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Derivative Instruments - Cash Flow Hedges (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) number_of_interest_rate_swaps | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jul. 31, 2018 USD ($)
Derivative [Line items] |  |  |  |  |
Number of interest rate swaps executed | number_of_interest_rate_swaps | 2 |  |  |  |
Issuance of long-term debt | $ 1,740.0 | $ 435.0 | $ 400.0 |  |
Proceeds received upon settlement recorded in OCI |  |  |  | $ 19.0 |
Reclassification within next 12 months | 1.8 |  |  |  |
Total senior notes issued in June 2015 |  |  |  |  |
Derivative [Line items] |  |  |  |  |
Issuance of long-term debt |  |  |  | $ 1,200.0 |
WEC Energy Group |  |  |  |  |
Derivative [Line items] |  |  |  |  |
Issuance of long-term debt | 600.0 | 0.0 | 0.0 |  |
WEC Energy Group | WEC 2007 Junior Notes due 2067 |  |  |  |  |
Derivative [Line items] |  |  |  |  |
Long-term debt outstanding | 500.0 | 500.0 |  |  |
Interest rate swaps |  |  |  |  |
Derivative [Line items] |  |  |  |  |
Amount of net derivative loss recognized in OCI | (2.9) | 0.0 | 0.0 |  |
Amount of net derivative gain reclassified from accumulated OCI to interest expense | $ 1.6 | $ 2.2 | $ 2.2 |  |
Interest rate swaps | WEC Energy Group |  |  |  |  |
Derivative [Line items] |  |  |  |  |
Number of interest rate swaps executed | number_of_interest_rate_swaps | 2 |  |  |  |
Interest rate swap notional value | $ 250.0 |  |  |  | $ 250.0
Interest rate swap fixed interest rate | 4.9765% |  |  |  |

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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=112277219&loc=d3e80748-113994 + Details
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