Commitments and Contingencies - Contingencies (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 AUD ($)	Oct. 24, 2017 lawsuits
Loss Contingencies [Line Items]
Number of Lawsuits \| lawsuits		3
Monto Coal Pty Limited
Loss Contingencies [Line Items]
LossContingencyDamagesSoughtValueMax	$ 18.0
Maximum \| Monto Coal Pty Limited
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value	$ 1,100.0

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14557-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14435-108349

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration