Fair Value Measurement - FV Hierarchy - Recurring and Non-recurring (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Investment securities available for sale	$ 1,136,389	$ 993,990
Single family mortgage servicing rights	235,997	226,113
Fair value of loans held for sale	502,288	656,334
Fair value of loans held for investment	19,042	17,988
Derivative assets	61,553	69,400
Liabilities:
Derivative liabilities	3,530	4,563
Residential Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	174,060	177,074
Commercial Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	29,476	25,536
Municipal Bonds [Member]
Assets:
Investment securities available for sale	619,934	467,673
Corporate Debt Securities [Member]
Assets:
Investment securities available for sale	51,075	51,122
Fair Value, Measurements, Recurring [Member]
Assets:
Single family mortgage servicing rights	235,997	226,113
Fair value of loans held for sale	502,288	656,334
Fair value of loans held for investment	19,042	17,988
Total assets	1,936,808	1,953,651
Liabilities:
Total liabilities	33,473	42,399
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	174,060	177,074
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	29,476	25,536
Fair Value, Measurements, Recurring [Member] \| Municipal Bonds [Member]
Assets:
Investment securities available for sale	619,934	467,673
Fair Value, Measurements, Recurring [Member] \| Collateralized Mortgage Obligations Residential [Member]
Assets:
Investment securities available for sale	182,037	191,201
Fair Value, Measurements, Recurring [Member] \| Collateralized Mortgage Obligations Commercial [Member]
Assets:
Investment securities available for sale	69,144	70,764
Fair Value, Measurements, Recurring [Member] \| Corporate Debt Securities [Member]
Assets:
Investment securities available for sale	51,075	51,122
Fair Value, Measurements, Recurring [Member] \| US Treasury Securities [Member]
Assets:
Investment securities available for sale	10,663	10,620
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets:
Single family mortgage servicing rights	0	0
Fair value of loans held for sale	0	0
Fair value of loans held for investment	0	0
Total assets	5	0
Liabilities:
Total liabilities	24	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Residential Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Commercial Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Municipal Bonds [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Collateralized Mortgage Obligations Residential [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Collateralized Mortgage Obligations Commercial [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Corporate Debt Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| US Treasury Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets:
Single family mortgage servicing rights	0	0
Fair value of loans held for sale	461,958	614,524
Fair value of loans held for investment	0	0
Total assets	1,614,280	1,648,154
Liabilities:
Total liabilities	33,431	42,032
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Residential Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	174,060	177,074
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Commercial Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	29,476	25,536
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Municipal Bonds [Member]
Assets:
Investment securities available for sale	619,934	467,673
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Collateralized Mortgage Obligations Residential [Member]
Assets:
Investment securities available for sale	182,037	191,201
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Collateralized Mortgage Obligations Commercial [Member]
Assets:
Investment securities available for sale	69,144	70,764
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate Debt Securities [Member]
Assets:
Investment securities available for sale	51,075	51,122
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| US Treasury Securities [Member]
Assets:
Investment securities available for sale	10,663	10,620
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets:
Single family mortgage servicing rights	235,997	226,113
Fair value of loans held for sale	40,330	41,810
Fair value of loans held for investment	19,042	17,988
Total assets	322,523	305,497
Liabilities:
Total liabilities	18	367
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Residential Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Commercial Mortgage-backed Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Municipal Bonds [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Collateralized Mortgage Obligations Residential [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Collateralized Mortgage Obligations Commercial [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Corporate Debt Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| US Treasury Securities [Member]
Assets:
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Eurodollar Futures [Member]
Assets:
Derivative assets	5
Liabilities:
Derivative liabilities	24
Fair Value, Measurements, Recurring [Member] \| Eurodollar Futures [Member] \| Level 1 [Member]
Assets:
Derivative assets	5
Liabilities:
Derivative liabilities	24
Fair Value, Measurements, Recurring [Member] \| Eurodollar Futures [Member] \| Level 2 [Member]
Assets:
Derivative assets	0
Liabilities:
Derivative liabilities	0
Fair Value, Measurements, Recurring [Member] \| Eurodollar Futures [Member] \| Level 3 [Member]
Assets:
Derivative assets	0
Liabilities:
Derivative liabilities	0
Fair Value, Measurements, Recurring [Member] \| Forward Sale Commitments [Member]
Assets:
Derivative assets	4,287	24,623
Liabilities:
Derivative liabilities	7,527	15,203
Fair Value, Measurements, Recurring [Member] \| Forward Sale Commitments [Member] \| Level 1 [Member]
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Forward Sale Commitments [Member] \| Level 2 [Member]
Assets:
Derivative assets	4,287	24,623
Liabilities:
Derivative liabilities	7,527	15,203
Fair Value, Measurements, Recurring [Member] \| Forward Sale Commitments [Member] \| Level 3 [Member]
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaptions [Member]
Assets:
Derivative assets	31	1
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaptions [Member] \| Level 1 [Member]
Assets:
Derivative assets	0	0
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaptions [Member] \| Level 2 [Member]
Assets:
Derivative assets	31	1
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaptions [Member] \| Level 3 [Member]
Assets:
Derivative assets	0	0
Fair Value, Measurements, Recurring [Member] \| Interest Rate Lock and Purchase Loan Commitments [Member]
Assets:
Derivative assets	27,154	19,586
Liabilities:
Derivative liabilities	18	367
Fair Value, Measurements, Recurring [Member] \| Interest Rate Lock and Purchase Loan Commitments [Member] \| Level 1 [Member]
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Interest Rate Lock and Purchase Loan Commitments [Member] \| Level 2 [Member]
Assets:
Derivative assets		0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Interest Rate Lock and Purchase Loan Commitments [Member] \| Level 3 [Member]
Assets:
Derivative assets	27,154	19,586
Liabilities:
Derivative liabilities	18	367
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaps [Member]
Assets:
Derivative assets	11,615	15,016
Liabilities:
Derivative liabilities	25,904	26,829
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaps [Member] \| Level 1 [Member]
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaps [Member] \| Level 2 [Member]
Assets:
Derivative assets	11,615	15,016
Liabilities:
Derivative liabilities	25,904	26,829
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swaps [Member] \| Level 3 [Member]
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	$ 0	$ 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&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 of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
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/presentationRef
-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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

+ 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Glossary Loan
-URI http://asc.fasb.org/extlink&oid=6517073

+ 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember
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_MajorTypesOfDebtAndEquitySecuritiesAxis=hmst_CollateralizedMortgageObligationsResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=hmst_UsTreasuryCollateralizedMortgageObligationsMember
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:

- 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_InterestRateSwaptionMember
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: