Revisions of Prior Period Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Loans and finance receivables	$ 1,207,609				$ 1,207,609		$ 1,169,407
Total assets	1,248,207				1,248,207		1,161,820
Accrued expenses and other liabilities	65,135				65,135		36,708
Total liabilities	915,243				915,243		859,335
Accumulated deficit	(186,997)				(186,997)		(196,959)
Total On Deck Capital, Inc. stockholders' equity	314,484				314,484		297,952
Total stockholders' equity	317,842	$ 311,596	$ 276,170	$ 266,113	317,842	$ 276,170	302,485	$ 264,842
Income Statement [Abstract]
Interest and finance income	105,641		92,209		211,440	178,438
Gross revenue	110,246		95,456		220,221	185,596
Net revenue	55,914		49,918		111,266	91,893
Income (loss) from operations, before provision for income taxes	3,964		4,612		11,032	2,036
Net Income (loss)	$ 2,168	$ 5,328	4,612	$ (2,576)	$ 7,496	2,036
Previously Reported
Statement of Financial Position [Abstract]
Loans and finance receivables							1,169,157
Total assets							1,161,570
Accrued expenses and other liabilities							34,654
Total liabilities							857,281
Accumulated deficit							(195,155)
Total On Deck Capital, Inc. stockholders' equity							299,756
Total stockholders' equity							304,289
Income Statement [Abstract]
Interest and finance income			92,371			178,740
Gross revenue			95,618			185,898
Net revenue			50,080			92,195
Income (loss) from operations, before provision for income taxes			4,774			2,338
Net Income (loss)			4,774			2,338
Restatement Adjustment
Statement of Financial Position [Abstract]
Loans and finance receivables							250
Total assets							250
Accrued expenses and other liabilities							2,054
Total liabilities							2,054
Accumulated deficit							(1,804)
Total On Deck Capital, Inc. stockholders' equity							(1,804)
Total stockholders' equity							$ (1,804)
Income Statement [Abstract]
Interest and finance income			(162)			(302)
Gross revenue			(162)			(302)
Net revenue			(162)			(302)
Income (loss) from operations, before provision for income taxes			(162)			(302)
Net Income (loss)			$ (162)			$ (302)