SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Cash Flows) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ 55.4	$ 236.1
Capital expenditures	(166.5)	(191.3)
Payments under ethylene long-term supply contracts	(461.0)	0.0
Payments under other long-term supply contracts	(75.8)	0.0
Proceeds from disposition of non-consolidated affiliate		20.0
Net investing activities	(703.3)	(171.3)
Long-term debt:
Borrowings	1,163.2	25.0
Repayments	(425.8)	(50.6)
Common stock repurchased and retired	0.0	(26.7)
Stock options exercised	0.5	1.5
Dividends paid	(63.1)	(65.7)
Debt issuance costs	(9.6)	0.0
Intercompany financing activities	0.0	0.0
Net financing activities	665.2	(116.5)
Effect of exchange rate changes on cash and cash equivalents	(0.3)	(0.2)
Net increase (decrease) in cash and cash equivalents	17.0	(51.9)
Cash and cash equivalents, beginning of year	220.9	178.8
Cash and cash equivalents, end of period	237.9	126.9
Parent Guarantor
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	219.1	120.1
Capital expenditures	(61.5)	(86.9)
Payments under ethylene long-term supply contracts	0.0
Payments under other long-term supply contracts	0.0
Proceeds from disposition of non-consolidated affiliate		20.0
Net investing activities	(61.5)	(66.9)
Long-term debt:
Borrowings	672.5	0.0
Repayments	(175.8)	(0.6)
Common stock repurchased and retired		(26.7)
Stock options exercised	0.5	1.5
Dividends paid	(63.1)	(65.7)
Debt issuance costs	(9.6)
Intercompany financing activities	(536.8)	(50.0)
Net financing activities	(112.3)	(141.5)
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	45.3	(88.3)
Cash and cash equivalents, beginning of year	11.6	92.0
Cash and cash equivalents, end of period	56.9	3.7
Issuer
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	0.0	0.0
Capital expenditures	0.0	0.0
Payments under ethylene long-term supply contracts	0.0
Payments under other long-term supply contracts	0.0
Proceeds from disposition of non-consolidated affiliate		0.0
Net investing activities	0.0	0.0
Long-term debt:
Borrowings	0.0	0.0
Repayments	0.0	(50.0)
Common stock repurchased and retired		0.0
Stock options exercised	0.0	0.0
Dividends paid	0.0	0.0
Debt issuance costs	0.0
Intercompany financing activities	0.0	50.0
Net financing activities	0.0	0.0
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, beginning of year	0.0	0.0
Cash and cash equivalents, end of period	0.0	0.0
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	(163.7)	116.0
Capital expenditures	(105.0)	(104.4)
Payments under ethylene long-term supply contracts	(461.0)
Payments under other long-term supply contracts	(75.8)
Proceeds from disposition of non-consolidated affiliate		0.0
Net investing activities	(641.8)	(104.4)
Long-term debt:
Borrowings	490.7	25.0
Repayments	(250.0)	0.0
Common stock repurchased and retired		0.0
Stock options exercised	0.0	0.0
Dividends paid	0.0	0.0
Debt issuance costs	0.0
Intercompany financing activities	536.8	0.0
Net financing activities	777.5	25.0
Effect of exchange rate changes on cash and cash equivalents	(0.3)	(0.2)
Net increase (decrease) in cash and cash equivalents	(28.3)	36.4
Cash and cash equivalents, beginning of year	209.3	86.8
Cash and cash equivalents, end of period	181.0	123.2
Eliminations
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	0.0	0.0
Capital expenditures	0.0	0.0
Payments under ethylene long-term supply contracts	0.0
Payments under other long-term supply contracts	0.0
Proceeds from disposition of non-consolidated affiliate		0.0
Net investing activities	0.0	0.0
Long-term debt:
Borrowings	0.0	0.0
Repayments	0.0	0.0
Common stock repurchased and retired		0.0
Stock options exercised	0.0	0.0
Dividends paid	0.0	0.0
Debt issuance costs	0.0
Intercompany financing activities	0.0	0.0
Net financing activities	0.0	0.0
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, beginning of year	0.0	0.0
Cash and cash equivalents, end of period	$ 0.0	$ 0.0

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[Line Items] for Supplemental Guarantor Financial Information Statements Of Cash Flows

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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