Fair Value of Financial Instruments (Servicing Rights) (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Minimum
Servicing Assets at Fair Value [Line Items]
Discount rate	30.00%	30.00%
Cost of service	0.04%	0.09%
Renewal rate	41.06%	46.20%
Default rate	10.63%	10.32%
Maximum
Servicing Assets at Fair Value [Line Items]
Discount rate	30.00%	30.00%
Cost of service	0.13%	0.14%
Renewal rate	51.83%	56.54%
Default rate	10.92%	10.75%
Weighted Average
Servicing Assets at Fair Value [Line Items]
Discount rate	30.00%	30.00%
Cost of service	0.12%	0.11%
Renewal rate	49.59%	50.14%
Default rate	10.70%	10.48%

Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Cost of Service

No definition available.

Name:	ondk_ServicingAssetsandServicingLiabilitiesatFairValueAssumptionsUsedtoEstimateFairValueCostofService
Namespace Prefix:	ondk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Default Rate

+ References

No definition available.

+ Details

Name:	ondk_ServicingAssetsandServicingLiabilitiesatFairValueAssumptionsUsedtoEstimateFairValueDefaultRate
Namespace Prefix:	ondk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Renewal Rate

+ References

No definition available.

+ Details

Name:	ondk_ServicingAssetsandServicingLiabilitiesatFairValueAssumptionsUsedtoEstimateFairValueRenewalRate
Namespace Prefix:	ondk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ 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 (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

+ Details

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

- 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

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

- 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: