Summary of Significant Accounting Policies - Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets:
Assets fair value	$ 114,262	$ 142,121
Level 1
Assets:
Assets fair value	26,081	9,785
Level 2
Assets:
Assets fair value	88,181	132,336
Cash equivalents | Money Market Funds
Assets:
Assets fair value	26,081	9,785
Cash equivalents | Money Market Funds | Level 1
Assets:
Assets fair value	26,081	9,785
Cash equivalents | Commercial Paper
Assets:
Assets fair value	7,564	12,248
Cash equivalents | Commercial Paper | Level 2
Assets:
Assets fair value	7,564	12,248
Short-term Investments | Municipal Bonds
Assets:
Assets fair value	7,289	9,945
Short-term Investments | Level 2 | Municipal Bonds
Assets:
Assets fair value	7,289	9,945
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	50,100	88,494
Short-term Investments | Corporate Debt Securities | Level 2
Assets:
Assets fair value	50,100	88,494
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	16,091	11,313
Short-term Investments | U.S. Government Agency Bonds | Level 2
Assets:
Assets fair value	16,091	11,313
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	7,137	10,336
Short-term Investments | Asset-backed securities | Level 2
Assets:
Assets fair value	$ 7,137	$ 10,336