Net Income (Loss) Per Common Share (Basic and Diluted Net Loss per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net Income (loss)	$ 5,520	$ (2,436)
Less: Net income (loss) attributable to noncontrolling interest	(338)	(518)
Net income (loss) attributable to On Deck Capital, Inc. common stockholders	$ 5,858	$ (1,918)
Denominator:
Weighted-average common shares outstanding, basic (in shares)	75,539,535	73,977,241
Net income (loss) per common share, basic (in dollars per share)	$ 0.08	$ (0.03)
Effect of dilutive securities (in shares)	3,575,502	0
Weighted-average common shares outstanding, diluted (in shares)	79,115,037	73,977,241
Net income (loss) per common share, diluted (in dollars per share)	$ 0.07	$ (0.03)
Anti-dilutive securities excluded (in shares)	4,863,474	12,163,804