SUPPLEMENTAL CASH FLOW INFORMATION - ADOPTION OF ACCOUNTING STANDARDS (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Provisions	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Operating activities
Other current assets	$ 81.3	$ 41.2	$ (7.1)	$ 103.2	$ (27.2)
Other, net	134.3	(19.2)	(197.4)	(53.6)	(207.6)
Net cash provided by operating activities	894.0	714.6	2,078.6	2,103.8	1,295.1
Investing activities
Withdrawal of restricted cash from rabbi trust for qualifying payments			0.0	0.0	0.0
Proceeds from the sale of investments held in rabbi trust	16.5	8.6	8.7	1.7	126.9
Purchase of investments held in rabbi trust			(3.7)	(59.2)	(60.2)
Integrys acquisition, net of cash acquired			0.0	0.0	(1,299.1)
Other, net	(0.7)	2.5	12.0	3.0	55.8
Net cash used in investing activities	(435.8)	(333.1)	(2,254.1)	(1,354.2)	(2,422.6)
Financing activities
Other, net	(0.3)	(0.6)	(6.5)	(13.6)	3.7
Net cash provided by (used in) financing activities	(445.1)	(380.8)	161.4	(845.7)	1,234.4
Net change in cash, cash equivalents, and restricted cash	13.1	0.7	(14.1)	(96.1)	106.9
Cash, cash equivalents, and restricted cash at beginning of period	58.6	72.7	72.7	168.8	61.9
Cash, cash equivalents, and restricted cash at end of period	71.7	73.4	58.6	72.7	168.8
2017 Form 10-K Cash Flows
Operating activities
Other current assets			(6.0)	103.1	(27.2)
Other, net			(197.5)	(53.8)	(209.1)
Net cash provided by operating activities			2,079.6	2,103.5	1,293.6
Investing activities
Withdrawal of restricted cash from rabbi trust for qualifying payments			19.5	26.6	1.4
Proceeds from the sale of investments held in rabbi trust			0.0	0.0	0.0
Purchase of investments held in rabbi trust			0.0	0.0	0.0
Integrys acquisition, net of cash acquired			0.0	0.0	(1,329.9)
Other, net			12.0	3.0	57.0
Net cash used in investing activities			(2,239.6)	(1,270.1)	(2,517.5)
Financing activities
Other, net			(6.5)	(13.6)	(18.9)
Net cash provided by (used in) financing activities			161.4	(845.7)	1,211.8
Net change in cash, cash equivalents, and restricted cash			1.4	(12.3)	(12.1)
Cash, cash equivalents, and restricted cash at beginning of period	38.9	37.5	37.5	49.8	61.9
Cash, cash equivalents, and restricted cash at end of period			38.9	37.5	49.8
Accounting Standards Update 2016-18 | Impact of ASU 2016-18
Operating activities
Other current assets			(1.1)	0.1	0.0
Other, net			0.1	0.2	1.5
Net cash provided by operating activities			(1.0)	0.3	1.5
Investing activities
Withdrawal of restricted cash from rabbi trust for qualifying payments			(19.5)	(26.6)	(1.4)
Proceeds from the sale of investments held in rabbi trust			8.7	1.7	126.9
Purchase of investments held in rabbi trust			(3.7)	(59.2)	(60.2)
Integrys acquisition, net of cash acquired			0.0	0.0	30.8
Other, net			0.0	0.0	(1.2)
Net cash used in investing activities		(7.5)	(14.5)	(84.1)	94.9
Financing activities
Other, net			0.0	0.0	22.6
Net cash provided by (used in) financing activities			0.0	0.0	22.6
Net change in cash, cash equivalents, and restricted cash			(15.5)	(83.8)	119.0
Cash, cash equivalents, and restricted cash at beginning of period	$ 19.7	35.2	35.2	119.0	0.0
Cash, cash equivalents, and restricted cash at end of period			$ 19.7	$ 35.2	$ 119.0
Accounting Standards Update 2016-15
Adoption of ASU 2016-15
Number of provisions in ASU 2016-15 | Provisions	8
Impact on financial statements from adoption	$ 0.0	$ 0.0