Business, Product and Geographical Area Information and Concentration of Risk - 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, 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 from External Customers, Income Taxes Paid, and Long-Lived Assets
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
Income taxes paid									3.6	1.3	1.2
Long-lived assets	611.8				571.4				611.8	571.4	530.9
Fabricated Products | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Revenues									1,509.6	1,337.3	1,278.6
Income taxes paid									1.6	1.2	0.7
Long-lived assets	581.7				541.2				581.7	541.2	499.5
Fabricated Products | Canada
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets
Revenues									76.3	60.2	52.0
Income taxes paid									2.0	0.1	0.5
Long-lived assets	$ 30.1				$ 30.2				$ 30.1	$ 30.2	$ 31.4