Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Revenues	$ 389.4	$ 393.1	$ 415.4	$ 388.0	$ 353.1	$ 332.8	$ 356.3	$ 355.3	$ 1,585.9		$ 1,397.5		$ 1,330.6
Goodwill impairment							18.4		0.0		18.4		0.0
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)					(37.2)
Cost of Goods and Services Sold	317.3	323.3	343.4	316.7	277.2	256.4	289.6	262.7	1,300.7	[1]	1,085.9	[1]	1,000.8	[1]
Lower of cost or market inventory write-down									0.0		0.0		4.9
Gross Profit	72.1	69.8	72.0	71.3	75.9	76.4	66.7	92.6
Operating income	36.9	34.9	34.7	37.1	41.9	41.5	12.5	59.3	143.6		155.2		181.1
Net income (loss)	$ 23.6	$ 21.7	$ 20.7	$ 25.7	$ (15.2)	$ 19.9	$ 4.7	$ 36.0	$ 91.7		$ 45.4		$ 91.7
Net income per common share:
Basic (in dollars per share)	$ 1.44	$ 1.31	$ 1.24	$ 1.54	$ (0.90)	$ 1.18	$ 0.28	$ 2.07	$ 5.53		$ 2.67		$ 5.15
Net Income (loss) per common share, Diluted:
Diluted (in dollars per share)	1.41	1.29	1.22	1.51	(0.90)	1.16	0.27	2.04	5.43		2.63		5.09
Dividends declared per common share (in dollars per share)	$ 0.55	$ 0.55	$ 0.55	$ 0.55	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 2.20		$ 2.00		$ 1.80

[1] See Note 7 for discussion of our adoption of ASU 2017-12 (as defined in Note 1) and the related reclassification of amounts previously presented in the Statements of Consolidated Income within Unrealized (gain) loss on derivative instruments and now included within Cost of products sold, excluding depreciation and amortization and other items.