Net Income (Loss) Per Common Share (Basic and Diluted Net Loss per Common Share) (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
Numerator:
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)
Less: Net income (loss) attributable to noncontrolling interest									(2,411)	(2,811)	(2,524)
Net income (loss) attributable to On Deck Capital, Inc. common stockholders									$ 27,681	$ (11,534)	$ (82,958)
Denominator:
Weighted-average common shares outstanding, basic (in shares)									74,561,019	72,890,313	70,934,937
Net income (loss) per common share, 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)
Effect of dilutive securities (in shares)									3,988,921	0	0
Weighted-average common shares outstanding, diluted (in shares)									78,549,940	72,890,313	70,934,937
Net income (loss) per common share, 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)
Anti-dilutive securities excluded (in shares)									5,423,547	11,410,980	15,580,272