Stockholders' Equity - Outstanding and Reserved Shares of Common Stock for Future Issuance (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Options outstanding (in shares)	1,810,580	3,369,732	5,818,435
Total reserved shares of common stock for future issuance (in shares)	54,725,897
2012 Equity Incentive Plan
Class of Stock [Line Items]
Total reserved shares of common stock for future issuance (in shares)	31,999,234
2012 Employee Stock Purchase Plan
Class of Stock [Line Items]
Total reserved shares of common stock for future issuance (in shares)	10,714,423
Stock options
Class of Stock [Line Items]
Options outstanding (in shares)	1,810,580
Restricted stock units
Class of Stock [Line Items]
RSUs (in shares)	10,201,660	11,403,341	12,222,282
Performance target (in percent)	100.00%