Net Loss per Share - Anti-dilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	14,016,000	10,463,000	13,167,000	11,421,000
Share issuances under equity incentive plans and ESPP
Anti-Dilutive Securities
Anti-dilutive securities (in shares)	7,340,000	3,783,000	6,491,000	4,741,000
Restricted shares
Anti-Dilutive Securities
Anti-dilutive securities (in shares)		4,000		4,000
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)			414,000	978,750