Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Assets:
Total	$ 139,218		$ 94,549
Liabilities:
Total	628		1,069
Interest rate swap contracts
Liabilities:
Total	628	[1]	1,069	[1]
Level 1
Assets:
Total	32,378		23,119
Level 2
Assets:
Total	106,840		71,430
Liabilities:
Total	628		1,069
Level 2 | Interest rate swap contracts
Liabilities:
Total	628	[1]	1,069	[1]
Cash equivalents | Money market funds
Assets:
Total	32,378		23,119
Cash equivalents | Commercial paper
Assets:
Total	14,395		35,856
Cash equivalents | Level 1 | Money market funds
Assets:
Total	32,378		23,119
Cash equivalents | Level 2 | Commercial paper
Assets:
Total	14,395		35,856
Short-term Investments | Municipal bonds
Assets:
Total	29,560
Short-term Investments | Money market funds
Assets:
Total	2,351
Short-term Investments | Commercial paper
Assets:
Total	44,290		34,826
Short-term Investments | U.S. government agency bonds
Assets:
Total	16,244		748
Short-term Investments | Level 2 | Municipal bonds
Assets:
Total	29,560
Short-term Investments | Level 2 | Money market funds
Assets:
Total	2,351
Short-term Investments | Level 2 | Commercial paper
Assets:
Total	44,290		34,826
Short-term Investments | Level 2 | U.S. government agency bonds
Assets:
Total	$ 16,244		$ 748

[1] As of June 30, 2013, the fair value of the interest rate swap contract was classified on our Balance Sheet in “Other current liabilities”. As of December 31, 2012, the fair value of the interest rate swap contracts were classified on our Balance Sheet in “Other non-current liabilities”.