Rate Matters	12 Months Ended
Dec. 31, 2019
Regulated Operations [Abstract]
Rate Matters	Rate Matters - MGE Energy and MGE. a.Rate Proceedings. In December 2018, the PSCW approved a settlement agreement between MGE and intervening parties in the then pending rate case. The settlement decreased electric rates by 2.24%, or $9.2 million, in 2019. MGE will maintain this rate level for 2020, with the exception that MGE's electric rates were adjusted by the 2020 Fuel Cost Plan. The decrease in electric rates reflects the ongoing impacts of the Tax Act. Lower fuel costs and an increase in rate base from renewable generation assets further impacted the rate change. In 2020, electric rates will decrease 0.84%, or $3.4 million, as approved by the PSCW in December 2019 in the 2020 Fuel Cost Plan. The settlement agreement increased gas rates by 1.06%, or $1.7 million, in 2019 and 1.46%, or $2.4 million, in 2020. The gas increase covers infrastructure costs. It also reflects the impacts of the Tax Act. The return on common stock equity for 2019 and 2020 is 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 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. MGE did not file a base rate case for 2018. 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- or 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. These costs are subject to the PSCW's annual review of fuel costs completed in the year following the deferral. The PSCW issued final decisions in the 2018, 2017, and 2016 fuel rules proceedings for MGE to refund additional fuel savings incurred to its retail electric customers over a one-month period. MGE returned $9.5 million, $4.2 million, and $6.2 million of electric fuel-related savings in October 2019, 2018, and 2017, respectively. There was no change to the refund in the fuel rules proceedings from the amount MGE deferred in the previous respective years. In December 2017, the PSCW approved a surcharge for 2018 electric fuel-related costs. The surcharge increased electric retail revenue in 2018 by $0.5 million or 0.13%. As of December 31, 2019, MGE has deferred $1.5 million of 2019 fuel-related savings. These costs will be subject to the PSCW's annual review of 2019 fuel costs, expected to be completed in 2020. c.2018 Tax Reform. Customer rates approved for 2018 reflected an income tax rate of 35 percent. In January 2018, the PSCW issued an order directing Wisconsin investor-owned utilities to defer the over-collection of income tax expense as a result of the decrease in tax rate to 21 percent. The PSCW issued an order in May 2018 to return to customers the estimated 2018 over-collection of income tax expense. The decision included a one-time bill credit on customer bills to reflect the estimate of the over-collection for January through June 2018, along with a volumetric bill credit which began in July 2018 and continued through the remainder of 2018 for the estimated remaining annual amount. MGE returned $8.2 million to customers through bill credits as of December 31, 2018. In August 2019, the PSCW issued a decision on the 2018 tax reform proceedings for MGE to refund the remaining 2018 overcollection of income tax expense to its retail customers as a one-time bill credit. MGE returned $3.2 million in September 2019. There was no change to the refund order from the amount MGE deferred as of December 31, 2018.

Rate Matters

12 Months Ended

Dec. 31, 2019

Rate Matters - MGE Energy and MGE.

a.Rate Proceedings.

In December 2018, the PSCW approved a settlement agreement between MGE and intervening parties in the then pending rate case. The settlement decreased electric rates by 2.24%, or $9.2 million, in 2019. MGE will maintain this rate level for 2020, with the exception that MGE's electric rates were adjusted by the 2020 Fuel Cost Plan. The decrease in electric rates reflects the ongoing impacts of the Tax Act. Lower fuel costs and an increase in rate base from renewable generation assets further impacted the rate change. In 2020, electric rates will decrease 0.84%, or $3.4 million, as approved by the PSCW in December 2019 in the 2020 Fuel Cost Plan. The settlement agreement increased gas rates by 1.06%, or $1.7 million, in 2019 and 1.46%, or $2.4 million, in 2020. The gas increase covers infrastructure costs. It also reflects the impacts of the Tax Act. The return on common stock equity for 2019 and 2020 is 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 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.

MGE did not file a base rate case for 2018.

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- or 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. These costs are subject to the PSCW's annual review of fuel costs completed in the year following the deferral.

The PSCW issued final decisions in the 2018, 2017, and 2016 fuel rules proceedings for MGE to refund additional fuel savings incurred to its retail electric customers over a one-month period. MGE returned $9.5 million, $4.2 million, and $6.2 million of electric fuel-related savings in October 2019, 2018, and 2017, respectively. There was no change to the refund in the fuel rules proceedings from the amount MGE deferred in the previous respective years.

In December 2017, the PSCW approved a surcharge for 2018 electric fuel-related costs. The surcharge increased electric retail revenue in 2018 by $0.5 million or 0.13%.

As of December 31, 2019, MGE has deferred $1.5 million of 2019 fuel-related savings. These costs will be subject to the PSCW's annual review of 2019 fuel costs, expected to be completed in 2020.

c.2018 Tax Reform.

Customer rates approved for 2018 reflected an income tax rate of 35 percent. In January 2018, the PSCW issued an order directing Wisconsin investor-owned utilities to defer the over-collection of income tax expense as a result of the decrease in tax rate to 21 percent.

The PSCW issued an order in May 2018 to return to customers the estimated 2018 over-collection of income tax expense. The decision included a one-time bill credit on customer bills to reflect the estimate of the over-collection for January through June 2018, along with a volumetric bill credit which began in July 2018 and continued through the remainder of 2018 for the estimated remaining annual amount. MGE returned $8.2 million to customers through bill credits as of December 31, 2018.

In August 2019, the PSCW issued a decision on the 2018 tax reform proceedings for MGE to refund the remaining 2018 overcollection of income tax expense to its retail customers as a one-time bill credit. MGE returned $3.2 million in September 2019. There was no change to the refund order from the amount MGE deferred as of December 31, 2018.