Net Loss Per Common Share - Schedule Of Anti-Dilutive Common Stock Excluded From Computation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	20,248	23,950
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	7,494	7,829
Common stock issuable under the 2018 Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	0	3,983
Common stock issuable under the 2020 Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	3,983	3,983
Common stock issuable under the 2024 Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	3,970	3,970
Unvested restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	4,108	3,544
Common stock potentially issuable for ESPP purchases
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	482	433
Common stock held by the NQDC
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	211	208