Statements of Consolidated Income - USD ($) shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Net sales	$ 331.9	$ 320.6	$ 334.9	$ 343.2	$ 316.6	$ 336.4	$ 367.2	$ 371.7	$ 1,330.6	$ 1,391.9	$ 1,356.1
Cost of products sold:
Cost of products sold, excluding depreciation and amortization and other items	252.4	254.7	250.4	262.0	251.0	267.3	294.8	302.3	1,019.5	1,115.4	1,117.5
Lower of cost or market inventory write-down	0.0	0.0	0.0	4.9	2.6	0.0	0.0	0.0	4.9	2.6	0.0
Unrealized (gain) loss on derivative instruments	(1.8)	(2.0)	(10.9)	(4.0)	(4.3)	1.7	1.5	4.5	(18.7)	3.4	10.4
Depreciation and amortization									36.0	32.4	31.1
Selling, general, administrative, research and development									105.0	88.1	81.4
Net periodic postretirement benefit cost (income) relating to VEBAs – Note 6									3.4	2.4	(23.7)
(Gain) loss on removal of Union VEBA net assets – Note 6									(0.1)	493.4	0.0
Total selling, general, administrative, research and development									108.3	583.9	57.7
Other operating charges, net									2.8	0.1	1.5
Total costs and expenses									1,152.8	1,737.8	1,218.2
Operating income (loss)	45.3	29.8	57.9	44.8	35.2	40.5	37.0	(458.6)	177.8	(345.9)	137.9
Other (expense) income:
Interest expense									(20.3)	(24.1)	(37.5)
Other (expense) income, net – Note 14									(10.3)	(1.8)	6.7
Income (loss) before income taxes									147.2	(371.8)	107.1
Income tax (provision) benefit									(55.5)	135.2	(35.3)
Net income (loss)	$ 24.5	$ 14.9	$ 26.0	$ 26.3	$ 13.3	$ 22.1	$ 20.2	$ (292.2)	$ 91.7	$ (236.6)	$ 71.8
Net income (loss) per common share:
Basic (in dollars per share)	$ 1.39	$ 0.84	$ 1.45	$ 1.47	$ 0.76	$ 1.29	$ 1.19	$ (16.85)	$ 5.15	$ (13.76)	$ 4.02
Diluted (in dollars per share)	$ 1.37	$ 0.82	$ 1.43	$ 1.44	$ 0.73	$ 1.21	$ 1.11	$ (16.85)	$ 5.09	$ (13.76)	$ 3.86
Weighted-average number of common shares outstanding (in thousands):
Basic (in shares)									17,813	17,201	17,818
Diluted (in shares)									18,033	17,201	18,593
Dividends declared per common share (in dollars per share)									$ 1.8	$ 1.6	$ 1.4