Balance Sheet Items - Asset Retirement Obligation (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020 USD ($) site	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) site	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of structural fill sites with maintenance and monitoring subsequent to closure \| site	2		2
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Opening balance	$ 12,987	$ 24,218	$ 15,131	$ 26,065
Liabilities incurred	0	0	0	1,017
Liabilities settled	(2,201)	(3,575)	(4,533)	(6,782)
Accretion	162	302	350	645
Ending balance	10,948	20,945	10,948	20,945
Less: current portion	(5,845)	(14,126)	(5,845)	(14,126)	$ (9,944)
Non-current portion	$ 5,103	$ 6,819	$ 5,103	$ 6,819	$ 5,187

Number Of Structural Fill Sites With Maintenance And Monitoring Subsequent To Closure

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

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- Definition

Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Amount of asset retirement obligations incurred during the period.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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