Long-Lived Asset Impairments (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
Dec. 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Carlisle Assets [Member]
Impairment Effects on Earnings Per Share [Line Items]
Mine carrying value	$ 1,800	$ 1,800	$ 1,800
Asset impairment		0	65,700
Cost to close mine		3,000
Equipment redeployed		23,000
Exit and disposal costs		500
Bulldog Mine [Member]
Impairment Effects on Earnings Per Share [Line Items]
Asset impairment	9,200
Hourglass Sands LLC [Member]
Impairment Effects on Earnings Per Share [Line Items]
Mine carrying value	$ 1,900	$ 1,900	1,900
Asset impairment			$ 2,900

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Undiscounted amount of the accrual for environmental loss contingencies expected to be paid in the remainder of the fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Includes, but is not limited to, one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and termination benefits associated with an ongoing benefit arrangement. Excludes expenses associated with special or contractual termination benefits, a discontinued operation or an asset retirement obligation.

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Mineral properties, net of adjustments.

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