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LONG TERM DEBT (Details) $ in Millions	9 Months Ended
	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

X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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