Common Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended						12 Months Ended								1 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2011 Maximum [Member]	Jan. 31, 2011 Minimum [Member]	Jan. 31, 2013 Performance Units [Member]	Jan. 31, 2012 Performance Units [Member]	Jan. 31, 2011 Performance Units [Member]	Jan. 31, 2010 Performance Units [Member]	Dec. 31, 2012 Performance Units [Member]	Dec. 31, 2011 Performance Units [Member]	Dec. 31, 2010 Performance Units [Member]	Dec. 31, 2012 Stock Options [Member]	Dec. 31, 2011 Stock Options [Member]	Dec. 31, 2010 Stock Options [Member]	Dec. 31, 2012 Non-Vested Options [Member]	Dec. 31, 2011 Non-Vested Options [Member]	Jan. 31, 2013 Restricted Stock [Member]	Sep. 30, 2012 Restricted Stock [Member]	Dec. 31, 2012 Restricted Stock [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2010 Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value																	$ 3.78
Common Stock, Shares Authorized	325,000,000	325,000,000
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	$ 22.0	$ 28.5	$ 35.1							$ 16.3	$ 24.1	$ 26.0	$ 2.7	$ 2.6	$ 7.6					$ 3.0	$ 1.8	$ 1.5
Related Tax Benefit										7.0	9.7	4.3								0	0.8	0.7
Restricted stock activity
Outstanding																		188,222	192,558	192,558
Granted, Number of Shares						239,120	346,570	435,690	555,830									74,290		94,959
Granted, Weighted-Average Market Price																				$ 34.46
Released, Number of Shares																				(93,250)
Released, Weighted-Average Market Price																				$ 29.87
Forfeited, Number of Shares																				(6,045)
Forfeited, Weighted-Average Market Price																				$ 31.00
Outstanding																				188,222	192,558
Common Equity (Textuals)
Unrecognized total compensation costs related share based compensation										13.7						1.0				2.6
Months, on a weighted-average basis, expected for recognizing total compensation costs related to non-vested stock options										19 months						21 months			21 months
Restricted stock vesting period in years													3 years					3 years		10 years
Intrinsic Value										19.3	26.7	12.6								3.5	2.5	2.3
Actual tax benefit realized for the tax deductions from option exercises/settlement	0	14.3	24.1
Requirement to maintain common equity ratio by Wisconsin Electric				53.50%	48.50%
Requirement to maintain common equity ratio by Wisconsin Gas				50.00%	45.00%
Share-based Compensation Arrangement by Share-based Payment Award, Period of Accrual for Costs Related to Performance Units										3 years
Common Stock, Shares, Outstanding	229,039,456	230,486,804
Related Tax Benefit	$ 8.8	$ 11.4	$ 14.1