Common Stock - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Mar. 31, 2019	Dec. 31, 2018	Jul. 31, 2018
Stockholders Deficit [Line Items]
Shares of common stock issued	18,227,000	17,744,000
Shares of common stock reserved for issuance for:
Total shares of common stock issued and reserved for issuance	22,746,000	22,159,000
Common Stock Warrants Outstanding
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	105,000	105,000
Stock Options Outstanding
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	2,670,000	2,711,000
Restricted Stock Units Outstanding
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	43,000	43,000
Shares Available for Grant Under Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	1,457,000	1,312,000
Shares Available for Sales Under Employee Stock Purchase Plan
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	244,000	244,000	200,000