EMPLOYEE BENEFITS-IMPACT OF ADOPTION OF ASU 2017-07 (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operation and maintenance	$ 511.9	$ 504.5	$ 2,056.1	$ 2,171.3	$ 1,710.7
Other income, net	7.5	18.3	73.7	66.6	60.3
2017 Form 10-K Income Statement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operation and maintenance			2,047.0	2,185.5	1,709.3
Other income, net			64.6	80.8	58.9
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-service cost components of net benefit cost	$ (7.2)
Accounting Standards Update 2017-07 | Impact of ASU 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-service cost components of net benefit cost		$ (2.6)
Other operation and maintenance			9.1	(14.2)	1.4
Other income, net			$ 9.1	$ (14.2)	$ 1.4