Stockholders' Equity - Schedule of Reserve for Authorized but Unissued Shares of Common Stock (Detail)	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for issuance	7,005,645
Common stock options and restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for issuance	3,885,220
Common stock employee purchase plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for issuance	3,120,425