Servicing Rights (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Transfers and Servicing [Abstract]
Serviced unpaid principal balance	$ 160.0	$ 181.0
Unpaid principal balance for loans sold during period		72.5	$ 368.3
Servicing revenue	$ 0.8	$ 1.1	$ 1.2

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

No definition available.

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