Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Initial Public Offering [Member]	Series A, B and C Redeemable Convertible Preferred Stock [Member]	Series A, B and C Redeemable Convertible Preferred Stock [Member] Series B Preferred Stock [Member]	Series A, B and C Redeemable Convertible Preferred Stock [Member] Series C Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Initial Public Offering [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Initial Public Offering [Member]	Accumulated other comprehensive income (loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2013	$ (31,536)		$ 37,709						$ 139			$ (31,675)
Balances, Shares at Dec. 31, 2013			37,750,000			1,622,761
Shares issued during period, Amount		$ 93,970		$ 14,970	$ 37,890		$ 1			$ 93,969
Shares issued during period, Shares				9,999,999	8,973,905		5,750,000
Issuance of common stock from exercise of stock options, Amount	40								40
Issuance of common stock from exercise of stock options, Shares						87,475
Vesting of restricted stock, Amount	14								14
Vesting of restricted stock, Shares						138,108
Issuance of common stock in payment of consultant fees, Amount	127								127
Issuance of common stock in payment of consultant fees, Shares						15,872
Stock-based compensation expense	2,512								2,512
Accretion of redeemable convertible preferred stock to redemption value			$ 2,294
Accretion of redeemable convertible preferred stock to redemption value	(2,294)								(935)			(1,359)
Conversion of redeemable convertible preferred stock to common stock, Amount	92,863		$ (92,863)			$ 2			92,861
Conversion of redeemable convertible preferred stock to common stock, Shares			(56,723,904)			18,007,575
Net loss	(33,811)											(33,811)
Balances at Dec. 31, 2014	121,885					$ 3			188,727			(66,845)
Balances, Shares at Dec. 31, 2014						25,621,791
Shares issued during period, Amount		129,171								129,171
Shares issued during period, Shares							2,628,571
Issuance of common stock from exercise of stock options, Amount	603								603
Issuance of common stock from exercise of stock options, Shares						417,475
Vesting of restricted stock, Amount	17								17
Vesting of restricted stock, Shares						128,051
Issuance of common stock under employee stock purchase plan	127								127
Issuance of common stock under employee stock purchase plan, Shares						3,852
Issuance of common stock in payment of consultant fees, Amount	1,211								1,211
Issuance of common stock in payment of consultant fees, Shares						23,809
Stock-based compensation expense	15,176								15,176
Net loss	(94,495)											(94,495)
Balances at Dec. 31, 2015	173,695					$ 3			335,032			(161,340)
Balances, Shares at Dec. 31, 2015						28,823,549
Shares issued during period, Amount		$ 329,541					$ 1			$ 329,540
Shares issued during period, Shares							8,220,786
Issuance of common stock from exercise of stock options, Amount	1,011								1,011
Issuance of common stock from exercise of stock options, Shares						124,557
Vesting of restricted stock, Amount	11								11
Vesting of restricted stock, Shares						42,781
Issuance of common stock under employee stock purchase plan	510								510
Issuance of common stock under employee stock purchase plan, Shares						10,499
Purchase of treasury stock, Amount	(17)							$ (17)
Purchase of treasury stock, Shares								346
Stock-based compensation expense	22,855								22,855
Unrealized loss on available-for-sale securities	(102)										$ (102)
Net loss	(158,987)											(158,987)
Balances at Dec. 31, 2016	$ 368,517					$ 4		$ (17)	$ 688,959		$ (102)	$ (320,327)
Balances, Shares at Dec. 31, 2016						37,222,172		346