Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	4.50%
Total	$ 842,314	$ 816,231
OnDeck Asset Securitization Trust II | Secured Debt
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	3.80%
Debt	$ 225,000		$ 225,000
OnDeck Account Receivables Trust 2013-1 | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	4.20%
Debt	$ 114,425		117,664
Receivable Assets of OnDeck, LLC | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	4.70%
Debt	$ 111,569		113,631
OnDeck Asset Funding II LLC | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	5.50%
Debt	$ 106,413		109,568
Prime OnDeck Receivable Trust II | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	4.60%
Debt	$ 116,812		108,816
Loan Assets of OnDeck, LLC | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	4.20%
Debt	$ 106,625		100,000
Corporate Debt | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	5.50%
Debt	$ 20,000		0
Other Agreements
Debt Instrument [Line Items]
Weighted Average Interest Rate at March 31, 2019	5.60%
Debt	$ 49,351		47,318
Debt
Debt Instrument [Line Items]
Debt	850,195		821,997
Deferred debt issuance cost	(7,881)		(5,766)
Total	$ 842,314		$ 816,231