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Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 USD ($) Segment | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | May 01, 2020 USD ($) | Apr. 28, 2020 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |
Number of operating segments | Segment | 1 |  |  |  |
Effect of LIFO Inventory Liquidation on Income | $ 0.2 | $ (3.6) |  |  |
Excess of current cost over the stated LIFO value of inventory | 8.4 | 11.9 |  |  |
Interest expense capitalized as construction in progress | 1.0 | 1.6 | $ 1.7 |  |
Non-cash asset impairment charges | 0.5 | 26.1 | 1.4 |  |
Workers' compensation charge | 0.6 |  |  |  |
Undiscounted workers' compensation liabilities | 29.8 | 29.2 |  |  |
Discounted workers' compensation liabilities |  | $ 26.6 |  |  |
Discount rate used in estimating liabilities for worker compensation claims |  | 1.75% |  |  |
Accrued employee healthcare benefits | 3.1 | $ 3.5 |  |  |
Advertising costs | 0.2 | 0.4 | 0.3 |  |
Research and development costs | $ 9.1 | 10.5 | 9.8 |  |
Accounting Standards Update ("ASU") No. 2016-13 |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Adopted | true |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect | true |  |  |  |
ASU No. 2018-15 |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Adopted | true |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect | true |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Transition Option Elected [Extensible List] | us-gaap:AccountingStandardsUpdate201815ProspectiveMember |  |  |  |
Fabricated Products | Idled Equipment |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Non-cash asset impairment charges | $ 0.5 | $ 0.9 | $ 1.4 |  |
Maximum |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Number of days to maturity for debt investment securities to be classified as cash and cash equivalents | 90 days |  |  |  |
Period over which accounts receivable is due, days | 90 days |  |  |  |
Minimum |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Number of days in maturity for debt investment securities to be classified as short-term investments | 90 days |  |  |  |
Period over which accounts receivable is due, days | 30 days |  |  |  |
Senior Notes Due 2025 | Senior Notes |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Debt principal amount | $ 350.0 |  |  | $ 50.0 | $ 300.0

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