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Acquisitions - Purchase Price Allocation for Integrys Acquisition (Details) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 29, 2015
Assets acquired |  |  |  |
Goodwill |  | $ 7.3 | $ 0.0 |
Integrys |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Deferred taxes related to goodwill |  | 0.0 |  |
Assets acquired |  |  |  |
Current assets |  |  |  | $ 1,060.1
Property, plant, and equipment, net |  |  |  | 7,107.4
Goodwill | $ 2,604.3 |  |  |
Other long-term assets |  |  |  | 2,830.5
Liabilities assumed |  |  |  |
Current liabilities |  |  |  | (1,320.7)
Long-term debt |  |  |  | (2,943.6)
Other long-term liabilities |  |  |  | (3,703.8)
Preferred stock of subsidiary |  |  |  | $ (51.1)
Total purchase price | $ 5,583.1 |  |  |
Accounting Standards Update 2015-16 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Impact of adoption of ASU 2015-16 |  | $ 0.0 |  |

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