Mortgage Servicing Rights (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Changes in the fair value of MSRs
Balance at beginning of period			$ 154,405	$ 131,537	$ 131,537
Additions from servicing retained loan sales			22,473		56,049
Addition from purchases					5,618
Reductions from bulk sales					(895)
Changes in fair value			4,127		(37,904)
Fair value of MSRs at end of period	$ 181,005		181,005		154,405
Total loans serviced	16,789,534		$ 16,789,534		$ 16,330,100
Pledged collateral			77.00%		81.00%
Mortgage Servicing Rights Sensitivity Analysis
Change in fair value of mortgage servicing rights	(5,445)	$ (11,177)	$ 4,127	(20,038)
Gain (loss) on sale of mortgage servicing rights		(8)		(90)
Realized and unrealized (losses) gains from hedging instruments	253	672	(1,445)	1,969
Gain (loss) on mortgage servicing rights, net	(5,192)	(10,513)	2,682	(18,159)
Servicing income, net
Contractual servicing fees	11,458	9,978	34,322	27,356
Late and Ancillary fees	182	117	500	275
Subservicing and other costs	(1,516)	(1,603)	(5,377)	(4,056)
Servicing fees, net	10,124	$ 8,492	29,445	$ 23,575
Government
Changes in the fair value of MSRs
Total loans serviced	3,820,752		3,820,752		$ 2,834,680
Conventional
Changes in the fair value of MSRs
Total loans serviced	12,966,922		12,966,922		13,493,463
NonQM
Changes in the fair value of MSRs
Total loans serviced	1,860		1,860		1,957
Mortgage servicing rights
Mortgage Servicing Rights Sensitivity Analysis
Fair value of MSRs	181,005		181,005		154,405
Prepayment Speed, Decrease in fair value from 10% adverse change	(1,846)		(1,846)		(5,643)
Prepayment Speed, Decrease in fair value from 20% adverse change	(3,914)		(3,914)		(11,275)
Prepayment Speed, Decrease in fair value from 30% adverse change	(6,216)		(6,216)		(16,807)
Discount Rate, Decrease in fair value from 10% adverse change	(5,357)		(5,357)		(5,461)
Discount Rate, Decrease in fair value from 20% adverse change	(10,346)		(10,346)		(10,555)
Discount Rate, Decrease in fair value from 30% adverse change	$ (15,001)		$ (15,001)		$ (15,316)

Amount of gain (loss) on sale or disposal of mortgage servicing rights (MSRs).

No definition available.

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- Definition

Amount of gain (loss) on mortgage servicing rights, net of change in fair value during the period.

+ References

No definition available.

+ Details

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- Definition

Represents the percentage of pledged collateral.

+ References

No definition available.

+ Details

Name:	imh_PercentageOfPledgedCollateral
Namespace Prefix:	imh_
Data Type:	num:percentItemType
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- References

No definition available.

+ Details

Name:	imh_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

The value of new servicing assets, subsequently measured at fair value, acquired during the current period through purchases of financial assets.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total amount of contractually specified servicing fees, late fees, and ancillary fees net of servicing costs that are earned for each period for which results of operations are presented.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

the amount of subservicing and other costs associated with servicing financial assets.

+ References

No definition available.

+ Details

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Namespace Prefix:	imh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

All amounts that, per contract, are due to the servicer in exchange for servicing the financial asset and would no longer be received by a servicer if the beneficial owners of the serviced assets (or their trustees or agents) were to exercise their actual or potential authority under the contract to shift the servicing to another servicer. Depending on the servicing contract, those fees may include some or all of the difference between the interest rate collected on the asset being serviced and the rate to be paid to the beneficial owners of the asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

+ Details

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- 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

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- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

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- Definition

The carrying amount (fair value) as of the balance sheet date of the transferor's interests in transferred financial assets (typically securitized receivables, generally not involved in the collection of principal) following transfer of significant portions of such financial instruments to other parties. Such interests could be ancillary cash flows and interest-only strips.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

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- Definition

All amounts that are due to the servicer of a financial asset as a result of the debt holder not meeting the monthly payment terms.

+ References

+ Details

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Balance Type:	credit
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- Definition

The adjustment to the carrying value of the rights retained or purchased to service mortgages. These adjustments are made when the estimate of the fair value is changed. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (a)(6),(7)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This is the principal amount outstanding for securitized loans only (across all types of loans).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

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Balance Type:	credit
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- Definition

The hypothetical financial impact of a 10 percent adverse change of the discount rate on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References