Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 source	Dec. 31, 2011
Financial assets subject to fair value measurements and the necessary disclosures
Number of pricing sources	2
Total available-for-sale securities	$ 183,915	$ 264,667
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	126,570	127,559
Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	57,345	137,108
Total
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	183,915	264,667
Total | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	126,570	127,559
Total | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	57,345	137,108
Level 1
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	126,570	127,559
Level 1 | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	126,570	127,559
Level 1 | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Level 2
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	57,345	137,108
Level 2 | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Level 2 | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	57,345	137,108
Level 3
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Level 3 | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Level 3 | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Nonrecurring basis
Investment in non-marketable security
Carrying Value	2,000	2,000
Nonrecurring basis | Level 3
Investment in non-marketable security
Fair Value	$ 2,000	$ 2,000