Net Loss Per Common Share (Basic and Diluted Net Loss per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss	$ (2,436)	$ (11,602)
Less: net loss attributable to noncontrolling interest	518	544
Net income (loss) attributable to On Deck Capital, Inc. common stockholders	$ (1,918)	$ (11,058)
Denominator:
Weighted-average common shares outstanding, basic and diluted (in shares)	73,977,241	71,854,287
Net loss per common share, basic and diluted (in dollars per share)	$ (0.03)	$ (0.15)