Net Loss Per Share - Earnings Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Denominator
Weighted-average common shares outstanding - basic and diluted (in shares)	47,058,209	46,731,663	46,166,660	45,229,279	44,472,672	43,973,428	43,234,655	42,858,756	46,302,656	43,640,408	41,618,838
Basic and diluted net loss per share (in dollars per share)	$ (0.35)	$ (0.34)	$ (0.18)	$ (0.17)	$ (0.17)	$ (0.25)	$ (0.50)	$ (0.22)	$ (1.04)	$ (1.15)	$ (1.07)
Class A Common Stock
Numerator
Net loss									$ (38,661)	$ (38,733)	$ (33,016)
Denominator
Weighted-average common shares outstanding - basic and diluted (in shares)									37,190,224	33,758,623	30,929,899
Basic and diluted net loss per share (in dollars per share)									$ (1.04)	$ (1.15)	$ (1.07)
Class B Common Stock
Numerator
Net loss									$ (9,473)	$ (11,338)	$ (11,410)
Denominator
Weighted-average common shares outstanding - basic and diluted (in shares)									9,112,432	9,881,785	10,688,939
Basic and diluted net loss per share (in dollars per share)									$ (1.04)	$ (1.15)	$ (1.07)