Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Assets fair value	$ 22,695	$ 37,073
Level 1
Assets:
Assets fair value	8,564	4,392
Level 2
Assets:
Assets fair value	14,131	32,681
Cash equivalents | Money Market Funds
Assets:
Assets fair value	8,564	4,392
Cash equivalents | Commercial Paper
Assets:
Assets fair value	2,794	9,078
Cash equivalents | Level 1 | Money Market Funds
Assets:
Assets fair value	8,564	4,392
Cash equivalents | Level 2 | Commercial Paper
Assets:
Assets fair value	2,794	9,078
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	5,018	16,357
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	2,904	3,724
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	3,415	3,522
Short-term Investments | Level 2 | Corporate Debt Securities
Assets:
Assets fair value	5,018	16,357
Short-term Investments | Level 2 | U.S. Government Agency Bonds
Assets:
Assets fair value	2,904	3,724
Short-term Investments | Level 2 | Asset-backed securities
Assets:
Assets fair value	$ 3,415	$ 3,522