Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Assets fair value	$ 181,504	$ 161,280
Cash equivalents | Money Market Funds
Assets:
Assets fair value	6,531	35,730
Cash equivalents | Commercial Paper
Assets:
Assets fair value	24,826	17,245
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	109,140	77,898
Short-term Investments | Municipal Bonds
Assets:
Assets fair value		2,763
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	26,513	16,201
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	14,494	11,443
Level 1
Assets:
Assets fair value	6,531	35,730
Level 1 | Cash equivalents | Money Market Funds
Assets:
Assets fair value	6,531	35,730
Level 2
Assets:
Assets fair value	174,973	125,550
Level 2 | Cash equivalents | Commercial Paper
Assets:
Assets fair value	24,826	17,245
Level 2 | Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	109,140	77,898
Level 2 | Short-term Investments | Municipal Bonds
Assets:
Assets fair value		2,763
Level 2 | Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	26,513	16,201
Level 2 | Short-term Investments | Asset-backed securities
Assets:
Assets fair value	$ 14,494	$ 11,443