FAIR VALUE MEASUREMENT - Schedule of Fair Value Hierarchy Measurement (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Fair value of debt securities available for sale	$ 978,843	$ 1,072,093
Single family mortgage servicing rights	61,206
Derivative assets	10,455	27,639
Liabilities:
Derivative liabilities	1,641	2,785
Residential mortgage-backed securities
Assets:
Fair value of debt securities available for sale	33,467	51,046
Commercial mortgage-backed securities
Assets:
Fair value of debt securities available for sale	42,050	45,184
Municipal bonds
Assets:
Fair value of debt securities available for sale	534,401	564,703
Corporate debt securities
Assets:
Fair value of debt securities available for sale	19,849	15,222
Agency debentures
Assets:
Fair value of debt securities available for sale		1,846
Recurring
Assets:
Fair value of loans held for sale	135,942	194,643
Fair value of loans held for investment	4,507	7,108
Single family mortgage servicing rights	61,206	49,966
Total assets	1,195,301	1,359,699
Liabilities:
Total liabilities	11,550	24,232
Recurring \| Residential mortgage-backed securities
Assets:
Fair value of debt securities available for sale	33,467	51,046
Recurring \| Commercial mortgage-backed securities
Assets:
Fair value of debt securities available for sale	42,050	45,184
Recurring \| Collateralized mortgage obligations residential
Assets:
Fair value of debt securities available for sale	184,360	234,909
Recurring \| Collateralized mortgage obligations commercial
Assets:
Fair value of debt securities available for sale	141,324	159,183
Recurring \| Municipal bonds
Assets:
Fair value of debt securities available for sale	534,401	564,703
Recurring \| Corporate debt securities
Assets:
Fair value of debt securities available for sale	19,849	15,222
Recurring \| U.S. Treasury securities
Assets:
Fair value of debt securities available for sale	23,392
Recurring \| Agency debentures
Assets:
Fair value of debt securities available for sale		1,846
Recurring \| Eurodollar futures
Liabilities:
Derivative liabilities	9	4
Recurring \| Forward sale commitments
Assets:
Derivative assets	1,419	1,035
Liabilities:
Derivative liabilities	1,017	3,714
Recurring \| Interest rate lock commitments
Assets:
Derivative assets	4,612	17,395
Liabilities:
Derivative liabilities	3	3
Recurring \| Interest rate swaps
Assets:
Derivative assets	8,772	17,459
Liabilities:
Derivative liabilities	10,521	20,511
Level 1 \| Recurring
Assets:
Fair value of loans held for sale	0	0
Fair value of loans held for investment	0	0
Single family mortgage servicing rights	0	0
Total assets	0	0
Liabilities:
Total liabilities	9	4
Level 1 \| Recurring \| Residential mortgage-backed securities
Assets:
Fair value of debt securities available for sale	0	0
Level 1 \| Recurring \| Commercial mortgage-backed securities
Assets:
Fair value of debt securities available for sale	0	0
Level 1 \| Recurring \| Collateralized mortgage obligations residential
Assets:
Fair value of debt securities available for sale	0	0
Level 1 \| Recurring \| Collateralized mortgage obligations commercial
Assets:
Fair value of debt securities available for sale	0	0
Level 1 \| Recurring \| Municipal bonds
Assets:
Fair value of debt securities available for sale	0	0
Level 1 \| Recurring \| Corporate debt securities
Assets:
Fair value of debt securities available for sale	0	0
Level 1 \| Recurring \| U.S. Treasury securities
Assets:
Fair value of debt securities available for sale	0
Level 1 \| Recurring \| Agency debentures
Assets:
Fair value of debt securities available for sale		0
Level 1 \| Recurring \| Eurodollar futures
Liabilities:
Derivative liabilities	9	4
Level 1 \| Recurring \| Forward sale commitments
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Level 1 \| Recurring \| Interest rate lock commitments
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Level 1 \| Recurring \| Interest rate swaps
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Level 2 \| Recurring
Assets:
Fair value of loans held for sale	135,942	194,643
Fair value of loans held for investment	0	0
Single family mortgage servicing rights	0	0
Total assets	1,122,462	1,282,520
Liabilities:
Total liabilities	11,538	24,225
Level 2 \| Recurring \| Residential mortgage-backed securities
Assets:
Fair value of debt securities available for sale	31,030	48,417
Level 2 \| Recurring \| Commercial mortgage-backed securities
Assets:
Fair value of debt securities available for sale	42,050	45,184
Level 2 \| Recurring \| Collateralized mortgage obligations residential
Assets:
Fair value of debt securities available for sale	184,360	234,909
Level 2 \| Recurring \| Collateralized mortgage obligations commercial
Assets:
Fair value of debt securities available for sale	141,324	159,183
Level 2 \| Recurring \| Municipal bonds
Assets:
Fair value of debt securities available for sale	534,401	564,703
Level 2 \| Recurring \| Corporate debt securities
Assets:
Fair value of debt securities available for sale	19,772	15,141
Level 2 \| Recurring \| U.S. Treasury securities
Assets:
Fair value of debt securities available for sale	23,392
Level 2 \| Recurring \| Agency debentures
Assets:
Fair value of debt securities available for sale		1,846
Level 2 \| Recurring \| Eurodollar futures
Liabilities:
Derivative liabilities	0	0
Level 2 \| Recurring \| Forward sale commitments
Assets:
Derivative assets	1,419	1,035
Liabilities:
Derivative liabilities	1,017	3,714
Level 2 \| Recurring \| Interest rate lock commitments
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Level 2 \| Recurring \| Interest rate swaps
Assets:
Derivative assets	8,772	17,459
Liabilities:
Derivative liabilities	10,521	20,511
Level 3 \| Recurring
Assets:
Fair value of loans held for sale	0	0
Fair value of loans held for investment	4,507	7,108
Single family mortgage servicing rights	61,206	49,966
Total assets	72,839	77,179
Liabilities:
Total liabilities	3	3
Level 3 \| Recurring \| Residential mortgage-backed securities
Assets:
Fair value of debt securities available for sale	2,437	2,629
Level 3 \| Recurring \| Commercial mortgage-backed securities
Assets:
Fair value of debt securities available for sale	0	0
Level 3 \| Recurring \| Collateralized mortgage obligations residential
Assets:
Fair value of debt securities available for sale	0	0
Level 3 \| Recurring \| Collateralized mortgage obligations commercial
Assets:
Fair value of debt securities available for sale	0	0
Level 3 \| Recurring \| Municipal bonds
Assets:
Fair value of debt securities available for sale	0	0
Level 3 \| Recurring \| Corporate debt securities
Assets:
Fair value of debt securities available for sale	77	81
Level 3 \| Recurring \| U.S. Treasury securities
Assets:
Fair value of debt securities available for sale	0
Level 3 \| Recurring \| Agency debentures
Assets:
Fair value of debt securities available for sale		0
Level 3 \| Recurring \| Eurodollar futures
Liabilities:
Derivative liabilities	0	0
Level 3 \| Recurring \| Forward sale commitments
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Level 3 \| Recurring \| Interest rate lock commitments
Assets:
Derivative assets	4,612	17,395
Liabilities:
Derivative liabilities	3	3
Level 3 \| Recurring \| Interest rate swaps
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities		$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansHeldForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122492-111745

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=125521744&loc=d3e122625-111746

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

+ Details

Name:	us-gaap_ServicingAssetAtFairValueAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AgencySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=hmst_CollateralizedMortgageObligationsResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=hmst_CollateralizedMortgageObligationsCommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=hmst_UsTreasuryCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EurodollarFutureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: