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Commitments and Contingencies - Contingencies (Details) - 9 months ended Sep. 30, 2018 $ in Millions, $ in Billions	AUD ($)	USD ($)
Maximum | Monto Coal Pty Limited
Loss Contingency [Abstract]
Damages sought, value	$ 1.8
Berenergy Corporation vs. the Company | Berenergy
Loss Contingency [Abstract]
Loss contingency accrual		$ 13.1
Reasonable Value of Oil Reserves		$ 1.0

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- Definition

Reasonable Value of Oil Reserves

+ References

No definition available.

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- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

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- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
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-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
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-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

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