Rate Matters	12 Months Ended
Rate Matters	Dec. 31, 2017
Regulated Operations [Abstract]
Rate Matters	Rate Matters - MGE Energy and MGE. a. Rate Proceedings. 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 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 covered 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 allowed MGE to reflect any differential between transmission costs reflected in rates and actual costs incurred in its next rate case filing. In July 2015, the PSCW approved MGE's request to extend the current accounting treatment for transmission related costs through 2016, conditioned upon MGE not filing a base rate case for 2016. MGE did not file a base rate case for 2016. On December 23, 2014, the PSCW authorized MGE to increase 2015 rates for retail electric customers by 3.8% or $15.4 million and to decrease gas rates by 2.0% or $3.8 million. The increase in retail electric rates cover costs associated with the construction of emission-reduction equipment at Columbia, improvements and reliability of the state's electric transmission system, fuel and purchased power related to coal delivery costs, partially offset by lower cost as a result of market conditions for pension and postretirement benefit costs. The authorized return on common stock equity was 10.2%. The PSCW also approved changes to customer rates and rate design for gas service that became effective January 1, 2015. Gas rate design consists of a fixed monthly customer charge and a variable charge tied to actual usage, in addition to the separate charge through the PGA for natural gas commodity costs. The change shifted more of the rate recovery to the monthly charge, reflecting the related fixed costs of providing gas services, and reduced the variable usage-based charge. Thus, gas net income is expected to be more evenly distributed during the year and less sensitive to weather. 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 August 2015, the PSCW approved a $0.00256/kWh fuel credit that began on September 1, 2015, and continued throughout 2016. MGE returned $2.6 million of electric fuel-related savings to customers through bill credits during the period from September 1, 2015, through December 31, 2015. MGE returned $8.3 million of electric fuel-related savings during the year ended December 31, 2016. In July 2016, the PSCW issued a final order in the fuel rules proceedings requiring MGE to refund additional fuel savings incurred during 2015 and 2016 to its retail electric customers over a one-month period. In September 2016, MGE returned $15.5 million to customers through bill credits. 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 0.13%. As of December 31, 2017, MGE has deferred $4.2 million of 2017 fuel-related savings. These costs will be subject to the PSCW's annual review of 2017 fuel costs, expected to be completed in 2018.