Summary of Significant Accounting Policies (Details 3) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Debt Securities In Cash Equivalents, Maximum Period Until Maturity	0 years 3 months
Fuel Cost Tolerance Range Set by Regulators, Percent	2.00%
Regulatory liabilities	$ 868.3	$ 915.9
Fair value of stock options
Dividend yield	3.90%	3.90%	3.70%
Expected volatility	19.00%	19.00%	20.30%
Expected life (years)	5 years 11 months	5 years 6 months	5 years 11 months
Expected forfeiture rate	2.00%	2.00%	2.00%
Weighted average fair value of our stock options granted	$ 3.34	$ 3.17	$ 3.36
Other Income and Deductions, Net
AFUDC - Equity	35.3	59.4	32.5
Gain on Property Sales	2.7	2.4	4.4
Other, net	(3.2)	0.9	3.3
Other Income and Deductions, net	34.8	62.7	40.2
Allowance For Funds Used During Construction
AFUDC - Debt	14.7	24.7	13.5
AFUDC - Equity	35.3	59.4	32.5
Inventories
Fossil Fuel	165.5	169.2
Materials and Supplies	121.9	114.1
Natural Gas in Storage	73.3	98.7
Total	360.7	382.0
Summary Of Significant Accounting Policies (Additional) (Textuals) [Abstract]
Symmetrical Fuel Cost Tolerance Range Set By Regulatory Authority	Our retail electric rates in Wisconsin are established by the PSCW and include base amounts for fuel and purchased power costs. Beginning in January 2011, the electric fuel rules in Wisconsin allow us to defer, for subsequent rate recovery or refund, any under-collection or over-collection of fuel costs that are outside of the symmetrical fuel cost tolerance, which the PSCW set at plus or minus 2% of the approved fuel cost plan. The deferred amounts are subject to an excess revenues test. Our retail gas rates include monthly adjustments which permit the recovery or refund of actual purchased gas costs. We defer any difference between actual gas costs incurred (adjusted for a sharing mechanism) and costs recovered through rates as a current asset or liability. The deferred balance is returned to or recovered from customers at intervals throughout the year.
Depreciation percentage of average depreciable utility plant	2.90%	2.80%	2.80%
Maximum duration to recover outstanding regulatory assets	8 years
Minimum duration to recover outstanding regulatory assets	1 year
Maximum regulatory current assets and current liabilities amortization period	1
Impairment charges related to goodwill	0
Long-lived assets held for sale	$ 0
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration of options from time of grant	10 years
Fair value of stock options
Weighted average fair value of our stock options granted	$ 34.88