Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets:
Assets fair value	$ 194,644	$ 161,280
Level 1
Assets:
Assets fair value	43,029	35,730
Level 2
Assets:
Assets fair value	151,615	125,550
Cash equivalents | Money Market Funds
Assets:
Assets fair value	43,029	35,730
Cash equivalents | Commercial Paper
Assets:
Assets fair value	58,481	63,890
Cash equivalents | Level 1 | Money Market Funds
Assets:
Assets fair value	43,029	35,730
Cash equivalents | Level 2 | Commercial Paper
Assets:
Assets fair value	58,481	63,890
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	39,435	31,253
Short-term Investments | Corporate Equity Securities
Assets:
Assets fair value	2,769
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	37,273	16,201
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	12,244	11,443
Short-term Investments | Level 2 | Corporate Debt Securities
Assets:
Assets fair value	39,435	31,253
Short-term Investments | Level 2 | Corporate Equity Securities
Assets:
Assets fair value	2,769
Short-term Investments | Level 2 | U.S. Government Agency Bonds
Assets:
Assets fair value	37,273	16,201
Short-term Investments | Level 2 | Asset-backed securities
Assets:
Assets fair value	12,244	11,443
Short-term investments, Municipal Bonds
Assets:
Assets fair value	1,413	2,763
Short-term investments, Municipal Bonds | Level 2
Assets:
Assets fair value	$ 1,413	$ 2,763