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Balance Sheet Information (Details) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Total available-for-sale securities          
Amortized Cost $ 322,479,000   $ 322,479,000   $ 192,158,000
Gains in Accumulated Other Comprehensive Income 99,000   99,000   76,000
Losses in Accumulated Other Comprehensive Income (90,000)   (90,000)    
Estimated Fair Value 322,488,000   322,488,000   192,234,000
Short-term investments , unrealized loss position fair value          
Less than 12 months, fair value 66,328,000   66,328,000    
Total, fair value 66,328,000   66,328,000    
Short-term investments, gross unrealized loss          
Less than 12 months, gross unrealized loss (90,000)   (90,000)    
Total, gross unrealized loss (90,000)   (90,000)    
Total proceeds from the sale of available-for-sale securities 150,500,000 76,800,000 422,903,000 176,303,000  
Gross realized gains on available-for-sale securities     100,000    
Estimated fair value of contractual maturities of available-for-sale debt securities          
Due in one year or less 230,968,000   230,968,000    
Due in 1-2 years 53,295,000   53,295,000    
Due in 2-3 years 38,225,000   38,225,000    
Total available-for-sale securities 322,488,000   322,488,000   192,234,000
United States treasuries
          
Total available-for-sale securities          
Amortized Cost 180,101,000   180,101,000   184,102,000
Gains in Accumulated Other Comprehensive Income 63,000   63,000   76,000
Losses in Accumulated Other Comprehensive Income (3,000)   (3,000)    
Estimated Fair Value 180,161,000   180,161,000   184,178,000
Short-term investments , unrealized loss position fair value          
Less than 12 months, fair value 8,882,000   8,882,000    
Total, fair value 8,882,000   8,882,000    
Short-term investments, gross unrealized loss          
Less than 12 months, gross unrealized loss (3,000)   (3,000)    
Total, gross unrealized loss (3,000)   (3,000)    
Government agency securities
          
Total available-for-sale securities          
Amortized Cost 55,657,000   55,657,000   8,056,000
Gains in Accumulated Other Comprehensive Income 7,000   7,000    
Losses in Accumulated Other Comprehensive Income (1,000)   (1,000)    
Estimated Fair Value 55,663,000   55,663,000   8,056,000
Short-term investments , unrealized loss position fair value          
Less than 12 months, fair value 7,999,000   7,999,000    
Total, fair value 7,999,000   7,999,000    
Short-term investments, gross unrealized loss          
Less than 12 months, gross unrealized loss (1,000)   (1,000)    
Total, gross unrealized loss (1,000)   (1,000)    
Corporate bonds
          
Total available-for-sale securities          
Amortized Cost 74,778,000   74,778,000    
Gains in Accumulated Other Comprehensive Income 29,000   29,000    
Losses in Accumulated Other Comprehensive Income (86,000)   (86,000)    
Estimated Fair Value 74,721,000   74,721,000    
Short-term investments , unrealized loss position fair value          
Less than 12 months, fair value 49,447,000   49,447,000    
Total, fair value 49,447,000   49,447,000    
Short-term investments, gross unrealized loss          
Less than 12 months, gross unrealized loss (86,000)   (86,000)    
Total, gross unrealized loss (86,000)   (86,000)    
Commercial paper
          
Total available-for-sale securities          
Amortized Cost 11,943,000   11,943,000    
Estimated Fair Value $ 11,943,000   $ 11,943,000