Derivative Instruments - Cash Flow Hedges (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) number_of_interest_rate_swaps	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jul. 31, 2018 USD ($)
Derivative [Line items]
Number of interest rate swaps executed | number_of_interest_rate_swaps	2
Issuance of long-term debt	$ 1,740.0	$ 435.0	$ 400.0
Proceeds received upon settlement recorded in OCI				$ 19.0
Reclassification within next 12 months	1.8
Total senior notes issued in June 2015
Derivative [Line items]
Issuance of long-term debt				$ 1,200.0
WEC Energy Group
Derivative [Line items]
Issuance of long-term debt	600.0	0.0	0.0
WEC Energy Group | WEC 2007 Junior Notes due 2067
Derivative [Line items]
Long-term debt outstanding	500.0	500.0
Interest rate swaps
Derivative [Line items]
Amount of net derivative loss recognized in OCI	(2.9)	0.0	0.0
Amount of net derivative gain reclassified from accumulated OCI to interest expense	$ 1.6	$ 2.2	$ 2.2
Interest rate swaps | WEC Energy Group
Derivative [Line items]
Number of interest rate swaps executed | number_of_interest_rate_swaps	2
Interest rate swap notional value	$ 250.0				$ 250.0
Interest rate swap fixed interest rate	4.9765%