Fair Value of Financial Instruments - Valuation Techniques And Unobservable Inputs Applied (Details) - Level 3 $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($)
Minimum
Unobservable input
Loss severities (as a percent)	6.60%
Maximum
Unobservable input
Loss severities (as a percent)	86.70%
Weighted Average
Unobservable input
Discount rates (as a percent)	4.20%
Loss severities (as a percent)	41.40%
Securitized mortgage borrowings
Valuation techniques
Estimated fair value of liabilities	$ (3,296,242)
Securitized mortgage borrowings \| Minimum
Unobservable input
Default rates (as a percent)	0.01%
Securitized mortgage borrowings \| Maximum
Unobservable input
Default rates (as a percent)	4.00%
Securitized mortgage borrowings \| Weighted Average
Unobservable input
Default rates (as a percent)	1.50%
Securitized mortgage borrowings \| DCF \| Minimum
Unobservable input
Loss severities (as a percent)	3.00%
Securitized mortgage borrowings \| DCF \| Maximum
Unobservable input
Loss severities (as a percent)	25.00%
Long-term debt \| DCF
Valuation techniques
Estimated fair value of liabilities	$ (46,738)
Unobservable input
Discount rates (as a percent)	10.00%
Long-term debt \| DCF \| Weighted Average
Unobservable input
Discount rates (as a percent)	10.00%
Securitized mortgage collateral \| DCF
Valuation techniques
Estimated fair value of assets	$ 3,302,192
Securitized mortgage collateral \| DCF \| Minimum
Unobservable input
Prepayment rates (as a percent)	2.30%
Securitized mortgage collateral \| DCF \| Maximum
Unobservable input
Prepayment rates (as a percent)	21.00%
Securitized mortgage collateral \| DCF \| Weighted Average
Unobservable input
Prepayment rates (as a percent)	6.90%
Mortgage servicing rights \| DCF
Valuation techniques
Estimated fair value of assets	$ 181,005
Mortgage servicing rights \| DCF \| Minimum
Unobservable input
Discount rates (as a percent)	9.00%
Prepayment rates (as a percent)	6.70%
Mortgage servicing rights \| DCF \| Maximum
Unobservable input
Discount rates (as a percent)	15.20%
Mortgage servicing rights \| DCF \| Weighted Average
Unobservable input
Discount rates (as a percent)	10.20%
Prepayment rates (as a percent)	9.30%
Mortgage servicing rights \| Derivative liabilities, net, securitized trusts \| DCF \| Maximum
Unobservable input
Prepayment rates (as a percent)	88.80%
Interest rate lock commitments. net (IRLCs) \| Market pricing
Valuation techniques
Estimated fair value of assets	$ 3,818
Interest rate lock commitments. net (IRLCs) \| Market pricing \| Minimum
Unobservable input
Pull-through rate (as a percent)	8.60%
Interest rate lock commitments. net (IRLCs) \| Market pricing \| Maximum
Unobservable input
Pull-through rate (as a percent)	99.90%
Interest rate lock commitments. net (IRLCs) \| Market pricing \| Weighted Average
Unobservable input
Pull-through rate (as a percent)	74.60%

No definition available.

Name:	imh_FairValueInputsAssetsAndLiabilitiesQuantitativeInformationLineItems
Namespace Prefix:	imh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the pull-through rate, used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	imh_FairValueInputsPullThroughRate
Namespace Prefix:	imh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsLossSeverity
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsPrepaymentRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsProbabilityOfDefault
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

- Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=imh_SecuritizedMortgageBorrowingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=imh_DiscountedCashFlowValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_LongTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=imh_SecuritizedMortgageCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=imh_MortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=imh_DerivativeLiabilitiesNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ValuationTechniqueAxis=imh_MarketPricingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: