Net Sales (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Net sales	$ 1,269.3	$ 679.2	$ 3,529.0	$ 1,976.5
Emulsion Polymers, Latex [Member]
Disaggregation of Revenue [Line Items]
Net sales	315.6	183.2	877.6	567.3
Engineered Materials [Member]
Disaggregation of Revenue [Line Items]
Net sales	230.8	50.0	477.5	135.2
Base Plastics [Member]
Disaggregation of Revenue [Line Items]
Net sales	393.3	240.1	1,119.3	649.1
Polystyrene [Member]
Disaggregation of Revenue [Line Items]
Net sales	274.8	167.3	855.0	505.9
Feedstocks [Member]
Disaggregation of Revenue [Line Items]
Net sales	54.8	38.6	199.6	119.0
United States [Member]
Disaggregation of Revenue [Line Items]
Net sales	275.0	116.9	629.5	341.9
United States [Member] | Emulsion Polymers, Latex [Member]
Disaggregation of Revenue [Line Items]
Net sales	86.3	54.1	228.2	167.2
United States [Member] | Engineered Materials [Member]
Disaggregation of Revenue [Line Items]
Net sales	102.4	8.0	174.1	25.3
United States [Member] | Base Plastics [Member]
Disaggregation of Revenue [Line Items]
Net sales	82.3	52.8	216.5	143.5
United States [Member] | Feedstocks [Member]
Disaggregation of Revenue [Line Items]
Net sales	4.0	2.0	10.7	5.9
Europe [Member]
Disaggregation of Revenue [Line Items]
Net sales	703.8	348.9	2,022.0	1,052.7
Europe [Member] | Emulsion Polymers, Latex [Member]
Disaggregation of Revenue [Line Items]
Net sales	153.5	79.7	431.4	254.0
Europe [Member] | Engineered Materials [Member]
Disaggregation of Revenue [Line Items]
Net sales	89.6	13.8	190.8	37.4
Europe [Member] | Base Plastics [Member]
Disaggregation of Revenue [Line Items]
Net sales	247.3	126.8	699.2	366.1
Europe [Member] | Polystyrene [Member]
Disaggregation of Revenue [Line Items]
Net sales	162.6	96.9	511.7	301.8
Europe [Member] | Feedstocks [Member]
Disaggregation of Revenue [Line Items]
Net sales	50.8	31.7	188.9	93.4
Asia-Pacific [Member]
Disaggregation of Revenue [Line Items]
Net sales	265.0	192.4	807.0	531.5
Asia-Pacific [Member] | Emulsion Polymers, Latex [Member]
Disaggregation of Revenue [Line Items]
Net sales	73.0	48.1	211.1	140.9
Asia-Pacific [Member] | Engineered Materials [Member]
Disaggregation of Revenue [Line Items]
Net sales	35.9	28.1	108.1	72.3
Asia-Pacific [Member] | Base Plastics [Member]
Disaggregation of Revenue [Line Items]
Net sales	43.9	40.9	144.5	94.5
Asia-Pacific [Member] | Polystyrene [Member]
Disaggregation of Revenue [Line Items]
Net sales	112.2	70.4	343.3	204.1
Asia-Pacific [Member] | Feedstocks [Member]
Disaggregation of Revenue [Line Items]
Net sales		4.9		19.7
Rest of World [Member]
Disaggregation of Revenue [Line Items]
Net sales	25.5	21.0	70.5	50.4
Rest of World [Member] | Emulsion Polymers, Latex [Member]
Disaggregation of Revenue [Line Items]
Net sales	2.8	1.3	6.9	5.2
Rest of World [Member] | Engineered Materials [Member]
Disaggregation of Revenue [Line Items]
Net sales	2.9	0.1	4.5	0.2
Rest of World [Member] | Base Plastics [Member]
Disaggregation of Revenue [Line Items]
Net sales	$ 19.8	$ 19.6	$ 59.1	$ 45.0