Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	12 Months Ended
Jun. 30, 2018 USD ($) facility	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)
Summary of significant accounting policies
Proceeds received from sale of equity method investment	$ 0.0	$ 0.0	$ 6.3
Note receivables recorded from sale of equity method investment			12.6
Proceeds from note receivable from the sale of equity method investment	6.3	6.3	0.0
Discounts, rebates, returns and allowances	26.5	23.8	29.8
Research and development expenditures	19.3	16.9	16.3
Inventory accounted under method other than LIFO	138.6	107.3
Capitalized software amortization expense	5.1	5.2	$ 5.5
Carrying value of computer software	$ 101.8	$ 71.0
Number of former operating facility site that the liability for future costs is discounted to present value (facility) \| facility	1
Liability for future costs discount amortization period	20 years
Environmental expenditures, discount rate	3.00%	3.00%
Environmental expenditures, liability	$ 11.0	$ 11.0
Customer \| Sales Revenue \| Arconic
Summary of significant accounting policies
Concentration risk	12.00%	11.00%	13.00%
Minimum \| Computer software
Summary of significant accounting policies
Estimated useful life	3 years
Minimum \| Intangible Assets
Summary of significant accounting policies
Estimated useful life	5 years
Maximum \| Computer software
Summary of significant accounting policies
Estimated useful life	7 years
Maximum \| Intangible Assets
Summary of significant accounting policies
Estimated useful life	30 years

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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