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Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Benefit Plans
Company contribution to 401(k) savings plan for Wisconsin Energy Corporation management employees hired after December 31, 2014 (percentage)	6.00%
Pension Costs
Change in benefit obligation
Obligation at January 1	$ 3,058.8	$ 3,083.0
Service cost	44.6	45.4	$ 30.4
Interest cost	121.8	130.8	94.3
Participant contributions	0.0	0.0
Plan amendments	0.0	(3.0)
Actuarial loss (gain)	162.6	71.7
Benefit payments	(224.1)	(269.1)
Obligation at December 31	3,163.7	3,058.8	3,083.0
Change in fair value of plan assets
Beginning balance at January 1	2,709.2	2,755.1
Actual return on plan assets	368.7	199.4
Employer contributions	113.0	23.8
Participant contributions	0.0	0.0
Benefit payments	(224.1)	(269.1)
Ending balance at December 31	2,966.8	2,709.2	2,755.1
Funded status at December 31	(196.9)	(349.6)
OPEB Costs
Change in benefit obligation
Obligation at January 1	818.4	842.0
Service cost	24.1	26.1	20.7
Interest cost	32.9	37.0	26.7
Participant contributions	13.4	16.4
Plan amendments	(36.4)	(18.9)
Actuarial loss (gain)	12.9	(36.5)
Benefit payments	(48.8)	(49.1)
Federal subsidy on benefits paid	2.0	1.4
Obligation at December 31	818.5	818.4	842.0
Change in fair value of plan assets
Beginning balance at January 1	773.5	749.8
Actual return on plan assets	95.9	51.5
Employer contributions	7.5	4.9
Participant contributions	13.4	16.4
Benefit payments	(48.8)	(49.1)
Ending balance at December 31	841.5	773.5	$ 749.8
Funded status at December 31	$ 23.0	$ (44.9)

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Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan.

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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Percentage company contributes to the 401(k) savings plan for Wisconsin Energy Corporation's management employees hired after December 31, 2014.

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