Earnings Per Common Share (Basic and Diluted Net Loss per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income (loss)	$ 9,497	$ (4,532)	$ 11,835	$ (18,703)
Less: Net income (loss) attributable to noncontrolling interest	(272)	(458)	(1,807)	(2,073)
Net income (loss) attributable to On Deck Capital, Inc. common stockholders	$ 9,769	$ (4,074)	$ 13,642	$ (16,630)
Denominator:
Weighted-average common shares outstanding, basic (in shares)	74,715,592	73,272,085	74,362,211	72,613,221
Net income (loss) per common share, basic (in dollars per share)	$ 0.13	$ (0.06)	$ 0.18	$ (0.23)
Effect of dilutive securities (in shares)	4,656,899	0	3,952,508	0
Weighted-average common shares outstanding, diluted (in shares)	79,372,491	73,272,085	78,314,719	72,613,221
Net income (loss) per common share, diluted (in dollars per share)	$ 0.12	$ (0.06)	$ 0.17	$ (0.23)
Anti-dilutive securities excluded (in shares)	3,020,562	11,813,427	5,169,484	11,813,427