The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Self Insurance (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) item mi	Aug. 31, 2010 USD ($)
Property, Plant and Equipment [Line Items]
Area of self-insured mining units \| mi		22
Mining equipment at historical cost			$ 258,000
Restricted cash	$ 3,811	$ 2,817
High Efficiency Units [Member]
Property, Plant and Equipment [Line Items]
Number of mining units with self-insured equipment \| item		10