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	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	13,394	23,479	20,028	23,479
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	5,829	7,517	7,388	7,517
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	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	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)	0	3,970	3,970	3,970
Unvested RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of potentially issuable shares (in shares)	3,360	3,411	4,029	3,411
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)	222	407	453	407
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)	0	208	205	208