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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ 4,358	$ (4,532)	$ (2,569)	$ (11,602)	$ (36,460)	$ (17,173)	$ (18,708)	$ (13,141)	$ (14,345)	$ (85,482)	$ (2,231)
Less: net loss attributable to noncontrolling interest									2,811	2,524	958
Net income (loss) attributable to On Deck Capital, Inc. common stockholders									$ (11,534)	$ (82,958)	$ (1,273)
Denominator:
Weighted-average common shares outstanding, basic and diluted (in shares)									72,890,313	70,934,937	69,545,238
Net loss per common share, basic and diluted (in dollars per share)									$ (0.16)	$ (1.17)	$ (0.02)