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LONG TERM DEBT (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2017 USD ($)
WEC Energy Group | 2007 Junior Notes (unsecured), due 2067 |
Debt Instrument [Line Items] |
Unsecured Long-term Debt, Noncurrent | $ 500.0
Interest rate percentage in excess of LIBOR | 2.1125%
MERC |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 120.0
MERC | MERC Senior Notes 3.11% due July 15, 2027 |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 40.0
Interest rate on long-term debt | 3.11%
MERC | MERC Senior Notes 3.41% due July 15, 2032 |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 40.0
Interest rate on long-term debt | 3.41%
MERC | MERC Senior Notes 4.01% due July 15, 2047 |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 40.0
Interest rate on long-term debt | 4.01%
MGU |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 90.0
MGU | MGU Senior Notes 3.11% due July 15, 2027 |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 30.0
Interest rate on long-term debt | 3.11%
MGU | MGU Senior Notes 3.41% due July 15, 2032 |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 30.0
Interest rate on long-term debt | 3.41%
MGU | MGU Senior Notes 4.01% due July 15, 2047 |
Debt Instrument [Line Items] |
Proceeds from Issuance of Debt | $ 30.0
Interest rate on long-term debt | 4.01%
Integrys | MERC |
Debt Instrument [Line Items] |
Repayment of long-term debt from parent | $ 78.0
Integrys | MGU |
Debt Instrument [Line Items] |
Repayment of long-term debt from parent | $ 71.0

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