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SHORT-TERM DEBT AND LINES OF CREDIT - SHORT-TERM BORROWINGS (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Short-term borrowings
Issuance of term loan		$ 0.5	$ 340.0
WEC Senior Notes 0.80% due 2024 \| WEC Energy Group
Short-term borrowings
Debt instrument stated interest rate percentage				0.80%
Commercial paper
Short-term borrowings
Commercial paper outstanding		$ 1,508.4			$ 1,436.9
Weighted average interest rate on amounts outstanding		0.16%			0.21%
Average amount of commercial paper outstanding during the period		$ 1,415.4
Weighted-average interest rate on amounts outstanding during the period		0.17%
Term loan
Short-term borrowings
Operating expense loans outstanding		$ 0.5			$ 0.0
Term loan \| WEC Energy Group
Short-term borrowings
Term loan outstanding		$ 0.0			$ 340.0
Weighted average interest rate on amounts outstanding					0.99%
Issuance of term loan	$ 340.0
Length of term loan	364 days