Supplementary Financial Statement Information - Revenue (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
Disaggregation of Revenue [Line Items]
Net sales	$ 1,677.1	$ 1,735.6	$ 1,746.6	$ 1,658.9	$ 1,495.6	$ 1,494.3	$ 1,557.2	$ 1,533.4	$ 6,818.2	$ 6,080.5	$ 5,882.5
Non-US [Member]
Disaggregation of Revenue [Line Items]
Net sales									634.8	627.1	655.6
Carbon [Member]
Disaggregation of Revenue [Line Items]
Net sales									4,409.1	4,034.5	4,014.5
Stainless And Electrical [Member]
Disaggregation of Revenue [Line Items]
Net sales									1,793.8	1,687.6	1,654.1
Other Product Lines [Member]
Disaggregation of Revenue [Line Items]
Net sales									615.3	358.4	213.9
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Net sales									4,284.7	3,951.5	3,906.8
Infrastructure and Manufacturing Market [Member]
Disaggregation of Revenue [Line Items]
Net sales									1,049.1	948.0	936.7
Distributors and Converters Market [Member]
Disaggregation of Revenue [Line Items]
Net sales									$ 1,484.4	$ 1,181.0	$ 1,039.0