Segment and Geographical Area Information Segment and Geographical Area Information, Schedule of net sales, income taxes paid, and long-lived assets, by geographical area (Details) - USD ($) $ 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
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers	$ 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
Income taxes paid									1.3	1.2	2.3
Long-lived assets	571.4				530.9				571.4	530.9	495.4
Fabricated Products
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									1,397.5	1,330.6	1,391.9
Long-lived assets	566.8				526.1				566.8	526.1	490.5
Fabricated Products | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									1,337.3	1,278.6	1,321.3
Income taxes paid									1.2	0.7	0.6
Long-lived assets	536.6				494.7				536.6	494.7	459.6
Fabricated Products | Canada
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									60.2	52.0	70.6
Income taxes paid									0.1	0.5	1.7
Long-lived assets	30.2				31.4				30.2	31.4	30.9
All Other
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	4.6				4.8				4.6	4.8	4.9
All Other | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	$ 4.6				$ 4.8				$ 4.6	$ 4.8	$ 4.9