Net Income (Loss) 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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income (loss)	$ 8,142	$ 9,338	$ 15,638	$ 11,374
Less: Net income (loss) attributable to noncontrolling interest	(542)	(272)	(3,007)	(1,807)
Net income (loss) attributable to On Deck Capital, Inc. common stockholders	$ 8,684	$ 9,610	$ 18,645	$ 13,181
Denominator:
Weighted-average common shares outstanding, basic (in shares)	75,433,008	74,715,592	75,722,117	74,362,211
Net income (loss) per common share, basic (in dollars per share)	$ 0.12	$ 0.13	$ 0.25	$ 0.18
Effect of dilutive securities (in shares)	2,325,273	4,656,899	2,854,782	3,952,508
Weighted-average common shares outstanding, diluted (in shares)	77,758,281	79,372,491	78,576,899	78,314,719
Net income (loss) per common share, diluted (in dollars per share)	$ 0.11	$ 0.12	$ 0.24	$ 0.17
Anti-dilutive securities excluded (in shares)	8,184,291	3,020,562	6,628,241	5,169,484