Net Loss per Share - Anti-dilutive Securities (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	12,389,000	14,144,000	12,417,000	12,903,000
Share issuances under equity incentive plans and ESPP
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	5,713,000	7,468,000	5,741,000	6,227,000
Share issuances upon the conversion of convertible senior notes
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	6,676,000	6,676,000	6,676,000	6,676,000
Performance-based vesting
Anti-Dilutive Securities
Anti-dilutive securities (in shares)			0	468,000