Net loss per share - Potentially Dilutive Securities Excluded From Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share
Potentially dilutive securities, excluded from computation of diluted net income (loss) per share (in shares)	6,655	6,728
Employee stock options
Antidilutive Securities Excluded From Computation Of Earnings Per Share
Potentially dilutive securities, excluded from computation of diluted net income (loss) per share (in shares)	3,145	3,940
Restricted stock units
Antidilutive Securities Excluded From Computation Of Earnings Per Share
Potentially dilutive securities, excluded from computation of diluted net income (loss) per share (in shares)	2,419	2,377
Market-based restricted stock units
Antidilutive Securities Excluded From Computation Of Earnings Per Share
Potentially dilutive securities, excluded from computation of diluted net income (loss) per share (in shares)	677	0
Employee stock purchase plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share
Potentially dilutive securities, excluded from computation of diluted net income (loss) per share (in shares)	414	411