Debt, Net - Components of recorded interest expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Amortization of debt issuance costs	$ 335	$ 672	$ 1,972	$ 2,048
Other interest expense	8	0	40	0
Capitalized interest	(654)	(44)	(1,831)	(551)
Total interest expense, debt	4,449	19,887	43,458	64,846
2.0 Lien Notes
Debt Instrument [Line Items]
Interest expense, debt	0	7,816	12,902	28,537
1.5 Lien Notes
Debt Instrument [Line Items]
Interest expense, debt	0	5,139	8,635	18,763
1.25 Lien Notes
Debt Instrument [Line Items]
Interest expense, debt	0	3,352	6,218	5,241
First Lien Agreement
Debt Instrument [Line Items]
Interest expense, debt	0	2,867	4,575	10,022
Sprott Credit Agreement
Debt Instrument [Line Items]
Interest expense, debt			6,009	0
Subordinated Notes
Debt Instrument [Line Items]
Interest expense, debt	2,120	0	4,797	0
Promissory Note
Debt Instrument [Line Items]
Interest expense, debt	$ 0	$ 85	$ 141	$ 786

Amount of the cost of borrowed funds accounted for as interest expense for debt classified as other.

No definition available.

Name:	hymc_InterestExpenseDebtOther
Namespace Prefix:	hymc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of amortization expense attributable to debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(8))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of interest costs capitalized disclosed as an adjusting item to interest costs incurred.

+ References

No definition available.

+ Details

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

- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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

- Definition

Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph 8
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_The20LienNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_The15LienNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_The125LienNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_FirstLienAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_SprottCreditAgreementNoncurrentPortionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_SubordinatedNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=hymc_PromissoryNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: