Net Loss Per Common Share (Basic and Diluted Net Loss per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income (loss)	$ (36,460)	$ (17,173)	$ (18,708)	$ (13,141)	$ (5,144)	$ 3,507	$ 4,748	$ (5,343)	$ (85,482)	$ (2,231)	$ (18,708)
Accretion of dividends on redeemable convertible preferred stock									0	0	(12,884)
Less: net loss attributable to noncontrolling interest									2,524	958	0
Net loss attributable to On Deck Capital, Inc. common stockholders									$ (82,958)	$ (1,273)	$ (31,592)
Weighted-average common shares outstanding, basic and diluted (in shares)									70,934,937	69,545,238	52,556,998
Net loss per common share, basic and diluted (in dollars per share)									$ (1.17)	$ (0.02)	$ (0.60)