Schedule I - Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financial instruments
Long-term debt, including current portion	$ 9,994.0	$ 8,746.6
Carrying amount
Financial instruments
Long-term debt, including current portion	10,335.7	9,561.7
Fair value
Financial instruments
Long-term debt, including current portion	10,554.9	10,341.9
WEC Energy Group
Financial instruments
Long-term notes receivable from UMERC	150.0	50.0
WEC Energy Group | Carrying amount
Financial instruments
Long-term notes receivable from UMERC	150.0	50.0
Long-term debt, including current portion	2,190.8	1,892.3
WEC Energy Group | Fair value
Financial instruments
Long-term notes receivable from UMERC	145.5	49.5
Long-term debt, including current portion	$ 2,132.8	$ 1,941.5