Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 SqMi	Dec. 31, 2010	Dec. 31, 2009
Derivative collateral [Abstract]
Derivative collateral	$ 3.0	$ 0.5
Receivables [Abstract]
Late payment charge on unpaid receivables	1.00%
Electric territory	316
Gas territory	1,631
Inventories [Abstract]
Emission allowance balances	0.2	0.4
Renewable energy credit allowance balances	0.3	2.4
Purchased gas adjustment [Abstract]
Purchased gas adjustment over collected	0.9	4.8
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%	8.77%
AFUDC rate on certain Columbia environmental costs	100.00%	100.00%	100.00%
Capitalized software [Abstract]
Capitalized software	14.5	12.5
Capitalized software amortization	1.8	1.6
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	$ 12.9	$ 12.6	$ 13.7
Electric [Member]
Composite straight-line depreciation rates [Line Items]
Composite straight-line depreciation rates	3.20%	3.30%	3.60%
Gas [Member]
Composite straight-line depreciation rates [Line Items]
Composite straight-line depreciation rates	1.60%	1.70%	3.30%
Non Regulated Energy [Member]
Composite straight-line depreciation rates [Line Items]
Composite straight-line depreciation rates	2.30%	2.30%	2.50%
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%