OPERATING REVENUES - OTHER OPERATING REVENUES (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Operating Revenues
Revenues	$ 2,286.5	$ 2,304.5
Other operating revenues
Disaggregation of Operating Revenues
Revenues	(3.5)
Other operating revenues \| Alternative revenues
Disaggregation of Operating Revenues
Revenues	(16.1)
Other operating revenues \| Late payment charges
Disaggregation of Operating Revenues
Revenues	11.4
Other operating revenues \| Leases
Disaggregation of Operating Revenues
Revenues	$ 1.2

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration