Fair Value Measurements - Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Warrant liabilities	$ 302		$ 397		$ 302	$ 397
Change in the estimated fair value of warrant liabilities
Balance at December 31, 2017			397
Increase in fair value (net of expired warrants)	(420)	$ 1,060	2,524	$ 512
Warrant exercise			0	$ (38,389)
Balance at September 30, 2018	302		302
Silicon Valley Bank | Debt Financing Agreement and Term Loan
Change in the estimated fair value of warrant liabilities
Debt instrument, fair value					17,300	17,300
Recurring
Assets:
Assets, fair value					75,162	0
Recurring | Level 1
Assets:
Assets, fair value					18,684	0
Recurring | Level 2
Assets:
Assets, fair value					56,478	0
Recurring | Level 3
Assets:
Assets, fair value					0	0
Warrants
Liabilities:
Warrant liabilities	0		1,014		0	1,014
Change in the estimated fair value of warrant liabilities
Balance at December 31, 2017			1,014
Increase in fair value (net of expired warrants)			2,524
Warrant exercise			(3,538)
Balance at September 30, 2018	0		0
Warrants | Recurring
Liabilities:
Warrant liabilities	0		1,014		0	1,014
Change in the estimated fair value of warrant liabilities
Balance at December 31, 2017			1,014
Balance at September 30, 2018	0		0
Warrants | Recurring | Level 1
Liabilities:
Warrant liabilities	0		0		0	0
Change in the estimated fair value of warrant liabilities
Balance at December 31, 2017			0
Balance at September 30, 2018	0		0
Warrants | Recurring | Level 2
Liabilities:
Warrant liabilities	0		1,014		0	1,014
Change in the estimated fair value of warrant liabilities
Balance at December 31, 2017			1,014
Balance at September 30, 2018	0		0
Warrants | Recurring | Level 3
Liabilities:
Warrant liabilities	0		0		0	0
Change in the estimated fair value of warrant liabilities
Balance at December 31, 2017			0
Balance at September 30, 2018	$ 0		$ 0
Money market funds | Recurring
Assets:
Assets, fair value					18,684	0
Money market funds | Recurring | Level 1
Assets:
Assets, fair value					18,684	0
Money market funds | Recurring | Level 2
Assets:
Assets, fair value					0	0
Money market funds | Recurring | Level 3
Assets:
Assets, fair value					0	0
Repurchase agreements | Recurring
Assets:
Assets, fair value					10,002	0
Repurchase agreements | Recurring | Level 1
Assets:
Assets, fair value					0	0
Repurchase agreements | Recurring | Level 2
Assets:
Assets, fair value					10,002	0
Repurchase agreements | Recurring | Level 3
Assets:
Assets, fair value					0	0
Commercial paper | Recurring
Assets:
Assets, fair value					21,832	0
Commercial paper | Recurring | Level 1
Assets:
Assets, fair value					0	0
Commercial paper | Recurring | Level 2
Assets:
Assets, fair value					21,832	0
Commercial paper | Recurring | Level 3
Assets:
Assets, fair value					0	0
Corporate notes | Recurring
Assets:
Assets, fair value					13,047	0
Corporate notes | Recurring | Level 1
Assets:
Assets, fair value					0	0
Corporate notes | Recurring | Level 2
Assets:
Assets, fair value					13,047	0
Corporate notes | Recurring | Level 3
Assets:
Assets, fair value					0	0
U.S. government securities | Recurring
Assets:
Assets, fair value					3,173	0
U.S. government securities | Recurring | Level 1
Assets:
Assets, fair value					0	0
U.S. government securities | Recurring | Level 2
Assets:
Assets, fair value					3,173	0
U.S. government securities | Recurring | Level 3
Assets:
Assets, fair value					0	0
U.S. asset-backed securities | Recurring
Assets:
Assets, fair value					8,424	0
U.S. asset-backed securities | Recurring | Level 1
Assets:
Assets, fair value					0	0
U.S. asset-backed securities | Recurring | Level 2
Assets:
Assets, fair value					8,424	0
U.S. asset-backed securities | Recurring | Level 3
Assets:
Assets, fair value					$ 0	$ 0