Servicing Rights (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Transfers and Servicing [Abstract]
Serviced unpaid principal balance	$ 233.6		$ 233.6		$ 345.9
Unpaid principal balance for loans sold during period	87.5	$ 169.0	284.9	$ 404.1
Servicing revenue	$ 0.3	$ 1.3	$ 0.9	$ 2.7

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