Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Assets fair value	$ 137,677	$ 161,541
Liabilities:
Liabilities fair value		154
Interest rate swap contract
Liabilities:
Liabilities fair value		154	[1]
Level 1
Assets:
Assets fair value	8,740	13,761
Level 2
Assets:
Assets fair value	128,937	147,780
Liabilities:
Liabilities fair value		154
Level 2 | Interest rate swap contract
Liabilities:
Liabilities fair value		154	[1]
Cash equivalents | Money Market Funds
Assets:
Assets fair value	8,740	13,761
Cash equivalents | Money Market Funds | Level 1
Assets:
Assets fair value	8,740	13,761
Cash equivalents | Commercial Paper
Assets:
Assets fair value	11,379	19,629
Cash equivalents | Commercial Paper | Level 2
Assets:
Assets fair value	11,379	19,629
Short-term Investments | Municipal Bonds
Assets:
Assets fair value	27,564	29,106
Short-term Investments | Corporate Debt Securities
Assets:
Assets fair value	66,915	76,786
Short-term Investments | Asset-backed securities
Assets:
Assets fair value	9,040	4,209
Short-term Investments | Level 2 | Municipal Bonds
Assets:
Assets fair value	27,564	29,106
Short-term Investments | Level 2 | Corporate Debt Securities
Assets:
Assets fair value	66,915	76,786
Short-term Investments | Level 2 | Asset-backed securities
Assets:
Assets fair value	9,040	4,209
Short-term Investments | U.S. Government Agency Bonds
Assets:
Assets fair value	14,039	18,050
Short-term Investments | U.S. Government Agency Bonds | Level 2
Assets:
Assets fair value	$ 14,039	$ 18,050

[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.