Stockholders' Equity - Shares of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Options outstanding (shares)	20,409,340
Total reserved for future issuance (shares)	105,511,561	95,940,598
Stock options and RSUs
Class of Stock [Line Items]
Options outstanding (shares)	47,538,097	62,082,821
Total reserved for future issuance (shares)	49,277,465	28,449,336
Available for ESPP
Class of Stock [Line Items]
Total reserved for future issuance (shares)	8,695,999	5,408,441	2,589,991