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Acquisitions - Consideration Transferred for Integrys Acquisition (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended |  |  |
--- | --- | --- | --- | ---
Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 29, 2015
Business Acquisition [Line Items] |  |  |  |
Integrys common shares outstanding at June 29, 2015 |  | 315,574,624 | 315,614,941 |
ATC |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Equity interest in ATC |  |  |  | 60.00%
Integrys |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Percentage of Integrys common shares acquired |  |  |  | 100.00%
Integrys common shares outstanding at June 29, 2015 |  |  |  | 79,963,091
Exchange ratio | 1.128 |  |  |
Wisconsin Energy Corporation shares issued for Integrys shares | 90,187,884 |  |  |
Closing price of Wisconsin Energy Corporation 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 |  |  |
Number of Wisconsin Energy Corporation fractional shares that were paid in cash |  |  |  | 10,483
Dollar value of Wisconsin Energy Corporation fractional shares paid in cash |  |  |  | $ 0.5
Integrys | ATC |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Equity interest in ATC |  |  |  | 34.00%

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