Servicing Rights (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Transfers and Servicing [Abstract]
Serviced unpaid principal balance	$ 157.0		$ 157.0		$ 222.0
Unpaid principal balance for loans sold during period	9.1	$ 77.2	50.2	$ 197.3
Servicing revenue	$ 0.6	$ 0.1	$ 0.9	$ 0.5

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