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RELATED PARTIES - OTHER TRANSACTIONS (Details) - USD ($) $ in Millions	Jan. 01, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
WPS Investments, LLC
Related parties
Equity method investment, ownership interest (as a percent)				10.37%
ATC
Related parties
Equity method investment, ownership interest (as a percent)					34.00%
Equity method investment transfer to affiliated company	$ 67.2
Transfer of deferred income tax related to ATC to affiliated company	$ 41.9
Accounts receivable for services provided to ATC		$ 1.9	$ 1.2
Accounts receivable from ATC for transmission infrstructure upgrades		2.8	0.0
Accounts payable to ATC for network transmission services		$ 9.1	$ 8.8