Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Assets fair value	$ 16,958	$ 175,128
Cash equivalents | Money Market Funds
Assets:
Assets fair value	14,828	3,544
Cash equivalents | Commercial Paper
Assets:
Assets fair value		32,467
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value		124,182
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	1,542	1,547
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	588	13,388
Level 1
Assets:
Assets fair value	14,828	3,544
Level 1 | Cash equivalents | Money Market Funds
Assets:
Assets fair value	14,828	3,544
Level 2
Assets:
Assets fair value	2,130	171,584
Level 2 | Cash equivalents | Commercial Paper
Assets:
Assets fair value		32,467
Level 2 | Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value		124,182
Level 2 | Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	1,542	1,547
Level 2 | Short-term Investments | Asset-backed securities
Assets:
Assets fair value	$ 588	$ 13,388