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Summary of Significant Accounting Policies (Details) (USD $) | 0 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 20, 2013 | Dec. 31, 2013 sqmi | Dec. 31, 2012 | Dec. 31, 2011
Basis of Presentation |  |  |  |
Stock split declared December 20, 2013, ratio (3:2) | 1.5 |  |  |
Receivable - Margin Account [Abstract] |  |  |  |
Derivative asset, collateral offset |  | $ 200,000 | $ 0 |
Trade Receivables, Allowance for Doubtful Accounts, and Concentration Risk |  |  |  |
Late payment charge on upaid receivables |  | 1.00% |  |
Electric territory (in square miles) |  | 316 |  |
Gas territory (in square miles) |  | 1,649 |  |
Inventories |  |  |  |
Emissions allowance included in inventory |  | 100,000 | 100,000 |
Renewable energy credit allowance included in inventory |  | 600,000 | 500,000 |
Purchased Gas Adjustment Clause |  |  |  |
Purchased gas adjustment, over (under) collected |  | 1,700,000 | (100,000) |
Allowance for Funds Used During Construction |  |  |  |
Authorized AFUDC rate |  | 50.00% | 50.00% | 50.00%
Rate at which AFUDC was capitalized |  | 8.21% | 8.36% | 8.36%
Capitalized Software Costs |  |  |  |
Capitalized software |  | 20,900,000 | 14,500,000 |
Capitalized software amortization |  | 1,500,000 | 1,300,000 |
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 |  | $ 13,800,000 | $ 13,500,000 | $ 12,900,000
Treasury Stock |  |  |  |
Treasury stock outstanding (in shares) |  | 0 |  |
Minimum [Member] | Capitalized Software [Member] |  |  |  |
Capitalized Software Costs |  |  |  |
Property, plant, and equipment, useful life |  | 5 years |  |
Maximum [Member] | Capitalized Software [Member] |  |  |  |
Capitalized Software Costs |  |  |  |
Property, plant, and equipment, useful life |  | 10 years |  |
Columbia Environmental Project [Member] | PSCW [Member] |  |  |  |
Allowance for Funds Used During Construction |  |  |  |
Authorized AFUDC rate |  | 50.00% | 100.00% | 100.00%
Electric [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Composite straight-line depreciation rates |  | 2.70% | 2.90% | 3.20%
Gas [Member] |  |  |  |
Property, Plant, and Equiptment |  |  |  |
Composite straight-line depreciation rates |  | 1.70% | 1.70% | 1.60%
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% |  |

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