COMMON EQUITY - STOCK-BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 4.6	$ 3.4	$ 4.8
Related tax benefit	1.8	1.4	1.9
Stock options | WEC Energy Group stock-based compensation plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	0.6	0.5	0.0
Restricted stock | WEC Energy Group stock-based compensation plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	0.7	1.4	0.0
Performance stock rights | WEC Energy Group stock-based compensation plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	3.3	1.5	0.0
Performance stock rights | Integrys stock-based compensation plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	0.0	0.0	1.3
Restricted stock units | Integrys stock-based compensation plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 0.0	$ 0.0	$ 3.5