Common Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended				1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015	Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
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	$ 21.9	$ 19.0	$ 22.0
Common Equity (Textuals)
Actual tax benefit realized for the tax deductions from option exercises/settlement	19.9	17.8	0
Common Stock, Shares, Outstanding	225,517,339	225,962,959
Related Tax Benefit	8.8	7.6	8.8
Maximum [Member]
Common Equity (Textuals)
Requirement to maintain common equity ratio by Wisconsin Electric				53.50%
Requirement to maintain common equity ratio by Wisconsin Gas				52.00%
Minimum [Member]
Common Equity (Textuals)
Requirement to maintain common equity ratio by Wisconsin Electric				48.50%
Requirement to maintain common equity ratio by Wisconsin Gas				47.00%
Performance Units [Member]
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	15.4	12.7	16.3
Related Tax Benefit	4.8	5.3	7.0
Restricted stock activity
Granted, Number of Shares					195,365	233,735	239,120	346,570
Common Equity (Textuals)
Unrecognized total compensation costs related share based compensation	12.6
Months, on a weighted-average basis, expected for recognizing total compensation costs related to non-vested stock options	20 months
Intrinsic Value	13.2	14.8	19.3
Stock Options [Member]
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	3.7	3.9	2.7
Non-Vested Options [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		$ 3.38
Common Equity (Textuals)
Unrecognized total compensation costs related share based compensation	2.1
Months, on a weighted-average basis, expected for recognizing total compensation costs related to non-vested stock options	19 months
Restricted Stock [Member]
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	2.8	2.4	3.0
Related Tax Benefit	1.0	1.3	0
Restricted stock activity
Outstanding	150,698			155,479	155,479	150,698
Granted, Number of Shares	71,504				60,164
Granted, Weighted-Average Market Price	$ 40.96
Released, Number of Shares	(63,509)
Released, Weighted-Average Market Price	$ 33.02
Forfeited, Number of Shares	(3,214)
Forfeited, Weighted-Average Market Price	$ 38.47
Outstanding	155,479	150,698
Common Equity (Textuals)
Unrecognized total compensation costs related share based compensation	2.8
Months, on a weighted-average basis, expected for recognizing total compensation costs related to non-vested stock options	20 months
Restricted stock vesting period in years						3 years
Intrinsic Value	$ 2.7	$ 4.0	$ 3.5