Summary of Significant Accounting Policies - Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
Assets fair value	$ 142,121	$ 161,541
Liabilities:
Liabilities fair value		154
Cash equivalents | Money Market Funds
Assets:
Assets fair value	9,785	13,761
Cash equivalents | Commercial Paper
Assets:
Assets fair value	12,248	19,629
Short-term Investments | Municipal Bonds
Assets:
Assets fair value	9,945	29,106
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	88,494	76,786
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	11,313	18,050
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	10,336	4,209
Level 1
Assets:
Assets fair value	9,785	13,761
Level 1 | Cash equivalents | Money Market Funds
Assets:
Assets fair value	9,785	13,761
Level 2
Assets:
Assets fair value	132,336	147,780
Liabilities:
Liabilities fair value		154
Level 2 | Cash equivalents | Commercial Paper
Assets:
Assets fair value	12,248	19,629
Level 2 | Short-term Investments | Municipal Bonds
Assets:
Assets fair value	9,945	29,106
Level 2 | Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	88,494	76,786
Level 2 | Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	11,313	18,050
Level 2 | Short-term Investments | Asset-backed securities
Assets:
Assets fair value	10,336	4,209
Interest rate swap contract
Liabilities:
Liabilities fair value		154	[1]
Interest rate swap contract | Level 2
Liabilities:
Liabilities fair value		$ 154	[1]

[1] As of December 31, 2013, the fair value of the interest rate swap contract was classified on our Balance Sheet in other current liabilities.