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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Dec. 31, 2012
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Cash and cash equivalents $ 264,008 $ 210,799 $ 247,666 $ 384,557
Short-term investments 222,954 281,538    
Assets and liabilities measured on a recurring basis | Level 1 | U.S. treasuries
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Cash and cash equivalents 25,000      
Short-term investments 90,558 130,977    
Assets and liabilities measured on a recurring basis | Level 2
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Derivative instrument   907    
Assets and liabilities measured on a recurring basis | Level 2 | Commercial paper
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Cash and cash equivalents 3,999      
Short-term investments   11,947    
Assets and liabilities measured on a recurring basis | Level 2 | Corporate debt
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Short-term investments 69,198 77,601    
Assets and liabilities measured on a recurring basis | Level 2 | Government agency securities
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Short-term investments 63,198 61,013    
Assets and liabilities measured on a recurring basis | Level 3
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Contingent consideration   (29,368)    
Assets and liabilities measured on a recurring basis | Fair Value
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Derivative instrument   907    
Contingent consideration   (29,368)    
Assets and liabilities measured on a recurring basis | Fair Value | U.S. treasuries
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Cash and cash equivalents 25,000      
Short-term investments 90,558 130,977    
Assets and liabilities measured on a recurring basis | Fair Value | Commercial paper
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Cash and cash equivalents 3,999      
Short-term investments   11,947    
Assets and liabilities measured on a recurring basis | Fair Value | Corporate debt
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Short-term investments 69,198 77,601    
Assets and liabilities measured on a recurring basis | Fair Value | Government agency securities
        
Major categories of assets and liabilities measured on a recurring basis, at fair value        
Short-term investments $ 63,198 $ 61,013