Quarterly Financial and Market Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Quarterly Financial Information [Line Items]
Net sales	$ 295.8	$ 286.9	$ 378.7	$ 541.0	$ 564.3	$ 572.5	$ 609.0	$ 609.9	$ 1,502.4	$ 2,355.7	$ 2,189.1
Gross margin	30.4	13.5	54.9	140.9	146.5	157.9	168.8	167.2	239.7	640.4	580.8
Operating income (loss)	(20.4)	(37.6)	6.4	65.7	97.4	109.9	115.1	102.8	$ 14.1	$ 425.2	$ 371.2
Net income (loss)	$ (19.4)	$ 9.7	$ (1.0)	$ 42.4	$ 73.2	$ 80.3	$ 80.9	$ 72.2
Net income per common share:
Basic	$ (0.23)	$ 0.12	$ (0.01)	$ 0.51	$ 0.87	$ 0.94	$ 0.95	$ 0.85
Diluted	(0.23)	0.12	(0.01)	0.50	0.86	0.93	0.94	0.84
Maximum
Market price:
Common Stock Value during the Period	56.00	46.71	47.91	79.89	80.29	85.33	80.88	72.65
Minimum
Market price:
Common Stock Value during the Period	$ 31.71	$ 32.57	$ 26.75	$ 29.51	$ 73.31	$ 77.01	$ 68.00	$ 56.18