Statements of Operations and Comprehensive Loss - USD ($) $ 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
Income Statement [Abstract]
Revenue									$ 0	$ 0	$ 0
Operating expenses:
Research and development	$ 4,311	$ 3,825	$ 3,161	$ 2,942	$ 3,092	$ 2,605	$ 2,086	$ 2,493	14,239	10,276	7,832
General and administrative	2,730	2,601	1,791	1,647	1,356	1,627	3,464	1,446	8,769	7,893	5,059
Total operating expenses	7,041	6,426	4,952	4,589	4,448	4,232	5,550	3,939	23,008	18,169	12,891
Loss from operations	(7,041)	(6,426)	(4,952)	(4,589)	(4,448)	(4,232)	(5,550)	(3,939)	(23,008)	(18,169)	(12,891)
Interest income	176	167	29	32	6	10	16	14	404	46	13
Net loss	(6,865)	(6,259)	(4,923)	(4,557)	(4,442)	(4,222)	(5,534)	(3,925)	(22,604)	(18,123)	(12,878)
Accretion of redeemable convertible preferred stock to redemption value			(21)	(20)	(19)	(19)	(19)	(18)	(41)	(75)	(71)
Net loss attributable to common stockholders	$ (6,865)	$ (6,259)	$ (4,944)	$ (4,577)	$ (4,461)	$ (4,241)	$ (5,553)	$ (3,943)	$ (22,645)	$ (18,198)	$ (12,949)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.47)	$ (0.45)	$ (10.98)	$ (10.58)	$ (10.31)	$ (9.84)	$ (12.95)	$ (9.25)	$ (3.04)	$ (42.35)	$ (32.31)
Weighted average common shares outstanding—basic and diluted	14,720,734	13,939,950	450,495	432,728	432,413	430,929	428,870	426,487	7,443,078	429,686	400,713
Comprehensive loss:
Net loss	$ (6,865)	$ (6,259)	$ (4,923)	$ (4,557)	$ (4,442)	$ (4,222)	$ (5,534)	$ (3,925)	$ (22,604)	$ (18,123)	$ (12,878)
Other comprehensive loss:
Unrealized loss on investments, net of tax of $0	(27)	(6)			1	(3)	(5)	7	(33)		(4)
Total other comprehensive loss	(27)	(6)			1	(3)	(5)	7	(33)		(4)
Total comprehensive loss	$ (6,892)	$ (6,265)	$ (4,923)	$ (4,557)	$ (4,441)	$ (4,225)	$ (5,539)	$ (3,918)	$ (22,637)	$ (18,123)	$ (12,882)