Balance Sheet Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Total available-for-sale securities
Amortized Cost	$ 340,547		$ 340,547		$ 192,158
Gains in Accumulated Other Comprehensive Income	39		39		76
Losses in Accumulated Other Comprehensive Income	(254)		(254)
Estimated Fair Value	340,332		340,332		192,234
Short-term investments , unrealized loss position fair value
Less than 12 months, estimated fair value	119,219		119,219
Total, estimated fair value	119,219		119,219
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss	(254)		(254)
Total, gross unrealized loss	(254)		(254)
Total proceeds from the sale of available-for-sale securities	171,200	55,900	272,449	99,533
Estimated fair value of contractual maturities of available-for-sale debt securities
Due in one year or less	247,937		247,937
Due in 1-2 years	32,271		32,271
Due in 2-3 years	60,124		60,124
Total investments in debt securities	340,332		340,332		192,234
United States treasuries
Total available-for-sale securities
Amortized Cost	196,063		196,063		184,102
Gains in Accumulated Other Comprehensive Income	27		27		76
Losses in Accumulated Other Comprehensive Income	(28)		(28)
Estimated Fair Value	196,062		196,062		184,178
Short-term investments , unrealized loss position fair value
Less than 12 months, estimated fair value	44,881		44,881
Total, estimated fair value	44,881		44,881
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss	(28)		(28)
Total, gross unrealized loss	(28)		(28)
Corporate bonds
Total available-for-sale securities
Amortized Cost	75,837		75,837
Gains in Accumulated Other Comprehensive Income	4		4
Losses in Accumulated Other Comprehensive Income	(213)		(213)
Estimated Fair Value	75,628		75,628
Short-term investments , unrealized loss position fair value
Less than 12 months, estimated fair value	66,351		66,351
Total, estimated fair value	66,351		66,351
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss	(213)		(213)
Total, gross unrealized loss	(213)		(213)
Government agency securities
Total available-for-sale securities
Amortized Cost	64,647		64,647		8,056
Gains in Accumulated Other Comprehensive Income	8		8
Losses in Accumulated Other Comprehensive Income	(13)		(13)
Estimated Fair Value	64,642		64,642		8,056
Short-term investments , unrealized loss position fair value
Less than 12 months, estimated fair value	7,987		7,987
Total, estimated fair value	7,987		7,987
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss	(13)		(13)
Total, gross unrealized loss	(13)		(13)
Commercial paper
Total available-for-sale securities
Amortized Cost	4,000		4,000
Estimated Fair Value	$ 4,000		$ 4,000