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Cash and Cash Equivalents and Investments (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Cash and Cash Equivalents
    
Fair Value    
Amortized Cost $ 23,254 $ 30,423
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 23,254 30,423
Certificates of Deposit
    
Fair Value    
Amortized Cost 551 551
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 551 551
U.S. Treasury Securities
    
Fair Value    
Amortized Cost 151,542 192,211
Gross Unrealized Gains 31 24
Gross Unrealized Losses 0 (1)
Estimated Fair Value 151,573 192,234
Total Short-term Investments
    
Fair Value    
Amortized Cost 152,093 192,762
Gross Unrealized Gains 31 24
Gross Unrealized Losses 0 (1)
Estimated Fair Value 152,124 192,785
Total Cash and Cash Equivalents and Investments
    
Fair Value    
Amortized Cost 175,347 223,185
Gross Unrealized Gains 31 24
Gross Unrealized Losses 0 (1)
Estimated Fair Value $ 175,378 $ 223,208