Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ 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
Goodwill impairment									$ 18.4	$ 0.0	$ 0.0
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)	$ (37.2)
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, 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
Gross Profit	75.9	76.4	66.7	92.6	81.3	67.9	95.4	80.3
Operating income	40.0	39.8	11.4	59.5	45.3	29.8	57.9	44.8	150.7	177.8	(345.9)
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)
Net Income (loss) per common share, Diluted:
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)
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
Fabricated Products
Goodwill impairment			$ (18.4)						$ (18.4)
Net sales									1,397.5	$ 1,330.6	$ 1,391.9
Operating income									$ 201.3	$ 229.6	$ 190.8