Schedule I - Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Notes to parent company financial statements
Long-term debt, including current portion	$ 11,858.3	$ 10,335.7
Carrying amount
Notes to parent company financial statements
Long-term debt, including current portion	11,858.3	10,335.7
Fair value
Notes to parent company financial statements
Long-term debt, including current portion	13,035.9	10,554.9
WEC Energy Group
Notes to parent company financial statements
Long-term notes receivable from UMERC	0.0	150.0
WEC Energy Group | Carrying amount
Notes to parent company financial statements
Long-term notes receivable from UMERC	0.0	150.0
Long-term debt, including current portion	2,541.6	2,190.8
WEC Energy Group | Fair value
Notes to parent company financial statements
Long-term notes receivable from UMERC	0.0	145.5
Long-term debt, including current portion	$ 2,619.4	$ 2,132.8