LOANS AND CREDIT QUALITY - TDRs (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan	14	9	21	35
Recorded investment - TDR	$ 4,289	$ 2,053	$ 6,744	$ 8,748
Related charge-offs - TDR	$ 0	$ 0	$ 0	$ 0
Consumer loans
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan		9		33
Recorded investment - TDR		$ 2,053		$ 6,945
Related charge-offs - TDR		$ 0		$ 0
Consumer loans \| Single family
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan	14		21
Recorded investment - TDR	$ 4,289		$ 6,744
Related charge-offs - TDR	$ 0		$ 0
Commercial and Industrial Sector \| Commercial loans
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan		0		2
Recorded investment - TDR		$ 0		$ 1,803
Related charge-offs - TDR		$ 0		$ 0
Interest rate reduction
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan	10	8	15	23
Recorded investment - TDR	$ 3,284	$ 1,642	$ 4,599	$ 4,878
Related charge-offs - TDR	$ 0	$ 0	$ 0	$ 0
Interest rate reduction \| Consumer loans \| Single family
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan	10	8	15	23
Recorded investment - TDR	$ 3,284	$ 1,642	$ 4,599	$ 4,878
Related charge-offs - TDR	$ 0	$ 0	$ 0	$ 0
Payment restructure
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan	4	1	6	12
Recorded investment - TDR	$ 1,005	$ 411	$ 2,145	$ 3,870
Related charge-offs - TDR	$ 0	$ 0	$ 0	$ 0
Payment restructure \| Commercial loans \| Owner occupied commercial real estate
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan		0		1
Recorded investment - TDR		$ 0		$ 678
Related charge-offs - TDR		$ 0		$ 0
Payment restructure \| Commercial loans \| Commercial business
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan		0		1
Recorded investment - TDR		$ 0		$ 1,125
Related charge-offs - TDR		$ 0		$ 0
Payment restructure \| Consumer loans \| Single family
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan	4	1	6	10
Recorded investment - TDR	$ 1,005	$ 411	$ 2,145	$ 2,067
Related charge-offs - TDR	$ 0	$ 0		$ 0
Payment restructure \| Commercial and Industrial Sector \| Commercial loans
Financing Receivable, Impaired [Line Items]
Number of loan modifications - TDR \| loan		0		2
Recorded investment - TDR		$ 0		$ 1,803
Related charge-offs - TDR		$ 0		$ 0

Financing Receivable, Modifications, Recorded Investment At Period End

No definition available.

Name:	hmst_FinancingReceivableModificationsRecordedInvestmentAtPeriodEnd
Namespace Prefix:	hmst_
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_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=us-gaap_CommercialAndIndustrialSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=hmst_OwnerOccupiedCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: