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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers	$ 331.9	$ 320.6	$ 334.9	$ 343.2	$ 316.6	$ 336.4	$ 367.2	$ 371.7	$ 1,330.6	$ 1,391.9	$ 1,356.1
Income taxes paid									1.2	2.3	3.5
Long-lived assets	530.9				495.4				530.9	495.4
Fabricated Products
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									1,330.6	1,391.9	1,356.1
Long-lived assets	526.1				490.5				526.1	490.5
Fabricated Products | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									1,278.6	1,321.3	1,254.0
Income taxes paid									0.7	0.6	2.1
Long-lived assets	494.7				459.6				494.7	459.6
Fabricated Products | Canada
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers									52.0	70.6	102.1
Income taxes paid									0.5	1.7	$ 1.4
Long-lived assets	31.4				30.9				31.4	30.9
All Other
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	4.8				4.9				4.8	4.9
All Other | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	$ 4.8				$ 4.9				$ 4.8	$ 4.9