Long-Term Debt - Tremont Promissory Note Payable - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2020 USD ($)
Promissory Note \| Tremont \| Nevada Environmental Response Trust
Debt Instrument [Line Items]
Repaid remaining principal balance of note	$ 2.0

The cash outflow for a borrowing supported by a written promise to pay an obligation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

Name:	us-gaap_RepaymentsOfNotesPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration