Common Stock - Shares of common stock reserved for issuance (Details) - shares	Sep. 30, 2017	Dec. 31, 2016
Options outstanding	7,896,734	9,835,992
Unvested early exercised stock options		2,471
Shares available for issuance under the plan	2,521,805	4,199,415
Shares available for purchase under Employee Stock Purchase Plan	24,650,151	25,088,827
Total	24,650,151	25,088,827
Unvested restricted stock units
Unvested restricted stock outstanding	13,241,111	10,474,975
2014 Employee Stock Purchase Plan | Employees Stock Purchase Plan
Shares available for purchase under Employee Stock Purchase Plan	990,501	575,974
Total	990,501	575,974