Schedule I - Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statements of comprehensive income
Net income attributed to common shareholders	$ 239.0	$ 266.8	$ 241.6	$ 452.5	$ 243.9	$ 234.3	$ 235.7	$ 420.1	$ 1,199.9	$ 1,134.0	$ 1,059.3
Other comprehensive loss, net of tax									(2.7)	(1.5)	(5.5)
Comprehensive income attributed to common shareholders									1,197.2	1,132.5	1,053.8
Derivatives accounted for as cash flow hedges
Derivatives accounted for as cash flow hedges
Net derivative loss, net of tax benefit of $1.6, $1.3, and $0.8, respectively									(4.3)	(3.5)	(2.1)
Reclassification of net (gain) loss to net income, net of tax									1.5	(0.8)	(1.2)
Cash flow hedges, net									(2.8)	(4.3)	(1.7)
Derivatives accounted for as cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
Derivatives accounted for as cash flow hedges
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	1.6
Defined benefit plans
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	1.6
Defined benefit plans
Defined benefit plans
Pension and OPEB adjustments arising during the period, net of tax expense (benefit) of $(0.2), $1.0, and $(1.2), respectively									(0.5)	2.6	(3.1)
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax									0.6	0.2	0.3
Defined benefit plans, net									0.1	2.8	(3.8)
Defined benefit plans | Cumulative Effect, Period of Adoption, Adjustment
Derivatives accounted for as cash flow hedges
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	1.0
Defined benefit plans
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	1.0
WEC Energy Group
Statements of comprehensive income
Net income attributed to common shareholders									1,199.9	1,134.0	1,059.3
Other comprehensive income (loss) from subsidiaries, net of tax									0.2	2.2	(2.8)
Other comprehensive loss, net of tax									(2.7)	(1.5)	(5.5)
Comprehensive income attributed to common shareholders									1,197.2	1,132.5	1,053.8
WEC Energy Group | Derivatives accounted for as cash flow hedges
Derivatives accounted for as cash flow hedges
Net derivative loss, net of tax benefit of $1.6, $1.3, and $0.8, respectively									(4.3)	(3.5)	(2.1)
Reclassification of net (gain) loss to net income, net of tax									1.5	(0.8)	(1.2)
Cash flow hedges, net									(2.8)	(4.3)	(1.7)
WEC Energy Group | Derivatives accounted for as cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
Derivatives accounted for as cash flow hedges
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	1.6
Defined benefit plans
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	1.6
WEC Energy Group | Defined benefit plans
Defined benefit plans
Pension and OPEB adjustments arising during the period, net of tax expense (benefit) of $(0.2), $1.0, and $(1.2), respectively									(0.4)	0.4	(0.9)
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax									0.3	0.2	0.2
Defined benefit plans, net									(0.1)	0.6	(1.0)
WEC Energy Group | Defined benefit plans | Cumulative Effect, Period of Adoption, Adjustment
Derivatives accounted for as cash flow hedges
Cumulative effect adjustment from adoption of ASU 2018-02									0.0	0.0	0.3
Defined benefit plans
Cumulative effect adjustment from adoption of ASU 2018-02									$ 0.0	$ 0.0	$ 0.3