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Fair Value Measurement of Assets and Liabilities (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
    
Cash equivalents    
Total cash equivalents $ 5,929 $ 30,921
Available-for-sale:    
Total long-term marketable securities 10,820 13,495
Total short-term marketable securities 36,391 28,267
Total financial assets 53,140 72,683
Fair Value, Measurements, Recurring [Member] | Government agency securities [Member]
    
Available-for-sale:    
Total long-term marketable securities 3,738 3,738
Total short-term marketable securities 2,180 6,912
Fair Value, Measurements, Recurring [Member] | Municipal bonds [Member]
    
Available-for-sale:    
Total long-term marketable securities 4,716 5,969
Total short-term marketable securities 11,732 8,384
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
    
Available-for-sale:    
Total long-term marketable securities 2,366 3,788
Total short-term marketable securities 5,041 4,321
Fair Value, Measurements, Recurring [Member] | Corporate debentures / bonds [Member]
    
Available-for-sale:    
Total short-term marketable securities 17,438 8,612
Fair Value, Measurements, Recurring [Member] | Marketable equity securities [Member]
    
Available-for-sale:    
Total short-term marketable securities   38
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
    
Cash equivalents    
Total cash equivalents 5,645 25,137
Fair Value, Measurements, Recurring [Member] | US Treasury securities [Member]
    
Cash equivalents    
Total cash equivalents 284 5,784
Level 1 [Member]
    
Cash equivalents    
Total cash equivalents 5,645 25,137
Available-for-sale:    
Total long-term marketable securities      
Total short-term marketable securities    38
Total financial assets 5,645 25,175
Level 1 [Member] | Government agency securities [Member]
    
Available-for-sale:    
Total long-term marketable securities      
Total short-term marketable securities      
Level 1 [Member] | Municipal bonds [Member]
    
Available-for-sale:    
Total long-term marketable securities      
Total short-term marketable securities      
Level 1 [Member] | Certificates of deposit [Member]
    
Available-for-sale:    
Total long-term marketable securities      
Total short-term marketable securities      
Level 1 [Member] | Corporate debentures / bonds [Member]
    
Available-for-sale:    
Total short-term marketable securities      
Level 1 [Member] | Marketable equity securities [Member]
    
Available-for-sale:    
Total short-term marketable securities   38
Level 1 [Member] | Money market funds [Member]
    
Cash equivalents    
Total cash equivalents 5,645 25,137
Level 1 [Member] | US Treasury securities [Member]
    
Cash equivalents    
Total cash equivalents      
Level 2 [Member]
    
Cash equivalents    
Total cash equivalents 284 5,784
Available-for-sale:    
Total long-term marketable securities 10,820 13,495
Total short-term marketable securities 36,391 28,229
Total financial assets 47,495 47,508
Level 2 [Member] | Government agency securities [Member]
    
Available-for-sale:    
Total long-term marketable securities 3,738 3,738
Total short-term marketable securities 2,180 6,912
Level 2 [Member] | Municipal bonds [Member]
    
Available-for-sale:    
Total long-term marketable securities 4,716 5,969
Total short-term marketable securities 11,732 8,384
Level 2 [Member] | Certificates of deposit [Member]
    
Available-for-sale:    
Total long-term marketable securities 2,366 3,788
Total short-term marketable securities 5,041 4,321
Level 2 [Member] | Corporate debentures / bonds [Member]
    
Available-for-sale:    
Total short-term marketable securities 17,438 8,612
Level 2 [Member] | Marketable equity securities [Member]
    
Available-for-sale:    
Total short-term marketable securities     
Level 2 [Member] | Money market funds [Member]
    
Cash equivalents    
Total cash equivalents      
Level 2 [Member] | US Treasury securities [Member]
    
Cash equivalents    
Total cash equivalents $ 284 $ 5,784