Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets:
Assets fair value	$ 126,297	$ 142,121
Level 1
Assets:
Assets fair value	4,894	9,785
Level 2
Assets:
Assets fair value	121,403	132,336
Cash equivalents | Money Market Funds
Assets:
Assets fair value	4,894	9,785
Cash equivalents | Money Market Funds | Level 1
Assets:
Assets fair value	4,894	9,785
Cash equivalents | Commercial Paper
Assets:
Assets fair value	21,798	12,248
Cash equivalents | Commercial Paper | Level 2
Assets:
Assets fair value	21,798	12,248
Short-term Investments | Municipal Bonds
Assets:
Assets fair value	1,352	9,945
Short-term Investments | Level 2 | Municipal Bonds
Assets:
Assets fair value	1,352	9,945
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	64,086	88,494
Short-term Investments | Corporate Debt Securities | Level 2
Assets:
Assets fair value	64,086	88,494
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	26,462	11,313
Short-term Investments | U.S. Government Agency Bonds | Level 2
Assets:
Assets fair value	26,462	11,313
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	7,705	10,336
Short-term Investments | Asset-backed securities | Level 2
Assets:
Assets fair value	$ 7,705	$ 10,336