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Summary of Significant Accounting Policies (Details-1) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Trade Receivables, Allowance for Doubtful Accounts, and Concentration Risk |  |  |  |
Late payment charge on upaid receivables | 1.00% |  |  |
Inventories |  |  |  |
Renewable energy credit allowance included in inventory | $ 600,000 |  | $ 700,000 |
Purchased Gas Adjustment Clause |  |  |  |
Purchased gas adjustment, over (under) collected | 1,900,000 |  | 1,200,000 |
Regional Transmission Organizations |  |  |  |
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting | 75,600,000 |  | 90,300,000 | $ 87,900,000
Allowance for Funds Used During Construction |  |  |  |
AFUDC - equity funds | 2,300,000 |  | 3,300,000 | 1,200,000
AFUDC - borrowed funds | 800,000 |  | 1,100,000 | 400,000
Capitalized Software Costs |  |  |  |
Capitalized software | 24,600,000 |  | 15,100,000 |
Capitalized software amortization | 5,100,000 |  | 4,700,000 | 3,300,000
Capitalized Software Hosting Agreement Costs |  |  |  |
Capitalized Computer Software Hosting Agreement Net | 14,700,000 |  | 9,200,000 |
Capitalized Computer Software Hosting Agreement Amortization | 1,400,000 |  | 100,000 | 0
Capitalized Computer Software Hosting Agreement Accumulated Amortization | 1,400,000 |  | 100,000 |
Impairment of Long-Lived Assets |  |  |  |
Impairment of long-lived assets | $ 0 |  | 0 | 0
Excise taxes |  |  |  |
License fee tax rate, electric, retail sales | 3.19% |  |  |
License fee tax rate, electric, resale by purchaser | 1.59% |  |  |
License fee tax rate, natural gas | 0.97% |  |  |
License fee tax expense | $ 13,900,000 |  | $ 14,400,000 | $ 14,100,000
Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase agreement, maximum exposure | 10,000,000.0 |  |  |
2020 [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase-to-Maturity Transactions, loans due | 280,000 |  |  |
2021 [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase-to-Maturity Transactions, loans due | 267,000 |  |  |
2022 [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase-to-Maturity Transactions, loans due | 250,000 |  |  |
2023 [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase-to-Maturity Transactions, loans due | 181,000 |  |  |
2024 [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase-to-Maturity Transactions, loans due | 70,000 |  |  |
Thereafter [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
Repurchase-to-Maturity Transactions, loans due | $ 119,000 |  |  |
Minimum [Member] |  |  |  |
Capitalized Software Hosting Agreement Costs |  |  |  |
Capitalized software hosting agreement, useful life | 5 years |  |  |
Minimum [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
MGE guarantee to the financial institution, term | 1 year |  |  |
Minimum [Member] | Capitalized Software [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Property, plant, and equipment, useful life | 5 years |  |  |
Maximum [Member] |  |  |  |
Capitalized Software Hosting Agreement Costs |  |  |  |
Capitalized software hosting agreement, useful life | 10 years |  |  |
Maximum [Member] | Chattel Paper Agreements [Member] |  |  |  |
Transfers and Servicing |  |  |  |
MGE guarantee to the financial institution, term | 10 years |  |  |
Maximum [Member] | Capitalized Software [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Property, plant, and equipment, useful life | 15 years |  |  |
PSCW [Member] |  |  |  |
Allowance for Funds Used During Construction |  |  |  |
Authorized AFUDC rate | 50.00% |  | 50.00% | 50.00%
Authorized AFUDC rate - Signficiant projects (100%) | 100.00% |  | 100.00% | 100.00%
Rate at which AFUDC was capitalized | 7.00% |  | 7.87% | 7.87%
Saratoga Wind [Member] |  |  |  |
Allowance for Funds Used During Construction |  |  |  |
AFUDC - equity funds | $ 4,100,000 |  |  |
Saratoga Wind [Member] | PSCW [Member] |  |  |  |
Allowance for Funds Used During Construction |  |  |  |
Authorized AFUDC rate - Signficiant projects (100%) | 100.00% |  |  |
Electric [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Composite straight-line depreciation rates | 3.60% | [1] | 2.90% | 3.00%
Gas [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Composite straight-line depreciation rates | 2.10% |  | 2.10% | 2.10%
Non Regulated Energy [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Composite straight-line depreciation rates | 2.30% |  | 2.30% | 2.30%
MGE Power Elm Road [Member] |  |  |  |
Principles of Consolidation |  |  |  |
Ownership percentage by parent | 100.00% |  |  |
MGE Power West Campus [Member] |  |  |  |
Principles of Consolidation |  |  |  |
Ownership percentage by parent | 100.00% |  |  |
[1] | In the December 2018 rate settlement, the PSCW approved new depreciation rates for Columbia effective January 1, 2019.

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