Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Gross revenue	$ 109,480	$ 102,999	$ 95,618	$ 90,280	$ 87,741	$ 83,666	$ 86,651	$ 92,892	$ 398,376	$ 350,950	$ 291,317
Net revenue	58,396	52,169	50,080	42,114	42,040	32,719	41,984	35,082	202,760	151,825	108,492
Net income (loss)	13,436	9,497	4,774	(2,436)	4,358	(4,532)	(2,569)	(11,602)	$ 25,270	$ (14,345)	$ (85,482)
Net income (loss) attributable to On Deck Capital, Inc. common stockholders	$ 14,040	$ 9,770	$ 5,790	$ (1,918)	$ 5,096	$ (4,074)	$ (1,498)	$ (11,058)
Net income (loss) per share attributable to On Deck Capital, Inc. common stockholders:
Basic (in dollars per share)	$ 0.19	$ 0.13	$ 0.08	$ (0.03)	$ 0.07	$ (0.06)	$ (0.02)	$ (0.15)	$ 0.37	$ (0.16)	$ (1.17)
Diluted (in dollars per share)	$ 0.18	$ 0.12	$ 0.07	$ (0.03)	$ 0.07	$ (0.06)	$ (0.02)	$ (0.15)	$ 0.35	$ (0.16)	$ (1.17)