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Long-Term Debt and Capital Lease Obligations - Long-term Debt (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | May 15, 2017 | Dec. 31, 2016 | Aug. 01, 2016
Future maturities of long-term debt outstanding |  |  |  |
2018 | $ 838.4 |  |  |
2019 | 360.1 |  |  |
2020 | 686.9 |  |  |
2021 | 338.8 |  |  |
2022 | 40.7 |  |  |
Thereafter | 7,336.0 |  |  |
Total | $ 9,600.9 |  |  |
WEC Energy Group junior notes (unsecured), 3.53% due 2067 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Debt instrument, basis spread on variable rate | 2.1125% |  |  |
Debt instrument interest rate stated percentage rate | 3.53% | 6.25% |  |
WEC Energy Group |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Current portion of long-term debt | $ 300.0 |  | $ 0.0 |
Future maturities of long-term debt outstanding |  |  |  |
2018 | 300.0 |  |  |
2020 | 400.0 |  |  |
Thereafter | 1,200.0 |  |  |
Total | 1,900.0 |  |  |
WEC Energy Group | WEC Energy Group senior notes (unsecured), 6.20% due 2033 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 200.0 |  | 200.0 |
Debt instrument interest rate stated percentage rate | 6.20% |  |  |
WEC Energy Group | WEC Energy Group junior notes (unsecured), 3.53% due 2067 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 500.0 |  | 500.0 |
Debt instrument interest rate stated percentage rate | 3.53% |  |  |
WPS | WPS senior notes 5.65% matured November 2017 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Repayments of debt | $ 125.0 |  |  |
Debt instrument interest rate stated percentage rate | 5.65% |  |  |
PGL | PGL first and refunding mortgage bonds (secured), 3.77% due 2047 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Debt instrument interest rate stated percentage rate | 3.77% |  |  |
Proceeds from issuance of debt | $ 100.0 |  |  |
We Power | We Power Notes (secured, nonrecourse), 4.91% due 2015-2030 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Debt instrument interest rate stated percentage rate | 4.91% |  |  |
Current portion of long-term debt | $ 5.9 |  |  |
Long-term debt, gross | $ 101.0 |  |  |
We Power | We Power Notes (secured, nonrecourse), 6.00% due 2015-2033 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Debt instrument interest rate stated percentage rate | 6.00% |  |  |
Current portion of long-term debt | $ 4.9 |  |  |
Long-term debt, gross | $ 121.5 |  |  |
We Power | We Power Notes (secured, nonrecourse), 5.209% due 2015-2030 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Debt instrument interest rate stated percentage rate | 5.209% |  |  |
Current portion of long-term debt | $ 11.4 |  |  |
Long-term debt, gross | $ 194.1 |  |  |
We Power | We Power Notes (secured, nonrecourse), 4.673% due 2015-2031 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Debt instrument interest rate stated percentage rate | 4.673% |  |  |
Current portion of long-term debt | $ 8.9 |  |  |
Long-term debt, gross | 162.4 |  |  |
Integrys Holding Inc | Integrys senior notes (unsecured), 4.17% due 2020 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 250.0 |  | 250.0 |
Debt instrument interest rate stated percentage rate | 4.17% |  |  |
Integrys Holding Inc | Integrys junior notes (unsecured), 6.00% due 2073 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 400.0 |  | 400.0 |
Debt instrument, basis spread on variable rate | 3.22% |  |  |
Debt instrument interest rate stated percentage rate | 6.00% |  |  |
Long-term debt, gross | $ 400.0 |  |  |
MERC |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Proceeds from issuance of debt | 120.0 |  |  |
MERC | MERC senior notes (unsecured), 3.11% due 2027 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 40.0 |  | 0.0 |
Debt instrument interest rate stated percentage rate | 3.11% |  |  |
Proceeds from issuance of debt | $ 40.0 |  |  |
MERC | MERC senior notes (unsecured), 3.41% due 2032 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 40.0 |  | 0.0 |
Debt instrument interest rate stated percentage rate | 3.41% |  |  |
Proceeds from issuance of debt | $ 40.0 |  |  |
MERC | MERC senior notes (unsecured), 4.01% due 2047 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 40.0 |  | 0.0 |
Debt instrument interest rate stated percentage rate | 4.01% |  |  |
Proceeds from issuance of debt | $ 40.0 |  |  |
MGU |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Proceeds from issuance of debt | 90.0 |  |  |
MGU | MGU senior notes (unsecured), 3.11% due 2027 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 30.0 |  | 0.0 |
Debt instrument interest rate stated percentage rate | 3.11% |  |  |
Proceeds from issuance of debt | $ 30.0 |  |  |
MGU | MGU senior notes (unsecured), 3.41% due 2032 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 30.0 |  | 0.0 |
Debt instrument interest rate stated percentage rate | 3.41% |  |  |
Proceeds from issuance of debt | $ 30.0 |  |  |
MGU | MGU senior notes (unsecured), 4.01% due 2047 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 30.0 |  | 0.0 |
Debt instrument interest rate stated percentage rate | 4.01% |  |  |
Proceeds from issuance of debt | $ 30.0 |  |  |
Bluewater Gas Storage, LLC | Bluewater Gas Storage senior notes (unsecured), 3.76% due 2047 |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term debt, unsecured | $ 125.0 |  | $ 0.0 |
Debt instrument interest rate stated percentage rate | 3.76% |  |  |
Proceeds from issuance of debt | $ 125.0 |  |  |
Current portion of long-term debt | 2.3 |  |  |
Long-term debt, gross | 125.0 |  |  |
WE |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Long-term pollution control bond | 80.0 |  |  | $ 67.0
Integrys Holding Inc | MERC |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Repayments of debt | 78.0 |  |  |
Integrys Holding Inc | MGU |  |  |  |
Debt Disclosure [Abstract] |  |  |  |
Repayments of debt | $ 71.0 |  |  |

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