ACQUISITIONS - FORWARD ACQUISITION (Details) - Forward Wind Energy Center $ in Millions	1 Months Ended
	Oct. 31, 2017 USD ($) wind_turbines utility MW	Sep. 30, 2017
WPS
Business Acquisition [Line Items]
Percentage of Forward Wind Energy Center's output purchased by WPS		44.60%
Subsequent event
Business Acquisition [Line Items]
Number of wind turbines at Forward Wind Energy Center | wind_turbines	86
Capacity of Foward Wind Energy Center | MW	129
Approximate purchase price	$ 174
Subsequent event | WPS
Business Acquisition [Line Items]
Number of utilities along with WPS that entered in an agreement to purchase Forward Wind Energy Center | utility	2
Approximate purchase price	$ 78
WPS's share of Forward Wind Energy Center's purchase price	44.60%