Acquisition Consideration Transferred (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Jun. 30, 2015	Sep. 30, 2015	Jun. 29, 2015	Dec. 31, 2014
Acquisition
Percentage of Integrys common shares acquired			100.00%
Equity interest in ATC			60.00%
Integrys common shares outstanding at June 29, 2015		315,684,451		225,517,339
Exchange ratio	1.128
Wisconsin Energy Shares issued for Integrys shares	90,187,884
Number of Wisconsin Energy fractional shares that were paid in cash			10,483
Dollar value of Wisconsin Energy fractional shares paid in cash			$ 0.5
Closing price of Wisconsin Energy common shares on June 29, 2015			$ 45.16
Fair value of common stock issued			$ 4,072.9
Cash paid per share of Integrys shares outstanding	$ 18.58
Fair value of cash paid for integrys shares			1,486.2
Consideration attributable to settlement of equity awards, net of tax			24.0
Total cash consideration paid for acquisition			$ 1,510.2
Total purchase price	$ 5,583.1
Integrys
Acquisition
Equity interest in ATC			34.00%
Integrys common shares outstanding at June 29, 2015			79,963,091