Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Assets:
Total	$ 155,325		$ 94,549
Liabilities:
Total	377		1,069
Interest rate swap contracts
Liabilities:
Total	377	[1]	1,069	[1]
Cash equivalents | Money market funds
Assets:
Total	33,365		23,119
Cash equivalents | Commercial paper
Assets:
Total	20,798		35,856
Short-term Investments | Municipal bonds
Assets:
Total	29,353
Short-term Investments | Commercial paper
Assets:
Total	48,574		34,826
Short-term Investments | U.S. government agency bonds
Assets:
Total	23,235		748
Level 1
Assets:
Total	33,365		23,119
Liabilities:
Total
Level 1 | Interest rate swap contracts
Liabilities:
Total		[1]
Level 1 | Cash equivalents | Money market funds
Assets:
Total	33,365		23,119
Level 1 | Cash equivalents | Commercial paper
Assets:
Total
Level 1 | Short-term Investments | Municipal bonds
Assets:
Total
Level 1 | Short-term Investments | Commercial paper
Assets:
Total
Level 1 | Short-term Investments | U.S. government agency bonds
Assets:
Total
Level 2
Assets:
Total	121,960		71,430
Liabilities:
Total	377		1,069
Level 2 | Interest rate swap contracts
Liabilities:
Total	377	[1]	1,069	[1]
Level 2 | Cash equivalents | Money market funds
Assets:
Total
Level 2 | Cash equivalents | Commercial paper
Assets:
Total	20,798		35,856
Level 2 | Short-term Investments | Municipal bonds
Assets:
Total	29,353
Level 2 | Short-term Investments | Commercial paper
Assets:
Total	48,574		34,826
Level 2 | Short-term Investments | U.S. government agency bonds
Assets:
Total	$ 23,235		$ 748

[1] As of September 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”.