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Summary of Significant Accounting Policies - Principles of Consolidation (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Uniti [Member]
Noncontrolling Interest [Line Items]
Equity method investment ownership percentage	50.00%
Related party accounts receivable	$ 1.4	$ 0.2
Allegheny & Tsingshan Stainless
Noncontrolling Interest [Line Items]
Equity method investment ownership percentage	50.00%
Related party accounts receivable	$ 0.0	$ 0.1
Shanghai STAL Precision Stainless Steel Co Ltd [Member] | Allegheny Technologies Inc
Noncontrolling Interest [Line Items]
Joint venture ownership percentage	60.00%

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