Summary of Significant Accounting Policies (Details 3) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
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	$ 830.6	$ 879.1
Fair value of stock options
Dividend yield	3.80%	3.70%	3.90%
Expected volatility	18.00%	18.00%	19.00%
Expected life (years)	5 years 10 months	5 years 11 months	5 years 11 months
Expected forfeiture rate	2.00%	2.00%	2.00%
Weighted average fair value of our stock options granted	$ 4.18	$ 3.45	$ 3.34
Other Income and Deductions, Net
AFUDC - Equity	5.6	18.3	35.3
Gain on Property Sales	7.5	0.8	2.7
Other, net	0.3	(0.3)	(3.2)
Other Income and Deductions, net	13.4	18.8	34.8
Allowance For Funds Used During Construction
AFUDC - Debt	2.3	7.7	14.7
AFUDC - Equity	5.6	18.3	35.3
Inventories
Fossil Fuel	125.6	117.7
Materials and Supplies	150.2	133.9
Natural Gas in Storage	124.8	77.8
Total	400.6	329.4
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. 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 under-collected 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%	2.90%	2.90%
Maximum duration to recover outstanding regulatory assets	8 years
Minimum duration to recover outstanding regulatory assets	1 year
Impairment charges related to goodwill	0
Long-lived assets held for sale	0
Treasury Grant	$ 17.4	$ 48.0	$ 0
Grant Revenue Received	76.2
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	$ 41.03