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Summary of Significant Accounting Policies, Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018
Significant accounting policies |  |  |
Retained earnings | $ 111.7 | $ 85.5 | $ 95.6
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | 586,137 |  |
Workers Compensation Liability Undiscounted | $ 26.1 | $ 24.8 |
Workers' Compensation Discount, Percent | 2.75% | 2.25% |
Accrued liabilities for employee healthcare benefits | $ 3.1 | $ 3.5 |
Fabricated Products |  |  |
Significant accounting policies |  |  |
Excess of current cost over the stated LIFO value of inventory | 32.3 | $ 24.3 |
Deferred Bonus [Member] |  |  |
Significant accounting policies |  |  |
Accrued Bonuses | 4.5 |  |
Accounting Standards Update 2016-01 [Member] |  |  |
Significant accounting policies |  |  |
Retained earnings |  |  | 0.4
Deferred Tax Assets, Net |  |  | 0.2
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09 |  |  |
Significant accounting policies |  |  |
Retained earnings | $ (14.6) |  | 10.1
Deferred Tax Assets, Net |  |  | $ 3.3

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