Revision of Prior Period Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Loans and finance receivables	$ 1,265,312				$ 1,169,407				$ 1,265,312	$ 1,169,407
Total assets	1,304,583				1,161,820				1,304,583	1,161,820
Accrued expenses and other liabilities	70,110				36,708				70,110	36,708
Total liabilities	993,909				859,335				993,909	859,335
Accumulated deficit	(169,002)				(196,959)				(169,002)	(196,959)
Total On Deck Capital, Inc. stockholders' equity	294,000				297,952				294,000	297,952
Total stockholders' equity	296,246				302,485				296,246	302,485	$ 264,888	$ 262,964
Income Statement [Abstract]
Interest and finance income									428,423	382,944	334,040
Net interest income										397,741	350,415
Net interest revenue, after credit provision										202,125	151,290
Income (loss) from operations, before provision for income taxes									20,122	24,635	(14,880)
Net income (loss)	$ 7,997	$ 8,142	$ 2,168	$ 5,328	$ 13,260	$ 9,338	$ 4,612	$ (2,576)	$ 23,635	$ 24,635	$ (14,880)
Net income (loss) per share attributable to On Deck Capital, Inc. common stockholders:
Net income (loss) per common share, basic (in dollars per share)	$ 0.13	$ 0.12	$ 0.06	$ 0.08	$ 0.18	$ 0.13	$ 0.08	$ (0.03)	$ 0.38	$ 0.36	$ (0.17)
Net income (loss) per common share, diluted (in dollars per share)	$ 0.13	$ 0.11	$ 0.05	$ 0.07	$ 0.17	$ 0.12	$ 0.07	$ (0.03)	$ 0.36	$ 0.34	$ (0.17)
Previously Reported
Statement of Financial Position [Abstract]
Loans and finance receivables					$ 1,169,157					$ 1,169,157
Total assets					1,161,570					1,161,570
Accrued expenses and other liabilities					34,654					34,654
Total liabilities					857,281					857,281
Accumulated deficit					(195,155)					(195,155)
Total On Deck Capital, Inc. stockholders' equity					299,756					299,756
Total stockholders' equity					304,289					304,289
Income Statement [Abstract]
Interest and finance income										383,579	$ 334,575
Net interest income										398,376	350,950
Net interest revenue, after credit provision										202,760	151,825
Income (loss) from operations, before provision for income taxes										25,270	(14,345)
Net income (loss)										$ 25,270	$ (14,345)
Net income (loss) per share attributable to On Deck Capital, Inc. common stockholders:
Net income (loss) per common share, basic (in dollars per share)										$ 0.37	$ (0.16)
Net income (loss) per common share, diluted (in dollars per share)										$ 0.35	$ (0.16)
Restatement Adjustment
Statement of Financial Position [Abstract]
Loans and finance receivables					250					$ 250
Total assets					250					250
Accrued expenses and other liabilities					2,054					2,054
Total liabilities					2,054					2,054
Accumulated deficit					(1,804)					(1,804)
Total On Deck Capital, Inc. stockholders' equity					(1,804)					(1,804)
Total stockholders' equity					$ (1,804)					(1,804)
Income Statement [Abstract]
Interest and finance income										(635)	$ (535)
Net interest income										(635)	(535)
Net interest revenue, after credit provision										(635)	(535)
Income (loss) from operations, before provision for income taxes										(635)	(535)
Net income (loss)										$ (635)	$ (535)