Rate Matters	6 Months Ended
Rate Matters	Jun. 30, 2018
Regulated Operations [Abstract]
Rate Matters	Rate Matters - MGE Energy and MGE. a. Rate Proceedings. In July 2018, MGE filed an application with the PSCW to decrease electric rates and increase gas rates in 2019 and 2020. MGE filed its application through a settlement agreement between MGE and intervening parties in the rate case, subject to PSCW approval. The settlement proposal would decrease electric rates by 1.94% in 2019. This amount is subject to change based on the final outcome of MGE's 2019 Fuel Cost Plan. MGE will maintain this rate level for 2020, with the exception that MGE will file a 2020 Fuel Cost Plan in 2019 and MGE's rates will decrease or increase accordingly. The decrease reflects the ongoing tax impacts of the Tax Act and the addition of lower-cost renewable generation capacity. The settlement proposal would increase gas rates by 1.06% in 2019 and 1.46% in 2020. The requested increase covers infrastructure costs. It also reflects the impacts of the Tax Act. The proposed return on common stock equity for 2019 and 2020 will be 9.8% based on a capital structure consisting of 56.6% common equity in 2019 and 56.1% common equity in 2020. In December 2016, the PSCW authorized MGE, effective January 1, 2017, to decrease 2017 rates for retail electric customers by 0.8% or $3.3 million on an annual basis and to increase rates for retail gas customers by 1.9% or $3.1 million on an annual basis. The decrease in retail electric rates was attributable to declining fuel and purchased power costs. The increase in retail gas rates covers costs associated with MGE's natural gas system infrastructure improvements. The authorized return on common stock equity for 2017 was 9.8% based on a capital structure consisting of 57.2% common equity. The PSCW also approved MGE's request to extend the current accounting treatment for transmission related costs through 2018. This accounting treatment allows MGE to reflect any differential between transmission costs reflected in rates and actual costs incurred in its next rate case filing. b. Fuel Rules. Fuel rules require the PSCW and Wisconsin utilities to defer electric fuel-related costs that fall outside a symmetrical cost tolerance band around the amount approved for a utility in its annual fuel proceedings. Any over/under recovery of the actual costs is determined in the following year and is then reflected in future billings to electric retail customers. The fuel rules bandwidth is currently set at plus or minus 2%. Under fuel rules, MGE would defer costs, less any excess revenues, if its actual electric fuel costs exceeded 102% of the electric fuel costs allowed in its latest rate order. Excess revenues are defined as revenues in the year in question that provide MGE with a greater return on common equity than authorized by the PSCW in MGE's latest rate order. Conversely, MGE is required to defer the benefit of lower costs if actual electric fuel costs were less than 98% of the electric fuel costs allowed in that order. In July 2017, the PSCW issued a final order in the fuel rules proceedings requiring MGE to refund additional fuel savings realized during 2015 and 2016 to its retail electric customers over a one-month period. In October 2017, MGE returned $6.2 million to customers through bill credits. In December 2017, the PSCW approved a surcharge for 2018 electric fuel-related costs. The surcharge will increase electric retail revenue in 2018 by $0.5 million or 0.1%. In July 2018, the PSCW issued a preliminary decision in the fuel rules proceedings for MGE to refund $4.2 million of additional fuel savings realized during 2017 to its retail electric customers over a one-month period in the second half of 2018. There was no change to the refund in the fuel rules proceedings from the amount MGE deferred as of December 31, 2017. As of June 30, 2018, MGE has deferred $5.2 million of 2018 fuel savings. The 2018 fuel savings will be subject to the PSCW's annual review of 2018 fuel costs, expected to be completed in 2019. As of June 30, 2017, MGE had deferred $1.8 million of 2017 fuel savings.