Statements of Consolidated Income - USD ($) shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net sales	$ 353.1	$ 332.8	$ 356.3	$ 355.3	$ 331.9	$ 320.6	$ 334.9	$ 343.2	$ 1,397.5	$ 1,330.6	$ 1,391.9
Cost of products sold:
Cost of products sold, excluding depreciation and amortization and other items	282.6	267.2	277.7	277.8	252.4	254.7	250.4	262.0	1,105.3	1,019.5	1,115.4
Lower of cost or market inventory write-down					0.0	0.0	0.0	4.9	0.0	4.9	2.6
Unrealized (gain) loss on derivative instruments	(5.4)	(10.8)	11.9	(15.1)	(1.8)	(2.0)	(10.9)	(4.0)	(19.4)	(18.7)	3.4
Depreciation and amortization									39.7	36.0	32.4
Selling, general, administrative, research and development									97.5	105.0	88.1
Net periodic postretirement benefit cost relating to Salaried VEBA									4.5	3.4	2.4
(Gain) loss on removal of Union VEBA net assets – Note 4									0.0	(0.1)	493.4
Total selling, general, administrative, research and development									102.0	108.3	583.9
Goodwill impairment									18.4	0.0	0.0
Other operating charges, net									0.8	2.8	0.1
Total costs and expenses									1,246.8	1,152.8	1,737.8
Operating income (loss)	40.0	39.8	11.4	59.5	45.3	29.8	57.9	44.8	150.7	177.8	(345.9)
Other (expense) income:
Interest expense									(22.2)	(20.3)	(24.1)
Other income (expense), net – Note 11									4.5	(10.3)	(1.8)
Income (loss) before income taxes									133.0	147.2	(371.8)
Income tax (provision) benefit									(87.6)	(55.5)	135.2
Net income (loss)	$ (15.2)	$ 19.9	$ 4.7	$ 36.0	$ 24.5	$ 14.9	$ 26.0	$ 26.3	$ 45.4	$ 91.7	$ (236.6)
Net income (loss) per common share:
Basic (in dollars per share)	$ (0.90)	$ 1.18	$ 0.28	$ 2.07	$ 1.39	$ 0.84	$ 1.45	$ 1.47	$ 2.67	$ 5.15	$ (13.76)
Diluted (in dollars per share)	(0.90)	1.16	0.27	2.04	1.37	0.82	1.43	1.44	$ 2.63	$ 5.09	$ (13.76)
Weighted-average number of common shares outstanding (in thousands):
Basic (in shares)									16,996	17,813	17,201
Diluted (in shares)									17,259	18,033	17,201
Dividends declared per common share (in dollars per share)	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 2	$ 1.8	$ 1.6