the ownership transfer agreement. As of September 30, 2017, MGE classified $6.7 million of Columbia assets as held-for-sale on the consolidated balance sheets. The assets recognized as held-for-sale are subject to a partial sale of plant assets to WPL, expected to occur in January 2018.