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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers	$ 316.6	$ 336.4	$ 367.2	$ 371.7	$ 338.0	$ 338.9	$ 344.1	$ 335.1	$ 1,391.9	$ 1,356.1	$ 1,297.5
Income taxes paid									2.3	3.5	2.1
Long-lived assets	495.4				454.9				495.4	454.9
Fabricated Products
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									1,391.9	1,356.1	1,297.5
Long-lived assets	490.5				450.0				490.5	450.0
Fabricated Products | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									1,321.3	1,254.0	1,204.7
Income taxes paid									0.6	2.1	1.2
Long-lived assets	459.6				432.6				459.6	432.6
Fabricated Products | Canada
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									70.6	102.1	92.8
Income taxes paid									1.7	1.4	$ 0.9
Long-lived assets	30.9				17.4				30.9	17.4
All Other
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	4.9				4.9				4.9	4.9
All Other | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	$ 4.9				$ 4.9				$ 4.9	$ 4.9