Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 SqMi	Dec. 31, 2011	Dec. 31, 2010
Derivative collateral [Abstract]
Derivative collateral	$ 0	$ 3,000,000
Receivables [Abstract]
Late payment charge on unpaid receivables	1.00%
Electric territory	316
Gas territory	1,631
Inventories [Abstract]
Emission allowance balances	100,000	200,000
Renewable energy credit allowance balances	500,000	300,000
Purchased gas adjustment [Abstract]
Purchased gas adjustment over (under) collected	(100,000)	900,000
Allowance for funds used during construction [Abstract]
AFUDC rate	50.00%	50.00%	50.00%
Rate at which AFUDC was capitalized	8.36%	8.36%	8.36%
AFUDC rate on certain Columbia environmental costs	100.00%	100.00%	100.00%
Capitalized software [Abstract]
Capitalized software	14,500,000	14,500,000
Capitalized software amortization	1,300,000	1,800,000
Excise taxes [Abstract]
Electric license fee tax rate for retail sales	3.19%
Electric license fee tax rate for resale	1.59%
Natural gas license fee tax rate	0.97%
License fee tax expense	13,500,000	12,900,000	12,600,000
Long lived assets [Abstract]
Impairment of long lived assets	$ 0
Treasury stock, shares [Abstract]
Treasury stock, shares	0
Electric [Member]
Composite straight-line depreciation rates [Line Items]
Composite straight-line depreciation rates	2.90%	3.20%	3.30%
Gas [Member]
Composite straight-line depreciation rates [Line Items]
Composite straight-line depreciation rates	1.70%	1.60%	1.70%
Non Regulated Energy [Member]
Composite straight-line depreciation rates [Line Items]
Composite straight-line depreciation rates	2.30%	2.30%	2.30%
MGE Power Elm Road [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent	100.00%
MGE Power West Campus [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent	100.00%