Nuclear Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Fuel Disposal [Abstract]
Fuel disposal fee charge (in dollars per KWh)	$ 0.001
DOE fuel disposal assessments included in fuel expense	11,000,000		13,000,000		12,000,000
Cumulative fuel disposal assessments paid to DOE	422,300,000
Initial payment received persuant to DOE Settlement	100,000,000
Regulatory Plant Decommissioning Recovery [Abstract]
Number of years of extended operation at Monticello nuclear plant (in years)	20Y
Number of authorized canisters filled and placed in Monticello dry-cask nuclear storage facility	10
Number of authorized canisters in Monticello dry-cask nuclear storage facility	30
Number of nuclear reactors at Prairie Island	2
Number of years of extended operation at Prairie Island nuclear plant (in years)	20Y
Number of authorized canisters filled and placed in Prairie Island dry-cask nuclear storage facility	29
Number of additional authorized canisters for the Prairie Island dry-cask nuclear storage facility	35
Percentage of total obligation for decommissioning expected to be funded by the external decommissioning trust fund (in hundredths)	100.00%
Assumed annual escalation rate of decommissioning costs ( in hundredths)	2.89%
Assumed rate of return on external decommissioning trust fund, net of tax (in hundredths)	6.30%
Authorized amount withdrawn from escrow not yet refunded to Wisconsin and Michigan customers associated with the Monticello nuclear plant	5,900,000
Assets held in external decommissioning trust	1,300,000,000		1,400,000,000
The number of decommissioning scenarios required to be presented	5
Funded Status of Nuclear Decommissioning Obligation [Abstract]
Estimated decommissioning cost obligation (2008 dollars)	2,308,196,000		2,308,196,000
Effect of escalating costs (to 2011 and 2010 dollars, respectively, at 2.89 percent per year)	205,960,000		135,342,000
Estimated decommissioning cost obligation (in current dollars)	2,514,156,000		2,443,538,000
Effect of escalating costs to payment date (2.89 percent per year)	2,602,207,000		2,672,825,000
Estimated future decommissioning costs (undiscounted)	5,116,363,000		5,116,363,000
Effect of discounting obligation (using risk-free interest rate)	(3,187,914,000)		(3,856,516,000)
Discounted decommissioning cost obligation	1,928,449,000		1,259,847,000
Assets held in external decommissioning trust	1,336,431,000		1,350,630,000
Underfunding / (overfunding) of external decommissioning fund compared to the discounted decommissioning obligation	592,018,000		(90,783,000)
Annual decommissioning recorded as depreciation expense [Abstract]
Externally funded	0		934,000		2,849,000
Internally funded (including interest costs)	(456,000)		(777,000)		(884,000)
Net decommissioning expense recorded	$ (456,000)	[1]	$ 157,000	[1]	$ 1,965,000	[1]

[1] Decommissioning expense does not include depreciation of the capitalized nuclear asset retirement costs.