Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Assets fair value	$ 22,363	$ 37,073
Cash equivalents | Money Market Funds
Assets:
Assets fair value	4,030	4,392
Cash equivalents | Commercial Paper
Assets:
Assets fair value		9,078
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	12,456	16,357
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	1,549	3,724
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	4,328	3,522
Level 1
Assets:
Assets fair value	4,030	4,392
Level 1 | Cash equivalents | Money Market Funds
Assets:
Assets fair value	4,030	4,392
Level 2
Assets:
Assets fair value	18,333	32,681
Level 2 | Cash equivalents | Commercial Paper
Assets:
Assets fair value		9,078
Level 2 | Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	12,456	16,357
Level 2 | Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	1,549	3,724
Level 2 | Short-term Investments | Asset-backed securities
Assets:
Assets fair value	$ 4,328	$ 3,522