Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets:
Assets fair value	$ 168,662	$ 161,280
Level 1
Assets:
Assets fair value	22,253	35,730
Level 2
Assets:
Assets fair value	146,409	125,550
Cash equivalents | Money Market Funds
Assets:
Assets fair value	22,253	35,730
Cash equivalents | Commercial Paper
Assets:
Assets fair value	20,492	17,245
Cash equivalents | Level 1 | Money Market Funds
Assets:
Assets fair value	22,253	35,730
Cash equivalents | Level 2 | Commercial Paper
Assets:
Assets fair value	20,492	17,245
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	83,978	77,898
Short-term Investments | Corporate Equity Securities
Assets:
Assets fair value	2,476
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	26,540	16,201
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	11,513	11,443
Short-term Investments | Level 2 | Corporate Debt Securities
Assets:
Assets fair value	83,978	77,898
Short-term Investments | Level 2 | Corporate Equity Securities
Assets:
Assets fair value	2,476
Short-term Investments | Level 2 | U.S. Government Agency Bonds
Assets:
Assets fair value	26,540	16,201
Short-term Investments | Level 2 | Asset-backed securities
Assets:
Assets fair value	11,513	11,443
Short-term investments, Municipal Bonds
Assets:
Assets fair value	1,410	2,763
Short-term investments, Municipal Bonds | Level 2
Assets:
Assets fair value	$ 1,410	$ 2,763