OTHER INFORMATION - Narrative (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Expenses incurred related to capital structure	$ 41.8
Letter of credit
Debt Instrument [Line Items]
Letters of credit outstanding	24.7
Cash collateral	$ 25.4

Amount of assets pledged to secure a debt instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Name:	us-gaap_DebtInstrumentCollateralAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant