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

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Commitments and Contingencies - Contingencies (Details) - 3 months ended Mar. 31, 2018 $ in Millions, $ in Millions	AUD ($) defendant	USD ($)
Monto Coal Pty Limited
Loss Contingency [Abstract]
Ownership Percentage In Subsidiaries		51.00%
Number of defendants \| defendant	3
Minimum \| Monto Coal Pty Limited
Loss Contingency [Abstract]
Damages sought, value	$ 15.6
Maximum \| Monto Coal Pty Limited
Loss Contingency [Abstract]
Damages sought, value	$ 1,800.0
Berenergy Corporation vs. the Company \| Berenergy
Loss Contingency [Abstract]
Loss contingency accrual		$ 13.1