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Summary of Significant Accounting Policies (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Receivable - Margin Account [Abstract] |  |  |
Receivable, margin account balance, net of collateral posted against derivative positions | $ 2,300,000 | $ 2,200,000 |
Derivative liability, collateral offset | $ 1,000,000 | 2,200,000 |
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 | $ 300,000 | 800,000 |
Transfers and Servicing |  |  |
Cumulative effect of new accounting guidance, retained earnings |  | (142,000) |
Purchased Gas Adjustment Clause |  |  |
Purchased gas adjustment, over (under) collected | 800,000 | 1,200,000 |
Capitalized Software Costs |  |  |
Capitalized software | 12,000,000 | 8,400,000 |
Capitalized software amortization | 2,200,000 | 1,600,000 | $ 1,500,000
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 | $ 14,700,000 | $ 14,600,000 | $ 13,800,000
Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase agreement, maximum exposure | 10,000,000 |  |
Repurchase-to-Maturity Transactions, loans due | 3,700,000 |  |
2016 [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase-to-Maturity Transactions, loans due | 706,000 |  |
2017 [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase-to-Maturity Transactions, loans due | 476,000 |  |
2018 [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase-to-Maturity Transactions, loans due | 508,000 |  |
2019 [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase-to-Maturity Transactions, loans due | 477,000 |  |
2020 [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase-to-Maturity Transactions, loans due | 445,000 |  |
Thereafter [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
Repurchase-to-Maturity Transactions, loans due | $ 1,059,000 |  |
Minimum [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
MGE guarantee to the financial institution, term | 1 year |  |
Minimum [Member] | Capitalized Software [Member] |  |  |
Capitalized Software Costs |  |  |
Property, plant, and equipment, useful life | 5 years |  |
Maximum [Member] | Chattel Paper Agreements [Member] |  |  |
Transfers and Servicing |  |  |
MGE guarantee to the financial institution, term | 10 years |  |
Maximum [Member] | Capitalized Software [Member] |  |  |
Capitalized Software Costs |  |  |
Property, plant, and equipment, useful life | 10 years |  |
PSCW [Member] |  |  |
Allowance for Funds Used During Construction |  |  |
Authorized AFUDC rate | 50.00% | 50.00% | 50.00%
Rate at which AFUDC was capitalized | 7.93% | 8.21% | 8.21%
Columbia Environmental Project, SCR [Member] | PSCW [Member] |  |  |
Allowance for Funds Used During Construction |  |  |
Authorized AFUDC rate | 100.00% | 100.00% | 50.00%
Electric [Member] |  |  |
Property, Plant, and Equiptment |  |  |
Composite straight-line depreciation rates | 2.60% | 2.60% | 2.70%
Gas [Member] |  |  |
Property, Plant, and Equiptment |  |  |
Composite straight-line depreciation rates | 1.70% | 1.70% | 1.70%
Non Regulated Energy [Member] |  |  |
Property, Plant, and Equiptment |  |  |
Composite straight-line depreciation rates | 2.40% | 2.40% | 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% |  |

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