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Acquisitions - Approval Conditions for Integrys Acquisition (Details) - Integrys - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 29, 2015
Earnings sharing mechanisms |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Expense for earnings sharing mechanisms |  | $ 2.9 | $ 24.4 |
WE |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Duration of earnings cap condition imposed by the PSCW (in years) | 3 years |  |  |
Percentage of first 50 basis points of additional utility earnings shared with customers | 50.00% |  |  |
ROE in excess of authorized amount (as a percent) |  |  |  | 0.50%
WG |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Duration of earnings cap condition imposed by the PSCW (in years) | 3 years |  |  |
Percentage of first 50 basis points of additional utility earnings shared with customers | 50.00% |  |  |
ROE in excess of authorized amount (as a percent) |  |  |  | 0.50%
PGL |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Duration of base rate freeze condition imposed by the ICC (in years) | 2 years |  |  |
NSG |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Duration of base rate freeze condition imposed by the ICC (in years) | 2 years |  |  |

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