Investments in Unconsolidated Affiliates (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) item	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) item	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | item	1		1
Equity in earnings of unconsolidated affiliates	$ 30.1	$ 14.4	$ 53.0	$ 24.2
Assets
Current assets	2,112.1		2,112.1		$ 1,517.1
Total assets	4,504.7		4,504.7		2,845.2
Liabilities
Current liabilities	814.0		814.0		533.3
Total noncurrent liabilities	2,860.9		2,860.9		$ 1,721.6
Summarized Financial Information, Net Income
Gross profit	220.0	23.4	408.9	81.1
Net income (loss)			223.1	(164.7)
Americas Styrenics
Summarized Financial Information, Net Income
Net sales	495.6	225.4	918.5	547.5
Gross profit	74.5	31.3	139.9	38.4
Net income (loss)	$ 63.6	$ 20.1	$ 114.6	$ 11.8