Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Assets fair value	$ 137,724	$ 181,504
Cash equivalents | Money Market Funds
Assets:
Assets fair value	3,790	6,531
Cash equivalents | Commercial Paper
Assets:
Assets fair value	18,138	24,826
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	92,749	109,140
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	8,905	26,513
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	14,142	14,494
Level 1
Assets:
Assets fair value	3,790	6,531
Level 1 | Cash equivalents | Money Market Funds
Assets:
Assets fair value	3,790	6,531
Level 2
Assets:
Assets fair value	133,934	174,973
Level 2 | Cash equivalents | Commercial Paper
Assets:
Assets fair value	18,138	24,826
Level 2 | Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	92,749	109,140
Level 2 | Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	8,905	26,513
Level 2 | Short-term Investments | Asset-backed securities
Assets:
Assets fair value	$ 14,142	$ 14,494