Net Income (Loss) Per Common Share (Basic and Diluted Net Loss per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net Income (loss)	$ 2,168	$ 5,328	$ 4,612	$ (2,576)	$ 7,496	$ 2,036
Less: Net income (loss) attributable to noncontrolling interest	(2,127)		(1,016)		(2,465)	(1,535)
Net income (loss) attributable to On Deck Capital, Inc. common stockholders	$ 4,295		$ 5,628		$ 9,961	$ 3,571
Denominator:
Weighted-average common shares outstanding, basic (in shares)	76,137,751		74,385,446		75,840,604	74,182,929
Net income (loss) per common share, basic (in dollars per share)	$ 0.06		$ 0.08		$ 0.13	$ 0.05
Effect of dilutive securities (in shares)	2,763,850		3,902,821		3,173,153	3,603,819
Weighted-average common shares outstanding, diluted (in shares)	78,901,601		78,288,267		79,013,757	77,786,748
Net income (loss) per common share, diluted (in dollars per share)	$ 0.05		$ 0.07		$ 0.13	$ 0.05
Anti-dilutive securities excluded (in shares)	6,747,782		5,174,846		5,591,794	5,351,219