Quarterly Financial Information (unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 98,079	$ 96,947	$ 94,260	$ 94,467	$ 93,958	$ 87,589	$ 79,964	$ 74,357	$ 383,753	$ 335,869	$ 287,841
Net interest income, after credit provision	54,048	53,851	51,309	51,176	54,105	48,487	46,671	38,064	210,384	187,328	134,915
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) attributable to On Deck Capital, Inc. common stockholders	$ 9,310	$ 8,684	$ 4,295	$ 5,666	$ 13,864	$ 9,610	$ 5,628	$ (2,058)
Net income (loss) per share attributable to On Deck Capital, Inc. common stockholders:
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)
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)