Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 MW	Dec. 31, 2011	Dec. 31, 2010
Columbia Commitments [Abstract]
Columbia environmental project commitments	$ 73,400,000
Capital Contribution Commitment [Abstract]
Investment capital contribution commitment	2,000,000
Venture Debt Investment Commitment [Abstract]
Venture debt investment commitment	1,000,000
Venture debt remaining investment commitment	500,000
Green Energy Projects Commitment [Abstract]
Green energy projects expenditures	300,000
Lease Commitments [Abstract]
Rental expense under operating leases	2,600,000	3,200,000	3,200,000
Smart Grid Investment Grant [Abstract]
Smart Grid grant approval amount	5,500,000
Total cost of Smart Grid projects	11,000,000
Smart grid expenditures to date	8,400,000
Purchase power agreement [Abstract]
Purchase power agreement with Osceola Windpower II, LLC - MW	50
Chattel Paper Purchase Agreement [Abstract]
Chattel paper purchase agreement maximum interest	10,000,000
Outstanding interest in chattel paper receivables	4,500,000	4,300,000	3,600,000
Servicing asset recognized	200,000	200,000	100,000
Chattel paper gain on sales of loans	100,000	100,000	100,000
Servicing asset amortized	100,000
Chattel paper loan sales	1,200,000	1,500,000	500,000
Chattel paper agreement principal payments	1,200,000	800,000	700,000
Energy Efficiency and Renewables [Abstract]
Minimum electricity generated from renewables	10.00%
Air Quality Regulations [Abstract]
Minimum capacity to be regulated by CAIR - MW	25
CAIR phase one emission reductions for NOx	55.00%
CAIR phase one emission reductions for SO2	40.00%
CAIR phase two emission reductions for NOx	65.00%
CAIR phase two emission reductions for SO2	70.00%
Coal-fired electric generating units - MW requirement	150
Minimum amount of fleet-wide mercury reduction in Wisconsin - Phase two	90.00%
Columbia Units [Member]
Columbia Units [Line Items]
Generating capability - MW	225
Jointly owned plant's share of MGE's net generating capability	29.00%
Ownership interest in jointly owned plant	22.00%
WPL ownership interest of Columbia	46.20%
WPSC ownership interest of Columbia	31.80%
Columbia Certificate Of Authority [Abstract]
MGE's share of total capital expenditures	140,000,000
One Year From Balance Sheet Date [Member]
Operating expense purchase contracts [Abstract]
Coal commitments	21,890,000
Natural Gas Commitments [Abstract]
Natural gas transportation and storage commitments	17,219,000
Natural gas supply commitments	9,765,000
Purchase power commitments	50,713,000
Other commitments	926,000
Total operating expense purchase contracts	100,513,000
WPDES Permit [Abstract]
WPDES permit payment	300,000
Wind Easements Commitment [Abstract]
Wind easements commitment	100,000
Chattel Paper Agreement Principal Payments [Abstract]
Principal payment commitments	689,000
Lease Commitments [Abstract]
Lease Commitments	2,026,000
Smart Grid Investment Grant [Abstract]
Smart grid commitments to date	1,300,000
More Than One And Within Two Years From Balance Sheet Date [Member]
Operating expense purchase contracts [Abstract]
Coal commitments	9,450,000
Natural Gas Commitments [Abstract]
Natural gas transportation and storage commitments	16,947,000
Natural gas supply commitments	0
Purchase power commitments	49,880,000
Other commitments	239,000
Total operating expense purchase contracts	76,516,000
WPDES Permit [Abstract]
WPDES permit payment	300,000
Wind Easements Commitment [Abstract]
Wind easements commitment	100,000
Chattel Paper Agreement Principal Payments [Abstract]
Principal payment commitments	547,000
Lease Commitments [Abstract]
Lease Commitments	1,557,000
More Than Two And Within Three Years From Balance Sheet Date [Member]
Operating expense purchase contracts [Abstract]
Coal commitments	6,340,000
Natural Gas Commitments [Abstract]
Natural gas transportation and storage commitments	16,793,000
Natural gas supply commitments	0
Purchase power commitments	48,050,000
Other commitments	0
Total operating expense purchase contracts	71,183,000
Green Energy Projects Commitment [Abstract]
Green energy projects commitment	100,000
WPDES Permit [Abstract]
WPDES permit payment	300,000
Wind Easements Commitment [Abstract]
Wind easements commitment	100,000
Chattel Paper Agreement Principal Payments [Abstract]
Principal payment commitments	862,000
Lease Commitments [Abstract]
Lease Commitments	1,082,000
More Than Three And Within Four Years From Balance Sheet Date [Member]
Operating expense purchase contracts [Abstract]
Coal commitments	1,623,000
Natural Gas Commitments [Abstract]
Natural gas transportation and storage commitments	9,218,000
Natural gas supply commitments	0
Purchase power commitments	49,037,000
Other commitments	0
Total operating expense purchase contracts	59,878,000
Green Energy Projects Commitment [Abstract]
Green energy projects commitment	100,000
WPDES Permit [Abstract]
WPDES permit payment	300,000
Wind Easements Commitment [Abstract]
Wind easements commitment	100,000
Chattel Paper Agreement Principal Payments [Abstract]
Principal payment commitments	790,000
Lease Commitments [Abstract]
Lease Commitments	791,000
More Than Four And Within Five Years From Balance Sheet Date [Member]
Operating expense purchase contracts [Abstract]
Coal commitments	1,040,000
Natural Gas Commitments [Abstract]
Natural gas transportation and storage commitments	197,000
Natural gas supply commitments	0
Purchase power commitments	49,833,000
Other commitments	0
Total operating expense purchase contracts	51,070,000
WPDES Permit [Abstract]
WPDES permit payment	300,000
Wind Easements Commitment [Abstract]
Wind easements commitment	100,000
Chattel Paper Agreement Principal Payments [Abstract]
Principal payment commitments	376,000
Lease Commitments [Abstract]
Lease Commitments	679,000
More than Five Years from Balance Sheet Date and Thereafter [Member]
Lease Commitments [Abstract]
Lease Commitments	$ 8,606,000