Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Statutory Accounting Practices
Restricted Cash Current Customer Deposits	$ 1.1
Restricted cash held in trust related to the tax exempt bonds		$ 6.0
Interest costs capitalized	18.6	11.9
Mineral rights	259.8	290.3
Deferred finance costs, net	1.2	1.9
Deferred finance costs, current	0.7	$ 0.7
Accounting Standard Update 2020 06 [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member]
Statutory Accounting Practices
Convertible Debt	$ 155.3
Minimum
Statutory Accounting Practices
Term of contract	1 year
Maximum
Statutory Accounting Practices
Term of contract	7 years
Exploration and production equipment \| Minimum
Statutory Accounting Practices
Estimated useful life	1 year
Exploration and production equipment \| Maximum
Statutory Accounting Practices
Estimated useful life	16 years
Building and building improvements \| Minimum
Statutory Accounting Practices
Estimated useful life	3 years
Building and building improvements \| Maximum
Statutory Accounting Practices
Estimated useful life	20 years
Mining properties and mineral rights \| Minimum
Statutory Accounting Practices
Term of contract	10 years
Mining properties and mineral rights \| Maximum
Statutory Accounting Practices
Term of contract	50 years

Amount of restricted customer deposit for performance bid bond, classified as current.

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Amount of restricted cash held in trust related to tax exempt bonds, classified as current.

+ References

No definition available.

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Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

+ References

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Amount, after accumulated amortization, of debt issuance costs classified as current. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of interest capitalized during the period.

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Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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- Definition

Carrying amount at the balance sheet date of mineral rights, or rights to extract a mineral from the earth or to receive payment in the form of a royalty for the extraction of minerals, net of amortization.

+ References

No definition available.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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