Servicing Rights (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Transfers and Servicing [Abstract]
Serviced unpaid principal balance	$ 141.0		$ 141.0		$ 222.0
Unpaid principal balance for loans sold during period	5.3	$ 87.5	55.5	$ 284.9
Servicing revenue	$ 0.4	$ 0.3	$ 1.3	$ 0.9

Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Loans Sold During Period, Unpaid Principal Balance

No definition available.

Name:	ondk_ServicingAssetsandServicingLiabilitiesatFairValueAssumptionsUsedtoEstimateFairValueLoansSoldDuringPeriodUnpaidPrincipalBalance
Namespace Prefix:	ondk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration