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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, 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
Class of Stock [Line Items]                      
Net loss $ (14,321) $ (14,073) $ (10,196) $ (5,836) $ (7,516) $ (12,891) $ (11,525) $ (12,045) $ (44,426) $ (43,977) $ (43,399)
Weighted average common shares outstanding - basic and diluted (in shares) 42,108,764 41,815,139 41,429,691 41,108,611 40,872,772 40,762,960 40,593,908 40,451,668 41,618,838 40,671,133 39,852,624
Basic and diluted net loss per share (in dollars per share) $ (0.34) $ (0.34) $ (0.25) $ (0.14) $ (0.18) $ (0.32) $ (0.28) $ (0.30) $ (1.07) $ (1.08) $ (1.09)
Class A Common Stock                      
Class of Stock [Line Items]                      
Net loss                 $ (33,016) $ (31,644) $ (30,075)
Weighted average common shares outstanding - basic and diluted (in shares)                 30,929,899 29,265,605 27,617,350
Basic and diluted net loss per share (in dollars per share)                 $ (1.07) $ (1.08) $ (1.09)
Class B Common Stock                      
Class of Stock [Line Items]                      
Net loss                 $ (11,410) $ (12,333) $ (13,324)
Weighted average common shares outstanding - basic and diluted (in shares)                 10,688,939 11,405,528 12,235,274
Basic and diluted net loss per share (in dollars per share)                 $ (1.07) $ (1.08) $ (1.09)