Segment and Geographical Area Information Segment and Geographical Area Information, Information by Geographical Area (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers	$ 338.0		$ 338.9	$ 344.1	$ 335.1	$ 311.3		$ 319.9	$ 328.9	$ 337.4	$ 1,356.1		$ 1,297.5		$ 1,360.1
Income taxes paid											3.5		2.1		1.8
Long-lived assets	454.9	[1]				429.3	[1]				454.9	[1]	429.3	[1]
Fabricated Products
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers											1,356.1		1,297.5		1,360.1
Long-lived assets	450.0	[1]				424.8	[1]				450.0	[1]	424.8	[1]
Fabricated Products | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers											1,254.0		1,204.7		1,256.5
Income taxes paid											2.1		1.2		0.5
Long-lived assets	432.6	[1]				409.5	[1]				432.6	[1]	409.5	[1]
Fabricated Products | Canada
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Net sales to unaffiliated customers											102.1		92.8		103.6
Income taxes paid											1.4		0.9		1.3
Long-lived assets	17.4	[1]				15.3	[1]				17.4	[1]	15.3	[1]
All Other
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	4.9	[1]				4.5	[1]				4.9	[1]	4.5	[1]
All Other | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Long-lived assets	$ 4.9	[1]				$ 4.5	[1]				$ 4.9	[1]	$ 4.5	[1]

[1] Long-lived assets represent Property, plant and equipment, net.