Cash, Cash Equivalents and Short-term Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Cash equivalents:
Total cash and cash equivalents, Amortized Cost	$ 110,277	$ 65,983	$ 58,293	$ 22,652
Total cash and cash equivalents, Unrealized Gains
Total cash and cash equivalents, Unrealized Losses
Total cash and cash equivalents, Estimated Fair Value	110,277	65,983
Short-term investments:
Cash, cash equivalents and short-term investments, Amortized Cost		108,856
Cash, cash equivalents and short-term investments, Unrealized Gains		34
Cash, cash equivalents and short-term investments, Unrealized Losses		(25)
Cash, cash equivalents and short-term investments, Estimated Fair Value		108,865
Cash [Member]
Cash equivalents:
Total cash and cash equivalents, Amortized Cost	59,845	59,033
Total cash and cash equivalents, Unrealized Gains
Total cash and cash equivalents, Unrealized Losses
Total cash and cash equivalents, Estimated Fair Value	59,845	59,033
Total cash equivalents [Member]
Cash equivalents:
Total cash and cash equivalents, Amortized Cost		6,950
Total cash and cash equivalents, Estimated Fair Value		6,950
Total short-term investments [Member]
Short-term investments:
Total short-term investments, Amortized Cost		42,873
Total short-term investments, Unrealized Gains		34
Total short-term investments, Unrealized Losses		(25)
Total short-term investments, Estimated Fair Value		42,882
Commercial paper [Member]
Cash equivalents:
Total cash and cash equivalents, Amortized Cost		2,749
Money market mutual funds [Member]
Cash equivalents:
Total cash and cash equivalents, Amortized Cost	50,432	4,201
Total cash and cash equivalents, Unrealized Gains
Total cash and cash equivalents, Unrealized Losses
Total cash and cash equivalents, Estimated Fair Value	50,432	4,201
Commercial paper [Member]
Cash equivalents:
Total cash and cash equivalents, Estimated Fair Value		2,749
Short-term investments:
Total short-term investments, Amortized Cost		3,645
Total short-term investments, Unrealized Gains		1
Total short-term investments, Unrealized Losses		(1)
Total short-term investments, Estimated Fair Value		3,645
Certificate of deposit [Member]
Short-term investments:
Total short-term investments, Amortized Cost		750
Total short-term investments, Unrealized Losses		(4)
Total short-term investments, Estimated Fair Value		746
Municipal securities [Member]
Short-term investments:
Total short-term investments, Amortized Cost		33,568
Total short-term investments, Unrealized Gains		30
Total short-term investments, Unrealized Losses		(6)
Total short-term investments, Estimated Fair Value		33,592
Corporate bonds [Member]
Short-term investments:
Total short-term investments, Amortized Cost		4,910
Total short-term investments, Unrealized Gains		3
Total short-term investments, Unrealized Losses		(14)
Total short-term investments, Estimated Fair Value		$ 4,899