Supplementary Financial Statement Information - Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Net sales	$ 1,445.7	$ 1,535.5	$ 1,680.5	$ 1,697.7	$ 1,677.1	$ 1,735.6	$ 1,746.6	$ 1,658.9	$ 6,359.4	$ 6,818.2	$ 6,080.5
Non-US [Member]
Disaggregation of Revenue [Line Items]
Net sales									569.4	634.8	627.1
Carbon [Member]
Disaggregation of Revenue [Line Items]
Net sales									4,143.6	4,409.1	4,034.5
Stainless And Electrical [Member]
Disaggregation of Revenue [Line Items]
Net sales									1,617.5	1,793.8	1,687.6
Other Product Lines [Member]
Disaggregation of Revenue [Line Items]
Net sales									598.3	615.3	358.4
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Net sales									4,172.2	4,284.7	3,951.5
Infrastructure and Manufacturing Market [Member]
Disaggregation of Revenue [Line Items]
Net sales									1,006.0	1,049.1	948.0
Distributors and Converters Market [Member]
Disaggregation of Revenue [Line Items]
Net sales									$ 1,181.2	$ 1,484.4	$ 1,181.0