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SEGMENT INFORMATION (Details) - USD ($) $ in Millions	3 Months Ended						6 Months Ended	9 Months Ended		95 Months Ended
SEGMENT INFORMATION (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Segment Reporting Information [Line Items]
Sales	$ 1,872.4			$ 1,554.9				$ 5,311.1	$ 4,648.5
Income (loss) before taxes:
Income (loss) before taxes	261.4			60.4				363.2	56.5
Environmental income (expense)	110.8			(1.8)				104.1	(6.2)
Restructuring charges	(3.3)	$ (6.4)	$ (4.0)	(9.2)	$ (8.5)	$ (8.2)		(13.7)	(25.9)	$ (200.9)
Acquisition-related costs	(0.4)			(1.1)				(1.0)	(12.5)
Other operating (expense) income	(1.7)			0.0				6.4	(0.1)
Interest expense	(59.2)			(53.1)				(184.0)	(158.0)
Interest income	0.3			0.4				1.1	1.0
Non-operating pension income	5.4			8.4				16.2	25.5
Gain (Loss) on Disposition of Property Plant Equipment								(1.9)	(0.4)
Recoveries From Third Parties Of Costs Incurred And Expensed	110.0			0.0				110.0	0.0
Environmental Possible Additional Contingent Environmental Liabilities	10.0							10.0		10.0
Gain in Other Operating (Expense) Income due to Insurance Recoveries							$ 8.0
Schedule of Equity Method Investments [Line Items]
Earnings (losses) of non-consolidated affiliates	0.4			0.5				(20.2)	1.5
BayGas non-cash impairment charge								21.5
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	1,872.4			1,554.9				5,311.1	4,648.5
United States
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	1,047.4			915.8				3,006.1	2,658.4
Europe
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	331.2			240.9				906.2	786.0
Other foreign
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	493.8			398.2				1,398.8	1,204.1
Chlor Alkali Products and Vinyls Segment
Segment Reporting Information [Line Items]
Sales	1,051.1			881.2				3,005.9	2,583.2
Income (loss) before taxes:
Income (loss) before taxes	210.8			129.7				490.7	270.0
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	1,051.1			881.2				3,005.9	2,583.2
Chlor Alkali Products and Vinyls Segment \| United States
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	682.8			589.5				1,956.0	1,706.6
Chlor Alkali Products and Vinyls Segment \| Europe
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	40.1			31.6				136.2	92.7
Chlor Alkali Products and Vinyls Segment \| Other foreign
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	328.2			260.1				913.7	783.9
Epoxy Segment
Segment Reporting Information [Line Items]
Sales	647.3			489.9				1,794.4	1,549.5
Income (loss) before taxes:
Income (loss) before taxes	31.1			(1.7)				33.8	(11.0)
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	647.3			489.9				1,794.4	1,549.5
Epoxy Segment \| United States
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	209.1			159.8				589.9	488.0
Epoxy Segment \| Europe
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	289.3			206.8				763.0	685.1
Epoxy Segment \| Other foreign
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	148.9			123.3				441.5	376.4
Winchester Segment
Segment Reporting Information [Line Items]
Sales	174.0			183.8				510.8	515.8
Income (loss) before taxes:
Income (loss) before taxes	10.3			17.2				34.1	61.3
Restructuring charges										$ (27.1)
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	174.0			183.8				510.8	515.8
Winchester Segment \| United States
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	155.5			166.5				460.2	463.8
Winchester Segment \| Europe
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	1.8			2.5				7.0	8.2
Winchester Segment \| Other foreign
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	16.7			14.8				43.6	43.8
Corporate/Other
Income (loss) before taxes:
Environmental income (expense)	110.8			(1.8)				104.1	(6.2)
Other corporate and unallocated costs	(42.7)			(28.4)				(124.5)	(87.6)
Restructuring charges	(3.3)			(9.2)				(13.7)	(25.9)
Acquisition-related costs	(0.4)			(1.1)				(1.0)	(12.5)
Gain (Loss) on Disposition of Property Plant Equipment	1.7							1.7
Caustic soda \| Chlor Alkali Products and Vinyls Segment
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	539.2			442.7				1,571.1	1,272.5
Chlorine, chlorine derivatives and other co-products \| Chlor Alkali Products and Vinyls Segment
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	511.9			438.5				1,434.8	1,310.7
Phenolics and allylics \| Epoxy Segment
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	345.6			227.1				891.8	759.1
Epoxy resins \| Epoxy Segment
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	301.7			262.8				902.6	790.4
Commercial \| Winchester Segment
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	118.2			128.8				335.3	364.2
Military and law enforcement \| Winchester Segment
Revenue from Contract with Customer, Excluding Assessed Tax [Abstract]
Sales	$ 55.8			$ 55.0				$ 175.5	$ 151.6