Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Changes in the Company's asset retirement obligations
Beginning asset retirement obligations	$ 26,256	$ 20,640
Liabilities incurred during period	3,566	2,920
Liabilities settled during period	(708)	(430)
Revisions in estimated cash flows	442	1,836
Accretion expense	1,530	1,290
Ending asset retirement obligations	$ 26,256	$ 20,640	$ 31,086	$ 26,256
Less: current asset retirement obligations			(1,350)	(1,176)
Long-term asset retirement obligations			$ 29,736	$ 25,080

Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

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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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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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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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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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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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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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