Net Loss per Share - Anti-dilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	14,739,000	14,016,000	13,242,000	13,167,000
Share issuances under equity incentive plans and ESPP
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	8,063,000	7,340,000	6,566,000	6,491,000
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	414,000