SUPPLEMENTAL CASH FLOW INFORMATION (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Supplemental Cash Flow Information [Abstract]
Cash (paid) for interest, net of amount capitalized	$ (26.1)	$ (27.5)
Cash (paid) for income taxes, net	(8.6)	(18.5)
Significant noncash transactions
Accounts payable related to construction costs	13.0	33.4
Transfer of assets from affiliates
Significant noncash transactions
Accounts payable related to assets transferred from affiliates	5.5	0.0
WPSI
Significant noncash transactions
Transfer of ownership in WPSI to another subsidiary of Integrys	0.0	67.2
UMERC
Significant noncash transactions
Transfer of net assets to UMERC	$ 0.0	$ 21.1

Future cash outflow to pay for purchases of fixed assets that have occurred.

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