Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets:
Assets fair value	$ 161,280	$ 142,121
Level 1
Assets:
Assets fair value	35,730	9,785
Level 2
Assets:
Assets fair value	125,550	132,336
Cash equivalents | Money Market Funds
Assets:
Assets fair value	35,730	9,785
Cash equivalents | Commercial Paper
Assets:
Assets fair value	63,890	12,248
Cash equivalents | Level 1 | Money Market Funds
Assets:
Assets fair value	35,730	9,785
Cash equivalents | Level 2 | Commercial Paper
Assets:
Assets fair value	63,890	12,248
Short-term Investments | Municipal Bonds
Assets:
Assets fair value	2,763	9,945
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	31,253	88,494
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	16,201	11,313
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	11,443	10,336
Short-term Investments | Level 2 | Municipal Bonds
Assets:
Assets fair value	2,763	9,945
Short-term Investments | Level 2 | Corporate Debt Securities
Assets:
Assets fair value	31,253	88,494
Short-term Investments | Level 2 | U.S. Government Agency Bonds
Assets:
Assets fair value	16,201	11,313
Short-term Investments | Level 2 | Asset-backed securities
Assets:
Assets fair value	$ 11,443	$ 10,336