Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2014
Minimum
Mar. 31, 2014
Maximum
Mar. 31, 2014
Cash and Cash Equivalents
Dec. 31, 2013
Cash and Cash Equivalents
Mar. 31, 2014
Short-term investments
Mar. 31, 2014
Long-term investments
Mar. 31, 2014
Cash
Cash and Cash Equivalents
Dec. 31, 2013
Cash
Cash and Cash Equivalents
Mar. 31, 2014
Money market funds
Cash and Cash Equivalents
Dec. 31, 2013
Money market funds
Cash and Cash Equivalents
Mar. 31, 2014
Available-for-sale investments
Mar. 31, 2014
Available-for-sale investments
Cash and Cash Equivalents
Mar. 31, 2014
Available-for-sale investments
Short-term investments
Mar. 31, 2014
Available-for-sale investments
Long-term investments
Mar. 31, 2014
Asset-backed securities
Mar. 31, 2014
Asset-backed securities
Long-term investments
Mar. 31, 2014
Corporate bonds
Mar. 31, 2014
Corporate bonds
Short-term investments
Mar. 31, 2014
Corporate bonds
Long-term investments
Mar. 31, 2014
Commercial paper
Mar. 31, 2014
Commercial paper
Cash and Cash Equivalents
Mar. 31, 2014
Commercial paper
Short-term investments
Mar. 31, 2014
Measured on a recurring basis
Mar. 31, 2014
Measured on a recurring basis
Level 1
Dec. 31, 2013
Measured on a recurring basis
Level 1
Mar. 31, 2014
Measured on a recurring basis
Level 1
Money market funds
Dec. 31, 2013
Measured on a recurring basis
Level 1
Money market funds
Mar. 31, 2014
Measured on a recurring basis
Level 2
Mar. 31, 2014
Measured on a recurring basis
Level 2
Asset-backed securities
Mar. 31, 2014
Measured on a recurring basis
Level 2
Corporate bonds
Mar. 31, 2014
Measured on a recurring basis
Level 2
Commercial paper
Dec. 31, 2013
Carrying amount
Mar. 31, 2014
Carrying amount
Cash
Dec. 31, 2013
Carrying amount
Cash
Mar. 31, 2014
Carrying amount
Money market funds
Dec. 31, 2013
Carrying amount
Money market funds
Mar. 31, 2014
Fair value
Cash
Fair Value Measurements                                                                            
Cash and Cash Equivalents               $ 8,576 $ 5,533 $ 15,510 $ 79,013                               $ 15,510 $ 79,013         $ 84,546 $ 8,576 $ 5,533 $ 15,510 $ 79,013 $ 8,576
Adjusted Cost                       61,531       4,466   37,085     19,980                                  
Unrealized Gains                       4           3     1                                  
Unrealized Losses                       (37)       (1)   (34)     (2)                                  
Fair Value                         2,799 36,592 22,107   4,465   19,412 17,642   2,799 17,180           61,498 4,465 37,054 19,979            
Total Cash and Cash Equivalents and Investments, Adjusted Cost 85,617                                                                          
Total assets, fair value       $ 26,885 $ 84,546 $ 36,592 $ 22,107                                 $ 85,584 $ 15,510 $ 79,013                        
Maturity period of long-term investments   1 year 2 years