Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Statutory Accounting Practices
Interest costs capitalized	$ 2.9	$ 0.3
Mineral rights	360.9	338.1
Deferred finance costs, net	3.0	4.7
Deferred finance costs, current	$ 2.0	$ 2.2
Exploration and production equipment \| Minimum
Statutory Accounting Practices
Estimated useful life	1 year
Exploration and production equipment \| Maximum
Statutory Accounting Practices
Estimated useful life	18 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	18 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, after accumulated amortization, of debt issuance costs classified as current. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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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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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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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.

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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.

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