Restructuring (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 26, 2023	Oct. 25, 2023	Apr. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restructuring Cost and Reserve [Line Items]
Asset Optimization and Corporate Restructuring Subtotal				$ 14.4		$ 14.4
Restructuring Charges				$ 4.0		$ 17.5	$ (1.0)
Restructuring, Incurred Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]				Selling, General and Administrative Expense	Selling, General and Administrative Expense	Selling, General and Administrative Expense	Selling, General and Administrative Expense
Gain on sale of property held-for-sale						$ 14.4
Restructuring Reserve [Roll Forward]
Accrued charges/Balance at beginning of period						13.3
Expenses						18.3
Deductions						(15.8)
Accrued charges/Balance at end of period				$ 15.8		15.8
MEXICO
Restructuring Cost and Reserve [Line Items]
Proceeds received on sale of property			$ 19.0
Transformational Restructuring Program
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				(0.4)		(0.4)	$ 0.1
Cumulative life-to-date charges				8.4		8.4
Other Restructurings
Restructuring Cost and Reserve [Line Items]
Restructuring Charges							(1.1)
Discontinuing styrene production at the Company's Terneuzen, the Netherlands plant
Restructuring Cost and Reserve [Line Items]
Additional estimated restructuring charges	$ 68.0	$ 58.0
Accelerated Depreciation
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				3.2
Expected restructuring charges				5.8		5.8
Accelerated Depreciation | Discontinuing styrene production at the Company's Terneuzen, the Netherlands plant
Restructuring Cost and Reserve [Line Items]
Additional estimated restructuring charges	21.0	19.0
Accelerated Depreciation | Asset restructuring plan
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				(12.9)		(4.6)
Employee termination benefits
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				9.6
Restructuring Reserve [Roll Forward]
Accrued charges/Balance at beginning of period						13.3
Expenses						8.1
Deductions						(5.6)
Accrued charges/Balance at end of period				15.8		15.8
Employee termination benefits | Corporate Restructuring Program
Restructuring Cost and Reserve [Line Items]
Asset Optimization and Corporate Restructuring Subtotal				8.4		8.4
Employee termination benefits | Transformational Restructuring Program
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				(0.4)		(0.4)	$ 0.1
Cumulative life-to-date charges				8.4		8.4
Employee termination benefits | Transformational Restructuring Program | Maximum
Restructuring Cost and Reserve [Line Items]
Expected restructuring charges				1.0		1.0
Employee termination benefits | Discontinuing styrene production at the Company's Terneuzen, the Netherlands plant
Restructuring Cost and Reserve [Line Items]
Additional estimated restructuring charges	5.0	3.0
Employee termination benefits | Asset restructuring plan
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				0.3		(0.3)
Decommissioning, contract termination costs and severance benefits to affected employees | Discontinuing styrene production at the Company's Terneuzen, the Netherlands plant
Restructuring Cost and Reserve [Line Items]
Expected cash amount	40.0	32.0
Contract terminations
Restructuring Reserve [Roll Forward]
Expenses						6.4
Deductions						(6.4)
Contract terminations | Discontinuing styrene production at the Company's Terneuzen, the Netherlands plant
Restructuring Cost and Reserve [Line Items]
Additional estimated restructuring charges	19.0	16.0
Contract terminations | Asset restructuring plan
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				2.0		6.3
Expected restructuring charges				13.8		13.8
Decommissioning and other
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				1.6
Expected restructuring charges				2.5		2.5
Restructuring Reserve [Roll Forward]
Expenses						3.8
Deductions						(3.8)
Decommissioning and other | Discontinuing styrene production at the Company's Terneuzen, the Netherlands plant
Restructuring Cost and Reserve [Line Items]
Additional estimated restructuring charges	$ 23.0	$ 20.0
Decommissioning and other | Asset restructuring plan
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				0.6		2.1
Expected restructuring charges				1.2		1.2
Plastic Solution | Asset restructuring plan
Restructuring Cost and Reserve [Line Items]
Expected restructuring charges				0.2		0.2
Plastic Solution | Accelerated Depreciation
Restructuring Cost and Reserve [Line Items]
Cumulative life-to-date charges				1.4		1.4
Plastic Solution | Employee termination benefits
Restructuring Cost and Reserve [Line Items]
Restructuring Charges						(0.4)
Cumulative life-to-date charges				3.0		3.0
Plastic Solution | Decommissioning and other
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				0.4		1.8
Cumulative life-to-date charges				1.8		1.8
Feedstocks | Asset restructuring plan
Restructuring Cost and Reserve [Line Items]
Expected restructuring charges				14.8		14.8
Feedstocks | Accelerated Depreciation
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				(12.9)		(7.7)
Cumulative life-to-date charges				27.4		27.4
Feedstocks | Employee termination benefits
Restructuring Cost and Reserve [Line Items]
Restructuring Charges						(0.2)
Cumulative life-to-date charges				3.7		3.7
Feedstocks | Contract terminations
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				2.0		6.3
Cumulative life-to-date charges				6.7		6.7
Feedstocks | Decommissioning and other
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				0.2		1.1
Cumulative life-to-date charges				4.4		4.4
Engineered Materials
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				(10.0)		3.5
Cumulative life-to-date charges				60.3		60.3
Engineered Materials | Accelerated Depreciation
Restructuring Cost and Reserve [Line Items]
Asset Optimization and Corporate Restructuring Subtotal				3.2		3.2
Restructuring Charges						3.1
Cumulative life-to-date charges				6.3		6.3
Engineered Materials | Employee termination benefits
Restructuring Cost and Reserve [Line Items]
Asset Optimization and Corporate Restructuring Subtotal				1.2		1.2
Restructuring Charges				0.3		0.3
Cumulative life-to-date charges				2.7		2.7
Engineered Materials | Decommissioning and other
Restructuring Cost and Reserve [Line Items]
Asset Optimization and Corporate Restructuring Subtotal				1.6		1.6
Restructuring Charges						(0.8)
Cumulative life-to-date charges				$ 2.9		$ 2.9