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Acquisitions - Impacts of Integrys Acquisition (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 29, 2015
Business Acquisition [Line Items] |  |  |  |
Severance expense | $ 29.4 |  |  |
ATC |  |  |  |
Business Combinations [Abstract] |  |  |  |
Equity interest in ATC |  |  |  | 60.00%
Integrys |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquisition costs |  | $ 3.5 | $ 107.6 |
Severance expense |  |  | 24.9 |
Severance payments |  | $ 7.5 | 16.9 |
Revenue attributable to Integrys |  |  | 1,416.8 |
Net income attributable to Integrys |  |  | 65.9 |
Integrys | ATC |  |  |  |
Business Combinations [Abstract] |  |  |  |
Equity interest in ATC |  |  |  | 34.00%
Integrys | Wisconsin |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Severance expense |  |  | 11.1 |
Integrys | Illinois |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Severance expense |  |  | 0.9 |
Integrys | Other States |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Severance expense |  |  | 0.1 |
Integrys | Corporate and Other |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Severance expense |  |  | $ 12.8 |

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