Statements of Consolidated Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Income Statement [Abstract]
Net sales	$ 356.3	$ 334.9		$ 711.6	$ 678.1
Cost of products sold:
Cost of products sold, excluding depreciation and amortization and other items	277.7	250.4		555.5	512.4
Lower of cost or market inventory write-down	0.0	0.0	$ 4.9	0.0	4.9
Unrealized loss (gain) on derivative instruments	11.9	(10.9)		(3.2)	(14.9)
Depreciation and amortization	9.5	9.0		19.1	17.7
Selling, general, administrative, research and development	26.3	27.5		50.0	53.6
Net periodic postretirement benefit cost relating to Salaried VEBA	1.1	0.9		2.2	1.7
Gain on removal of Union VEBA net assets	0.0	0.0		(1.3)	(0.1)
Total selling, general, administrative, research and development	27.4	28.4		50.9	55.2
Goodwill impairment	18.4	0.0		18.4	0.0	$ 0.0
Other operating charges, net	0.0	0.1		0.0	0.1
Total costs and expenses	344.9	277.0		640.7	575.4
Operating income	11.4	57.9		70.9	102.7
Other (expense) income:
Interest expense	(5.5)	(5.5)		(11.1)	(9.2)
Other (expense) income, net	1.0	(10.7)		1.6	(10.4)
Income before income taxes	6.9	41.7		61.4	83.1
Income tax provision	(2.2)	(15.7)		(20.7)	(30.8)
Net income	$ 4.7	$ 26.0		$ 40.7	$ 52.3
Net income per common share:
Basic (in dollars per share)	$ 0.28	$ 1.45		$ 2.37	$ 2.92
Diluted (in dollars per share)	$ 0.27	$ 1.43		$ 2.34	$ 2.87
Weighted-average number of common shares outstanding (in thousands):
Basic (shares)	17,003	17,871		17,193	17,867
Diluted (shares)	17,201	18,194		17,418	18,194
Cash dividends declared (in dollars per share)	$ 0.50	$ 0.45		$ 1.00	$ 0.90