Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets:
Assets fair value	$ 124,192	$ 142,121
Level 1
Assets:
Assets fair value	10,146	9,785
Level 2
Assets:
Assets fair value	114,046	132,336
Cash equivalents | Money Market Funds
Assets:
Assets fair value	10,146	9,785
Cash equivalents | Money Market Funds | Level 1
Assets:
Assets fair value	10,146	9,785
Cash equivalents | Commercial Paper
Assets:
Assets fair value	14,948	12,248
Cash equivalents | Commercial Paper | Level 2
Assets:
Assets fair value	14,948	12,248
Short-term Investments | Municipal Bonds
Assets:
Assets fair value	1,353	9,945
Short-term Investments | Level 2 | Municipal Bonds
Assets:
Assets fair value	1,353	9,945
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	69,620	88,494
Short-term Investments | Corporate Debt Securities | Level 2
Assets:
Assets fair value	69,620	88,494
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	17,662	11,313
Short-term Investments | U.S. Government Agency Bonds | Level 2
Assets:
Assets fair value	17,662	11,313
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	10,463	10,336
Short-term Investments | Asset-backed securities | Level 2
Assets:
Assets fair value	$ 10,463	$ 10,336