Available-for-sale Securities (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
summary of available-for-sale debt securities recorded in cash and cash equivalents, marketable securities, or restricted cash    
Amortized Cost $ 98,391 $ 61,112
Unrealized Gains 6 29
Unrealized Losses      
Estimated Fair Value 98,397 61,141
Certificates of deposit [Member]
    
summary of available-for-sale debt securities recorded in cash and cash equivalents, marketable securities, or restricted cash    
Amortized Cost 200 200
Unrealized Gains      
Unrealized Losses      
Estimated Fair Value 200 200
Money market funds [Member]
    
summary of available-for-sale debt securities recorded in cash and cash equivalents, marketable securities, or restricted cash    
Amortized Cost 27,517 9,238
Unrealized Gains      
Unrealized Losses      
Estimated Fair Value 27,517 9,238
U.S. Treasury securities [Member]
    
summary of available-for-sale debt securities recorded in cash and cash equivalents, marketable securities, or restricted cash    
Amortized Cost 70,674 51,674
Unrealized Gains 6 29
Unrealized Losses      
Estimated Fair Value $ 70,680 $ 51,703