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RELATED PARTIES - OTHER TRANSACTIONS (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2017	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2016
Integrys
Related parties
Liability related to income tax allocation		$ 4.1	$ 3.6
WPS Investments, LLC
Related parties
Equity method investment, ownership percentage				10.37%
ATC
Related parties
Equity method investment, ownership percentage				34.00%
Transfer of ownership in WPSI to another subsidiary of Integrys	$ 67.2
Transfer of deferred income tax related to ATC to affiliated company		41.9
Accounts receivable for services provided to ATC		0.7	0.5
Accounts payable to ATC for network transmission services		$ 9.0	$ 8.8