Total of property assets serviced as measured by UPB.

No definition available.

Name:	onit_AssetsServiced
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Derecognition Of Financing Liability

+ References

No definition available.

+ Details

Name:	onit_DerecognitionOfFinancingLiability
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Other financing liabilities.

+ References

No definition available.

+ Details

Name:	onit_OtherFinancingLiabilities
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Servicing Asset At Fair Value, Amount, Derecognition

+ References

No definition available.

+ Details

Name:	onit_ServicingAssetAtFairValueAmountDerecognition
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Servicing asset at fair value sales.

+ References

No definition available.

+ Details

Name:	onit_ServicingAssetAtFairValueSales
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Servicing assets at fair value, additions recognized from mortgage sales.

+ References

No definition available.

+ Details

Name:	onit_ServicingAssetsAtFairValueAdditionsRecognizedFromMortgageSales
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Servicing assets at fair value, net additions (sales) recognized

+ References

No definition available.

+ Details

Name:	onit_ServicingAssetsAtFairValueNetAdditionsSalesRecognized
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Unpaid Principal Balance, Mortgage Servicing Rights Sold, Subservicing Retained

+ References

No definition available.

+ Details

Name:	onit_UnpaidPrincipalBalanceMortgageServicingRightsSoldSubservicingRetained
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Unpaid Principal Balance Of Servicing Asset, Transferred

+ References

No definition available.

+ Details

Name:	onit_UnpaidPrincipalBalanceOfServicingAssetTransferred
Namespace Prefix:	onit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

+ Details

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

- 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
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481260/860-50-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-11

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-11

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-10

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481260/860-50-45-2

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481260/860-50-45-1

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) in fair value from changes in the inputs, assumptions, or model used to calculate the fair value of the contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(4)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Decrease in servicing assets subsequently measured at fair value resulting from conveyance of servicing rights to unrelated parties.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

+ Details

Name:	us-gaap_ServicingAssetAtFairValueDisposals
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) from changes in fair value classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(4)(ii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

+ Details

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

- Definition

Amount of increase (decrease) in fair value from changes classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(5)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

+ Details

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

- Definition

The increase (decrease) in the fair value of servicing assets that are subsequently measured at fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-3

+ Details

Name:	us-gaap_ServicingAssetAtFairValuePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=onit_FinancingLiabilityMortgageServicingRightsPledgedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=onit_OriginalRightstoMortgageServicingRightsAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	srt_CounterpartyNameAxis=onit_MAVAssetVehicleLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=onit_RithmCapitalCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=onit_OtherMSRCapitalPartnersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=onit_MSRAssetVehicleLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Mortgage Servicing - Schedule of Activity Related to MSRs - Fair Value Measurement Method (Details) - USD ($) $ in Millions			3 Months Ended				9 Months Ended
	Jun. 30, 2026	Jan. 05, 2026	Sep. 30, 2026	Jun. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance			$ 3,208.8		$ 2,825.3	$ 2,632.6	$ 2,825.3	$ 2,466.3
Sales			(136.7)				(157.2)
Sales						0.0		0.3
Recognized on the sale of residential mortgage loans			172.2			116.0	432.8	258.9
Purchases of MSRs			128.6			86.7	465.8	234.0
Servicing transfers and adjustments			(126.6)			0.0	(249.9)	(12.0)
Changes in fair value recognized in earnings:
Changes in valuation inputs or assumptions			193.3			(5.6)	266.7	1.6
Realization of cash flows (5)			(79.0)			(66.8)	(222.9)	(186.2)
Ending balance	$ 3,208.8		3,360.6	$ 3,208.8			3,360.6
Assets serviced			199,800.0				199,800.0		$ 191,100.0
Servicing Asset at Fair Value, Period Increase (Decrease)			114.3				43.8
Servicing assets at fair value, net additions (sales) recognized			37.5				491.5
MSR related financing liabilities, at fair value			613.1				613.1		842.0
Financing Liability Mortgage Servicing Rights Pledged [Member]
Changes in fair value recognized in earnings:
MSR related financing liabilities, at fair value			346.8				346.8		573.2
Original Rights to Mortgage Servicing Rights Agreements \| Financing Liability Mortgage Servicing Rights Pledged [Member]
Changes in fair value recognized in earnings:
MSR related financing liabilities, at fair value			38.5				38.5
Financing Liability Excess Servicing Spread [Member]
Changes in fair value recognized in earnings:
MSR related financing liabilities, at fair value			266.3				266.3		268.8
Financing Liability Mortgage Servicing Rights Pledged [Member]
Servicing Asset at Fair Value, Amount [Roll Forward]
Sales			(9.8)			(7.9)	25.2	22.1
Changes in fair value recognized in earnings:
Unpaid Principal Balance, Mortgage Servicing Rights Sold, Subservicing Retained		$ 1,300.0
Derecognition of financing liability			(126.6)			0.0	(249.7)	(12.2)
MAV Asset Vehicle LLC
Changes in fair value recognized in earnings:
RMSR			5,100.0				5,100.0
Servicing asset derocgnized			(85.1)				(85.1)
Rithm Capital Corp
Changes in fair value recognized in earnings:
RMSR			3,400.0				4,600.0
Servicing asset derocgnized			(41.5)				(58.7)
Rithm Capital Corp \| Financing Liability Mortgage Servicing Rights Pledged [Member]
Changes in fair value recognized in earnings:
Derecognition of financing liability			(41.5)				(58.7)
Other MSR Capital Partners
Changes in fair value recognized in earnings:
RMSR							5,900.0
Servicing asset derocgnized							(105.9)
Other MSR Capital Partners \| Financing Liability Mortgage Servicing Rights Pledged [Member]
Changes in fair value recognized in earnings:
Derecognition of financing liability	(105.9)
MSR Asset Vehicle LLC \| Financing Liability Mortgage Servicing Rights Pledged [Member]
Changes in fair value recognized in earnings:
Derecognition of financing liability			(85.1)				(85.1)
Fair Value Agency Mortgage Servicing Rights
Changes in fair value recognized in earnings:
Ending balance						2,762.9		2,762.9
Servicing Asset at Fair Value, Period Increase (Decrease)						(72.4)		(184.6)
Servicing assets at fair value, net additions (sales) recognized						$ 202.7		$ 481.3
Transferred MSR, Subject To Pledged Liability
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance					581.0		581.0
Changes in fair value recognized in earnings:
Ending balance			349.9				349.9
Assets serviced			22,600.0				22,600.0		38,600.0
Transferred MSR, Subject To Pledged Liability \| Rithm Capital Corp
Changes in fair value recognized in earnings:
Assets serviced			3,300.0				3,300.0
Fair Value GSE Mortgage Servicing Rights
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance					2,089.2		2,089.2
Changes in fair value recognized in earnings:
Ending balance			2,347.3				2,347.3
Assets serviced			146,700.0				146,700.0		143,500.0
Fair Value GNMA Mortgage Servicing Rights
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance					528.6		528.6
Changes in fair value recognized in earnings:
Ending balance			868.4				868.4
Assets serviced			40,000.0				40,000.0		28,100.0
Fair Value Non-Agency Mortgage Servicing Rights
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance					207.4		207.4
Changes in fair value recognized in earnings:
Ending balance			144.9				144.9
Assets serviced			13,100.0				13,100.0		$ 19,500.0
Fair Value Non-Agency Mortgage Servicing Rights \| Rithm Capital Corp
Changes in fair value recognized in earnings:
RMSR			3,400.0	$ 0.0	1,200.0		4,600.0
Fair Value Non-Agency Mortgage Servicing Rights \| Other MSR Capital Partners
Changes in fair value recognized in earnings:
Derecognition of financing liability	$ (126.6)
Financing Liability Excess Servicing Spread [Member]
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance					$ 373.5		373.5
Changes in fair value recognized in earnings:
Ending balance			$ 363.8				$ 363.8